!doctype html> Rotors Unlimited

Rotors Unlimited

Unthinking schoolchildren have long indulged in a curious classroom trick. The only prop needed is a foot ruler (“30 cm ruler” does not have the same ring to it), made of a light but reasonably rigid and shatterproof material such as wood. It is held sideways on with the fingertips and thumb of one hand, palm down, such that the lower edge is held by the thumb a fraction further away from your body than the upper edge. It is then launched by simultaneously releasing the thumb and giving a jerk downwards and forwards with the arm and wrist such that it flips away forwards while spinning rapidly backwards with a satisfyingly deep and powerful whirring sound. Surprisingly for those who have never seen it, the ruler does not crash to the floor but rather zooms away like a large, angry insect and may briefly gain height. Typically it banks progressively steeply and rapidly over to one side, curving in that direction like any aeroplane in a turn, moments later ending its short flight with a clattering dive to earth. Although the sideways tumble shows that such a simple device is not directionally stable, far more significantly the gain in height and the curving into the turn as it banks prove that it is undeniably creating lift. This classroom trick thus demonstrates the principle of the wing rotor, one of a large but little-known family of lifting rotors.

Lifting rotors

The aerodynamic lift on spinning objects has a long history of study. As a simple toy comprising two feathers stuck into a soft central hub and spun up by hand or wound string, the helicopter rotor is the most ancient form known. For much of the nineteenth century it was as strong a candidate for the practical flying machine as were fixed and flapping wings. Less well known is the fact that horizontal-axis rotors of various kinds also began to be studied in this period, and for a while looked as promising as the others. Indeed, the first aeroplane design believed to be inherently stable and safe in flight used auxiliary rotors.

Some types, as both main wings and auxiliary devices, have been rediscovered several times. Their discoverers and commentators have used a variety of terms to describe them, often in ways incompatible with each other; popular usages of one evocative word or another also run riot over any attempt at systematisation. I have therefore adopted what I feel to be a reasonably systematic naming convention of these rotor types and the aircraft which employ them. In particular, I avoid the somewhat ambiguous term "rotor wing" in favour of either the wing rotor (a particular form of horizontal-axis rotor) or the rotary wing in general. This last term is commonly defined as a helicopter-type rotor blade, but a more general meaning to describe any lifting rotor helps to disambiguate the rotor wing and cannot be said to be wrong. So if you come across these terms in other writings, be warned that they may not mean quite the same thing as they do here.

There are also many hybrid convertiplane and powered lift designs (often referred to incorrectly as rotor aeroplanes), in which a rotary wing stops lifting in forward flight and does something else instead, or stops rotating and becomes a fixed wing. For the sake of my own sanity, if for no other reason, these are mostly not discussed here.

Rotary wings

A rotary wing is a lifting rotor or wing which spins to create aerodynamic lift. Some have blades which stick out, forming a disc as they spin. Others have blades which lie along the axis, instead forming a cylinder (indeed, some are cylinders). A few kinds are more complicated, with surfaces spinning around secondary axes which rotate around the main axis. Some require forward motion to generate lift, just like a fixed wing. Others can create lift simply by spinning, allowing the craft to fly vertically. Together they form a weird and wonderful menagerie of flying machines, many of which have been let out of their cages and flown at one time or another but only a few of which are commonly domesticated. This note is a walk around the zoo.

All lifting surfaces, whether fixed, flapping or rotary, rely on the same fundamental principle to generate lift. Two components of airflow must be created, one across and over the surface and the other circulating around it; up over the leading edge, back and over the top, down behind and forwards again underneath. When the two flows are superimposed the effect is to accelerate the net airflow over the top and slow it underneath. According to Bernoulli's principle this creates pressure changes which, in turn according to Newton's laws, draw the air downwards and, in reaction, lift the wing up. The principle as a whole is known as the circulation theory of lift.

But different types of rotor create these two flows in different ways. A fixed wing must be thrust through the air and is shaped to generate its own circulation. Some rotors use their spin to push a rotary wing through the air, as a fixed wing spinning around one end. Others require the whole rotor to be thrust through the air by some other means. Some use the spin to create the circulation directly. Yet others combine things in ingenious and sometimes complicated ways.

A convenient way to start classifying this plethora of lifting rotors is by the alignment of the spin axis. Vertical-axis rotors are the familiar egg whisks of the helicopter and autogyro. A horizontal-axis rotor may have its axis aligned fore-and-aft to the direction of travel (longitudinally) or sideways-on (spanwise).

Vertical-axis rotors

A set of long blades spin in a horizontal plane known as the rotor disc, around a central hub> These blades thus fly through the air to create lift like any other wing. Indeed, the blades are sometimes referred to as rotary wings. These rotors are so dominant that the term rotorcraft is often used to denote them exclusively, although certification authorities tend leave the definition of a rotorcraft open to all types of lifting rotor. There are two main kinds of vertical-axis rotor, which look similar and can even do each other's jobs, but have different flight characteristics.

Helicopter rotor: Driven under power. Like any powered wing, it draws down the air flowing over it, creating a reaction which lifts the rotor up. Thus, air flows downwards through the rotor disk. Such craft can operate vertically. It's a bit complicated, but a net forward thrust may be achieved by tilting the rotor disc in the appropriate direction. The sideways thrust component is created not just from the tilt angle, but also from the cyclical airflow effects over the individual blades. There are two physical approaches to the tilt:

Autorotating rotor: Relies on the air blowing over them to make it spin. In free flight the blades act just like the wings of a glider, with the craft descending gently in similar fashion. This is necessary in order to extract work from the air and keep the rotor spinning. With these rotors, the air flows upwards through the rotor disc, which tilts the other way from powered rotors; for example when flying forwards the autogyro's disc angles backwards. Tethered kites and gyro gliders are wholly unpowered. Autogyros have a source of powered thrust to keep them moving and are capable of powered flight. They still have to keep moving, so they need a runway to get up flying speed and cannot hover; vertical takeoff or landing can only be carried out as a party trick. The autogyro is the lightest and most compact form of winged flying machine known and, since its development in the 1920s, has consequently retained a niche presence.

The gyrodyne is a hybrid in which the rotor can operate in either mode, taking off as a helicopter and flying under separate power as an autogyro. Like the remaining types in this list, it has proved no more than a curiosity.

Longitudinal-axis rotors

These are weird things, best to get them out of the way before digging deeper. The basic horizontal-axis rotor is just a propeller, a helicopter rotor tipped over, as on the V-22 Osprey in forward flight. It does not generate lift, only thrust. But the rotor can be made to generate lift in this position as well, as a radial-lift propeller. It does this in a similar way to a helicopter generating forward thrust, just laid on its side so it is upward thrust instead. The rotor requires forward motion to provide lift, it cannot hover. As with the helicopter, there are two ways to produce the necessary cycling of the blade pitch, by tiliting the rotation axis up or by providing cyclic pitch control. There are two broad classes of longitudinal-axis rotor, which differ more in the division between lift and thrust than in their principles of operation:

Radial-lift propeller: Primarily generates thrust, and can be used to create additional lift for short-takeoff (STOL) performance. The Vought V-173 tilted the main axis to obtain some vertical lift and STOL performance. Cyclic-pitch examples were fitted to its successor, the prototype Vought XF-5U Flapjack. But, in a stroke of bureaucratic stupidity, this was cancelled and broken up just as it had been prepared for its first flight.

Self-propelling wing: Primarily generates lift as a wing, and spins as a rotor to screw itself through the air. Such an aircraft is sometimes called a helicoplane. The Focke-Wulf Triebflügel VTOL interceptor proposal of WWII is probably the best known example; this convertiplane would have taken off vertically as a tail-sitter or helicopter, and then keeled over to the horizontal for forward flight as a helicoplane (unlike the Osprey with its additional fixed wing, the rotor would still be providing all the lift)./p>

Some rotors normally arranged spanwise, such as the wing rotor, have also been used as longitudinal auxiliary devices to a fixed main wing (see below).

Spanwise-axis rotors

This is the biggie, where all the complications and variations on a theme run riot. The rotors described above all have blades or wings sticking out radially from a central hub, but spanwise rotors do not. Instead, the blades run parallel to the main axis of rotation in a generally cylindrical arrangement. A craft using such a rotor as its primary lift may be called a rotor aeroplane (be warned that many people use this term indiscriminately to describe any novel convertiplane or stop-rotor they currently have the hots for). Spanwise-axis rotors can operate on any of three basic principles, with the Magnus type having three significant sub-types:

NACA/NASA in particular have studied a wide range of spanwise rotors on many occasions from the 1920s through to at least the 1990s, both as primary wings and as auxiliary high-lift devices.

Magnus rotors

When a spinning body passes through air at right angles to its axis of spin, it experiences a sideways force in the third dimension. Although the effect on musket, cannon and tennis balls had long been known, Gustav Magnus first observed it on a spinning cylinder in 1852. If air passes across the cylinder at right angles to its main axis, a force is exerted at right angles to both directions, in the third dimension. Lord Rayleigh soon adopted the idea to explain the swerve of tennis balls.

In the case of a lifting wing, the rotor is thrust through the air, with its axis spanwise to the air flow. It is given backspin (spun top-backwards) to promote the circulation of airflow around it, accelerating the flow above it backwards, and slowing the air underneath. This interaction of forward and circulatory flow components is the fundamental mechanism of lift creation in all winged aircraft. By spinning the rotor fast, the circulatory effect is increased and sufficient lift may be generated at low airspeeds to give an aircraft good STOL performance.

The Magnus effect applies to a wide variety of shapes, the rotating body does not need to be a cylinder or ball.

The cylindrical Magnus rotor and its direct derivatives require engine power to drive them; in a free airstream the cylinder is one of the few kinds of rotor which will not autorotate. However most other forms can autorotate.

The more significant lifting types are:

Flettner and Thom rotors

Flettner rotor: comprises a Magnus cylinder, with a disc endplate at each end to reduce loss of efficiency at the ends and significantly improve lift performance. The arrangement was suggested by Ludwig Prandtl, and first implemented in upright alignment as a ship's sail by Anton Flettner. The American Plymouth A-A-2004 floatplane had port and starboard Flettner rotors in place of the main wing and achieved short flights in 1924.

Thom rotor: Similar to the Flettner type, but has additional discs at intervals along the rotor. These additional discs can significantly enhance performance, it is thought by entraining more air into the circulatory pattern and/or reducing parasitic turbulence and vortices. However they also cause drag, so each type is the better choice in different circumstances.

Wing rotor

As used here, a wing rotor is a symmetrical aerofoil-style surface which spins about its long axis, like the ruler which introduced this essay.

Nature provides us with a natural wing rotor in the seed pod of the Robinia pseudoacacia, also known as the False Acacia or Locust Tree. In the 19th century James Clerk Maxwell and Friedrich Ahlborn studied how flat plates spontaneously spun and drifted sideways when dropped. From around 1901, JW Dunne investigated many horizontal-axis rotor forms and developed the planar wing rotor, both as a lifting rotary wing in its own right and as an auxiliary device to enhance the performance of a fixed wing. Many other variations, some tapered along their span, have since been studied, especially by NACA/NASA.

Among lifting rotors, the wing rotor is unusual in that it can be stopped to act as a conventional fixed flying surface though, being typically symmetrical fore-aft and essentially uncambered, it is not a very efficient wing in this mode. Dunne proposed such variable modes in 1904, suggesting that auxiliary wing rotors need be spun up only when required for additional stability in rough weather, or for enhanced control. NASA have proposed drones which fly as fixed-wing aircraft, then spin up to land as STOL wing rotors.

Cross-flow fan

The rotor or fan of a cross-flow fan comprises a set of blades running the length of a cylindrical form and radially aligned. Air can flow freely across the middle of the rotor. When placed within a suitable asymmetric duct and spun, the rotor creates a net airflow along it. The duct can be one-sided, with the fan only partially enclosed. The arrangement is familiar to an older generation as the rotating blade of the cylinder-type lawnmower (though not spiralled), and I have found such fans inside small domestic electric fan heaters. By varying the duct angle and aerodynamic profile to direct the air in the desired direction, STOL and even VTOL performance can be achieved. Two variations are of note:

FanWing: A proprietary cross-flow fan configuration in which the half-duct also acts as a lifting aerofoil surface. This arrangement can simultaneously develop both thrust and augmented lift. The project has demonstrated several successful drones.

Savonius rotor: Although not normally a lifting rotor, the Savonius type has a gap between its two curved blades, and so can be used in conjunction with a duct to form a cross-flow fan. In this configuration it has been tested as an auxiliary high-lift device. Savonius also wrote a book on lifting rotors (see bibliography). The plain Savonius rotor (without duct) is sometimes classed as a Magnus type, although it creates little side force.

Cycloidal rotors

If the complexity of Dunne's auxiliary wing rotors left Lord Rayleigh unimpressed, the cycloidal rotor would have given him fifty fits. He may have been familiar with it, as its invention goes back at least as far as 1828. It comprises set of horizontal lifting aerofoils parallel to and rotating around a spanwise horizontal axis, not unlike the cross-flow fan. However the slats are not fixed to the main rotor but are pivoted like the seats on the London Eye, so they do not rotate directly with the rotor. Typically they are linked together in some way and their pitch is varied cyclically with the overall rotation. An aircraft with a cycloidal rotor wing is called a Cyclogyro.

The idea is to drive the individual blades round and round through the air, and to angle them at each point in the cycle to obtain the desired lift and/or thrust. Like conventional rotorcraft, some are powered and provide both thrust and V/STOL performance, while others autorotate under the forward motion created by a separate thrust system. A cyclic pitch mechanism is used to maximise lift, and may be variable to optimise thrust in forward flight and/or lift for VTOL performance.

There is significant variation between designs. Some even employ a hybrid rotor comprising a cycloidal rotor around a concentric Flettner rotor. None has proved practical on aircraft, although the system is used in marine applications, to provide forward thrust as the cycloidal propeller. The orbiting flap concept places an eccentric single-blade cycloidal rotor or similar "undulator" in front of a fixed wing.

Auxiliary rotors

From time to time, researchers have studied the use of horizontal-axis rotors as auxiliary devices to a main fixed wing. The very first design for an inherently stable aeroplane was of this type. At the dawn of the twentieth century JW Dunne investigated many variations of Magnus rotors and related forms, eventually settling on the aerofoil-like wing rotor and even taking out a provisional patent on the more promising variations. He developed a stable wing which, instead of a tail had both spanwise and longitudinal horizontal-axis wing rotors. The rotors could be stopped in forward flight to reduce drag, and be deployed only when needed for stability or control. They conferred great stability, even allowing the wing to fly backwards. Later he reduced the auxiliary rotors to a single spanwise example at the front. In 1904 he corresponded with the President of the Aeronautical Society, Baden Baden-Powell, about it and they began planning to manufacture it. But then Baden-Powell went to America and the deal faded. Lord Rayleigh told Dunne he thought the contraption too complex to be practicable and so eventually Dunne abandoned the idea.

NACA/NASA have also explored many auxiliary rotor devices scattered about various places on and in the main wing; so far I have come across Flettner and wing rotors in profusion, with the odd Savonius type thrown in. They have even adapted a few aircraft and test-flown some combinations. Other researchers have thought up yet more, see Foshag and Boehler (1969) for a reasonably exhaustive study.


Updated 23 Nov 2021