MOTORLESS FLIGHT

By PHILIP WILLS

GLIDING, it is sometimes forgotten, was the first step to powered flying. Lilienthal and a host of others experimented with gliders in their attempts to solve the secrets of controlled flight till finally the "Wright brothers reached the stage when they were able to fit one of their gliders with an engine. Since then much has been discovered about upward air currents which has enabled motorless flight to develop into an art practically without bounds. That is suffi- ciently demonstrated by the existing records of which the following may be quoted :

Distance. Height. Duration. Miles. Feet. Hours.

International (all German) 165 .. 13,800 .. 36 British .. .. 55.8* .. 4,800 ..

74 It will be seen that the Germans, aided by a large govern- ment subsidy, are still well ahead of us, but a band of British amateur pilots has now appeared who may be expected to narrow the gap considerably before long.

The principles of gliding are so far only very vaguely understood. There are three questions which are always and immediately flung at one by a sightseer or visitor to a gliding display : How does it manage to stay up ? Is it any use ? Why don't you put just a small engine in it ? It is difficult adequately to answer these in the heat of the fray, because the answers are not entirely simple.

In the first place, why-does- it stay up ? It is extra- ordinary how difficult it is for people to realize that, granted the very remarkable fact that air blows to and fro over the earth's surface, it must obviously find it equally possible to blow up and down. These vertical air currents are just as important as are the horizontal ones. Everyone knows that cold air cannot hold dissolved as much water as can warm air ; the man who wears spectacles knows how, if he walks into a hot room the coldness of his spectacles will condense water out of the surrounding air and befog his vision. Now a rising current of warm air gets colder as it goes up, and finally its moisture condenses and forms a cloud. Thus clouds are the visible signposts capping a current of ascending air. Very beautiful they are too ; doubly so to a sailplane pilot ; for they are a sign that he is that day free of the skies.

If a wind blows into a wall or line of hills, it naturally has to climb over them before it can continue. And if, standing on top of such a line of hills, you were to di op a light handkerchief, you can readily imagine it would blow up above your head. Similarly a bird or light aircraft in this up-current is able to maintain height and soar up and down the ridge of the hill so long as the wind blows. Gulls can often be seen doing this along the cliffs in an on-shore sea-breeze.

The limiting factor is the "sinking speed" of the aircraft. If you launch an aeroplane or glider from a cliff 100 ft. high on a calm day, it will glide to the bottom, losing height steadily as it goes. It may take 25 seconds

*This' record was made by the author of this article himself on

March 18th, 1934, - • to get to the bottom ; if so, its sinking speed is, obviously, 4 ft. per second. (A good glider would have a sinking speed only -half this, but a good aeroplane would be three times as great.) Now if there is a wind blowing up the hill, and its vertical component is 4 ft./see., our aircraft will obviously not lose height at all ; if the vertical component of the wind is greater than this, the machine will gain height. And it will go on so doing until it attains a height at which the vertical velocity of the air in which it is flying drops to the equivalent of the machine's sinking speed. That height depends chiefly on what is known as the "lapse-rate."

Shortly, this is the temperature gradient of the air in a vertical plane. If at ground level the air temperature is 60° F., and at 1,000 ft. it is 56° F., the lapse-rate at that place and moment of time is 4° F. per 1,000 ft. Take the current of warm air rising up the hill ; it is forced up into a region where the surrounding air is colder. Hot air being lighter than cold, it will continue to rise until it attains the temperature of the surrounding air. As it goes up, it will get colder, not by losing heat to the surrounding air (air being a poor conductor of heat) but adiabatically, by virtue of its expansion, due to the lessening atmospheric pressure as it rises. This is a pretty constant affair ; in dry air a mass of rising warm air will cool at an approximate rate of 5° F. per 1,000 ft.

But supposing the lapse rate of the surrounding air is greater than this ? Then at 1,000 ft. it will still be warmer than the surrounding air, and will continue ascending indefinitely, until it finally condenses out its moisture and forms a cloud ; and even then it • may continue on above the cloud, having shed its moisture, until such time as it cools to the temperature of the outside air. Clouds formed by ascending currents of this kind have been observed as high as 26,000 ft. The existing height record for sailplanes is around 13,800 ft.

So now, you have your sailplane pilot, catapulted off the ground by a crew of half a dozen men with an elastic rope and soaring up and down his hill, and he sees coming at him downwind a line of clouds. He knows they mark the top of an aerial fountain ; he makes for the region underneath, hits the area of lift, starts circling in order to remain within the confines of the invisible pillar,-and rapidly climbs to a great height. From this he can glide to a distant cloud caused by -a rising current produced elsewhere (on unstable days the sky is full of them), and in this way great distances can lee covered.

Now for the second question : Is it any use ?

I have been rather contemptuously told by a visitor at the gliding club that we were a lot of children playing with life-size toys ; that we were no use. (Her own week-end amusement, I believe, was golf.) And actually, rnotorless flight is first and foremost a sport. But what a perfect one ! The longing to be able to indulge in silent, effortless flight has been at the heart of man for centuries. Motorless flight involves more fresh air and exercise than any other sport I know, it asks for, and teaches, patience, physical fitness, constructive ability, quickness of decision, courage, technical ability, a knowledge of meteorology,, and above all tenacity of purpose. And it usually takes place in superb country.

But it .does more than that ; it teaches many things which the power-plane pilot, in his comparatively clumsy, heavy, and vibrating machine can never learn. There was recently a story in the Press of a certain august personage who was making an official flight of inspection to the Near East. The pilot of his service plane approached a range of mountains, and suddenly found his machine in the grip of down currents so violent that after struggling with full engine for some minutes, he only just got the machine away from. the mountains before being forced on to the ground. I forget whether he eventually had to return to his starting- point or whether he found a way round ; anyway, he was congratulated on his bravery and skill in a tight corner. If that story is true as it stands, and- that pilot had been a member of the London Gliding Club, on his return he would have been laughed at. In Germany, where the value of motorless flight is fully realized,-, and -the many clubs are heavily subsidized, all commercial pilots are made to take a preliminary course in motorless flight. -Of course, a sailplane is flown in exactly the same way as is an aeroplane ; the controls are identical except that there is no engine throttle.

So: gliding is the perfect sport ; it turns out better pilots than any other method ; it has potential value to the meteorologist ; and fourthlY, it provides a cheap way for aerodynamical research. A really revolutionary design can be easily and cheaply tried out as a glider ; similarly a young and unrecognized designer, quite unable to afford to build an experimental aeroplane, which costs up to £10,000, can try his design as a glider at quite trivial cost, £200 being quite a reasonable figure. How many potential young designers have come to naught on this rock ?

Finally, about engines. This is really not a sensible question at all ; the best answer is " You're thirty years late ; the Wright brothers did that in 1905." It is rather like suggesting to the latest America Cup challenger that he might instal "just a small engine" for calm days. When you instal an engine, the machine becomes an aeroplane and vanishes into the category of aeroplanes, where it is subjected to all the official supervision and expense of its kind. But it immediately becomes too heavy and clumsy for soaring flight in any but the most exceptional conditions, and passes out of the scope of this article.