LEARNING TO GLIDE on the Downs near Dunstable, Bedfordshire. In these simple gliders, which are held back against the strain of a rubber rope and then released, men and women learn to glide as a preliminary to the higher art of soaring.
GLIDERS were used for the first step in the development of the aeroplane, as the force of gravity provides a convenient source of power when gliding down. The glider, without a heavy engine, is cheaper, lighter and safer, but slower, than the aeroplane.
The building of aeroplanes has now become well understood, and although it will always be necessary to test them, it is no longer necessary to try them without engines, as gliders. Gliding is now used only for the training of men who want to soar. Simple gliders are made for this purpose, and they can be launched by means of a rubber cord, which is stretched by a team of men while the glider is held back. When the glider is released, it slides forward and takes the air. The instructor adjusts the power of the launch by releasing the glider sooner or later. By this means the glider can be made to rise only a foot or two if its pilot is a beginner, or higher if the pilot is more experienced.
Soaring is practised as a sport in Great Britain, though it is used as a preliminary training for military purposes in some other countries. Man has wanted for centuries to soar. Soaring has become possible in the last few years, and man has learned to build efficient gliders which need little power to keep them in the air. This efficiency is shown by their slow rate of descent when gliding, their sinking speed being only two or three feet a second. When gliders are flown in regions where the air is rising as fast as, or faster than, their sinking speed, they will keep their height or they will rise. An efficient glider which has a low sinking speed and which is often able to do this is called a sailplane.
The King Kite is a typical sailplane. In the team of five sailplanes which represented Great Britain in the international soaring competitions in Germany during 1937, there were three King Kites, one of which was the best of the British team.
The object of the design of any sailplane is to reduce its resistance as much as possible. This resistance has three forms. First there is the aerodynamic resistance or induced drag, which is caused by the weight of the sailplane and is reduced as the span is increased. Secondly, there is the skin drag, due to skin friction, which is cut down by reducing the surface area of the sailplane as much as possible and by making the skin as smooth as possible. Thirdly there is eddy drag, which is reduced by making everything of streamline shape.
The fuselage is such a streamline shape. The only break in the streamlining is the front window of the pilot’s cockpit cover, which is generally made fairly steep so that the pilot can see easily through it. Even this is to be eliminated, and in future sailplanes the pilot will be able to see forward through transparent panels which will be put at the front of the fuselage.
The strength of the fuselage comes from the combination of the plywood skin, the spruce longerons (main longitudinal members) and the cross frames spaced at intervals along the fuselage. These are all glued together with casein cement so that they reinforce one another. The fuselage is egg-shaped in section, with the point downwards, and it has one longeron at the bottom and one at either side near the top. The landing load is taken by a skid in front and by a streamline protuberance near the tail. The skid, made of ash, is sprung on rubber blocks, but the protuberance is solid, as the tail is so light that a spring is not necessary. The gap between the skid and the fuselage is covered by a strip of leather to reduce the air resistance.
The cockpit of a modern sailplane is always enclosed. It is pleasant for the pilot to be able to fly in the open and feel the wind on his face, but the extra resistance of an open cockpit is too great, and a streamline roof made of transparent sheet is used. The instruments carried in the cockpit are an air speed indicator, altimeter, rate-of-climb indicator, compass and turn and bank indicator. In competitions and for record attempts a barograph is carried also to provide evidence of the height and duration of the flight.
The wing of the King Kite has a main spar in the deepest part of the wing section and a light spar near the trailing edge. All the bending strength is in the main spar, which is a box spar with heavy spruce flanges top and bottom and webs of plywood. The rear spar carries the hinges of the ailerons and the flaps. The wing is covered with plywood from the leading edge back to the rear spar, on the top and on the bottom surface. The ply covering forms a tube and, by its resistance to torsion, prevents the wing from twisting about the main spar.
The thickness of most of the plywood on the wing and the fuselage is about 1½ mm, though it varies from place to place according to the strength that is needed. The main spar, however, is much heavier than most of the rest of the structure, so that it may carry the bending moment caused by the great span of the wing. The flanges are about 100 mm by 60 mm near the roots.
Angle of Descent 1 in 23
The wing is prevented from breaking backwards or forwards by the combined strength of the rear spar, of the main spar and of the ply skin of the wing.
The fittings at the root of the wing are important, because they have to be strong enough to carry all the wing loads and they must also be easy to undo. After a long flight, the sailplane is taken to pieces, put in a light trailer and towed back behind a car to the flying ground. Thus the wing and tail fittings must be so designed that they can be quickly and easily dismantled in the field. The ailerons, flaps and elevators of the King Kite are all worked by an invisible mechanism which does not protrude into the air stream at all. The control cables act on levers which are all arranged within the wing or tail-plane. The joint is made like the joints of a lobster’s claw, so that there is virtually no gap between the moving part and the fixed part.
The landing speed of this sailplane with flaps down is thirty-five miles an hour, and the best gliding angle is found when the gliding speed is about forty-four miles an hour. At this speed the gradient of descent, or “angle”, is about 1 in 23.
There are several methods of starting a sailplane off. It can be started, as is the glider, by a pair of thick rubber ropes which are hooked on to the nose. Six men run out with each rope, and two men hold back on the tail. At a word from the pilot, the two men on the tail let go. The sailplane slides forward and takes off in a moment. The takeoff is generally arranged on the brow of a hill facing into the wind, so that the pilot goes straight into the upwind.
The sailplane can also be taken off by being towed by a car or by a high-speed winch. A long cable is attached to the nose of the sailplane by a quick-release mechanism, which can be worked at any moment by the pilot. The car tows, or the winch hauls in, fast enough for the sailplane to take off and climb. When the pilot has reached a height at which the cable is running down so steeply that it can give him no more useful pull, he releases it. By this means, a height of several hundred feet can be reached.
STREAMLINED SAILPLANE at the take-off in a German soaring competition. The streamline shape of the sailplane, and particularly of the fuselage is dictated by the need to reduce resistance as far as possible. Soaring is generally started from the top of a ridge facing the wind, the object being to have the benefit of the up-current.
The sailplane can also be towed up by an aeroplane. There is a quick release at either end of the cable so that either the pilot of the aeroplane or the pilot of the sailplane can release it. The pilot of the aeroplane taxies forward gently until the jig cable is drawn straight to its full length; he then opens up his engine, after the pilot of the sailplane has signalled that he is ready. Because of its lower stalling speed, the sailplane takes off first, and then flies along low down until the aeroplane is off. After the aeroplane has taken off, the sailplane always flies slightly above it to avoid its slipstream and the air disturbed by it.
If it is not possible to start a flight by hill soaring, the sailplane may be towed by an aeroplane to a height at which thermal soaring or cloud soaring is possible. Hill soaring is performed on a ridge facing the wind. The wind must be flowing over the ridge, and the sailplane flies in the up-current in front of the hill. The ridge ought to be at least 200 feet high and its gradient should be 1 in 5 or more.
When the pilot has got as high as he can above one hill, he may be able to reach another hill by gliding down to it if it is near enough. If this second hill is suitable, he can soar above it and gain height and then repeat the process. He can thus make a distance flight if he is in a suitably hilly district.
A duration flight, so far as the technique of flying is concerned, is easy as long as the wind blows against the ridge. The pilot flies to and fro in the up-current in front of the ridge.
It has recently been discovered that it is possible to soar in the small up-currents of warm air that are formed on a hot day when the sun heats the ground. These small up-currents are called thermal up-currents.
The sailplanes circle round in the thermal up-current, circling as tightly as is necessary to keep in the strongest part of it. An English pilot who did this in South Africa found that the vultures treated him as one of themselves. When he found a “thermal” and began to circle in it and rise, several vultures would appear and would circle round with him, sharing the benefits of the thermal which he had found. He sometimes saw a vulture circling and rising and, having flown to the place, was able to rise as well.
For thermal soaring, it is necessary to have a rate-of-climb indicator, an instrument which tells the pilot the rate at which he is gaining or losing height. The pilot watches this instrument as he flies and notes when it shows that he has begun to rise in a thermal. He may also feel the slight heave, as he enters the up-current. If the lift continues for several seconds, he judges that the thermal may be big enough for him to use, and he turns sharply and begins to fly in a circle. As he flies round, he adjusts his position, until he is in the middle of the thermal and his indicator shows a steady rate of rise all the time. Thermal up-currents seem to be more like huge rising bubbles of hot air than steady columns. The pilot will find, after a time, that the thermal gives out. He then flies along until he finds another one. It is easier to find thermals if there are several sailplanes flying together, for they can fly within sight of one another and all collect to share a thermal when one is seen to find it and begin circling in it.
SAILPLANES CIRCLING in a thermal current. This is an up-current caused by warm air rising and is used by sailplanes to attain height. Sailplanes, having entered a thermal current, circle round in it so that it will carry them up.
The pilot can make a distance flight by drifting with the wind as he busies himself with finding thermals and rising in them. To make a great flight in a special direction, however, he will rise in his thermal as quickly as possible and then glide down at high speed in the direction in which he wants to go. If it is a good day he will use only the strongest thermals and will pass those in which his climb would be slow. Thus he can make his way across or even against the wind.
The up-currents in clouds are stronger than those outside, because of the heat which is given out by the water vapour when it condenses. It is possible to rise faster and to get higher in a cloud.
A pilot generally flies with the help of his view of the Earth and sky. His senses are not acute enough to enable him to fly blind in a cloud. The turn and bank indicator and the air speed indicator, however, give him the information which he needs to do this. The sailplane must also be stable, to make blind flying as easy as possible, and strong enough for the rough conditions that may be met. However strong the sailplane is - and all sailplanes are as strong as aeroplanes - it would not be string enough to withstand the worst conditions that are possible.
The pilot must be able to retreat when he finds that the cloud is beginning to become “rough”. As the cloud probably is becoming rough only because the up-currents are becoming strong, it may not be possible to escape by gliding down fast, unless the glide becomes a dive at high speed, which is a strain on the sailplane. One way out of the difficulty is for the pilot to put the sailplane into a spin. The speed in a spin soon becomes constant and does not rise to a high enough degree to strain the structure.
Because of these difficulties, the pilot should not venture into a cloud unless his sailplane has been designed for cloud flying, and is equipped with instruments for blind flying, or unless he has a parachute which he can take to as a last resort if the sailplane breaks. The pilot can reach the cloud in several ways. He may be towed up by an aeroplane, he may climb up to a cloud by circling in thermals, or he may reach the cloud when hill soaring, if the cloud base is not too high above the top of the hill. When the pilot is near the cloud, he generally finds that the up-current improves, and he rises rapidly to the base of the cloud and into it.
When he is in the cloud, he is not likely to have any more difficulty in finding up-currents. He turns his attention to flying blind, to finding his way out of the side of a cloud from time to time, to seeing that he is in the best and strongest looking part of the cloud and to verifying where he is. A pilot cannot go soaring alone. He needs help to assemble the sailplane and to launch it; he needs a friend who will bring the trailer if he lands away from the flying ground; and, unless he can already fly an aeroplane, he must first learn to fly.
All these needs are met by gliding clubs, of which the London Gliding Club at Dunstable, Bedfordshire, is a good example. The club has a flying ground, with a hangar which houses the club gliders and sailplanes, and also the sailplanes owned by groups of members. There are a club house for meals and a bunk house for sleeping. The new members are taught first to glide and then to soar. When they have learnt to soar, they can use the club sailplanes. Groups of friends who wish to own a high-efficiency sailplane often club together and buy it. In this way the natural difficulties of learning to fly and of beginning flights without an engine are solved by co-operation.
AN ITALIAN SAILPLANE in a field near Roms. This craft has an open cockpit but cockpits of modern sailplanes are enclosed to improve the streamlining effect The latest sailplanes are designed with transparent panels at the front of the fuselage.