Some aerodynamic aspects of hang gliding Albrecht Fischer

Many sporting activities reflect new technological developments, sometimes to the extent of creating new

industries. Among the most striking is hang-gliding, now so popular that the production of the necessary

equipment is the fastest growing branch of the aviation industry in the USA. The design of the ultra-light aircraft

involves important aerodynamic principles and novel methods of construction. This article reviews the

development of hang gliding and the type of equipment in current use.

The legendary flights of Daedalus and Icarus epitomise man’s innate desire to fly but some three millennia were to pass before desire was translated into reality. Leaving aside the balloon flights initiated by the Montgolfier brothers in 1783 and the early gliding experiments of George Cayley around the middle of the nineteenth century, the first to achieve success was Otto Lilienthal (1848-96) who made his first successful flight in a glider in 1881 (figure 1). Although his approach was systematic and scientific, and he made more than 2000 flights, his gliders were intrinsically unstable because of the high centre of gravity; he was killed in a flying accident in 1896. At the time of his death he was engaged in experiments with powered flight, but in the event the first to achieve this, in 1903, were the Wright brothers, successors to a long line of experimenters with unmanned aircraft in the second half of the nineteenth century. This was, of course to be the main line of development, leading on through to the military aircraft of the First and Second World Wars to the huge jet-propelled airliners of modern aviation. A parallel line of development was that of light aircraft-such as the famous Moths in Britain and the Aeronca and Cubs in the USA for private owners, mainly as a sport. Finally, gliders have never lost their importance. Between the wars gliding became a popular pastime, stimulated in part by interest in Germany where limitations had been imposed by the Treaty of Versailles on the use of powered aircraft. During the last war gliders became of some importance in air-borne invasions and after it they remained popular for sporting purposes. Today, its appeal is similar to that of sailing: a sense of freedom and isolation and independence of mechanical aids. Increased prosperity and great leisure has brought gliding within the reach of many enthusiasts and it is a popular and well organised sport in many parts of the

Albrecht Fischer,

Was born in Ilmenau, East Germany, in 1928. As a physicist he worked for 15 years in research laboratories in the USA and he is now Professor of Electrical Engineering in the University of Dortmund. He has had a life-long interest in ultra-light gliders and powered aircraft and is himself a certified hang glider pilot.

This is a Euro-article, sponsored by the Commission of the European Communities through its Directorate-General for Scientific and Technical Information Management, which arranges the necessary translations. Such articles are published concurrently in some or all of the participating journals,

Umschau in W;ssenschaft und Technik in the Federal Republic of Germany, La Recherche in France, Naruur en Techniek in Holland and Technology lrelarrdin the Republic of Ireland.

Endaevour, New 9*.is*“ol”me 6, NO. 4.1981

(0 Pargamon Pram, Printed in Great Britain)

or60-9237/61/1U152/6/t02.00

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world. Yet is is still by no means a poor man’s sport. Modern gliders are sophisticated and correspondingly expensive; they commonly require the facilities of small airfields and powered aircraft or catapults for take-off, unless operating from a fixed base road transport is necessary. Thus many who would like to enjoy gliding in its traditional form cannot afford to do so, or at least to only a very limited extent by occasional hiring of equipment. Much the same may be said of sailing: to own, maintain, and pay for moorings for their own boat is beyond the means of many enthusiasts. Here an acceptable alternative has been found in surf-sailing, a sport which has enjoyed such a remarkable surge of popularity as to form the basis of a substantial new industry.

In flying an analogous development began with the advent of the light-weight hang glider (or delta glider) in the 1950s as a cheap but acceptable alternative to the traditional version. It originated in California, in the wake of the space programme, with the development by Rogallo of simple gliders to bring space vehicles back to earth. Four folding bars with fabric stretched between them and weighing only 10 kg (figure 2) could be converted cheaply into a man-carrying hang glider that could be set up or dismantled in a matter of minutes and carried easily on the shoulder or the car roof. Then canie the invention of the harness and steering bar. This enables the pilot, slung about 1 m under the glider, to steer by pressing his body weight on to the steering bar which is rigidly connected to the glider. By using the arm muscles to thrust the body weight to the left or right the glider can be made to turn accordingly and by thrusting the body weight forwards or backwards the glider can be made to dip or soar. Launching is achieved by simply running downhill into the wind until the glider takes off, at approximately 17 km/h. Landing is effected by gliding in towards the ground and ‘braking’ the glider at the last moment by pressing the steering bar forward and positioning the sail almost vertically so as to alight like a bird.

These simple gliders are made of thin-section, high- strength light metal tubes (Al/Mg/Zn/Cu alloy (7075-T6) with a tensile strength of some 600 N/mm2) and Dacron sail-cloth (weighing approximately 150 g/m’); they have a sail surface area of approximately 16 m2. They are used today only for training beginners (about one week’s tuition) their glide slope being only about 1:4. To achieve this simplicity of design many rules of aerodynamics had to be broken. For example, the pointed triangular form of the sail (which causes considerable induced drag), is really suitable only for supersonic aircraft. It is well known that below the sound barrier the smallest induced drag (edge turbulence) is achieved with a wing shape like an elongated ellipse.

Figure 1 Otto Lilienthal, 1896,

experimenting with a ‘slotted wing’

glider.

Furthermore, the two tunnel-shaped, billowing sections of the trailing edge of the sail to the right and left of the keel bar (figure 2) make the formation of an efficient wing section impossible. Additionally, the air turbulence behind the cross bar, behind the many bracing wires, and in particular behind the pilot, generate enormous parasite drag.

If a modern streamlined glider with a glide angle of 1:40 (the overall drag of which is roughly equivalent to that of a DIN A5 sheet (0.04 m2) positioned face-on to the airstream) had a human body slung beneath it on wires (approx. 0.2 m* equivalent surface) the glide slope would be reduced to something like 1: 18. This example shows that when constructing hang gliders it is pointless to strive for perfection in wing design. Modest performance is the price we pay for having direct contact with our element and of being able to dismantle our aircraft and take it home.

The simple Rogallo delta glider described above has another major disadvantage. When flown too quickly, the air stream does not enter the billowing sail diagonally from underneath, as at slow speeds, but almost from the front.

Figure 2 Rogallo-type training glider.

The front part of the sail is thus not filled with air and flaps like the sail of a boat turning into the wind (figure 3). The hang glider sail thus no longer provides lift at the front and consequently the glider drops its nose, gains further speed, and heads into a ‘flap induced’ dive from which there is no escape except bailing out. It is only in the last three years that the wearing of a parachute (3 kg) has been obligatory on flights more than 100 m above ground.

This defect was quickly corrected: from experience with rigid ‘flying wing’ type gliders it was realized that the missing elevator needed to be moved roward to the wing tips. A batten rigidly connected to the nose spar assembly was thus inserted into each wing tip to give a negative angle of attack to the part of the sail located behind the centre of force. If the hang glider now attempts to fly too fast the ‘rising moment’ of this permanently adjusted elevator increases with the square of the velocity and prevents flap- induced dives. This permanently adjusted elevator also has the effect, however, of producing less lift and more drag at that part of the wing. As a further measure to prevent ‘nose dip’ the section in the centre of the wing is given an S-curve which makes it resistant to ‘point of pressure’ phenomena, but again this is at the expense of lift. The lift coefficient (C J of hang glider wings is only approximately 1; good sailplane sections have C a = 1.8.

The flexible hang glider, resistant to sail flap, which was developed in this way from the simple Rogallo design (currently offering a glide angle of 1: 10 and a sink rate of 0.9 m/s) had remaining disadvantages. For example, an overbanked turn could easily cause tail spin. Any aircraft must have more lateral surface behind the centre of gravity than in front of it so that the ‘wind sock’ effect can come into play and bring back on course a glider which is slipping sideways. After a few accidents from this cause the ‘keel pocket’ was introduced. This was a piece of vertically positioned fabric between the keel bar and the sail, behind

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Figure 3 Sketch showing (a) normal flight and

(b) sail-flap induced dive.

the centre of gravity. It could also be used to store accessories, such as the carrying bag. This shows that aerodynamics experts shun this sport, leaving the design to laymen.

The Rogallo wing had become progressively less triangular and pointed in favour of the ideal elliptical shape. By carefully shaping the sail a progressively better section is obtained. Insertion of S-shaped battens made of aluminium tubing or fibre-glass reinforced plastics (similar to those used in the sails of racing yachts) into sleeves sewn on to the sail has a stiffening effect. The nose of the wing is rounded off by inserting a strip of elasticated foil, thus obtaining smoother air flow behaviour. Sailcloth is used on the top and under surfaces of the wing so that the cross bar is no longer exposed to the air stream.

Such sophisticated gliders permit today’s high performance flights. Flights over several hundred kilometres; ascents of 6000 metres above launch point; and flights of 24 hours’ duration have been achieved. Nevertheless, hang gliders are still not completely airworthy. Since the pilot is slung low below the glider it is possible that in very turbulent conditions (thermal currents are always turbulent) the glider will be suddenly braked. In the case of a deceleration of 2G the pilot must then keep the equivalent of twice his body weight attached to the steering bar. If he lets go, he will swing upwards and strike his head against the keel bar (with possible loss of consciousness) or against the sail, causing the glider to overturn. This phenomenon is known as ‘tuck’ and has also been the cause of accidents. The simple way of overcoming this problem is to change from the popular prone position to a sitting one, so that sudden turbulence would not catapult the pilot forward through the trapeze. The disadvantage, however, is that a sitting pilot offers more resistance to the air than a prone one, and he no longer has the feeling of flying head first like a bird, one of the attractions of the sport.

In spite of these difficulties the “flexible” hang glider (competition class FAI 1) which can be opened and closed like an umbrella has already established a secure place for itself. For the enthusiast there is no greater pleasure than to carry your glider, weighing only 17 kg and packed into a bundle 2 m long, up a mountain as you would a rucksack, to assemble it in a few minutes, take a short run and then glide off for hours at a speed of some 35 km/h. 154

‘Semi-rigid’ hang gliders The move away from the principle of the delta wing to other designs-no longer triangular but still foldable-was a logical one. Some of the most successful (Quicksilver and Fledge) are shown in figures 4 and 5. With the Fledge, simple steering by the shifting of weight is possible only about the transverse axis (climbing, diving). As these wings provide full lift right to their tips, shifting/weight sideways has no effect and variable air brakes located at both the wing tips must be used: they are operated by cables, a further complication. With gliders of the Quicksilver type the rudder at the back of the glider is operated by the

Figure 4 Quicksilver-a semi-rigid hang glider.

Figure 5

A powered version

of Fledge-

a semi-rigid glider.

traditional weight-shift method which in this case has no effect other than to transmit the movement back to the rudder by means of control cables attached to the pilot’s hips. In the case of the Fledge there are twist grips, similar to motorcycle throttle controls, attached to the control bar which turn the vertical fins attached to the wing tips, thus increasing their drag. This has the effect of turning the glider. Both fins operated together have the effect of an air brake: for example, when landing.

The aerodynamics of these semi-rigid gliders have been improved in recent years by covering the top and under surfaces of the wing with fabric, in conjunction with shaped sectional battens inserted in sleeves and elasticated foil-features which are now being copied for flexible gliders. This also conceals the cross bar. Unfortunately, other efforts to obviate the need for bracing wires are making slow progress as unbraced wings require thick, rigid, and consequently heavy spars. The new high-tensile lightweight composites (carbon-fibre, aramide), without which glider construction is now difficult to imagine, are not yet widely used in hang gliders owing to their cost. As mentioned earlier, the main cause of turbulence-induced drag, the pilot slung in the trapeze, in any event sets a limit on potential performance unless the pilot were to be enclosed in an inflatable streamlined suit and the cables and trapeze bars were similarly treated. Work on these developments is slow, as most hang glider pilots already obtain enough enjoyment from existing equipment. Cost is also an important consideration. Although hang gliding is relatively cheap, to own one’s equipment is comparable in cost with hiring as required a lightweight powered aircraft. In Germany, the latter costs approximately DM 140 per hour. By comparison, a complete hang gliding kit will cost around DM 5000, to which must be added repairs and depreciation.

Powered gliders Flying over flat country, gaining access to upcurrent areas over otherwise inaccessible mountains, or hopping between widely separated thermal currents calls for powered flight, if possible with an engine which can be stopped and started in the air.

Unfortunately, present-day hang gliders with their mediocre aerodynamics require an engine thrust of over 300N if reasonable rates of climb (say 1.5 m/s) are to be obtained. The conventional glider requires an engine capable of providing at least 7.4 kW, which with all its accessories weighs at least 15 kg. In addition there is the essential exhaust silencer which, if it is to be effective, will be fairly bulky. The propeller located near the pilot is a potential source of danger. Enclosing the propeller adds additional weight, and is cumbersome when transporting the glider on the ground. Battery-driven motors have a much poorer power-to-weight ratio than those fuelled by petrol, although noise and dirt are eliminated. The powered glider quickly comes to weigh over 40 kg and running starts and landings are no longer possible. The logical consequence is then the addition of an undercarriage which in turn means 15 kg extra weight and yet more parasite drag. The overall result is that the ‘ultra light’ plane now weighs 60 kg, and requires an airfield for take-off and landing (figure 6).

When designing these planes the aim must be to equip a modern high-performance glider with an efficient lightweight, two-stroke engine so simply and safely that flying is just as easy as before. The Soarmaster has been one of the most successful of this type: an 8 HP two-stroke

Figure 6 Specifications of the Kasper IL80 motorglider.

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Figure 7 A twin-engined version of the powered glider.

single cylinder engine (8000 rpm) drives a thrust propeller via reducing gear (3: 1) and a long transmission shaft is enclosed in a light metal tube. The entire power unit including rubber vibration dampers can be bolted on to the side of any hang glider in five minutes. The propeller is located right at the back, where the pilot’s legs normally do not reach, yet the centre of gravity of the glider can remain unchanged. One disadvantage is that when starting from foot on level ground the thrust axis, well above the pilot, tends to move much faster than the pilot can run, with a consequent risk of a head-stand. Additionally, sharp changes of course call for extra care owing to the gyroscopic moment of the propeller and the rotating parts of the engine. On the other hand the gyro effect tends to offset jolts in turbulent air. One way of overcoming this problem, namely the use of two engines rotating in opposite directions, is being tried by Lazair (figure 7). These developments introduce design problems. The engine is often controlled with a throttle clamped between the pilot’s teeth, as both hands are needed for steering and holding on.

Another recent development is that the engine and propeller are no longer rigidly attached to the aircraft but to the pilot, who is thus thrust forwards by the propeller and transfers his thrust via his harness to the glider. There is therefore less weight of glider to be steered, while the pilot is heavier. However, since the engine is now slung very low in order to keep the propeller clear of the bracing wires to the tail, the propeller comes close to the ground during take-off and landing. The pilot is thus obliged to fly in a sitting position and to use an undercarriage, the effect of which, as 156

we have already noticed, is a considerable increase in weight and drag.

In an attempt to provide a power source for a standard glider without the need for special modifications another ingenious method has been devised. A small 8 HP engine (with propeller which can be cased in if desired) is attached semi-rigidly on a long rod running diagonally up to the keel bar. The engine, located behind the pilot, provides thrust, via two thin rods, to the hips of the pilot, who lies downwards beneath the glider in the conventional manner. The pilot is thus pushed further forward through the trapeze, which means that the centre of gravity of the glider is not affected by flying with or without an engine, despite having the weight of the engine so far back. By locating the engine’s axis of thrust below the axis of the wing’s braking force a rising moment is created during foot launches from level ground (in contrast to the Soarmaster principle) and thus there is no longer a tendency to perform a headstand during take-off. Since the pivoted engine always turns in the opposite direction to the pilot, owing to the rods connected to the latter’s hips, the engine thrust gives an effect of power steering.

The aim of having a power package weighing less than 10 kg capable of being fitted to any glider, making it as free and independent as a bird, is a powerful incentive to invention. High-speed chainsaw engines weighing only 500 g per HP: ultra light reducing gears with glass fibre or aramide-reinforced transmission belts; cased-in propellers achieving 80 per cent efficiency, and lightweight resonance silencers seem to provide the most promising routes to

TOP OF SAIL

Figure 8 Construction of an ‘ultra light’glider.

success. Already, however, some considerable fiights have been achieved. A powered Fledge has flown from Los Angeles to New York in eight days with fifty stops and-despite a ban by the Swiss civil aviation authorities-a powered hang glider has flown over the Alps.

The ultra light glider In the USA-where‘flying birdcages’ with 30 HP engines can be built, sold, and flown without official interference -‘ultra-lights’ (ULs) are the fastest-growing branch of the aviation industry (figure 8). In Germany where independent developments have been few owing to restrictive legislation. ULs are permitted only as part of a test programme, despite the fact that the air space is less congested there than in the USA. A complicating factor is that because of low-level flights by F104 German Air Force fighters the maximum permitted height for hang gliders is 150m.

Although developed from the hang glider, most ULs cannot in practice be foot-launched: even if American manufacturers claim that they can be in order to exempt themselves from certain regulations. Like full-size gliders they require a trailer for movement on the ground. The best UL engine is currently a German two-stroke three-cylinder radial which develops 18 HP, weighs 15 kg, and costs DM 3500 excluding propeller. It is one of the first two-stroke radials to be developed. Unfortunately, no lightweight twin-

Figure 9 The slotted wing principle, first used by Lilienthal, which

orovides more lift.

rotor Wankel engine is yet available: this would, in principle, offer the advantage of being both very light and completely free of vibration. Possibly Japan will eventually supply such an engine.

Some hang gliding innovations derive from systems developed for early aeroplanes but since discarded. Thus it has been rediscovered that the Canard (Wright Brothers I903), Focke-Wulf 1927), in which the elevator assembly is located in front of and slightly higher than the main wing, cannot stall or get into a tailspin or nose-dive: unfortunately it is not completely safe against tail slide, a fact which was not mentioned.

Powered gliders with power units located aft are constructed on this principle (examples are Tomcat and Eagle. Some recent ultra light gliders utilize the slotted wing principle, giving extra lift, which was explored by Lilienthal (figure 9).

Two recent achievements indicate possible new lines of development. The first was the crossing of the English Channel in an ultra light aircraft powered by pedals; the second was a crossing in a similar aircraft deriving its power from solar cells. Of the two the latter is the most promising, particularly in view of the intensive research and development currently being directed to the improvement of solar cells in connection with the space programme and the search for alternatives to fossil fuel as a source of energy. For the moment, however, the very high cost and still low efficiencies of such cells rules out such applications except for very special purposes.

Bibliography Schwipps, W. ‘Lilienthal’. Arami Verlag, Berlin. 1979. Poynter, D. ‘Hang gliding’. Santa Barbara. 1978. Welch, A. and Welch, L. ‘The story of gliding’. John Murray, London.

1965. “Drachen flieger”, monthly magazine (colour), Ringier-Verlag, Miinchen (W-Germany).

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