Sailflying

I am better acquainted with Brazil, as I have been to every port in that marvellous country. Brazil produces over 2,000 different kinds of wood, some of which require no polish to make them shine like an exhibition radio cabinet.

This ship can go 2,200 miles up the Amazon river, the average width of which is three miles. At the eastern mouth the river is 90 miles wide. I have seen the fresh water of the Amazon 150 miles out to sea.

Extract from letter from F. Martin,

Wireless Operator.

The other day I picked up a German handbook dealing with the sport of sailflying. The first words of the introduction were " Segelflug ist Schon "—which is the best possible way in which one can introduce the subject of this fine sport. Sailplaning has something about it which no other sport has. It seems to satisfy one's every mood in some way or another—for flying a sailplane can be peaceful ; it can be very awe-inspiring. If you are amused by pushing a file or using a wood chisel, there is plenty of scope. If you will persist in being scientific—the science of meteorology is open to you. This can be studied from a new and extremely interesting angle. The acquisition of skill and knowledge in actually flying sailplanes is a most absorbing business. Fears have to be overcome, and as in all things, it is very pleasant to look back and laugh at what one was once a little afraid of.

How do you begin ? The best and quickest way of learning to fly gliders is to join a fortnight's gliding camp. These are held at most of the gliding clubs throughout the summer months, and the cost to take part, including board, is in the region of £10. If you cannot spare a whole fortnight at a time, a club can be joined in the normal way, and your training done at the week-ends. The usual entrance fee is 2 guineas and annual subscription 3 guineas. Much the best way to begin gliding, though, is to join a training camp. In the early stages continuity of instruction is most important for quick progress. At the end of a fortnight's training you will be at a stage of proficiency which would take at least six months by just flying at the week-ends.

I will not try to explain how one is taught to glide, as space does not permit. It will suffice to say that one goes 28

alone right from the start, and the process can be likened to the old game of jumping downstairs. One starts at the bottom, and gaining in skill and courage step by step, can soon launch oneself confidently from the top. As the pupil satisfies the instructor that he has reached each stage of the training successfully, he is promoted to more and more efficient machines. As a beginner is not very sure of himself, he starts his career on a glider with a poor gliding angle, and with slow response to the controls. He is thus given time • to change his mind in his efforts to control the machine, and the average pupil makes good use of this feature in the design.

At the end of a fortnight or six months, as the case may be, the machine you will be permitted to fly will be capable of soaring flight. That is, it will have a low sinking speed and a good gliding angle at 30 m.p.h. The sinking speed will be about 3 feet per second, and the gliding angle about 1 in 18.

Your training up to this stage will have only concerned itself with gliding—that is descending flight. You will have been taught to fly at the correct speed, and to make your turns smoothly. The next step is the problem of sustained flight, that is soaring.

The easiest way to explain the secret of soaring flight is this : a man starts running down a stationary escalator at a speed which gives him a vertical rate of descent of 3 feet per second. Someone starts up the escalator, and by varying the speed of it can either keep the man where he is or lift him back to the top.

It will be seen, then, that all one has to do to remain aloft in a sailplane is to find an area of air that is going up faster than your machine is coming down. As long as its speed is held at 30 m.p.h. your sinking speed is at its lowest, and the gliding angle good. If you fly too fast, your sinking speed will increase, your gliding angle will be too steep, and you will come down. If you fly too slowly, your machine will stall (that is, the wings will lose their lifting properties) and you will come down too. Therefore, the machine must be flown at its most economical speed, which is generally about 30 m.p.h.

Areas of rising air can be found in various places, and the first place that you will make their acquaintance will be over the brow of a hill. The hill chosen will be a range about 2 miles long, and facing the prevailing wind. Sutton Bank and the Dunstable Downs may be taken as typical soaring sites.

As the wind strikes the hill it is deflected over it and thus there is a belt of rising air along its brow. This rising air reaches roughly twice the height of the hill—at Dunstable the downs are about 250 feet high, and in a good breeze one can climb about 500 to 600 feet.

So therefore, as long as the sailplane is patrolled along, above the brow of the hill, where the rising air is strongest, it will stay up. Many people ask if this beating back and forth along a hill top does not become boring. It does not. Some days it is rough, other days the air will be dead smooth. One generally flies in the company of several other machines, and each pilot vies with the others to be the highest. When you have made your first soaring flight and taken the " C " test—that is, a flight of 5 minutes' duration above the starting point—your career as a pilot of motorless aircraft has begun. It is now up to you to get in as many hours of flight as you can. To fly confidently and naturally, to improve your turns and, in short, to feel as much at home in the air as possible, you must fly in all strengths of winds and make flights of longer and longer duration. You must practice landing a sailplane in as small a space as possible.

When you have done all this and have about 20 hours of flight behind you, you are ready for the next stage in your career. This is the art of thermal or convection soaring. When the sun shines on an area of ground which absorbs heat better than that surrounding it—for instance, a cornfield or a cluster of houses—a stream of warm air is given off. These rising currents are known to the sailplane pilot as thermals.

They may be compared to the smoke rising from a chimney, and bend with the wind in the same way. They are, of course, invisible and much larger in area. These streams of warm air rise until they reach their condensation level, and there form into cumulus cloud. This level is commonly known as cloud base, and in this country the height of it varies between 2,500 and 6,000 feet. Thermals frequently seem to be given off in " bubble " form, as they occur at regular intervals of perhaps 10 to 15 minutes, from the same birthplace. The area of warm air appears to collect, detach and float away. The technique of utilising thermals for soaring flight is extremely simple. In the sailplane dashboard is a very important instrument known as a variometer. This instrument shows the rate of climb or descent of the sailplane.

There are various forms of this apparatus, but the simplest and bests works in this way. A large thermos 30

bottle is connected by a rubber tube to the instrument. The instrument itself is a small oblong block of a transparent celluloid-like material called plexiglass. In this are bored two vertical holes about 3 inches long and I of an inch apart. These holes have a taper, and at the bottom and smallest end are about 1/32 of an inch in diameter. At the bottom of each long hole rests a small pip of plexiglass. The one on the left coloured green and the one on the right coloured red. The rubber tube from the thermos bottle is led to the back of the instrument, and ducts in the transparent block are arranged to give the following effect.

When the sailplane rises the air in the thermos tries to escape, owing to the decreased pressure outside. It is allowed to escape, but in doing so is made to lift the small green pip in the tapering tubes—in the same way as a ping-pong ball at a shooting range. The faster the machine rises the higher goes the pip, and to read off the rate there is a scale beside the tapering holes. The instrument is calibrated from 6 inches per second to 20 feet per second. When the sailplane descends the reverse of this operation is made to happen. The pressure outside the thermos bottle is increased, and air tries to get in. To do this it has to pass the red pip on the way, and in doing so lifts it. I have gone to great lengths to describe this instrument, as its function is extremely important in thermal soaring—indeed, without it, thermal flight would be nearly impossible. When one is flying well away from the ground there is little or no immediate indication if one is rising or falling, as there is no datum near to go by.

So, when you are soaring in the hill lift and the green pip begins to dance persistently in its tube, you have indication that you have flown into a thermal current.

If you continue to fly on a straight course, in a second or two you will fly out of it. The obvious thing to do, therefore, is to circle. With a little .practice a sailplane pilot learns to stay in the column of rising air in this way and can detach himself from the hill lift and climb up to the cloud base. If he can blind-fly—that is fly level by using an instrument known as a bank and turn—he enters the cloud and climbs very much higher. Owing to the condensation of the moisture in the thermal current, latent heat is liberated and this tends to increase the strength of the up-current inside the cumulus cloud. Heights of 25,000 feet have been reached recently in Germany by this means.

If the pilot has no blind-flying instruments, or has no knowledge of their use, he deserts his thermal which has

31