JFAT. How autogyros fly, are flown and were flown

Transcription

1 Thomas Pinnegar (S) JFAT How autogyros fly, are flown and were flown 4764 words

2 Autogyros were pioneered by Juan de la Cierva following the crash in 1919 of his aircraft which, after performing well in a competition organised by the Spanish military, stalled in slow flight near the ground and was wrecked 3,18 (a stall occurs when a wing travels too slowly for the air to smoothly pass over it and instead creates turbulent air around it meaning that the lifting force holding the aeroplane in the air virtually disappears 1 ). He set himself to develop an aircraft which could fly well at slow speeds and would not stall if flown too slowly 3,18,21. In experiments with models, he discovered that unpowered rotors could be made to spin (autorotate) by tilting the rotor rearwards in an airflow, and this would have a low lift-to-drag ratio at slow speeds 3 which made such a rotor ideal for Cierva's needs as it would be able to produce plenty of lift when travelling slowly this makes sense as the blades would have an airspeed greater than that of the aircraft and thus more lift. Before moving on to explain how autorotation occurs and how autogyros can fly and be controlled, it is important to understand how it is that conventional aircraft fly, both fixed and rotary wing, and to remove any of the commonly taught incorrect theories from the mind of the reader. One of the most prevalent 5 incorrect theory is the 'Equal Transit Theory' which argues that when looking at many aerofoils, we notice that the top surface is curved and the bottom surface is nearly flat; if we look at the airflow around this aerofoil, we see that the air travelling over the top must travel further than the air travelling along the bottom. If the air which is together when it is first affected by the aerofoil at the leading edge is also to be together some time later at the trailing edge, the air which has travelled the longer distance must have travelled faster; using Bernoulli's Principle - which states that the static pressure (the pressure measured perpendicular to the fluid flow) is lower where the fluid velocity is faster - shows that the pressure above the wing must be lower than the pressure beneath the wing thus creating a lifting force. There is a wealth of evidence to show that this is inconsistent: an angled flat plate creates lift in a flow of air, which naturally has an equal distance on both the top and bottom surfaces, and most early flying machines used thin aerofoils with parallel surfaces; also, symmetrical aerofoils are common, especially in aerobatic aircraft such as the Extra 300 4, and there are even aerofoils where the curved surface is on the bottom of the wing and the top is mostly flat, such as many supercritical airfoils used for flying at speeds just under the speed of sound 1 ; but most obviously, we often see aircraft flying upside down without being accelerated down to earth. The problems with this theory are due to the postulate that the air particles together at the leading edge must also be together at the trailing edge 1,5, which not only has been shown not to be true experimentally, but the theory normally proposed suggests that there is no net change in speed or direction of the air particles and thus no acceleration of the airflow, and therefore by Newton's second law (which can be expressed as ), there can be no force on the aerofoil 1. This latter reason is also the main reason for the 'half-venturi' theory to be unfounded 1, whose supporters argue that the wing acts like a venturi which is a device which speeds up air where it is constricted by the shape and thus the pressure there is reduced causing a lifting force however there has once again been no net change in the air's velocity. This theory also assumes that no lift is generated by the bottom surface of an airfoil 6, which

3 has been experimentally shown to be incorrect. We can also look at these applications of Bernoulli's Principle, where it is suggested that the acceleration of a section of air causes a lifting force, with Newton's first law in mind an object in motion will remain in motion at a constant speed in a straight line, and an object at rest will remain at rest, unless an external force is applied on it. We see that there must be a force acting on the air particles for them to accelerate initially in order to lower the pressure and therefore it is not the acceleration which causes the force on the wing, but the force on the wing which causes the acceleration of the air particles. It should also be noted that Bernoulli's Principle only applies when no work is being done on the fluid, which is not true when a real wing passes through it. 1 The last of these incorrect theories that shall be covered here is the theory based on the deflection of air particles by the underside of the wing as it passes through the air. If we consider the aerofoil shown in Fig.1, which is what would be seen in an air tunnel and can thus be used as a piece of experimental evidence, we see that the air changes its course before reaching the airfoil and follows the upper surface as well as the lower one this theory does not explain these phenomena, and when calculations are performed based on this theory the results achieved do not match experimental observations 1,7. The explanation most widely accepted as correct and consistent is, like so many other parts of classical physics, based heavily on Newton's laws. We can begin with a very simple thought experiment: imagine an aeroplane passing overhead, flying straight and level. Suddenly the wings fail at their roots, and break off at the fuselage; the aeroplane falls out of the sky. From this, we can deduce that the wings were providing a force upwards to counteract the downwards force which we know as gravity, which we noticed when the wings fell off. By Newton's third law, this upwards force requires an opposing force of equal strength in order for the aeroplane not to drop. This is provided by downwash, a net downwards acceleration of air particles by the wing. We can then know from Newton's second law that the amount of air diverted must produce a force equal to the weight of the aircraft which must therefore equal the mass of air diverted multiplied by the downwards acceleration of that air, which is the same as the mass of air per time multiplied by the vertical velocity of that air: This is a basic statement of what is known in rotary wing analysis as momentum theory 2, 3 as it bases calculations on the momenta of the air particles. Fig.1 shows a real wing with lift and a vector diagram showing how the effective angle of attack (α) and the initial wind speed affect the vertical velocity. It is important for the later discussion of autorotation to notice in the diagram that the effective angle of attack must be directly proportional to the vertical velocity of the air and thus the lifting force; it is also true that the initial wind speed is also directly proportional to the vertical velocity of the air and thus also to the lifting force 1. In practice, we increase the effective angle of attack by increasing the geometric angle of attack, which is the angle of the wing, rather than the downwash, relative to the initial wind direction; often a wing with zero geometric angle of attack will still have an effective angle of attack.

4 Fig. 1 1(from several parts) A wing with downwash Fig. 2 1 (Fig. 1.6) Forces on air and their reaction forces on the wing Newton's first law should be kept in mind when analysing the forces shown in Fig.2.At each stage along the path of an air particle as shown by the streamlines the particle will 'want' to keep moving in a straight line, but that would cause a reduction in pressure which alters the course of the particle by placing a force on it, as shown by the blue arrows in the diagram; naturally by Newton's third law, there is an equal and opposite reaction force on the wing for each of these forces on the air these are the black arrows. It should also be noted that the components of the forces forwards and backwards cancel out exactly and that the lift on the wing is always perpendicular to the initial air flow 1, otherwise aircraft would accelerate themselves without the need of an engine, or else be massively inefficient, depending on the direction the force might apply itself. In order for the air to be accelerated down, energy must be used, and therefore there is a power requirement either from an engine, or from gravity and this is noticed as induced drag 1,8. As well as induced drag, there is also a certain amount of drag caused by air particles colliding with the aircraft, friction between particles and the surface of the wing and aircraft, and the friction experienced due to the viscosity of the air which attempts to equalise the speeds of air particles next to each other; this is parasite drag 1,9. The induced drag is

5 proportional to 1/airspeed, so that an aircraft will require less power to maintain its lift when travelling faster; parasite drag is proportional to the airspeed cubed, so rises sharply with increasing speed 1. The seemingly crazy suggestion about a wing that can accelerate itself essentially describes autorotation, although there is a requirement for oncoming wind to provide the energy to sustain the rotation. An autogyro achieves this by having its rotor rotating in a different plane to that of the oncoming wind: early in Cierva's experiments with slow flight possibilities, he discovered that an unpowered rotor would only autorotate when tilted slightly back in an airstream. This, we will see, is the fundamental requirement for autorotation. Fig. 3 shows a typical autogyro in straight and level flight. Fig. 3 An autogyro in level flight showing the rotor's rearwards tilt It is clear from the above drawing that there is an angle between the plane of rotor rotation and the wind direction caused by forward flight. Fig. 4 shows the rotor blade as it is at the part of its rotation where it travels into the wind, with the whole diagram rotated such that the plane of rotation is horizontal on the paper (all references to horizontal and vertical until otherwise stated will be relative to the plane of rotation of the blades as in the following diagram):

6 Fig.4 1 a section from the driving region of a rotor blade in autorotation The bottom right section of the diagram shows the two velocity vectors for the wind being blown through the rotor and the wind caused by the rotor's speed due to its rotation, this obviously varies over the length of the rotor blade, but this diagram refers to the section called the driving region 1 which is the part we are mainly concerned with. As we know that the lift on a wing is always perpendicular to the initial wind direction, we must combine these vectors to give the overall initial wind direction, or the 'relative wind' as we shall call it. This, as well as the accompanying lift, is shown on the diagram in dark red. There is also a drag caused by the rotation of the rotor, just like the parasite drag on a fixed wing aircraft, this is called profile drag,2,3 and is marked on the diagram in bright red, this pulls the overall lifting force slightly backwards, shown in the same colour. We can see in the diagram that the overall lifting force on the rotor is slightly forwards of the axis of rotation, and thus there is a force on the blade pulling it forwards. As mentioned previously, the speed of the rotor increases with increasing radius. This naturally alters the amount of lift created, as the lift is proportional to the initial wind speed, but also this will affect the relative wind, as can be seen if the length of the 'wind due to rotor speed' arrow on the above diagram is altered. This means that near the rotor's root, the relative wind will have a steep angle of attack because the rotor speed is so small compared to the aircraft speed; this portion of the blade will stall as was briefly mentioned earlier, a stall occurs when a wing travels through the air at a high angle of attack with a slow speed, causing the air to no longer stay attached to the wing's surface, but instead creates turbulent

7 air behind the wing effectively producing very little lift with a high drag so any resultant aerodynamic force on the rotor will not be driving the blade around, but most likely to be almost horizontally backwards. This section of the blade is called the 'stall region'. The blade will gradually have more laminar (smooth and non-turbulent) flow until it follows the blade completely smoothly, and is under the conditions mentioned in the previous paragraph, creating the section known as the 'driving region' because it is where the forwards force on the rotor is created. As the aircraft speed becomes more negligible compared to the rotor speed near the tip of the blade, the relative wind becomes nearer to horizontal, so the lifting force on the blade which is always perpendicular to the relative wind gradually becomes more vertical. We know that the magnitude of the force of profile drag is caused by air particles transferring their momentum to a rotor, just like parasite drag on a fixed wing aircraft, and thus the profile drag increases with increasing airspeed; therefore over the length of the blade other than where it is stalling the profile drag increases gradually along its length. As the lift is very close to vertical at the tip, when the drag is added, the total aerodynamic force is slightly behind the rotor pulling it backwards this is called the 'driven region'. In stable flight these three sections will be in equilibrium such that the driving force will equal the drag and the rotor will spin at a constant speed. Having understood the mechanism by which autorotation occurs, we can see why the autogyro was the perfect solution to Cierva's problem: with only a very small aircraft speed, the rotor blades experience a comparatively large airspeed and angle of attack, and thus they can produce much greater amounts of lift than an equivalent fixed wing aircraft at low speeds. Unfortunately, Cierva's first full size autogyro was the C.1 could not fly due to interference between its counter-rotating rotors, however by early 1922 the C.3 was able to briefly hop 3,12 and by the next year, the C.2 could make slightly larger leaps. He began to notice that his 'autogiros' 'had a tendency to fall over sideways' 3. The reason for this problem was due to the forward motion of the aircraft causing the airflow over the blades on one side of the rotor to be faster than on the other as one side travels towards the airflow and one travels away from it, as shown in the diagram below (Fig. 5). This effect is known as the 'asymmetry of lift'.

8 Fig. 5 diagram showing the problem of asymmetry of lift and region of reversed flow This is a fundamental problem of a rotary wing when travelling in the plane of rotation, and naturally the extent of its effects are dependent on the pitch of the blades and wind speed. Cierva solved this problem, allegedly, at the opera 1, though not watching Don Quixote as many romanticise 10. He came up with a system whereby each blade had a hinge on at its root, allowing them some degree of vertical travel these are known as flapping hinges for obvious reasons. The way these work is that the blade on the side with more lift can now rise independently of the aircraft, and the blade on the retreating side can drop independently of the aircraft; the vertical movement now alters the relative wind such that the lift is reduced on the advancing side and increased on the retreating side so that the lift is even all around the rotor. This modification allowed the autogyro to first maintain controllable flight in The C.6 was demonstrated to the Royal Aircraft Establishment in ,10 and Cierva also set up an autogyro company in England that year 3,10, granting licences to other companies to manufacture some of his autogyros. A crash in 1927 due to fatigue at the blade root highlighted another fundamental problem with the rotors 3 : as the lift was continually altered by means of the flapping motion, the induced drag naturally fluctuated with it, which lead to rapidly changing forces in the plane of rotation 1. Cierva soon modified his design such that it included a hinge allowing horizontal movement of the rotor blades, known as 'lead-lag' hinges 1,2,3, which completed the development of the modern 'fully articulated rotor', as it is known.

9 Fig. 6 2 a fully articulated rotor (the feathering hinge shown here alters the pitch of the rotor which is used for control in all but the earliest rotorcraft) Cierva's articulated rotor is still the most usual solution to the asymmetry of lift problem for rotors with more than two blades; however when two are used, a teetering system is more usual. This has just one hinge at the rotor hub, allowing both blades to move rather like a seesaw; this works well because as one side rises to reduce the lift on the advancing side, the other falls to increase the lift on the retreating side. This system often does not incorporate lead-lag hinges 3, although they can be added on each blade. This solution is used on most microlight sized autogyros, as well as the famous Bell UH-1 'Huey'helicopter 13, and most rotorcraft with two blades. Fig. 7 2 (Fig (b)) a teetering (or semi-rigid) system The only other common solution to the problem uses 'rigid' blades which rely purely on the flexibility of each blade to naturally flap 2,3. These are generally more responsive than the other two systems and also lighter and less complex, however vibration and noise are significantly worse. 11 The asymmetry of lift is not the only fundamental problem to cause the retreating side of the rotor to have less lift than the advancing side; there is a portion of the retreating blade which meets oncoming air from the side which should normally be the trailing edge 1,2. This is inevitable with any amount of forward speed if the blade is idealised as going all the way to the hub: the hub is always stationary compared to the body of the aircraft, but the speed of the blade will increase with greater radius, and on the retreating side this will be backwards. If the blade is travelling backwards (relative to the aircraft) faster than the aircraft is travelling forwards (relative to the surrounding air), it will meet some air from its leading edge;

10 conversely, as the speed reduces with decreasing radius, at some point the blade will have an airspeed of zero, and thus moving closer to the hub, we find that the aircraft travels faster forwards than the blade travels backwards and there is therefore a reversed flow across that portion of the blade. This can be seen easily in Fig. 5 if one visualises the arrows on the diagram showing the retreating side changing length and considering the resultant wind. To measure of the size of this region of reversed flow, the idea of a rotor tip advance ratio was created, and that is the ratio of the velocity of the aircraft forwards to the speed of the blade backwards 2,3 ; this ratio is often known as a μ-number. This reversed flow limits the top speed of the aircraft because it is also compensated for by flapping down to reduce the lift on the advancing side; naturally as μ increases, the total lift must be reduced until it cannot support the weight of the aircraft. In order to pass this speed limit, some aircraft have been designed to use autorotation for their lift in slow flight and a small wing for high speed flight, examples include the CarterCopter (the first aircraft to reach μ1) 14, Fairey Rotodyne 15 and Gyrodyne 16 and McDonnell XV-1 17.Even where there is a small forwards airspeed on the blade, we find that the angle of attack is too great, especially when flapping, for proper laminar flow to be achieved and thus part of the blade will stall, this also limits the aircraft's top speed and is known as retreating blade stall. 1 Control in autogyros was initially by standard control surfaces as found on a fixed wing aircraft Cierva's C.6 was based on the fuselage of an Avro 504K 3,10. These controls consist of a throttle control for the engine as well as elevators, rudder and ailerons which work by altering the direction of airflow past them - elevators provide an upwash or downwash at the tail by means of a hinged flap, this then alters the pitch of the aircraft; ailerons work in pairs, using similar flaps, one goes up and the other down to create a rolling force; the rudder is another hinged flap at the rear which creates a force in exactly the same manner, but this time it is arranged vertically so that the air has its course altered to the side thus turning the aircraft in a manner known as 'yaw'. These controls naturally have less effect in the slow flight that the autogyro was designed to do well in because of the decreased air mass deflected per time, so an alternative means of control was sought. The C.19 Mk.V 20 had the first significant advance, with rudder and engine controls as normal, but the entire rotor hub could be tilted by means of a control stick beneath it extending into the cockpit, which was put into a production model in the C.30 1,18,22 ; this was a basic equivalent to a cyclic pitch control system, the type now standard for virtually all rotorcraft. Cyclic pitch variation was first used effectively by another Spaniard, Raul Pateras Pescara, who in 1924 built a helicopter whose blades could be warped to alter their pitch 3, similar in principle to the Wright brothers' famous wing-warping control in their 'Flyer' aircraft. Modern cyclic pitch systems normally involve a swashplate, which is similar in principle to a cam and follower mechanism, but where the movement transmitted is perpendicular to the plane of rotation. It consists of two circular plates placed around the axis of rotation, one stationary relative to the airframe and one rotating with the rotor. The blades are connected to the top rotating plate by a means such that when the swashplate is tilted, the blade turns in its feathering axis (shown in Fig. 6). Naturally, when the entire swashplate is moved, rather than just one side, the pitch of all of the blades can be altered at the same time; this is known as the collective pitch.

11 Fig. 8a 1 The swashplate without cyclic applied Fig. 8b 1 The swashplate with cyclic applied

12 The cyclic allows for the lift to be adjusted locally in the rotor disc, such that an autogyro will bank and pitch very much like a fixed wing aircraft. This is because the overall physical mechanisms are essentially the same, other than that one is a very frequent periodic alteration of local downwash whereas the other is continuous. Were a helicopter pilot to read that previous statement, he would be likely to claim that flying a helicopter, which also uses cyclic and collective pitch controls, is vastly different from flying an aeroplane. This would be a valid observation, however there is a significant difference between flying an autogyro and flying a helicopter. The pilot of a helicopter would instantly notice that in order to become airborne an autogyro must have some forward speed unless performing a jump takeoff (another of Cierva's ideas, first tried out on the C.30 1,18, in which the rotor is spun faster than necessary for a normal takeoff, by means of a clutch and mechanical linkage from the engine, with the blades of the rotor at zero effective angle of attack; when the clutch is disengaged, the pitch is rapidly increased and the aircraft will 'jump' into the air). It is not abnormal for some heavily laden helicopters to make use of a take-off roll 1, but in order to move a helicopter in any particular direction, the aircraft is tilted so that a component of the downwash provides thrust as well as lift an autogyro cannot do this as the autorotation is dependent upon the airspeed upwards through its rotor and will therefore start to lose height if the rotor is tilted forwards, like an aeroplane. Another major difference in the controls can be seen from the outside of an autogyro: there is no tail rotor (which is used to control yaw in a helicopter) to compensate for the torque of the main rotor. This is because there is no torque between the rotor and the fuselage of an autogyro: the reason for the tail rotor seen on most helicopters is because the rotor is driven by an engine in the fuselage and this makes the body 'want' to turn in the opposite direction to the rotor 1,19 ; this torque can be felt when a powerful handheld electric drill is turned on. As was demonstrated in the explanation of autorotation, an autogyro's rotor is not driven from the fuselage, but from aerodynamic forces on the rotor itself. This means that there is no torque from the fuselage to the rotor and thus the body will not be turned; in order to turn the body of an autogyro, a rudder is used, just as on an aeroplane, rather than by altering the thrust of a tail rotor as on a helicopter. Being such versatile contraption, it seems remarkable that we see so few autogyros in the skies today. There are a number of reasons for their decline though: their drag at high speeds is large enough to make long journeys or fast flight impractical, as does the previously discussed speed limit enforced by the region of reverse flow, and the ability of a helicopter to take off and land vertically makes it invaluable to many of its users. Also the apparent magic by which its rotors turn, contrary to common sense and without a clear cause for staying in the air leads people not to trust autogyros and thus prefer aeroplanes and helicopters whose flight is more plausible 18. This has not always been the case, however: before successful controllable flight was achievable from a helicopter, the autogyro was popular because of its short (or vertical jump) takeoffs and short landings, its mechanical simplicity, and its similarity to aircraft which allowed it to share their already well developed technology, including their engines helicopters require engines of much greater specific power 3 because the air in slow flight,

13 including take-off and landing, is sucked downwards even before being accelerated through the rotor disk 1. The autogyro's qualities even allowed an airmail service between the roof of the Post Office in Philadelphia and Camden airport in New Jersey 20,21 in By the early 1930s, Cierva had created his C.30 with its jump takeoffs and direct rotor control by tilting the hub, but it was only shortly after this that the problems that helicopters had faced were being solved satisfactorily and VTOL aircraft could finally be completely and reliably controlled, with many of the refinements originally designed for autogyros. Development continued through the Second World War and through to the 1960s, including the Fairey Rotodyne, the largest autogyro yet made, a short to medium haul airliner which first flew as an autogyro in , however no production models were ever built due to government budget cuts 22. Despite even this, since then, autogyros have remained firmly as microlight and ultralight 1 aircraft which rarely travel far and normally for recreational purposes. The decline of autogyros was also not helped by the death of Juan de la Cierva in 1936 after boarding a scheduled airline flight in foggy weather which went off course and stalled, crashing into a house and catching fire 23,24,25 ; this is one of the greatest ironies of aviation, as this was the exact problem Cierva set out to solve back in 1919.

14 Bibliography 1 Understanding Flight David F. Anderson and Scott Eberhardt 2 Rotary-Wing Aerodynamics W.Z. Stepniewski and C.N. Keys 3 Helicopter Theory Wayne Johnson