Aerodynamics of the Tailless Wing

Updated 6 Dec 2022

This tale is still emerging. While my cognizance of it at present lags even further, a bold and challenging thesis is suggesting itself. At present this essay smells rather of the modern fashion for post-truth journalism, being somewhat rude about the establishment in favour of one's chosen maverick. Like you, that is the bit that troubles me most, but it is an honest representation of my investigations to date and I would not hide it; it is my goad to do better. Besides, there have indeed been issues over which the establishment were, with hindsight, demonstrably pig-headed, and commentators both contemporary and subsequent have not been afraid to say so. With luck, this story will continue to grow in completeness, coherence and (who knows?) conviction.

Contents

Introduction

"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.

The tale of the tailless aeroplane is a remarkable one, of unsung genius, wilful ignorance, missed opportunities and seemingly 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 27 or so Dunne aeroplanes 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 throughout the 1930s to the 1980s and rediscovered 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.

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 Igor 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 it, and perhaps more. 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 I am exploring and building here. There is a parallel tale to unravel of the unswept flying wing and the likes of Charles Fauvel, but please, one thing at a time. My main thesis is already edgy enough and demanding of extraordinary evidence. Well, we shall have to see how extraordinary my evidence turns out to be.

Flying without a tail

Any aeroplane relying on a human pilot must, if it is to handle safely, be adequately stable and controllable about all three axes. Two of these, pitch (longitudinal) and yaw (directional), generally require additional stabilising tail surfaces, with control surfaces attached. But these add 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 (tailplane, foreplane or tandem wing) is described as 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. "Jack" Capper at Blair Atholl in 1907. Dunne's subsequent monoplanes had only partially-downturned wingtips, his subsequent biplanes had endplates for quite another reason. A generation later the Horten brothers produced a series of very clean monoplane flying-wing gliders, designed principally by Reimar and culminating in the Gotha/Horten 660 jet fighter. Most recently the Prandtl-D experimental drone has been flown by NASA engineers. None of these had any vertical tail surfaces. Even the wingtips of the Dunne monoplanes yielded minimal side area which, were it the only trick up his sleeve, was quite insufficient in itself to provide stability – as Northrop found out the hard way with his XP-56 "Black Bullet".

Longitudinal stability

For an aircraft to fly level, its centre of pressure (CP) must coincide with its centre of gravity. To climb, the CP is moved forward briefly to raise the nose, to dive it is moved back to lower the nose. Unfortunately, conventional aerofoils have a feature called camber, an upward bowing in the middle, which makes them asymmetrical and magnifies the movement of the CP. If unchecked, this causes a runaway effect and makes them inherently unstable. Conveniently, by adding a tail to provide a counteracting force, the runaway is checked and the plane as a whole can easily be stabilised. The tailplane must typically be set at a reduced angle of attack (AoA) to the airflow.

If there is to be no tail then there are several ways in which the wing itself can be made stable:

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, which prevent its widespread use. More often reflex camber, sweep and washout are used in combination, with results that form the technical meat of this story.

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, 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.

The problem of CG range and trim remains a severe limitation on the subsonic tailless aeroplane.

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, the reflex rear 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, lift and drag on the rearward wing are increased. The net result is directional instability. For such a wing, a tail fin is necessary to overcome the effect.

One solution, discussed later, is to combine 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 the forward fuselage or nacelle. Many designs need the fuselage or similar to protrude well forward, so that all the equipment and payload can be stacked in behind it 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 in a true fuselage-less flying wing than in 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 yaw coupling is to pitch, or even to both the others. 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. For example on a flying wing, significant yaw may build up unnoticed and the pilot unconsciously apply compensating control movements to keep the plane flying level in pitch and roll. When a manoeuvre is attempted, the control surfaces may already be too near their maximum deflection to act as required.

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.

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 aerodymanic 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 provide 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 once used and was lost.

Stalling and spinning

When a wing struggles to create enough lift, its nose goes up until the airflow can no longer follow it 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 (the designser 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. But if your wing is tailless, you have only your elevons. It is therefore vital that either the stall is confined to the wing root, or that it will naturally recover without pilot intervention, preferably both.

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 accumulates 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 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, 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 start 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.

The Zanonia fallacy

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 "washout" or reduction in the angle of incidence to the outer sections. Because the tips curve back from the centre section, 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 were José Weiss and Handley Page in the UK, and Igor 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. 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 and Page to a conventional straight wing as well as a tail. The Zanonia agenda had proved a miserable failure.

Unfortunately, that was not the last the world was going to hear of it. As early as 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, but the principle of applying positive camber at the front with reflex camber at the back was beginning to lose its bond with its origins and 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. His success may yet show that, despite being directionally unstable, the Zanonia is not inherently unsafe, but I need to study it properly. Sadly, most proponents of the "flying plank" have proved abysmal failures, and the Zanonia has another, more subtle legacy of death and destruction to which I shall return in due course.

JW Dunne

Their contemporary JW Dunne 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 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). He himself had a unique grasp of the theory at that 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.

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. In the slightest gust of wind the seeds would zigzag crazily all over the place as they flew. This was 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 was hopeless. He found a better solution in vultures and seagulls. These birds could soar for long periods, making no perceptible movement of their wings, while their small tails were not needed and were closed up and tucked away to minimise drag. Yet they could still manoeuvre adroitly when they wanted to. Dunne observed them more closely than most, watching the vultures create 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 drew seagulls in close by feeding them, allowing him to observe their manoeuvrings from close quarters. This 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.

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 produce a slight downforce, 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. 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.

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 it 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 nine 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 Bristol "Boxkite", 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 Bristol would, 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. Nevertheless, the outbreak of WWI saw Dunne working for the great armaments company Armstrong Whitworth, designing a new D.12 biplane for volume manufacture at their specially-built new aircraft factory. But War Office priorities were as blind to the Dunne as ever and 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. Had Dunne succeeded, many thousands of lives would have been saved and the RFC's compliment of airmen substantially increased.

Meanwhile Farnborough was working, despite Ministry disinterest, to incorporate what it could of stability and safety in its front-line machines. Had the Ministry 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.

The new misdirection: Hill, Lippisch and Northrop

Instinctive reflex

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 and Lippisch was hooked on the dream. But first he took time to master the novel art of the thick cantilever wing on a series of tailed Storch 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.

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. His last, giant flying-wing bombers of the early postwar period still did not fully tame the problems of pitch and yaw. Officially, the problem of undetected excessive yaw was the final straw. He never liked to sweep the wings very much and they were uncomfortably close to flying planks; I cannot help suspecting that this did not help either. After his death, his company would eventually resort to fly-by-wire computers to make a flyable bomber.

Both Lippisch and Northrop occasionally tried a simplified verson of Dunne's turned-down wingtips, but without success. 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 little more than a simple kink running along the chord of an otherwise flat wing. Perhaps this is symptomatic of the reason for their failure.

Prandtl's near-miss

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.

A Soviet interlude

Paralleling Hill, Lippisch and Northrop were a similar batch of Soviet designers and their bureaus; Chyeranovskii and Moskalyev in Russia and Kharkiv in Ukraine. Where Cheranovskii's BICh-1 was contemporary with the Hill's 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 sharply upwards, this can mimic reverse camber and stabilise the plane in pitch. In the 1970s 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. Rutan never made the 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 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 Horten

The Horten brothers were passionate about flying wings. Reimar Horten 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. However Horten was not an able enough mathematician to employ the theory until Alexander Lippisch published a simplified design method. In at least one significant aspect Horten went a step further, considering not just the distribution of lift across the span, but also that of the accompanying induced drag. He continually refined his theories and his designs, battling with the principles of stability and proverse yaw, his path eventually leading to the Horten 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 his successes with proverse yaw relied mainly on the drag properties of Frise ailerons. While in an Allied prison in 1945, he has claimed that he wrote his doctoral thesis on the subject of span loading and proverse yaw. I have yet to track it down and confirm whether this is true, but if it is then it was the first real step in understanding since the days of Dunne.

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.

In part due to the recent release of classified reports, the claim has arisen that Horten was able to design proverse yaw into his flying wings. Together with sweepback, this created a plane that was directionally stable. None of his earlier 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 later in life he was not above providing fins where necessary. His own accounts are somewhat self-contradictory, sometimes suggesting that he liked to keep his little trade secrets and gave them out only reluctantly.

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. In all this, Horten does not stand out from the general German wartime picture as unusually insightful.

If he was, as claimed and contrary to appearances, able to design proverse yaw into the wing itself and wrote his Ph.D thesis on the subject, that would mark him out as the first designer since pioneer JW Dunne to conquer the stability of the finless aeroplane. The appearance of fins on his designs invariably marked the presence of a forward fuselage or similar, so cannot be held against him. But his track record on safety unarguably falls short of Dunne's. Was this through lack of understanding or 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.

Blohm & Voss and Richard Vogt

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 close-coupled outboard tails still share many of the same issues. Intriguingly, Vogt was no stranger to Lippisch; the two of them had once co-authored a note on "Theoretische Grundlagen des Flugzeug-Berechnung" (Theoretical foundations of aircraft calculation), Flugsport 11, 1919, pp.200-672.

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.

By contrast, 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. Plohmann'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 so little 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 Émigré 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 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.

On the outbreak of WWII, the HP 75 remained unfinished. Lachmann was arrested as an enemy alien and sent to Canada. Hill too was recruited back into active work and 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 it cannot be entirely ruled 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 but 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.

Meanwhile the idiocy of its actions had 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 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, and the Manx was still earthbound. The Committee commissioned a series of tailless prototypes from companies who, frankly, had no idea what they were doing. Dunne's old colleagues and admirers should have known better.

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. Baynes, and the Slingsby company which had just built his successful Bat, would have been a far better bet. GAL built a series of gliders and powered types, with specific differences between them for comparison. 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 too far; this time it is washout that appears to have been wholly discarded, and in its place is a pronounced reverse camber. The effect on the stalling and spinning characteristics was profoundly for the worse.

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 subsequent daydreams of giant flying wings, 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. 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 itself at last 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 Tailless Aircraft Advisory Committee was long gone.

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, so could they. 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 vast forces unleashed by sonic shock waves, these three deaths were the exception that proves 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 sort of heroes. The British tailless fiasco had now reached its tragic climax and the curtain fell.

NACA to NASA

The Convair deltas

Towrds the end of WWII, Robert T. Jones at NACA (as NASA was originally called) had begun studying the thin-wing delta for supsersonic flight. He was already putting out a paper in January of 1945, before the US knew of even the thick-wing Lippisch DM-1. America's first delta-winged jet 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 to a thin-wing delta. With a symmetrical aerofoil, no tail plane would be needed, just a huge fin. Lippisch was involved in some of the discussions, but it seems that his role was more one of offering moral support than technical originality. A research prototype emerged as the XF-92A and flew in 1948.

By then, already in 1947 but too late for the XF-92A, Jones had 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 and, although the development cane too late for the Convair YF-102 prototypes, the effect can be seen on their subsequent F-102A Delta Dagger, F-106 Delta Dart and B-58 Hustler supersonic deltas. Conical camber has since been applied to further types, such as the conventional McDonnell-Douglas F-15 Eagle and the General Dynamics F-16XL tailless delta.

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) says that it was "to reduce inherent drag at high altitudes and to improve handling at the high angles of attack encountered during landing." Mason (1983) specifically that "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." Mason further noted that, "although the camber shape was developed based on supersonic flow theory, the main benefits of conical camber for drag reduction occurred at subsonic speeds." 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 the complex and troubled avionics systems let it down. In other words, the conical development improved both manoeuvrability and drag across a wide range of speeds. Around this time the UK began to revisit conical camber, if only for supersonic drag reduction. It would also later be incorporated in more conventional US types such as the F-15 Eagle.

Recalling how closely this conical treatment follows Dunne's prescription, we need not be too surprised to learn that it also helped with the inherently unpleasant stall behaviour of the swept symmetrical-aerofoil wing at low speeds. But there is nothing in the records to suggest that Jones and his colleagues knew anything of their pioneering predecessor, and indeed they arrived at the design via a very different route.

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.

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. 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. So please treat my own arguments with a touch of scepticism.

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, 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, shout through the phrasing and the choice of what not to mention. A hundred years' worth of analytical and experimental progress ought to count for something in the semantics of those glib brush-offs. 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.

Conclusion

Different designers have sought different benefits from the tailless aeroplane. Some want safety even in the stall, or ease of control, or both. Some want ultimate aerodynamic efficiency, lowering costs and improving payload-range, or increasing the operational ceiling. 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, perhaps even the flying wing, potentially offers its own characteristics to help achieve those ends.

But it is a capricious beast and you have to get things right. Although the basic theory had been worked out by 1913 and has gained only in mathematical accuracy since, 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?

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. Felix, even locked the controls, climbed out of the cockpit and went wing-walking. Nevertheless his 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.

It took Bowers' academic background, on top of many other modern advances, to finally understand bird flight and reveal the technical secrets of the all-wing and proverse yaw. They go a long way to explaining the unexpected "firecracker" performance of Dunne's monoplanes and give the lie to the contemporary criticism, still raised today, that the washed-out lift and wide span must render the wing inherently inefficient.

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. On 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 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.

What becomes clear from all this 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). 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 testing ill-conceived tailless experiments, throughout the mid-twentieth century, 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.

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 the aerodynamic 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. 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, amnually-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. 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.

But perhaps there is another reason, 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 mindset, Prandtl and Jones lacked in interest and Horten may or may not have lacked in application, 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 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, becomes an ever-increasing embarrassment to the mainstream aeronautical community.

Postscript

The beauty of Bowers' solution reminds me of yet another shunned genius, of Barnes Walls and his "wing-controlled aerodyne", a tailless variable-geometry concept which did away with not only the fin and rudder, but all supplementary control surfaces, with the wings and fuselage themselves interacting to provide all the control that was necessary. 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 designs have received great fame, it is superficial; his paper on the theory, which goes all the way back to his airship days, is yet another forgotten gem, though one which I have still to dig out. The independent and closely related discoveries in this field of his contemporary and fellow countryman LE Baynes are even less acknowledged.

The presence of a fuselage and variable wing sweep resolves the tailless wing's inherent practical problems of payload and trim (It was used by Hill on one of his Pterodactyls). 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 somewhat echoed by Baynes. While we may now have ultimate safety and ultimate efficiency combined, Nature teaches us that the practicalities of life demand a subtle compromise. The story of high promise and wilful ignorance may not be over yet.

Looking more to the future, one to watch is the TU Delft Flying-V project. 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. The key feature I highlight here is that it has only twist or washout; no reverse camber has been used. Model trials and airliner flight simulations have revealed no obvious vices, though it is early days yet.

Bibliography

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 cited 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.