Aerodynamics of the Tailless Wing

Updated 17 May 2024

The story of the tailless aeroplane is not what everybody assumes, not by a long chalk. As I began to uncover it, a bold and challenging thesis established itself. There have demonstrably been issues over which the establishment were, with hindsight, intractably pig-headed over the qualities of tailless aircraft. Commentators both contemporary and subsequent have not been afraid to say so – and been equally ignored for their pains. Academic historians have mostly followed the mainstream, eventually coming to show an unhealthy obsession with just two folk heroes; Wunderwaffen creator Alexander Lippisch and all-American Jack Northrop. It was never like that at all. As I continue to unearth more about the realities and complexities of the past, this remarkable story continues to grow in completeness and conviction.

"But the aeroplane does do these things, and if the theory does not give warranty to the practice, then it is the theory which is wrong." — JW Dunne to the Aeronautical Society of Great Britain, 23 April 1913.

" far he was ahead of his time. ... There does not seem to be much that John William Dunne did not foresee." — CM Poulsen, "Tailless Trials", Flight, 27 May 1943, pp.556-8.

“ saw this future machine and put the vision into practice!” — Geoffrey de Havilland, letter to Dunne, 1946.

The first Dunne D.7 in 1911



The tale of the tailless aeroplane is a remarkable one, of unsung genius, wilful ignorance, missed opportunities and avoidable tragedy. Only recently has it become possible to answer some of the questions that have long been in my mind and crystallise my suspicions, thanks largely to two resources; the purchase of the JW Dunne archive by the Science Museum and the tireless dedication of one of NASA's Chief Scientists, Albion Bowers. Although the latter owes little real debt to the former, the connection between their work is deep and compelling.

JW Dunne built the world's first certified stable aeroplane, the D.5, to much acclaim. It had a tailless swept wing, flew as far back as 1910, used its wing endplates not as stabilising fins but winglets, and needed no rudder. His design theory became public knowledge in 1913. None of the 29 Dunne tailless types flown ever killed anybody, a remarkable achievement for the pioneer era. His D.8 was quite possibly the aerodynamically safest aeroplane ever flown.

In stark contrast, almost all subsequent tailless designs have had unsafe characteristics to a greater or lesser extent and most have needed stabilising fins. The GAL.56 of 1944 was declared by the world's most prolific test pilot Eric "Winkle" Brown to be the most dangerous aeroplane he had ever flown; it soon went on to kill the world's most experienced tailless pilot, Robert Kronfeld. Yet the year before, Kronfeld had found the Baynes Carrier Wing (commonly known as the Baynes Bat) to be a docile and eminently practical glider. What did Dunne and Baynes know that the rest of the world chose to forget? What mistaken theory so beguiled the world for the next century yet failed to give warranty to their lethal practice?

A generation after Dunne, Reimar Horten worked steadily on the problem from the 1930s through to the 1980s, rediscovering much that had been forgotten. Like Dunne, he was studiously ignored by the mainstream and his discoveries all but forgotten once more. What on Earth did Dunne and Horten's colleagues and successors in the field think they had been doing for the near-century following Dunne's widely publicised first official certification?

Only a tiny handful of designers ever really knew what they were facing, understood the nature of the beast and kept safely within the limits of their own ignorance. Perhaps only Dunne achieved all three. He built that first certified stable machine largely through intuition driven by observation and endless trial-and-error. Reimar Horten rediscovered many of its secrets through somewhat more formal analysis coupled to that same intuition and created the first viable jet flying wing, though the evidence suggests that he never quite cracked the secret of the safe and stable aeroplane. Barnes Wallis too made significant new discoveries, though his work was cut short too soon to say if he had grasped it all.

The rest of those famous tailless names – GTR Hill, Alexander Lippisch, Jack Northrop and their ilk – remained unaccountably blind to the lessons of their forebears and contemporaries. Following the lead not of Dunne but of his debunked theoretical opponents Jose Weiss, Handley Page and Igo Etrich, their tailless aeroplanes never lost the need for a tail fin and, despite the presence of one or more such, gained the whole concept a reputation not for safety and stability but for caprice and lethality. The trail of dead test pilots many of them left in their wake is frankly horrific. Their global dominance amounts to a breathtaking institutionalised incompetence.

Then, over 80 years after Dunne rose to prominence, Bowers and his colleagues once more recognised the potential of the tailless wing, with a little help from an equally forgotten 1933 paper of Ludwig Prandtl, and began to re-evaluate, rediscover and advance the theory once more. The extent to which Horten presaged Bowers' findings is a topic of active debate. However Dunne anticipated pretty much all of the subsonic developments. His understanding is now largely vindicated by computer analyses and experimental drones, while some of his key design features still await rediscovery and re-evaluation.

At least, that is the thesis which has emerged from my studies. Such a claim is edgy enough to demand extraordinary evidence. Well, you may decide for yourself whether what follows is extraordinary enough. Most of the material here is presented in broadly chronological order. To help the non-technical reader make sense of it all, I shall begin with an intuitive introduction to tailless aircraft aerodynamics and design.

Flying without a tail

Tailless aeroplane.

Flying wing.

Any aeroplane must, if it is to handle safely, be adequately stable and controllable, and maintain trim or balance, about all three axes. Two of these axes, for pitch (longitudinal) and yaw (directional), generally employ additional stabilising tail surfaces, with control surfaces attached. But tails add weight, drag and complexity. It seems far more elegant to do away with that clumsy tail and set the wing free. If you watch seagulls, you will see that when soaring freely they tuck their tails away, bringing them out only for sharp manoeuvres, while some bats and most flying insects do not even have tails; there seems no reason why planes should be less efficient. A plane with no horizontal stabiliser (be it tailplane, foreplane or tandem wing) is said to be tailless. A tailless aeroplane without even a big fuselage to pack stuff into is a flying wing. Both these types usually keep one or more vertical fins. A flying wing without fins or anything more than a few small bulges to mar its lines has been called an all-wing design (the original German term nurflügel means "wing-only"). Even without the benefit of a tail, all such tailless types still have to maintain stability and control in both pitch and yaw. Banking a tailless plane into a turn brings its own special problems.

Only a very few designers have been able to do away with all such aids. Pioneer Lt. JW Dunne built his D.1 biplane glider at Farnborough and it was flown by his commanding officer Col. JE Capper at Blair Atholl in 1907. Dunne's subsequent monoplanes had partially-downturned wingtips, his subsequent biplanes had endplates for quite another reason. A generation later the Horten family produced a series of very clean monoplane flying-wing and all-wing gliders, designed principally by Reimar, and culminating in the Horten/Gotha 660 jet fighter. Most recently a series of Prandtl-D experimental drones has been flown by NASA engineers. None of these needed any vertical tail surfaces. (Dunne's endplates rendered his biplanes too stable, while the wingtips of his stable monoplanes yielded minimal side area which, were it the only trick up his sleeve, was quite insufficient in itself to provide stability, as Northrop later found out the hard way with his XP-56 "Black Bullet"). For these very few who made the grade, how was the trick pulled?

Longitudinal stability

For an aircraft to fly level, its centre of lift (CL) must coincide with its centre of gravity.* To climb, the CL is moved forward briefly to raise the nose, to dive it is moved back to allow the nose to drop. Unfortunately, conventional aerofoils have a feature called camber, an upward bowing in the middle, which makes them asymmetrical and magnifies the movement of the CL. If unchecked, this causes a runaway feedback effect that makes them inherently unstable. Conveniently, by adding a tail to provide a counteracting force, the feedback is opposed and the plane as a whole can easily be stabilised. The tailplane will typically be set at a reduced angle of attack (AoA) to the airflow. Alternatively, a canard "tail at the front" may be set at an increased angle of attack, or tandem wings set accordingly.

Aerofoils (from top): cambered, symmetrical, reflex.

If there is to be no additional surface, then the wing itself must be made stable. There are several ways in which this can be done:

Of these solutions, symmetrical aerofoils are not very efficient, though they have proved popular on aerobatic and early supersonic types, while forward sweep has some inherently undesirable characteristics, such as directional instability and a tendency to bend dangerously, which prevent its widespread use. More often reflex camber, backwards sweep and washout are used in combination, with results that form the technical meat of this story.

[The is an over-simplification, but it will do to illustrate the principles involved.]

**[Symmetrical aerofoils are often only stable over a narrow range, and different profiles can vary in their stabiity. A wing is only stable where the pitching moment decreases with AoA. For example the aerofoil NASA LS(1)-0013 has a small but non-zero and varying pitching moment. AAt around 120 mph (180 kph), the lift moment coefficient Cm about the quarter-chord point varies with both Reynolds number and angle of attack (AoA), between at least -0.1262 and 0.0076). LS(1)-0013 is stable at moderate AoA, but becomes unstable if the nose pitches up above about 8 deg. However the broad assumption of stability is good enough for the present purpose.]

Longitudinal trim

Any practical flying machine must be able to accommodate a range of loading conditions. Throughout the flight, as payload shifts or is dropped, and fuel from various scattered tanks is burned, its centre of gravity moves around. At the same time, its centre of lift moves under the influence of varying flight conditions. For stable flight, these two centres must be kept in close alignment. The plane must be trimmed for level flight on takeoff and periodically adjusted throughout the flight. Like the elevators, the trim tabs need a good moment arm to act effectively. Indeed, the small trim tabs necessary are usually fitted to the larger elevators, or the whole elevator is used for trimming.

But if there is no tail, how can this be done? Often it cannot, and many tailless aircraft suffer from a narrow CG range. They must be carefully loaded, the fuel tanks used up and/or munitions dropped in the right order, and so on. Any unusual payload may prove impossible or at best unsafe. In other words, they are of limited practicality.

The usual solution is to sweep the wings sharply, typically backwards, so that the wing tips can be far enough off-centre to act as a "tail at the end of the wing". But sharp sweepback brings other problems of sideways airflow and is aerodynamically inefficient at subsonic speeds. And even with all that, some types, such as Concorde, had to pump fuel to and fro to stay on an even keel. Another solution is to adjust the angle of wing sweep; it has been done, but carries a significant weight penalty.

In all this, supersonic planes benefit from being long and slender, offering viable locations for trim tabs and large shifts in fuel loading. But for the subsonic tailless aeroplane, the problem of CG range and trim remains a severe limitation on its suitability for many roles.

Directional stability

When you look at all the so-called tailless aeroplanes, primarily those of GTR Hill with his Pterodactyls, Alexander Lippisch with his Storks and Deltas, and Jack Northrop in the US, they almost invariably have to keep their stabilising fins or some equivalent surface. Yet sweepback in principle confers stability, so why the need for fins as well?

When a swept-back wing yaws, pushing one wing forward and the other back, the forward wing widens in effective span, increasing drag, while the rearward wing reduces in span and the accompanying drag, so the plane straightens itself out.

But reflex camber changes that; as one wing turns forward to meet the airflow edge-on, the reflex rear tightens its effective radius. This causes lift to reduce (just like raising an aileron). This in turn lessens the induced drag, encouraging the wing to move further forward still. Similarly, the reflex on the rearward wing becomes more gentle, so lift and drag increase. The net result is directional instability. For such a wing, a tail fin is necessary to overcome the effect. Sideways flow on the rearward wing also increases, which further enhances these effects.

Moreover, marginal stability is not really enough. Such a plane may pick up and maintain significant yaw, flying slightly crabwise. The pilot will unconsciously apply control compensation so that the plane flies level, and not realise what is happening. The control surfaces may end up too close to their maximum deflection, so that when a manoeuvre is attempted the controls cannot deliver; and worse, a confused pilot faced with such a surprise might react the wrong way, making the plane unsafe. This was a primary reason for the cancellation of the big Northrop bombers. So a reasonable margin of stability is important.

One solution, discussed later, is to introduce significant washout with droop (anhedral) at the tips, so that the surface actually creates a small amount of downforce. When the plane yaws, or is met by a side gust, the downforce and associated drag increase on the forward tip and decrease on the rearward tip, significantly enhancing the wing's natural stability.

Then there is the problem of control. With no fin, there is nowhere to put a conventional rudder, and even with a fin it is too far forward to have much effect. The answer is the drag rudder. A device capable of creating drag when operated is installed near each wing tip. Increasing the drag on one side then pulls that wing back, yawing the plane as required.

However there is one common feature which cannot be cleverly avoided by the aerodynamicist, and that is a forward fuselage or nacelle. Many designs need a fuselage or similar to protrude well forward, so that all the equipment and payload can be stacked in behind without upsetting the balance, and so that the pilot can sit in front of the wing with a clear view sideways and down. In such circumstances, an equivalent side area at the rear is needed to provide "weathercock" stability, or at least neutrality. A fin or two is often the least worst way of achieving this. Thus, a finless arrangement is more likely to be achieved on a true fuselage-less flying wing than on a mere tailless machine. The more determined proponents of low drag and external simplicity must therefore find mechanical engineering solutions to the problem of packing everything into the wing, and that often cannot be done.

Control coupling

A problem with some aircraft, especially swept-wing types, is where starting a move about one axis upsets the balance about another axis. Yaw-roll coupling, where the rudder also affects lateral roll and/or the ailerons also affect directional yaw, is known as Dutch roll. Sometimes the roll coupling is to pitch, or even between all three. As one movement follows another, dangerous oscillations can build up. Unless the pilot is experienced on the particular type, these effects can be confusing and lead to fatal mistakes.

Tailless aircraft, and those with swept wings, tend to be particularly vulnerable. Design features such as a low-set wing can help, and the designer needs to get things right. But this is a complex topic applicable to all aircraft, so I shall not dwell on it here.

Control balance

Tailless aeroplanes with little or no sweep behave in roll and yaw like any ordinary aeroplane of the same span, which may be quite large. However flying wings behave in pitch like any small aeroplane of the same overall length as the wing. The pilot must simultaneously handle a big, lumbering thing slow to respond in roll and yaw, but also a sprightly little thing that is quick, agile and sensitive in pitch. By contrast a long and slender supersonic delta may be almost the opposite – agile in roll but lumbering in pitch and yaw.

When performing a turn, it is necessary to smoothly coordinate pitch and roll, along with a touch of yaw (see later). The widely varying characteristics about each axis make the tailless aeroplane difficult to handle in the turn, and even more difficult to design for safe handling while turning and manoeuvring generally. Without any correction to the imbalance, the plane will be badly affected by Dutch roll and similar unpleasantnesses. Some palliative can be applied by making the roll control sensitive and powerful, with the pitch control sluggish and insensitive, but achieving these contrasting characteristics with a single pair of elevons can tax the less ingenious designer. Some successful high-performance sailplanes have followed this route, but such competition thoroughbreds are almost always a handful to fly and sailplane pilots must develop a high degree of expertise before they are safe in the things.

The sharper the sweep angle, the easier it is to achieve balance, but the less efficient the wing is aerodynamically. Since aerodynamic efficiency is something of a mantra among tailless designers, many have erred too far towards imbalance.

Adverse and proverse yaw in the turn

When banking into a turn, most aeroplanes have a built-in tendency to a phenomenon known as adverse yaw. The pilot operates the controls to lower the outer aileron and raise the inner. As the outer one is lowered this increases lift to raise the wing. But it also increases the associated drag, pulling the wing back. The inner wing behaves in the opposite way; raising its aileron reduces both lift and drag, so that wing skids forwards as it dips. The net effect is to yaw the nose away from the turn. This is adverse yaw.

For the plane to turn safely and responsively, it needs to head into the turn not away from it, that is to say a small amount of proverse yaw. The traditional solution is to provide a tail fin with a rudder. The fin helps to check the yawing motion, while the pilot uses the rudder to compensate for the adverse yaw and direct the nose into the turn.

Frise ailerons have a short projection of the lower surface, forward of the hinge. When the aileron is raised, the forward projection lowers into the airstream and creates drag. But when the aileron is lowered, the projection tucks up into the wing and the gap below it creates far less drag. Thus, the Frise aileron incorporates its own automatic drag rudder. These ailerons are commonplace and they do help a lot, but they can seldom fully counter the adverse yaw from the main aileron surface.

A better solution is to arrange for the main aileron surface itself to create that ideal small amount of proverse yaw, in which the banking action has the opposite effect on drag to the adverse phenomenon. In some quarters the quest for such proverse yaw has become something of its own Holy Grail, eagerly sought but ever out of reach since the day it was lost, and believed by many not to exist. For yes, it was a deliberate feature of the world's first certified stable aeroplane, but was soon forgotten.

Stalling and spinning

When a wing is overloaded or slows down too much, it struggles to create enough lift. Its nose goes up until the airflow can no longer follow the wing round and breaks away in turbulence, so that the wing suddenly and drastically loses lift, a phenomenon known as the stall. It is vital that the control surfaces remain outside the turbulent region or they will have no effect and cannot be used to recover from the stall. Often one wing stalls before the other, causing it to drop and the plane to enter a spin, a spiral dive in the stalled condition, from which recovery can be difficult or even impossible.

If you have a tail, the elevator and rudder will usually remain effective enough (though the designer may need to expend some effort in finding the best size and place for them). You can use them to correct the wing's attitude and restore lift; the ailerons can then stop the spinning. But if your wing is tailless, you have only your elevons and perhaps drag rudders. It is therefore vital that either the stall is confined to the wing root, or even that the machine will naturally recover without pilot intervention.

The problem with the tapered and/or swept wing is that the tip region tends to stall first. Taper reduces the Reynolds number at the tip, changing the flow characteristics at the critical angle of attack, while sweepback creates a sideways flow which accentuates towards the tip, causing the lifting flow to break away and the aileron to lose control authority.

Two features which help delay the stall are camber, especially at the leading edge, and, at the tip, washout. Both work by aligning the leading edge better with the incoming flow, thus delaying the point of separation. Once the plane has entered a spin, they also help with recovery. However designers often reduce the camber at the tip, thus exacerbating the problems, in order to reduce drag during the cruise.

One problem with relying on a separate tail is that the flow over the wing typically changes very suddenly, the plane snaps into a stall, most likely dropping a wing sharply at the same time and, if not immediately and skilfully corrected by the remaining aileron, entering a spin and plunging downwards. Sometimes there is no warning and the pilot is caught unawares. Another problem with the badly-placed tail is the "deep stall" in which the turbulent wing wake washes over the tail surfaces, rendering them useless.

These effects can be very dangerous and have at times been a major cause of death among pilots. The careful use of camber and washout can bring the onset of the stall inboard so that it begins at the wing root and, as it deepens, extends progressively out along the wing. Thus, there is no snap action, both ailerons remain responsive if the pilot needs them, and an inherently stable machine will be able to recover itself if the pilot lets it. Under this design condition, a tail is at best unnecessary and can actually be a liability. But sadly few tailless designers ever take this very fundamental and long-known safety lesson to heart.

Thick vs. thin

A perennial issue for aeroplane designers is how thick or thin to make the wing. The earliest pioneers used a single sheet of thin fabric, braced with wires and struts. As speed and understanding improved, the top and bottom surfaces would be separated to improve both lift and drag. A supporting framework was needed inside to maintain the shape, and this could also take some of the load, reducing the need for all that bracing.

If the wing is made thick enough, then you can fit all the bracing inside it. This creates a very clean, low-drag form known as the cantilever wing. But keeping it strong enough, especially for high-speed flight, carries a huge weight penalty. The thicker the wing, the lighter you can make it. In the extreme, a large thick wing may contain the crew, engines, payload and everything else, and it becomes a flying wing.

It turns out that, in general, thick wings perform better at low speeds and thin wings at high speeds. It's all about the tradeoff between curving the upper surface to draw the air downwards as much as possible and create lift, while avoiding flow separation towards the rear and the accompanying drag. High-lift devices can help the thinner wing at low speeds, but can be difficult to implement on tailless aircraft. At supersonic speeds the rules change though the outcome does not; in this case it is more about minimising the energy wasted by the shockwaves, and keeping the wings as thin as possible is more important than ever.

The Zanonia fallacy

Dunne's opening drawing of the Zanona seed,
which few people ever read past.

This particular fallacy has been, and remains, so widespread that I have given it its own section to deal with it.

The seed of the tropical Java cucumber, then known to botanists as Zanonia macrocarpa (though since re-classified as Alsomitra macrocarpa), has an extraordinary and unique property. Twin bracts of the seed ripen and dry to form a crescent-shaped wing, with the seed providing the nose weight needed for stable flight. Crucially, the wing is slightly curled up at the back. This creates a net washout to the outer sections. Because the tips curve back from the centre section, they are effectively swept and this washout provides stability in pitch. The seeds disperse by gliding away from the parent tree.

German investigator Friedrich Ahlborn studied this naturally stable all-wing and recognised its potential for use in aeroplanes, publishing his study in 1897. It inspired many early aeronauts, especially those who recognised the need for safe and manageable flying characteristics (a view which the Wright brothers disagreed with for a long time). They soon realised that much the same upturning could be seen in the wings of the common pigeons and crows they saw flapping about, and the design is also variously described in period literature as the crow or pigeon type.

Chief among its early proponents, during the first years of the twentieth century, were José Weiss and Handley Page in the UK, and Igo Etrich in Germany. Yet when these pioneers copied such wing forms, with or without a tailplane, they found it necessary to add a vertical tail fin before their planes would fly straight and safely. It seems those birds rely on their brains to provide the necessary "fly-by-wire" artificial stability when needed. Weiss returned to painting, Etrich to a conventional tail on his widely-manufactured Taube, and Page to a conventional straight wing as well as a tail. The Zanonia agenda had proved a miserable failure.

The truth was, the Zanonia pattern may be stable in pitch but it is directionally unstable. In the slightest gust of wind the seed will zigzag crazily all over the place. This is no doubt good for wider seed dispersal, or for a crow dodging in and out among the trees, but for a stable and safe aeroplane it is hopeless. Hence the need for a tail fin at the very least.

However, that was not the last the world was going to hear of it. From 1912, Frenchman René Arnoux began incorporating aerofoils with Zanonia type reflex along the whole of the trailing edge in a simple straight wing, to give it inherent stability. The problems of control, trim and directional stability remained and his agenda also failed, while the principle of applying positive camber at the front with reflex camber at the back was beginning to lose its bond with its origins. Arnoux would later put his talents to work for the altogether more successful Charles Fauvel. He too developed straight tailless wings with reverse camber, adding a graceful taper. Most retained a fuselage and tail. His most successful design was the twin-fin AV.36 glider, of which around 100 were built in the 1950s. For his later projects at least, he developed his own reflexed aerofoils. In the stall, breakaway and turbulence occurred in such a way as to induce a nose-down pitching tendency, which restored smooth airflow and control. This led to a characteristic gentle porpoising and the classic catastrophic stall was avoided. This was in large part down to the influence of the elevators, which were designed to be the first to stall. This destroyed the downforce they exerted to lift the nose, so the nose then dropped back down. In the US, Jim Marske has followed the Fauvel approach and even managed to damp out the porpoising, to produce unswept types which are claimed to be stall- and spin-proof. The tailfin remains essential. Their apparent success may yet show that, despite being directionally unstable, the Zanonia is not inherently unsafe and there is more than one way to fly both tailless and safe, but any analysis of the flight dynamics, even of the control balance about long vs. short axes, remains elusive. Most examples of this kind of "flying plank" have proved at best difficult to control in pitch; this is often not mentioned by high-performance glider pilots, as such thoroughbreds are expected to be on the edge of flyability, but do not be fooled by the absence of mere anecdotal evidence.

The "flying plank" experience shows that reflex camber may not be inherently unsafe on tailless types, just annoyingly in need of a tailfin, but the Zanonia has another, more subtle legacy of death and destruction to which I shall return in due course.

JW Dunne

Contemporary with the Zanonia pioneers was JW Dunne. He shared, even exceeded, their passion for a safe and stable aeroplane. He was a creative thinker and experimenter, not given to advanced mathematics (whatever some commentators claim). His approach was to make hundreds of small models and methodically tweak each one over and over to see how it flew. His writer friend HG Wells described them as being made of "cane, paper and elastic", though most of the later ones were just tiny paper planes less than 4 in (10 cm) in span and with one or two sewing pins for noseweight. Nevertheless in 1910 his D.5 became the world's first certified stable aeroplane, officially witnessed on behalf of the Aeronautical Society of Great Britain by none other than Orville Wright and his UK lawyer. Dunne rose to high office and honour in the Society, sitting on its ruling Council and earning too its first ever Certificate of Fellowship (now in the Science Museum archives). Through his endless investigations and sharp observations he had developed a uniquely advanced and sophisticated theory for the time, which he expounded to the society in 1913. It quickly became public knowledge, published not only in the Society's own journal but also serialised in Flight a few months later. Not being a mathematician, his understanding was almost wholly intuitive, born of watching seagulls, thinking everything over and experimenting with his endless models. He could confidently state in the ensuing extended discussion that, "The aeroplane does do these things, and if the theory does not give warranty to the practice, then it is the theory which is wrong." And yes, it is my thesis here that mainstream theory has remained lethally wrong throughout the following century and beyond.

Dunne was unimpressed by Ahlborn's Zanonia seed, having gone a rather different and more thorough route. He realised that the upturned trailing edges were what made the wing directionally unstable. He found a better model in vultures and seagulls. These birds can soar for long periods, making no perceptible movement of their wings, while their small tails are not needed and are closed up and tucked away to minimise drag. Yet they can still manoeuvre adroitly when they want to. Dunne observed them more closely than most, watching the vultures create rising thermals for themselves while in South Africa, and the seagulls wheeling above the ship on his voyages to and fro. Similar observations were made by contemporaries such as EH Hankin. Later, visiting the Swiss lakes, Dunne's sister May enticed seagulls in close by feeding them, enabling him to observe their manoeuvrings from close quarters. This meticulous close-up observation revealed that they banked into a turn by dipping down the leading edge of the outer wing. This had the same effect on lift as raising a trailing aileron. Indeed, the outer leading edge tended to have a permanent slight droop. This helped impart the same washout as seen on the Zanonia, but its effect on drag and directional yaw was quite different. Through experimenting with models Dunne discovered that, crucially, if the wing was swept then turning down the leading edge towards the tip made it stable in both pitch and yaw.

When he gave his address to the Aeronautical Society, the first thing he did was to debunk the Zanonia myth. The illustration and caption above are taken from that lecture. Since it is the first illustration in the transcript and nobody ever reads the text, it gave the impression that he was endorsing it and only went to create a second, parallel myth.

Dunne sent a model and a sketch to Lord Rayleigh.

In seeking to simplify the seagull's wing for manufacture, Dunne came up with a conical wing profile which washed out progressively along its span. He initially applied it to the delta wing, thus discovering late in 1904 what we now call the Rogallo wing. But such a delta is not very efficient. So in 1905 he successfully applied the conical development to a swept wing of constant chord. In fact he applied two cones on each side, one to set the washout of the outer leading edge and the other to set a "wash-in" to the inner trailing edge. This latter greatly helped in automatic recovery from the stall.

For control, rather than have adjustable leading edges like a seagull, or wing warping like the Wright brothers, he found that trailing-edge ailerons worked best. Turning down the leading edge even further at the tip would, under the right circumstances, produce a slight downforce, or negative lift. Now in a turn, when the outer aileron was lowered it reduced any downforce and accompanying drag so the wing slewed forwards, while raising the inner caused both the downforce and drag to increase, holding it back. The nose automatically swung into the turn. Proverse yaw had been discovered. This downforce was not generated all the time, but only under certain flight conditions. Many of its benefits remained present even when the net force on the tips was zero or positive. Remarks made elsewhere by Dunne show that he appreciated all these principles. By 1906 he was ready to build a full-size aeroplane, was taken on at Farnborough and his first manned glider flew as a military secret in 1907.

His 1909 patent, taken out as soon as the secret was declassified, shows how the conical development can be applied to both delta and swept planforms. All of his aeroplanes (save one civilian example designed while the cone was still secret) incorporated this swept wing with conical washout and potentially negative lift at the tips, thus also possessing proverse yaw in the turn and a remarkable ability to recover unaided when thrown about the sky.

Dunne's drawing of "reverse tangential gain"
and the resulting negative drag.

More astonishingly, his 1913 lecture even goes so far as to recognises the existence of forward thrust or negative drag from the wing outer sections at increased angles of attack, where washout is only slightly negative but the lift is still positive. He called the effect the "reserve tangential device" or "tangential gain" and it is depicted in Fig.14 of the transcript (see right). It helped restore some of the efficiency lost through the use of downforce. This phenomenon would sink without trace and fail to be rediscovered for the next hundred years! But even the most recent and enlightened rediscoverers do not seem to have yet realised how profoundly this affects the wing's behaviour in the approach to the stall, to render it far more benign that it might otherwise have been; this one is a hundred and ten years later and counting!

Since Dunne's aeroplanes were longitudinally stable they needed no tail plane, and since they were also directionally stable they needed no fin. The elevons moving together acted as elevators, while moving them differentially made them act as ailerons to roll the aircraft; carefully-balanced proverse yaw then took the plane into and round the turn. No separate rudder control was necessary either.

On this last, Dunne did however have one determined blind spot. He was convinced that the simplicity of his two-lever control system helped make it the safest that could be contrived. He was fully aware that it offered no ability to sideslip and that therefore takeoff and landing must always be made into the wind. Crosswind landings became a common cause of accidents with the Dunne type and he himself crashed at least one in this way. But he never accepted that an independent rudder control with foot pedals would reduce the accident rate for the novice pilot, a rare example perhaps of ignoring his own famous advice which prefaces this essay.

Most of Dunne's biplanes featured end plates at the tips and these are often mistaken for stabilising fins, even assumed by eminent authors to be fitted with rudders (No, I shan't embarrass you here). That understanding is quite false. His first successful biplane, the D.1, had no such endplates and demonstrated stable flight in 1907. One must recall that such surfaces characterise the boxkite, which had flown in the previous century, and some tailed unswept aeroplanes such as the canard Santos-Dumont 14-bis and several early Voisin types, and in none of those designs could the vertical wingtip surface contribute to stability. Dunne acknowledged that in the case of his swept wing the endplates did add stability, but his biplanes were on the whole criticised for being excessively stable; the fins only made that worse. He added endplates to his later biplanes in part for the same reasons that Voisin did, to reduce tip losses, but mainly because he was concerned with the effects of sideways flow induced by the wing sweep. He thought of these vertical surfaces as "side curtains", what we would nowadays call wing fences, to check the sideways flow. This both improved the overall efficiency of the wing and enhanced the effectiveness of the tip section, which included the control surfaces. The turning-down of the monoplane wing tips had a similar effect, although here he acknowledged that the additional side area was useful for stability. While he only hazily understood sideways flow and some of his intuitions have proved incorrect, he was nevertheless the first to both recognise it as a problem and develop a practical solution; his side curtains were the earliest precursors of what were later to be reinvented as wing fences and winglets.

There was another reason why Dunne wanted to do away with the tailplane. With a well-designed wing the stall would not be sudden and catastrophic but progressive and correctable, and a tailplane would add a discontinuity which could compromise that recovery. No other means of recovering from a stall was then known, and even after WWI it was a haphazard procedure strictly for daredevils. So a plane that could automatically recover itself without pilot intervention would be even better, and this was also one of his key goals. His biplanes achieved such automatic stability, while his monoplanes would initially "pancake" down in the stall and he never dared try it above 60 ft (20 m) or so for fear of it failing to recover and killing him. As it was, he destroyed so many examples while investigating their stall characteristics at low level that they began giving him nightmares.

The War Office was never more than ambivalent about Dunne's aeroplanes and, despite powerful advocacy among those who had direct experience of it, the armchair nay-sayers won the day. A single example was eventually evaluated at Farnborough. Its basic design was praised by its pilot, but it was immediately forgotten. The mainstream view was that its poor performance was down to its tailless layout, with the shape and size of the wing tips causing even more drag than a tail. This is of course the exact opposite of the truth, as everybody from the 1930s onwards knew perfectly well, but back in the day nobody could prove it. (In fact the D.8 was slow because it was designed with a low wing loading so that it could fly on an inadequate engine; by now it was obsolescent, as the Chair of Dunne's syndicate, the Rt. Hon. Marquess of Tullibardine, had publicly told everybody at the time of ordering but they had, as ever, promptly forgotten). Nevertheless, the outbreak of WWI saw Dunne working for the great armaments company Armstrong Whitworth, designing an updated D.11 biplane for volume manufacture at their specially-built new aircraft factory. But War Office priorities were as blind to the Dunne as ever and, in the rush to mobilise, the project was swept away for more conventional machines. Even after its cancellation, Dunne tried to get a basic trainer programme together by scavenging Europe for the hundreds of idle 70 hp Gnome engines made obsolete by the new 90 hp model used for war. Pilots were being trained on even more obsolete deathtraps and the attrition rate was appalling (For a masterful account, of all things get hold of a copy of WE Johns' Biggles Learns to Fly). Had Dunne been listened to, many thousands of pilots' lives would have been saved and the RFC's compliment of airmen substantially increased. The stricture to focus only on manufacturing current types as fast as possible was tragically counter-productive.

Meanwhile Farnborough was working, despite War Office disinterest, to incorporate what it could of stability and safety in its front-line machines. Had the War Office given this work greater recognition, again thousands fewer would have fallen from the skies and died in the days before pilots discovered how to recover from a spin. Those who sat back and forced the carnage in the air were as culpable as those who refused the Tommies machine guns and then set them charging en masse against the German nests.

Frederick Hill and the Belmont monoplanes

Frederick Hill (no known relation to other aeronautical Hills) also developed a stable wing, of tapered tailless form. A member of the UK based Birmingham Aero Club, he teamed up with FA Thompson and RE James to form the Belmont Aeroplane Company. It is not known if Hill was initially inspired by Dunne's 1909 patent, but in 1910 Belmont flew a quarter-scale glider to Hill's design. It flew stably and well, soaring upward on the wind and landing above its launch point. As such it was probably the largest free-flying tailless wing to date, being larger than any of Dunne's models.

Belmont quarter-scale glider

The Levis-Belmont monoplane

They went on to build a series of low-powered waterplanes capable only of skimming, then in 1914 moved to a landplane capable of flight. Bulit by local company Levis, the Levis-Belmont monoplane by now incorporated several of Dunne's patented ideas, not least a conical leading edge (although this cone had its apex at the tip). It also had a novel wing-warping control system involving variable washout/stability, which might or might not have worked. Unfortunately in 1915 it met an obstacle while taxiing for takeoff, and never flew. Its engine is on show (Object No. 1953/349) in the Science Museum, London.

Junkers and the thick flying wing

Drawings from Junkers' 1910 patent.

A more significant conceptual leap was made around the same time by German industrial designer Hugo Junkers. Up until then, wings were conceived of as thin, often just a single sheet. Junkers realised that as speeds and size increased, the fuselage and bracing would add more and more drag. But Dunne's design, published the year before, was a thin wing with pilot and engine hung beneath it. Junkers realised that you could fair in these and the bracing, thus reducing its drag, by enclosing it all within a deep wing profile. The thick monoplane wing was born.

In Junkers' patent, the fuselage was reduced to a short nacelle, projecting behind the trailing edge and formed as a lifting body with the same Zanonia-style reflex profile as the wing. It was therefore also the first true flying wing, as opposed to a mere tailless aeroplane.

These various features display radical innovation and Junkers deserves to be remembered not only as the father of the true flying wing but also of the thick-wing monoplane. It was a prophetic formula in many ways, with its various features copied many times. He would embody some of them in his own transport aircraft of the 1920s and a few subsequent experiments over the years, although neither he nor anybody else ever got to do the job fully.

Between them, by 1910 Dunne and Junkers had discovered all the basic aerodynamic principles that would underlie the design of the perfect wing. Realising it in flight would be another struggle altogether.

The new misdirection: Hill, Lippisch and Northrop

Instinctive reflex

RAF 34 aerofoil section.

During WWI, RFC pilot Capt. GTR Hill gained a profound sense of waste at the vast numbers of his colleagues killed in those uncontrolled aircraft crashes. After the war he studied aeronautics and determined to resurrect Dunne's quest for the safe and inherently stable aeroplane. He contacted Dunne, who sent him a drawing and a model of his latest thinking. Hill's first Pterodactyl followed Dunne's general plan but differed in much detail, abandoning the wingtip droop among other things. He wanted an aerofoil which was itself inherently stable and did not need all of Dunne's complicated subtleties which, the establishment argued, had made those machines inefficient. Hill knew from the work of Handley Page and others that turning up the trailing edge – reverse camber – was the only way to do this without using an equally inefficient symmetrical aerofoil. So he developed a mathematical technique for designing aerofoils with positive camber forward and reflex camber aft, such as to yield a stationary centre of pressure. Aerofoils such as RAF 34 were soon developed; the red line in the drawing shows the subtle reflex camber as the line flattens out towards the trailing edge. Hill used RAF 34 in his later Pterodactyls.

I suggest here that in doing so he discarded too much of Dunne and his seagulls, turning instead to Handley Page's and Arnoux's problematic Zanonia inheritance. Despite working right through until the 1950s, he never designed another machine that did not need a tail fin. When Page's company eventually returned to the tailless Manx, they too would fall back into their old ways.

The rot soon spread. Hermann Glauert was a British aerodynamicist with a German-born father. He was senior to Hill and was involved in the research programme which developed RAF 34. Not long after Hill published his theory, Glauert went on a visit to Germany and introduced it over there. He and German theoretician Walter Birnbaum soon developed it even further.

Meanwhile an up-and-coming young Alexander Lippisch had dreams of high-speed flying wings and was introduced to the basics of tailless flight by Friedrich Wenk, whom he helped build the revolutionary Weltensegler sailplane. A kind of halfway house between Hill and Arnoux, with a straight centre section and swept outer sections, the Weltensegler lacked much of Dunne's sophistication, was alarmingly difficult to fly, and proved one of the first tailless killers. Nevertheless it would set a trend (for example in 1930 Junkers would adopt its form for a projected thick flying wing airliner). Lippisch was hooked on the dream. But first he took time to master the novel art of the thick cantilever wing step by step, on a series of Storch swept-wing designs. At the end, he was just in time to pick up on Glauert and Birnbaum's work for his own tailless project, the Delta 1.

But Germany missed a trick. Dunne had been a great idol of many German pioneer aeronauts, a prophet rejected in his own country but much admired abroad. Back in the days before WWI, Lohner had even tried to poach him from Britain with promises of a whole design office and factory at his disposal, and when he had refused even to license his patents to them, had gone ahead anyway (though Lohner always kept the tail). But for the postwar generation of German engineers he had become a mere semi-mythical figure from the past, to be acknowledged with reverence but also total ignorance. The excising of his subtleties, perpetrated by Hill and Wenk, was not noticed and the Zanonia crow once more cackled from its perch.

Stubborn instinct

Lippisch would go on to abandon reflex camber (though for other reasons) and become both the "father of the delta aeroplane" and designer of the tailless swept-wing Messerschmitt Me 163 Komet rocket fighter of WWII. Nevertheless he continued to encounter many intractable problems and never really conquered the stability and handling issues, even with vertical fins; the Komet flew well within its designed flight envelope, but could be a real pig when it met the unexpected, such as during a bad landing, and it was far from the kind of dogfighter you could throw around the sky as you pleased. Many other advanced projects, drawing on his delta- and swept-wing work, appeared in German design offices around this time, but one has to question their viability. For example after the war Willy Messerschmitt designed the Spanish Hispano HA P-300 tailless delta glider, prototype for a supersonic fighter; the glider proved unstable in flight and when the jet project was moved to Egypt as the Helwan HA-300, Messerschmitt had to add a tailplane to make it fly right.

Northrop's YB-49 struggled to find enough fin area.
Note also the upward angle of the elevons during cruise.

Meanwhile Jack Northrop in America had been pursuing his own dream of the tailless swept flying wing. Although in occasional contact, even co-operation with his European counterparts, he followed his own ideas. He began by removing the fuselage but leaving the tail, mounted on a pair of thin booms. When he eventually dared remove the tail, the fuselage tended to return and, in a succession of experimental prototypes, like Lippisch he never really conquered the intractable problems, vertical fins and/or fuselage or no, and several of his planes killed their pilots or did their best to. By the time of his last, giant flying-wing bombers of the early postwar period he had moved to a symmetrical aerofoil with cusped upper and lower rear sections (possibly of the type known as a Joukowski transformation and seeking the elusive sprite of laminar flow). Though he allowed that washout could help, they still did not fully tame the problems of pitch and yaw. Although he believed they were adequate, he certainly never tamed the stall and accepted going into a spin as the norm. Officially the problem of undetected excessive yaw was the final straw leading to their cancellation, though his prior address to the RAeS in 1947 makes it clear he knew the need for a yaw indicator; there is a smell of the usual grabbing for a lame excuse in order to mask the politics of a bad descision. He never liked to sweep the wings very much and they were uncomfortably close to flying planks without the sophistication of a Fauvel or a Marske, nor did he ever show the slightest awareness of leading-edge droop; I cannot help suspecting that these blind spots did not help either. After his death, his company would eventually resort to fly-by-wire computers to make a flyable bomber (something that he had, remarkably, foreseen in his 1947 address, as the most practical way to fix the problems that he himself could not).

In that address, Northrop also noted a curious phenomenon, in that when the elevons were trimmed upwards this reduced adverse yaw when rolling. However it of course reduced the roll authority available. He did not mention that this tendency to trim the elevators up was deliberaltely introduced to help ensure stable flight with a symmetrical wing lacking sufficient sweep and washout. He did not appear to notice that this was the classic Zanonia feature which destabilised the plane in yaw, or that it tends to have an adverse effect on the stall characteristics. On other topics, he did offer useful insights, remarking on how endplate fins can unbalance the aerodynamics of the outer section, how leading-edge slats can be a mixed blessing, and more generally how even very small changes can turn a perfectly docile tailless plane into a killer and vice versa.

Both Lippisch and Northrop occasionally tried a simplified version of Dunne's turned-down wingtips, but not always successfully and each eventually abandoned the idea. Their approach broke Dunne's cardinal rule never to have a step-change in the lifting surface, and there appears little attempt to produce an integrated aerodynamic flow, being no more than a simple kink running along the chord of an otherwise flat wing. Perhaps this is symptomatic of the reason for their failure. Lippisch once noted a problem with eradicating flutter at high speeds, another phenomenon involving step changes in the flow regime.

Prandtl's near-miss

Elliptical and bell-shaped lift distributions.

In 1920, Ludwig Prandtl had famously published his elliptical lift distribution curve, which minimised induced drag, and thus achieved the maximum efficiency, for a wing of a given span. It had quickly become a byword in design circles and remains established dogma to this day. But then in 1933 he published the answer to a subtly different question; what is the optimal lift distribution giving the lowest induced drag for a given amount of lift rather than a given span? In other words, what was the most aerodynamically efficient, regardless of span? It turned out to be not elliptical but bell-shaped, with the lift tailing off to almost nothing before it flattened out to approach zero over the last section before the tip. Being more efficient even than the ellipse, it meant a further reduction in induced drag. Moving some of the lift inboard in this way also meant that the structural forces on the wing were reduced and, even though it had a wider span than the elliptical solution, it could nevertheless be made lighter, reducing overall weight and the lift needed to counter it, and thus yet further reducing the drag induced by that lift – a double-whammy on drag reduction. Lifting capacity is generally a greater determinant of a wing than its absolute span, while light weight is always high on everybody's agenda, so this curve was actually more significant than the elliptical one.

However Prandtl stopped at calculating the total induced drag, he did not go on to consider the drag distribution along the wing as he did for the lift. In this, he once again missed a forgotten subtlety of Dunne's – proverse yaw. Another point worth noting is that he applied his analysis essentially to a straight wing, tacitly assumed to have a tail, rather than to the tailless swept type.

Some recent writers have suggested that Beverley Shenstone, the aerodynamicist in charge of the Spitfire wing, recognised the importance of Prandtl's bell-shaped lift and designed it in. The wing itself was undeniably elliptical in shape, but it is a common mistake to assume that therefore the lift was distributed the same way, as it was very much not. For a start the aerofoil's thickness was reduced from around 13% at the root to as low as 6% at the tips, reducing the lift loading as it thinned. More unusually it also washed out progressively, to a small negative angle of -1/2 deg at the tips, so the lift distribution would have been closer to the bell shape than the elliptical. Shenstone was well travelled and well connected, he probably knew of Prandtl's work. However, other sources show no hint of any such overt design strategy, noting instead the benevolent stall characteristics the solution imbued. Frise ailerons were adopted to mitigate adverse yaw in their own right. It is conceivable that some deliberate disinformation was indulged in, to conflate the planform with the aerodynamics in a bid to keep the Spitfire's secret from the light of day. Shenstone himself was capable of pointing at the "ideal" wing shape while in the next breath explaining how its thickness and washout changed things, so that everybody has known ever since that the elliptical wing was what gave the Spitfire its superior performance and handling. Only, it wasn't the ellipse, it was the lift distribution derived from those more subtle features.+ It may be that Shenstone stumbled on a broadly similar lift distribution through a process of convergent evolution or unconscious emulation rather than any directed plan, and just not bothered to explode the "ideal" tag which so conveniently obfuscated the wing's secrets. We may never know. Nevertheless, whether imbued consciously or unconsciously, the Spitfire's supreme performance stands as testament to the soundness of Prandtl's bell-shaped curve.

+[Several attempts to improve the Spitfire's wing would later be made, the final one being renamed the Spiteful. All disregarded Shenstone's subtleties, and all fell short of the original perfect behaviour, especially in the stall.]

A Soviet interlude

Paralleling Hill, Lippisch and Northrop were a similar batch of Soviet designers and their bureaus; Chyeranovskii and Moskalyev in Russia and the Kharkiv Aviation Institute in Ukraine. Where Chyeranovskii's BICh-1 was contemporary with the Hills' first Pterodactyl in 1924, twenty years later in 1944 he was penning an unnamed twin-jet design of even more radical appearance than the Horten jet fighter being built at that time. I have little information on his aerofoils, bar a few photographs and some dubiously reconstructed drawings. However they suggest to me some unusual aerofoils, with his early designs appearing consistent with some reverse camber at the front, changing later to Hill-style reverse camber at the back. His designs of this period are not generally swept or washed-out and, even where he did adopt a delta planform with its inherent sweep, that last did not change. On or two of them, such as the BICh-20 Pionyer did prove pleasant to fly, suggesting that he was indeed doing something special to the underside of the foil, where the camera sees only black shadow. But, much like Hill, despite receiving significant recognition for his work, even on into the postwar era none of his designs would ever get beyond the experimental stage.

Aerofoils used by the others are even less clear, although one was said to be a modified RAF 38, a relatively conventional foil with the front two-thirds cambered and the trailing section symmetrical. Such a wing would be unstable, however these designs tended to feature large elevators; if the weight is well forward and the elevator trimmed noticeably upwards, this can mimic reverse camber and stabilise the plane in pitch. In 1970s America the Rutan Vari-Viggen relied on this technique, despite also having a foreplane. But in terms of performance it is a high-drag cop out, an admission of failure on the part of the designer. It was Rutan's first design and he never made the same mistake again.

Like everywhere else, a few other Soviet names appear in the records, surrounded by little more than ignorance and rumour. All in all, whether anybody knew what they were doing or not, the Soviet authorities doggedly mistrusted the tailless plane, again much as the British did, and it never saw operational service within the Union (unless one allows the Tu-144 SST with its retractable moustache).

The German mavericks

Alexander Soldenhoff

Soldenhoff was a German-speaking Swiss artist who, like Weiss in England, caught the aviation bug and while he was bitten by it painted only to raise money for his planes. He is for the most part a distraction here; his designs were idiosyncratic, in some ways building on Dunne's formula and in others departing from it. Working during the first decade or so of the great misdirection, he built some of his planes in Germany and others in Switzerland. He had mixed success with them, but his work is so obscure that what was good about them and what was bad cannot easily be discovered - he certainly did not know.

But one feature bears recording, if only for completeness. Like Dunne he sought a simple-to-fly "safety plane" and had the same blind spot over yaw control. His eventual solution was to fit small split rudder-like surfaces on the top of the wing, some distance in from the tip. They were too far forward to act as stabilisers or conventional rudders but, when operated, acted as drag rudders similarly to those which Hill and Northrop fitted horizontally to the wing trailing edge. At the same time this disturbed the airflow over the upper wing surface, reducing lift so that the wing drooped on that side and the plane banked into a neatly-synchronised turn.

The problem of plain roll control without yaw coupling would not be encountered by a novice in a stable aircraft and, unlike crosswind landings, it had never been a serious issue with the Dunne types, so there was no need to fit ailerons. However directional stability was often lacking when taxiing for takeoff or landing, so eventually he added conventional tip rudders as well. Although none of his planes ever seems to have needed to land in a crosswind, these extra rudders would have allowed it – provided they could be operated independently of the main split rudders.

Reimar and Gunhilde Horten

The Horten family were passionate about flying wings. Of the brothers, Reimar was the principal designer. He had studied under Prandtl in the 1920s. He designed and built his first flying wing in 1933, the same year that Prandtl published his bell-shaped curve. I have seen it said that Reimar was not an able enough mathematician to employ the theory until Alexander Lippisch published his simplified design method. However the Horten brothers had a sister, Gunhilde, who proved a competent aerodynamicists and mathematician. She carried out many of Reimar's design calculations for him. Why were her skills apparently not sufficient? Karl Nickel was another mathematician who became hooked when he flew the gliders, eventually both joining the enterprise and marrying Gunhilde. Might the couple have contributed more than just mathematics to the designs? Karl later wrote extensively on the subject, perhaps that may give a few hints.

In at least one significant aspect the Hortens went a step further, considering not just the distribution of lift across the span, but also that of the accompanying induced drag. Reimar continually refined his theories and his designs, battling with the principles of stability and proverse yaw, his path eventually leading to the Ho 229 jet fighter prototypes of 1945.

None of these designs ever needed fins, although it has to be said that some did not behave well and their successes with proverse yaw relied mainly on the drag properties of Frise ailerons. While in an Allied prison in 1945, Reimar is said to have written his doctoral thesis on the subject of span loading and proverse yaw, like Dunne some years after developing these ideas and putting them into practice. I have yet to track it down and confirm the details, but Reimar's is perhaps the first real step in understanding since the days of Dunne. Nevertheless, the Frise aierons remained necessary, so one must be cautious about such claims.

After the war, Reimar Horten applied to join the "paperclip" expatriate technologists in the US, and also in the UK, but was refused both. This despite working for the British for a time in Germany, in the immediate aftermath of war, and being interviewed by many in the UK industry. His rejection may in part be down to an unsympathetic summary of his knowledge by a British official interviewer. But still-raw anti-German sentiment proved the nail in the coffin; when Horten and Sir Richard Fairey came across each other, Fairey liked what he saw but his colleagues at Fairey Aviation refused to work with a German.

On the other hand, Lippisch was readily accepted into the US. One reason for this difference in perception of the two rivals was that Lippisch had fifteen years of solid financial backing and academic research programmes behind him, leading to a large pile of materials attesting to his standing, not to mention a revolutionary warplane. By comparison the Hortens had always struggled and much of their accumulated expertise was never given the resources to be properly written up, remaining in Reimar's head or notes or at best scattered across hard-to-find gliding journals. Wind tunnel funding had not been forthcoming so model testing had had to be replaced by glider flights, leading to the circular criticism that the use of gliders showed his work to be too trivial for a wind tunnel. His rather high-handed retaliation that, "I don't need experts, I am the expert" may have had some truth to it, but it lacked a visible evidential base and consequently was not believed.

Together with sweepback, proverse yaw creates a plane that is directionally stable. None of the earlier Horten designs used fins and some were indeed tame enough, though that was at least in large part thanks to their Frise ailerons. However Horten's mileage varied with different designs, only sometimes deliberately, and his accounts of them can be somewhat self-contradictory.

A comparison of wing root sections, found in contemporary technical drawings and museum specimens of their various designs, reveals that both Lippisch and Horten originally opted for reflex camber but, in the pursuit of transonic performance, by 1944 had joined Northrop in adopting symmetrical profiles. Junkers, whose flying wings originated back in the pioneer days and who occasionally still dabbled, broadly followed suit. When Horten returned to sailplanes after the war, he also returned to reflex camber. Later in life he was not above providing fins where necessary. They invariably marked the presence of a forward fuselage or similar, so cannot be held against his understanding. Nevertheless, Horten's designs do not stand out from the general German wartime picture as unusually insightful. His track record on safety unarguably falls short of Dunne's. But how much was this through lack of understanding and how much because, as so many high-performance competition aircraft designers have done, he deliberately put ultimate performance ahead of safety?

He would continue to refine his understanding and to design aircraft until the end of the 1980s. It is perhaps telling that one of his last, but unbuilt designs, the Piernifero 3 ultralight sailplane, had turned-down wingtips; his formula to date had evidently not entirely satisfied him, but had light finally dawned? Perhaps the persistence of the Zanonia's reflex camber in its aerofoil answers that question in the negative.

He eventually passed all his papers on to Edward Uden in Germany. According to Bowers, Uden went on to discover why Reimar had never really solved the problem of adverse yaw; he put the ailerons too far inboard. In a theoretical study, Uden found that the further out you place them the better, with all-flying tips being the most effective.

Thus, Uden became the first designer since pioneer JW Dunne to conquer the stability of the finless aeroplane.

Adolf Busemann

In the mid-1930s the rules of the game began to change. By then, the prospect of supersonic flight was being taken seriously. It was in 1935 that Adolf Busemann announced his discovery that sweeping the wing greatly improved supersonic performance. His work remained an academic curiosity for the next few years, until practical jet and rocket engines allowed the likes of Lippisch to push their designs into the transonic regime. It has been suggested that late in WWII he turned his attention to the delta wing, but I have not been able to confirm this. Either way, his discovery of the swept supersonic wing would go on to profoundly affect and reinvigorate the history of the tailless aeroplane.

Blohm & Voss and Richard Vogt

The B&V P 208.03.02 was refined in the wind tunnel.

But one German design team became well aware of proverse yaw. Working at Blohm & Voss during WWII, Chief Designer Richard Vogt and colleagues developed a semi-tailless wing in which the tips were moved aft on short booms to act as outboard horizontal stabilisers (OHS). This gave them more control authority than the wingtip location, without the adverse effects of greater sweepback. While not a pure tailless form, such a close-coupled outboard tail still shares many of the same issues. Intriguingly, Vogt was no stranger to Lippisch; the two of them had once co-authored a journal paper on the theoretical foundations of aircraft calculation.

B&V's first project to feature their new wing was for a single-engined pusher fighter, Projekt 208, which had no vertical fin or rudder. Vogt's deputy Hermann Pohlmann (1979) explains:

"The outer horizontal control surfaces can also be used as ailerons ... By angling the tail planes — they have to be adjusted downwards in anhedral and lift — the elevator can also take over the function of the rudder." (»Die außen liegenden Höhenleitwerke lassen sich gleichzeitig als Querruder mitbenutzen ... Durch Schrägstellung der Höhenleitwerke — sie müssen sinn- und wirkungsrichtig nach unten gestellt werden — kann das Höhenruder auch noch die Funktion des Seitenruders mit übernehmen.«)

Vogt and his head of wing design George Haag went to the trouble of commissioning a test aircraft for its control system from the Czech designer Skoda Kauba. But we do not know what that control system was.

These B&V designs have recently come under significant scrutiny. I have yet to get hold of Tipton (1995). Two studies, by Muller (2002) and Hagen (2019), concluded that the P 208 had insufficient vertical tail area. This conclusion implies that a serious fighter design presented to the authorities in fact failed in the basic necessities of flight. Yet it seems unlikely that B&V would not have obtained wind tunnel data to inform the aerodynamic design or would not have resolved any critical stability issues before putting their reputation behind the design. One must question whether such limited modern simulations, based on modern assumptions of perhaps less radical aerodynamic forms and not validated in the wind tunnel, are sufficiently convincing.

More positively, Hagen also found that the P 208 would have exhibited proverse yaw in the turn. The downturning of the tail in both pitch and anhedral are known to be conducive to proverse yaw, so the agreement between design and analysis is this time encouraging. But the problem is that they couple aileron and rudder action together (a problem which Dunne regarded as an advantage, but that is another story). So a pure yawing command would require some form of drag rudder. Pohlmann's remark makes more sense if it is referring to their action as proverse-yaw "rudders" in the turn. But we know neither the exact control provision for yaw, nor exactly what Pohlmann intended to convey. Also, we do not know the extent to which the B&V team were or were not aware of Prandtl's work but, given their very similar remarks on structural and aerodynamic efficiency, if they were not then they did a darn good job of rediscovering it and taking it a step forwards.

A series of jet-powered projects followed. To make room for the air intake these had larger noses. Moreover the stabilising effect of the pusher propeller's side area was now absent. These changes both had destabilising effects. The first, the P 209, was indeed found by Barnes (2012) to be directionally unstable. However giving it a modest increase in dihedral would have restored acceptable flying qualities. We do know that wind tunnel models of the next in the line, the P 212, were tested with and without small vertical fins added, and as a result it did end up with vestigial fins. Again, although the accuracy of limited modern analyses is arguable, the principles and tradeoffs were very evidently understood to a fair depth by B&V (Indeed one might argue that it was the obvious presence of this understanding, suggested so strongly by the designs and contemporary claims, which attracted the modern researchers).

At the end of the war, Vogt and his Head of Advanced Projects Hans Amtmann were hoovered up by the American "Operation Paperclip". However their outboard tail was studied primarily for its drag-reducing potential, especially in supersonic flight, and their work on stability, control and, possibly, proverse yaw faded once again into history.

The British fiasco

During this period, Britain's contribution descended from its leading position to become an absurd, embarrassing and ultimately disastrous fiasco.

In the mid 1930s Westland appointed a new head of design, WEW Petter. He was keen to stamp his own mark on the field and showed such weak enthusiasm for Hill's Pterodactyl that the Ministry gave up on it. The company, bereft of its sponsor, abandoned the whole idea too. Hill left in disgust for academia and faded temporarily from the scene.

Meanwhile Gustav Lachmann was a German emigré who had gained some knowledge of tailless design before he came over to work for Handley Page. Page and Lachmann had independently invented the leading-edge slat, a high-lift device which one might loosely interpret as Dunne's sharply cambered leading edge detached from the main wing in such a way as to reduce drag, and which could be retracted when not required. Lachmann now persuaded his employers to develop their own tailless research aircraft, the HP 75. This was the UK's chance to take Hill's work to the next level, but we blew it.

On the outbreak of WWII, the HP 75 remained unfinished. Lachmann was arrested as an enemy alien and sent to Canada. Hill was recruited back into active work – but then also sent to Canada, as a liaison officer. Just as the rest of the world was recognising that tailless aircraft were the natural high-speed vehicle for the jet engine then under development, and was throwing its weight into the relevant research, Britain had packed its only two experts in the field off across the Atlantic! Page eventually got Lachmann recalled and interned on the Isle of Man where he could be visited by company staff and, under these absurd conditions, allowed to resume what work he could. It was possibly this internment which gave rise to the HP 85's unofficial but highly appropriate name as the Manx, after the tailless cat native to that island.

Another tailless plane appeared around this time. LE Baynes conceived of a wing which could be strapped on top of a battle tank and used to air-tow it behind a suitable tug. A one-third scale glider was built. Its design adopted much good practice in the Dunne tradition, featuring a moderately swept and tapered wing with pronounced washout and endplate fins. There is no obvious sign of reverse camber, though I do not know enough about the design to entirely rule that out. Control was via elevons and rudders, with inboard landing flaps located forward of the trailing edge to minimise trim changes on deployment. As it happened the world's most experienced glider pilot Robert Kronfeld had also become its most experienced tailless pilot and was now with the RAF. In the summer of 1943 Flt. Lt. Kronfeld put the Bat through its paces and found it to be a docile and thoroughly airworthy glider. The programme however was ended, due to the lack of a suitable tank.

The Tailless Aircraft Advisory Committee

Why was the GAL-56a a killer?

The idiocy of its actions eventually percolated into the government's Ministry of Aircraft Production and, at much the same moment that Kronfeld was climbing into the Bat for the first time, it set up a Tailless Aircraft Advisory Committee (TAAC) to try and recover lost ground. Many of its members had known Dunne personally and become fans of his in varying degree, however only Handley Page had the slightest experience of tailless design, the Manx was still earthbound and Lachmann incarcerated. The Committee commissioned a series of tailless prototypes from companies who, frankly, had no idea what they were doing. I am puzzled by this, as Dunne's old colleagues and admirers should have known to pick better; perhaps their mindset would not admit that their mainstream understanding might still have its blind spots.

In particular, quite why they chose General Aircraft Ltd. (GAL) is unclear. The company had no experience with tailless aircraft and was busy making other companies' warplanes under contract. Charles W. Prower, one of their stressmen, speculates that GAL were chosen for their experience with heavy gliders. The design team were all Central European fugitives. They were led by Otto Wels, who had been chief engineer of Aero in Czechoslovakia. It seems quite likely that Prower's previous job interview with GTR Hill at Westland was about the team's only experience of tailless aircraft (Prower himself viewed them as too hard to get right, to be worth bothering with). It seems to me that Baynes, and the Slingsby company which had just built his successful Bat, would have been a far better bet. Be that as it may, during and after the war GAL designed and built a series of gliders and powered prototypes, with specific differences between them for comparison. They all appear to follow a formula of reflex camber inboard, long ailerons outboard and no washout. Those that flew also had twin endplate fins with conventional rudders. When we consider this formula in the light of Northrop's contemporary comments to the RAeS, we can begin to understand what happened next. The chief lesson learned was that if you have no idea what you are doing, you can quite easily kill the world's most experienced test pilot of tailless aircraft, poor Robert Kronfeld. You can also earn, from the world's most experienced and prolific test pilot of all, Royal Naval pilot Eric "Winkle" Brown, the epithet of the nastiest plane he ever flew. The evil machine concerned was the first of the brood, the GAL 56a. It looked not unlike a two-seat Bat with proper undercarriage. But its wing followed Hill, possibly too far; the effect of reflex camber, in place of washout, on the stalling and spinning characteristics proved disastrous. It stalled one day at Kronfeld's hands, and ended up in an inverted spin from which he could not recover.

Armstrong Whitworth built three examples of a large laminar-flow flying wing, the AW 52, in anticipation of an even larger jet bomber or transport. The first was a glider and the others jet powered. Like the Weltensegler and many follow-on daydreams of giant flying wings (including the Westland-Hill Pterodactyl VIII), the centre section was straight and only the outer wing sections swept. In other respects it followed a fairly typical form, with "Zanonia" reflex camber and modest washout - and endplate fins on the wing tips (I wonder whether Hill's posting to Canada had anything to do with it being offered to AW instead, but the TAAC minutes offer no hint). Ultimately it failed to advance the art. The project itself stumbled through the classic British refusal to pay for powerful enough engines and the impracticality of achieving laminar flow in an imperfect world. The design concept eventually lost credibility when Handley Page moved on to the opposite idea, the crescent wing with steeply swept centre section and lightly swept outer (and a tail), which had been developed in wartime Germany. By this time the TAAC would be long gone.

In passing, a further idiocy was enacted over Busemann. In his own words: "The English soldiers saw that America was getting all the German scientists, and they knew that the British needed some too. So they put me as one of the first scientists on the British list. But the people in Britain said, the thing that we need is not chiefs – what we need is Indians to help. ... After six months, there I was in several different places just talking about results that we got during the war. They brought me to a lot of universities, but nobody wanted me. So I finally asked my American friends whether there was still a chance to come to America. And then, because I did that by going to the American embassy, I was ... persona non grata. I was sent back home to Germany the same day, when they found out through the Secret Service that I had contacted the Americans." Three months later he left Germany once again, this time for the USA.

de Havilland's Dunne jet

Meanwhile, next up was de Havilland's DH 108, originally planned like the AW 52 as a sub-scale aerodynamic prototype of their forthcoming jet airliner. Geoffrey de Havilland knew Dunne of old and liked to goad his team with the remark that if Dunne could make it work, they could too. He did not slavishly follow Dunne or British tradition, but used a symmetrical aerofoil so that neither washout nor reverse camber was necessary. Its only concession to subtlety was retractable Handley Page slats on the outer leading edge. Together with the pronounced fuselage, this required a graceful swept tail. The jet's only problem was that it was so sleek and fast that its single engine propelled it to speeds which not only set the odd world record and broke the sound barrier in a dive, but also caused it to break up in the air. Three were built and all three killed their pilots. Brown described it as a "killer". Britain responded by recoiling from tailless aircraft for a decade or more. But its lethality was nothing to do with its tailless form; every nation around the world capable of hurling a plane forwards at such speeds was killing its test pilots, tail or no, through the huge forces unleashed by sonic shock waves; these three tailless deaths were the exception that proved the rule, they had nothing to do with the tailless design. Indeed, the tailless 108's critical value to high-speed flight research was evidenced in the manner that pilot after pilot stepped forward to fly the killer plane and gather that critical data. Geoffrey de Havilland himself lost his oldest son in the first of them and struggled deeply with his conscience to allow the others to take the same risk. For many years afterwards, every British fast jet that flew owed an incalculable debt to those three men. There are all sorts of heroes. However the British tailless swept wing fiasco had now reached its tragic climax and the curtain fell.

Barnes Wallis

After the end of the war, Barnes Wallis turned to the problem of the low-drag, high-speed jet aeroplane. He went back to basics, drawing heavily on his prewar airship days to develop equations which shed much light on the stability of the tailless aeroplane, light which his predecessors would have died for. He came up with such a radically new concept that he declared it was not an aeroplane at all but the "wing-controlled aerodyne". It took the form of an aft-mounted tailless, variable-geometry wing which does away with not only the fin and rudder, but all supplementary control surfaces. The only control movements are wing sweep and angle of incidence, independently for each wing. The wings and fuselage interact to provide all the control that is necessary; in a sense, the forward fuselage acts as the tail in both pitch and yaw. He flew two successful families of drone, Wild Goose and Swallow (pioneering drone technology in the process, but that is by the by). While his design for the supersonic Swallow has received much fame it is superficial; few have the slightest clue how it works. But, with traditional British lack of appreciation, it was all cancelled and came to nought. (The independent and closely related discoveries by Baynes and Hill in the field of variable geometry are even less acknowledged, although they worked along rather more conventional lines and would typically keep the tail.)

Light dawns at last

Leading-edge droop on the outer wing section
of a Gloster Javelin.

Although the UK abandoned the tailless swept wing for the time being, work on the delta wing from both Germany and (see below) America was compelling. To ensure stability, symmetrical aerofoils and no washout were used throughout. Fairey began work in 1946 with their thin-wing "VTO" small rocket-powered research models, graduating ten years later to a smashing of the world air speed record at over 1,000 mph by the Delta 2. Where the Delta 1 had been given a tail as an insurance policy, the pilot of the tailless Delta 2, Peter Twiss, was unequivocal in his criticism of its low-speed handling, and on one occasion crash-landed badly through no fault of his own. Avro followed a slightly different path from around 1948, with their thick-wing 707 research planes leading to the subsonic Vulcan bomber of 1952, also giving them much bigger wings to reduce their low-speed troubles well below safe landing and takeoff speeds. Meanwhile Gloster had been developing the smaller Javelin fighter, differing mainly in the presence of a tail plane. Despite this, it showed problems at the flight extremes and in 1953 the wing outer section was fitted with a drooped leading-edge glove which, one may note, mimicked conical camber. The Vulcan also turned out to suffer from such high-speed issues and was given a similar fix the next year. Avro claimed in their advertising that the high form drag of the large delta was outweighed by its benefits, not least a deep root chord allowing light weight and internal payload, no need for landing flaps or other high-lift devices, and a high maximum altitude. And given its outstanding success, even out-manoeuvring the Lightning interceptor at altitude on the one hand and winning US bombing competitions by flying nap-of-the-earth on the other, who are we to argue?

The fiasco was coming to an end. After the first crop of symmetrical-wing high-speed deltas had flown, during the mid 1950s the UK finally began to return to sanity and revisited conical camber proper, possibly on the back of previous NASA research (see below). Work started on its application to transonic and supersonic drag reduction. Of particular note is G.M. Roper, a lady working at the RAE, who studied supersonic camber and twist throughout the 1950s. Like Dunne, she applied the conical development to the upper leading edge area. The benefits for low-speed flight at high angles of attack, especially for the delta planform, were in due course rediscovered, perhaps inspired by the sharply-swept but otherwise conventional Short SB.5 of 1952, which almost inadvertently tested the effectiveness of leading-edge flaps (terminated inboard with unusual triangular root flaps). Leading-edge camber, also referred to as leading-edge droop, finally entered the mainstream, in due course appearing on Concorde, the British Aerospace EAP (Experimental Aircraft Programme) canard delta configuration technology demonstrator, and from there onto the Eurofighter Typhoon.

France learns quickly

RAAF Mirage III showing off its conical leading-edge
camber (the apparent double-delta tip is its shadow
on the underwing missile). [Source:Mediawiki]

During the war the French of course had little chance to study aerodynamics. Afterwards, they were as quick off the mark as anybody. Sidestepping the thick-wing delta, in 1953 Dassault fitted a symmetrical thin delta to a Mystère. This little MD 550 showed such promise that it was developed into a fighter and renamed the Mirage (and the MD 550 renamed the Mirage I). Trailing Convair (see below) by a couple of years, these first Mirages, including the prototype Mirage III fighter, all had symmetrical aerofoils. (It is sometimes claimed that Marcel Dassault was inspired by the remarkably similar Fairey Delta 2. This plane even underwent testing at Cazaux, France, where Dassault had a base. But this was in the same autumn of 1956 that the prototype Mirage took flight.) Still following the Convair experience, the production Mirage III was given conical leading-edge camber. This was primarily to delay the stall at high AoA and improve its low-speed handling. Dassault also investigated the use of leading-edge flaps for this purpose, but concluded that the camber was adequate, cheaper and had less to go wrong. It was found to reduce the maximum speed slightly, but not unacceptably so.

The Mirage III proved outstanding. It had well-balanced flight controls and an extremely high maximum roll rate. It became the classic fighter of the Western world, outselling all other postwar aircraft except its tailed Russian counterpart, the MiG 21, a success story if ever there was one. The Australian air force found it could still give a good account of itself against the much later conventionally-tailed American F-16 Fighting Falcon, which has since replaced it as the current classic. Whether its camber was conical and/or informed by the NACA work, as Convair was, I have yet to find out, although the fact that Dassault adopted it so soon after Convair smells of more than mere coincidence.

But the wing was not especially sophisticated in other ways; later variants added small canard foreplanes or "moustaches" to improve its high-AoA characteristics at both low and high speeds, while the next-generation Mirage 2000 would be given the more advanced leading-edge flaps instead of camber, in order to adapt the effective camber to the current flight conditions and thus eke out better performance.

It was all a far cry from the hopeless Nieuport-Dunne, a disastrous reworking of the equally remarkable Dunne D.8 by Nieuport, which they displayed at the 1913 Paris Salon.


The Convair deltas

Sunlight and shadow pick out the leading-edge conical
camber on a B-58 Hustler. [Source:Mediawiki]

Towards the end of WWII, Robert T. Jones at NACA (as NASA was originally called) had begun studying the thin-wing delta for supersonic flight. He was already putting out a paper in January of 1945, before the US knew of the thick-wing Lippisch DM-1 or of Busemann's current studies. America's first delta-winged jet, a tailless design, appeared almost by accident. In response to a 1945 USAAF requirement, Convair proposed a swept-wing design with a V tail. They then found that lengthening the root chord improved it and led directly on to a thin-wing delta of the type proposed by Jones. With a symmetrical aerofoil, no tail plane would be needed, just a huge fin. Lippisch had arrived in the US by then and was involved in some of the discussions, but his role was more one of offering moral support than technical originality. Busemann had also arrived at Langley, but I have yet to follow that up. A research prototype emerged as the XF-92A and flew in 1948. It was successful enough to give birth to the world's first production supersonic delta-winged jet project, the Convair F-102.

In 1947, too late for the XF-92A, Jones suggested giving the delta wing some camber and twist, and especially leading-edge camber, in order to reduce drag at both high and low speeds. The idea was rapidly developed by other researchers into the notion of conical camber, applied initially to the whole wing in classic biconical form but then refined down to only the leading edge. On a sharp-tipped delta this conical profile caused the trailing edge to curl downward at the tips. Although these refinements in turn came too late for the Convair YF-102 prototypes, they were applied to the production F-102A Delta Dagger. The first iteration (Case 10 or X) was somewhat conservative, but encouraging enough for later production to adopt a more pronounced conical camber (Case 20 or XX). According to Yenne (2009, p.62) this second uplift increased the operational ceiling by 10,000 ft and increased maximum speed at altitude. It also greatly improved manoeuvrability and handling at very low speeds. These three extremes – maximum altitude, sharp manoeuvres and minimum speed – all require a high angle of attack as the wing approaches the stall. In this regime, the conical camber both delayed the stall allowing a steeper angle and greater lift, while also reducing drag which in turn allowed higher maximum speed. This accounts for most of the benefits observed. Explanation for the improvement in handling goes back to Dunne's 1913 paper, although NACA and Convair had a finer grasp of the aerodynamic details than he. An even greater conical camber was from Day One built into Convair's subsequent supersonic deltas, the F-106 Delta Dart and B-58 Hustler supersonic deltas.

The conical camber is not always obvious at first glance. Because the upper leading-edge curvature naturally tightens outboard, this results in a somewhat similar curvature throughout, as the wing grades from maximum foil size at the root to maximum cone size at the tip. This appears to be because, unlike the UK pioneers, the conical camber is applied to the mean chord line and, when combined with the progressive scaling of the aerofoil, results in the upper leading-edge characteristic observed; the underside correspondingly develops a slight trough, which increases progressively outboard, ending in the downturned tip of the trailing edge. This trough can be quite evident on some machines. It offers an interesting counter-example to the widely-stated principle that the under surface of an aerofoil matters less than the upper surface. Effectively, by gouging out the undersurface in this way, control and stalling characteristics at the extremes of the flight envelope are dramatically affected.

It is sometimes claimed that conical camber causes excess drag at speeds well into the supersonic regime. This notion appears to have arisen from some analytical studies based on unduly primitive mathematical models of the aerodynamics. It is quite false. Mendenhall (1983, p.28) confirms Yenne's remarks that it was "to reduce inherent drag at high altitudes and to improve handling at the high angles of attack encountered during landing." It is possible that, in cruise at moderate altitudes where the angle of attack is low, it may have had little or no benefit but, as Mason (1983) notes, "no significant reductions in drag-due-to-lift were found in experiments" ... "there is little, if any, performance penalty associated with the conical flow assumption." When the F-106 met the Navy's new, conventional F-4 Phantom II in a flyoff, it was generally able to outfly its rival and only its complex and troubled avionics systems let it down.

Recalling how closely this conical treatment of an essentially thin wing follows Dunne's prescription (his first planes had no underside skin at all), we need not be too surprised to learn that it also helped with the inherently unpleasant stall behaviour of the swept thin symmetrical-aerofoil wing at low speeds, whether there was a tail behind it or not. But there is nothing in the records to suggest that Jones and his colleagues or successors knew anything of their pioneering predecessor, and indeed they arrived at the design via a very different route. As Mason also notes, "although the camber shape was developed based on supersonic flow theory, the main benefits of conical camber for drag reduction occurred at subsonic speeds."

Robert T. Jones and Prandtl

In 1950, around the time that Reimar Horten seems to have been making some modest advances in proverse yaw and with conical camber yet to appear on the F-102, Jones made a related rediscovery and further advance. He revisited the span loading problem according to various criteria, among other results deriving a bell-shaped curve similar to Prandtl's and with broadly similar criteria. In particular, he derived the lift distribution for minimum induced drag, for a given total lift and structural bending moment, while allowing the span to vary. He thus effectively rediscovered Prandtl's curve (of which he apparently had no knowledge).

Jones also went a step further beyond Prandtl, considering the induced drag in greater detail and deriving its spanwise distribution – probably trumping Reimar Horten's work with a further level of mathematical sophistication.

Conical camber and the F-16XL

The F-16XL had an exceptionally advanced and subtle
cranked-arrow wing. [Source:Mediawiki]

Conical camber was subsequently applied to further types. For example it was incorporated into the outer wing leading edges of the Mach 3+ Lockheed A-12/SR-71 "Blackbird" series and even the conventionally tailed F-15 Eagle.

Its value was further underlined and enhanced by the experimental F-16XL built by General Dynamics, as the Convair brand had now been absorbed into. This aircraft had a tailless compound delta or "cranked arrow" wing, especially developed "to significantly improve the benefits of wing camber as applied to a highly swept delta-wing planform."

The designers were also aware of the possibility of negative drag; Piccirillo (2014) remarks that correctly cambered wings were intended to generate "tailored leading-edge vortices ... [to] improve lift as well as provide some thrust in the direction of flight." Yes, that's negative drag, aka Dunne's reverse tangential gain, making its appearance once again. The wing's camber and twist were digitally optimised at the design stage to minimise drag during high-speed cruise and level acceleration. The outer sections had approximately 4.5 deg of washout. Conical camber was applied to the main wing section's leading edge, from the root to the kink where the sweep changes. Piccirillo also notes that the camber was "highly sophisticated" and that "This feature was designed to reduce drag at transonic and supersonic speeds and improve the acceleration characteristics of the aircraft." He also notes that the F-16XL "did not display the high induced drag at high lift and the adverse tail-trim characteristics that were usually associated with tailless delta-wing aircraft." and "any low-speed pitch-up tendency was eliminated." In other words, the benefits of conical camber were felt across the entire speed range, not just the extremes as previously understood.

Another intriguing development was the subsequent investigation which the F-16XL undertook into so-called vortex flaps, which were in effect the leading-edge flaps used by Dassault on the Mirage 2000. This time, Dassault were the ones ahead by a few years.

Bowers and the Prandtl-D

Several decades after Jones, Albion Bowers and colleagues at the NASA Armstrong Flight Research Center began revisiting the bell lift distribution and the swept flying wing. Bowers, motivated by the need for efficient flight in minimising impact on the environment, had followed the example of so many early pioneers and gone back to basics, first studying bird flight. Like Dunne before him, he put the observed behaviour ahead of the current theory. He realised that the accumulated data gave the lie to the traditional mantra of elliptical lift distribution, nor could the latter explain how birds can turn well without a vertical tail fin. Presently he unearthed Prandtl's forgotten bell curve.

By 1994 Bowers had realised that a bell distribution could in theory imbue a swept wing with proverse yaw in the turn, making a fin unnecessary. Around this time he took on board the work of Reimar Horten and Edward Uden on proverse yaw, and deepened the analysis. In 2013 he and his colleagues flew an experimental all-wing drone, the first "Prandtl-D", which successfully demonstrated its ability to fly stably and controllably, taking advantage of negative drag and proverse yaw to dispense with any tail surface whatsoever. More drones followed.

Bowers has stated his belief that none of his predecessors, not even Dunne, had seen the possibility of proverse yaw in the bell-shaped lift distribution. Consequently he does not believe that he was beaten to its practical realisation – in a manned aeroplane to boot – by just over a century. It is possible that he did not study Dunne's lecture closely enough, and of course he has not been privy to the reams of documents in Dunne's archive, nevertheless, you might be wise to treat my own arguments with a touch of scepticism - but I hope your interest will be sufficiently piqued to go check out the archive for yourself!

Where Dunne sought inherent stability, Bowers sought efficiency. His analyses have certainly taken our understanding forwards, not only in systematising, confirming and quantifying Dunne's empiricism and intuitions but also in refining his rather crude arguments requiring downforce at the tips to a more nuanced relationship between airflow deflection, washout and the resulting lift and drag. The Prandtl-D was the first aircraft ever to embody Prandtl's mathematical description of Dunne's experimental discoveries. In this, Bowers has succeeded in unifying the solution to the two parallel, century-long quests for stability and efficiency.

And yet Bowers complains of another of Dunne's bugbears – no matter his professional standing, the mainstream refuse to take his discoveries seriously. This despite his rising to become the research centre's Chief Scientist (just as Dunne sat on the ruling Council of the Aeronautical Society of Great Britain). It's the same old story; we don't quite understand what you insist on banging on about, it contradicts our own entrenched theories, so we refuse to believe the benefits you claim; here is a list of the same old objections you just debunked but we skipped that, the idea needs more research anyway. He has described it as "a lost cause". All this despite a century of international research, the demonstrable success of tailless supersonic aircraft and the rock-solid data gathered by Bowers.

I might have more sympathy for the mainstream if their criticisms of Bowers did not read as if they had copy-pasted their predecessors' criticisms of Dunne word for word from some period journal. The same not-invented-here psychology, the same Great Man syndrome, the same diversionary tactics from one's own ignorance of what one has not studied on the basis that it would "obviously be a waste of time to read such rubbish", and the choice of what not to mention. A hundred years' worth of well-documented analytical and experimental progress ought to count for something more than a glib and vacuous brush-off. That it does not is telling, damning even.

There is at least a ray of hope. Bowers's social skills appear at least a little more effective than Dunne or Horten's. Before he retired he year-on-year built up a community of interns who will hopefully, as their careers percolate through to the mainstream of aeronautics, carry the truth with them.

The near future

The Northrop Grumman B-21 Raider

Northrop seem to have finally learned, be it by accident or design. The new Northrop Grumman B-21 Raider stealth bomber offers an intriguing endorsement of this analysis. Hundreds if not thousands of iterative computer designs were undertaken, and are visibly seen to have converged on these classic historical features that are generally ignored or forgotten by the mainstream, despite the exact parallel having already occurred with the F-16XL programme.

The tailored airflow over the centre section echoes Reimar Horten's deep concern for the subtleties of the "mitteneffekt" (middle effect), which he never entirely resolved. He also wanted to improve the engine installation to a more conformal design.

Those intakes remind me somewhat of the NACA duct used for many small auxiliary intakes in the cold war era and trialled as an engine intake on the North American YF-93. No doubt the B-21 version is far more advanced, but the principle appears to be the same, and is I suspect the secret to decelerating the leading-edge flow at transonic speeds and thus enabling economical supercruise.

The wing outer sections have pronounced washout and conical leading-edge droop. As we have seen, these features may be traced back through the majority of successful tailless swept wings, right back to the Dunne machines of the pioneer era and his 1909 patent. Jack Northrop studiously ignored the lessons to be learned and, while the F-16XL began the recovery, NASA only publicly caught up in this millennium with Jonathan Bowers' PRANDTL-D. Clearly, the key benefit for the digitally-controlled B-21 is not inherent stability (that can be done digitally) but Prandtl's minimal weight and drag. Once you sweep the wing and tailor it for minimum drag, stability and proverse yaw come with that package. All the way through from Dunne to Bowers, the mainstream believed that these design features increased drag. Now, we see that they actually enhance the B-21's low-drag characteristics. Welcome at last, Jack Northrop's ghost, to the world of NASA and General Dynamics.

The question I ask myself now is, did Northrop Grumman methodically take forward all these long-known but also long-obscure aerodynamic features so studiously disregarded by their founder, or did their computers arrive at something they knew nothing of beforehand? Perhaps they studied the F-16XL, if not then perahps some of Bowers' former interns will one day join the dots. Either way, the B-21 looks like becoming the latest vindication of all those maverick forebears.

The TU Delft Flying-V

Another one to watch is the TU Delft Flying-V, conceived by Justus Benad. A sharply-swept flying wing, its constant-chord main section and reduced-sweep tip section hark back to the Blohm & Voss outboard-tail days. But it is unique in having no aerodynamic discontinuity, other than the change in sweep, between wing and tip surface. As such it represents a kind of halfway house between a tailless wing and an outboard tail, with a dash of the lifting body thrown in. The key feature I want to highlight here is that it has only twist or washout; no leading-edge droop or reverse camber appear to have been used; neither additional virtue nor vice designed in. Model trials and airliner flight simulations have revealed no obvious vices, though I am concerned that its steep angle of sweep may lead to inefficiencies from sideways flows, or perhaps high tip loadings and a tendency to flutter similar to that which put Lippisch off.

There is a perhaps more significant issue over the planform. Earlier I compared the English Electric Lightning to the Avro Vulcan. In planform, the Lightning can be seen as a delta wing with a sharp gouge out of the trailing edge. Its contemporary, the Saab J35D Draken interceptor, was a tailless double-delta type with much the same engine. The Lightning gave much the same performance, but for far less range because it needed twice as many engines! It was heavier, with a higher wing loading, short on fuel, and less manoeuvrable at altitude. The Vulcan filled in the gouge to avoid these and related problems, with Avro taking out a series of full-page advertisements to explain why. Given the remarkable qualities of both Vulcan and Draken, one has to wonder whether the same treatment would benefit the Flying-V. But passenger cabins are not weapon loads, and it is early days yet.

(Unrelated to the tailless design is the positioning of the engines just above and behind the wing. They sit in the disturbed airflow at the onset of the stall, leading to loss of thrust just when you need it most. Then, there is the risk of them breaking free in a crash and falling forward into the passenger cabin, necessitating an extensive high-strength mounting framework; the Lockheed Tri-Star's central mounting was stressed to 40G. A more conventional position below the leading edge might be wiser, trading off the much lighter mounting for a stalky undercarriage and sacrificing some of those paper efficiency gains.)


Compare the Dunne D.7 at the top of this essay with the NASA PRANDTL-D below and ask yourself, apart from a little streamlining and structural engineering, and the move of modelling from paper to digital, what have we really learned in the last 100 years? And why did everything go so horribly wrong in between?

Different designers have sought different benefits from the tailless aeroplane. Some want safety even in the stall, or ease of control, or both. Others want ultimate aerodynamic efficiency, lowering costs and improving payload-range, or increasing the operational ceiling. Yet others seek high speed for minimal drag. Many of the features discussed here will help towards one end or another, whether with or without a tail. The tailless approach, even the flying wing, offers its own characteristics to help further those ends.

But the tailless aircraft can be a capricious beast and you have to get things right. Although the basic theory had been worked out by 1913 and for the next half-century gained only in mathematical accuracy, the ease with which a mediocre design could be salvaged by slapping on a tail meant that nobody in the mainstream ever bothered to take the time to get things right. Or, to put it another way, if you stopped to do so then you were soon left behind in the technology race.

The benefits of inherently stable flight are obvious enough for low-tech craft unaided by complex computerised flight systems, such as primary trainers and light sports types. And of course in the days before pilots discovered how to pull out of a spin, it would have been a massive life-saver. But what are the benefits of proverse yaw?

Cmdt. Félix discusses the D.8 in which he flew
the Channel in a storm and went wing-walking.

Dunne conquered both stability and proverse yaw, and his biplanes at least were unstallable. A Dunne machine was the first to fly through a storm and arrive at its destination unscathed, crossing the English Channel and making headlines around the world in 1913, only four years after Blériot first did so on the calmest of days. Its pilot, Cmdt. Félix, even locked the controls, climbed out of the cockpit and went wing-walking over Paris. Nevertheless the Dunne aeroplanes would have benefited from rudders of some kind, not to correct but to induce sideslip. This manoeuvre is not only useful in losing height while travelling only a short distance when needed, but is also essential in landing across the wind. Dunne learned to fly downwind of his landing site, then turn into the wind for the run in. On occasion he was forced into a crosswind landing and, unable to stop his plane crabbing sideways over the ground, crash-landed badly. The obvious solution on his biplanes, so obvious that many assume it to have been done as a matter of course, would have been to add independent rudders to the endplates. Swinging one rudder out would induce both a side force and a drag force. Some later twin-finned tailless aircraft have successfully included such an arrangement. But he was too fixated on simplicity of control for the untrained pilot, ever to consider adding a rudder bar.

Horten's unbraced monoplanes seemingly took Dunne's formula to its ultimate conclusion. Yet unlike Dunne he never really conquered proverse yaw, relying on Frise ailerons and drag rudders to coordinate turns. His understanding remains debatable. It is unfortunate that when questioned he changed his tune on occasion for one reason or another, while Lee's and Bowers' accounts of him are a little too apt to reference each other in a disturbing circularity. I need to track down the truth about that Ph.D thesis, which he obtained from Göttingen in 1946.

Can the death toll and general record of accidents and incidents in tailless aeroplanes tell us anything? No Dunne machine ever lost pilot, passenger or bystander. Hill's record too, though smaller, is blameless but lacking in certain areas. Lippisch, Horten, Northrop and a good many imitators – well into the supersonic era – have far sorrier track records. In a bid to get to the bottom of this I have been compiling a spreadsheet of every tailless type ever flown, as well as a few other salient designs. It is early days yet and I have yet to fill in enough data for a significant result, but there is undoubtedly a correlation between design approaches and fatalities. A decent amount of sweep with washout is a sound basis for subsonic success, just as Dunne taught. At high speeds a symmetrical aerofoil behaves well - providing it also has washout or some form of outboard high-lift device for takeoff and landing. The big creators of handling problems are shallow sweep leading to a very short airframe, and/or reflex camber; even all-flying tip surfaces will not fix these problems, as Hill unwittingly demonstrated several times. Building out the fuselage too far forward without an adequate tail fin, or placing the CG too far aft, are also dangerous, though they are not really tailless wing issues per se.

The evidence supports Dunne's thesis. He did not do away with the stabilising tail, rather he moved it out to the ends of the wing in order to do away with the aerodynamic discontinuity between the two surfaces. This in turn was done in order to tame the stall, to ensure that the plane would always recover from any disturbance however drastic. His recipe works: the practice does give warranty to the theory. No other recipe does.

What also emerges from the record is that in the subsonic regime, the establishment's cold-shouldering of the stable, unstallable aeroplane and of the underlying theory has resulted in countless thousands of avoidable deaths. Had Dunne types been used as primary trainers in WWI, as he proposed, the huge death toll among trainee RFC pilots would have been drastically cut. Moreover, the planes would have lasted longer and the acute shortage relieved far sooner. Had his principles been incorporated with greater urgency and completeness in the observation, reconnaissance and bomber types, with or without tails (and he patented both), further great savings would have been made. Fighting scouts are more of a moot point, for the manoeuvrability of the safe wing has never been properly investigated (though the Spitfire goes a long way to reconciling the two and some of the early tailless delta jets such as the Douglas F4D Skyray proved outstanding dogfighters). And throughout the middle decades of the twentieth century, the lives of so many test pilots would not have been sacrificed in vain. The truth was there to see, and had been since 1913. Had it been taken seriously by the mainstream and followed up, all those individuals who lost their lives to the folly might yet have been saved. It was all an utterly avoidable waste. The wilful institutional ignorance of Dunne's life-saving discoveries, generation after generation after generation, is the truly shocking scandal of the whole affair.

If you doubt this analysis, then consider again the work of NACA/NASA and Convair/General Dynamics, dragging conical camber and washout into mainstream manufacture. The studies which led directly to the F-16XL advanced the art by leaps and bounds, rendering the technical basics unquestionable. It all goes a long way to explaining the unexpected "firecracker" performance of Dunne's monoplanes and permanently explodes the contemporary criticisms, still raised today, that washed-out lift, leading-edge droop and more-than-minimal span must render a wing inherently inefficient.

It took Uden's and Bowers' academic background, on top of many other modern advances, to finally quantify Dunne's intuitive understanding of bird flight, and reveal the technical secrets of the all-wing and proverse yaw.

But the majority of these secrets, from conical camber to negative drag and proverse yaw, never found their way into the textbooks (that I have ever found), and so only a handful of academics and initiates ever learned of their benefits. Most professionals have never even heard of them, never mind realised that in the fast jet world they are now the mainstream; creeping out of digital optimisations even when the aerodynamicists have no idea why.

The advent of fly-by-wire changed the landscape. Whether tailed or fuselaged or no, provided the basic design is viable, nowadays most if not all of the desired flight characteristics can be programmed into a flight control system. Some years after Northrop's death, his company succeeded with the B-2 stealth bomber by using computers to make up for its aerodynamic shortfalls. So why bother with s subtleties any more?

The technical answer is threefold. Firstly, the quest for ultimate aerodynamic cleanliness is also the quest for ultimate efficiency, yielding the lightest weight and lowest drag for any given payload – and hence also the maximum speed and range, lowest operating cost and least damage to the environment with any given engine. The more informed fast jet designers have known this for decades, the rest of us have some catching-up to do. Secondly, proverse yaw can greatly improve the handling characteristics of such highly efficient airframes, especially when safety is paramount, such as in the stall or when landing in a crosswind. This is of obvious benefit to the pilot of lower-cost, manually-flown types, while even for complex, sophisticated and expensive digital flight systems, it can significantly reduce the workload demanded of the control computer, yielding a faster time to market and a lighter, cheaper, more robust system. Then again, the plane must still fly and land safely even if the computers get knocked offline. Thirdly, the military quest for stealth is also the quest to remove any and every extraneous feature from the physical shape. That includes not only the tail, but also the flashes of reflection produced by control surfaces. Proverse yaw helps too in this last, as does an efficient and controllable airframe requiring minimal control movements; here, tailless is better than tailed, the flying wing better still, the finless all-wing stealthiest of all.

And perhaps there is another reason why all this matters to us, one which stirs the deepest passions and drives its protagonists the hardest whether they be of mathematical or practical bent. It is, simply, the sheer beauty and sense of awe that arises through the conceptual simplicity of the all-wing form and the elegance of its ultimate realisation.

While Dunne lacked in analytics, Prandtl and Jones lacked in interest and Horten lacked in his aileron positioning, the truly practical all-wing aircraft remained elusive. Bowers has brought the all-wing closer to its ultimate perfection, in which only the inherent drawbacks of fitting everything inside a slim aerofoil, and then balancing it within a very narrow range, now restrict the roles in which it can at last fulfil its promise. He has since retired, but others are already beginning to fly new drones and push his work forwards.

Where ignoring Dunne cost thousands of lives throughout WWI and on into the jet age, ignoring Bowers is costing the environment every day from here on in. In an age where Greta Thunberg can become a superstar just by telling the truth, this century-long institutional cold-shouldering, of the safety and wellbeing of humankind and planet alike, becomes an ever-increasing embarrassment to the mainstream aeronautical community.

While Bowers may at last have combined ultimate safety and ultimate efficiency, Nature teaches us that the practicalities of life demand subtle compromises. Wallis showed us that the presence of a fuselage and variable wing sweep resolves the tailless wing's inherent practical problems of payload and trim, even that a tail fin is not necessary in order to compensate for the side area of the forward fuselage, if you get things right. I am minded too of the seagull, which tucks its tail out of the way while soaring and only deploys it when manoeuvring sharply (an approach explored by Baynes' variable-geometry studies); the story of high promise and wilful ignorance may not be over quite yet.

The new millennium is bringing a sea change in our approach to knowledge and understanding, in the open-minded questioning and reappraisal of any and all twentieth-century dogma. Social media and #metoo have a lot to do with that, but the phenomenon has spread far more widely; the dark fin-de-siècle malaise of the millennium has given way to a new zeitgeist, a new spirit, of bright hopes in the face of adversity. And as we have seen, reappraisal of the tailless dogma is long overdue. Let us go to it with a will.

NASA PRANDTL-D No. 2 in 2016. [Source:Mediawiki]


In order to follow the text, this (somewhat incomplete) bibliography is presented broadly in order of the discussion, rather than the usual alphabetical by author. Several of the sources are in German; I make no apology for this, as a significant part of the story took place there and has yet to make it into English.

Basic theory