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Aviation History
1938
1938 - 2019.PDF
JULY 14, 1938. FLIGHT, c The Minimoa sailplane in which the author of this article secured the British height record of over 10,000ft. feat which would quickly confuse all but the best aeroplane pilots. Then he is not rich enough to buy (and, anyway, does not go fast enough to drive) the various but heavy instruments which the power pilot uses to reduce fatigue whilst in cloud. It seems possible that even power pilots underestimate the difficulties of blind flying. The thing is that it is quite easy unless and until you get into trouble, when in the twinkling of an eye it becomes very difficult indeed. A sudden attack of vertigo, unless one is already trained to overcome it, makes it literally impossible to retain con trol. One can call to mind many accidents which prob ably arose from rough conditions encountered inside clouds; a recent case, no doubt, was the fatal accident involving the R.A.F. boxing team in South Africa; and such crashes induce in one a feeling of wonder that the "B" pilot's licence does not require recovery from spins or high-speed spiral dives under the hood, or, as in more advanced countries, any knowledge of sail-flying. Incidentally, one quite reliable source of news produced a semi-official theory that the South African crash might have been caused by the machine hitting a vulture in the middle of the cloud. It does not seem widely known that even birds are unable to fly blind for long. They, too, evidently experience vertigo induced by the Eustachian tubes of the inner ear (irreverently called the Nasturtiums) and blindfold pigeons have been dropped from an aero plane and in every case have eventually gone into high speed spiral dives and the like, ending up in desperation with a parachute descent with the wings outstretched at a large dihedral angle. Few birds can be seen flying even on a misty day, and I cannot believe any have ever been encountered in clouds. Stop Me and . . . The size of the hailstones which fairly commonly emerge from South African clouds prove that up-currents of over 200 m.p.h. must occur inside them. So it is likely that a knowledge of the meteorology of motorless flying is of greater assistance to the blind pilot than a knowledge of ornithology. The primary reason for the violent up-currents existing in clouds is to be found in the physical laws concerning the Latent Heat of Evaporation. It requires more heat to convert a pound of water at 100 deg. C. into a pound of steam than it does to heat that water from normal tem peratures up to the boiling point. This is reversible ; all this heat is liberated again when the steam condenses. Similarly, when the water vapour in a rising, cooling current of air eventually condenses and forms a cloud, there is a release of heat which starts the air rising again. If the up-current so caused reaches a speed greater than 8m./sec, any raindrops which form will be split up and carried upwards again, and this generates electricity in a big way. So the presence of a thunderstorm indicates up- currents of over 8m. /sec. Hail is formed in rather the same way ; the individual hailstones have each fallen and been lifted by up-currents several times during their formation, like the balls on jets of water one sees at fair-ground shooting galleries. Hail stones weighing over 1 lb. each occur in South Africa. The up-current needed to lift such a lump of ice can be imagined but not negotiated. Observe the smoke rising from a cigarette. For a short distance it ascends smoothly in a steady stream, but at a definite point this stream suddenly breaks up and dissi pates in an extremely turbulent way. It is possible that strong up-currents inside clouds may in certain circum stances behave in the same way, the break-down of stead ' flow depending on their speed and cross-section. What armour, then, should the sailplane enthusiast don to tackle these redoubtable adversaries ? In the first place, he clearly needs a very strong machine. Sailplanes designed for cloud-flying have factors up to 12. But here comes a difficulty. In order to give good lateral control at speeds near the stall, considerable wash-out is necessary on the wingtips. In inverted flight this wash-out becomes, in effect, wash-in, so that at high speeds whilst inverted the entire weight of the machine may be imposed on the wing- tips, inevitably producing structural failure. It is probably true that the sailplane pilot in a modern machine is more or less safe in cloud-flying whatever happens, so long as he remains right way up. But if he meets gusts of sufficiently sharp a gradient to invert the machine (and, no doubt, such things do occur in certain clouds) it is clearly possible for any pilot, however skilled, to qualify as a Caterpillar. Therefore, having bought a suitably rugged machine, the next two items are a parachute, and as much theory and practice of gliding meteorology as one can get. There seems to be no alternative to actual cross-country thermal flying as the method of obtaining this knowledge. One must learn to distinguish as far as possible between a "live" and a "dead" cumulus, i.e., one which is still forming or one which is merely dissolving ; to decide where the up-currents forming it are to be found ; and to obtain some idea as to which clouds are likely to be relatively friendly and which must be avoided. If by the time one reaches this stage one has not died of old age (and, indeed, with the growth of technical facilities in this country the sailplane pilot's evolution has recently enormously accelerated), one then requires a fore- and-aft level to act as an A.S.I.—should the latter chance to ice up in cloud—and a turn-and-bank indicator. This should be an electrical one for choice, and, surprisingly enough, there is an English make which is at least as good as, if not better than, any foreign instrument specially designed for sailplane work which I have seen. One should then have a lot of lessons under the hood at the local light aeroplane club. When you have once been spun under the hood and have recovered your senses sufficiently to think, you at last realise the nature of the problem you are up against. Those whose have not had this nauseating experience can not appreciate the problem, often deny its existence (thus claiming a superiority even to the birds) and so may occa sionally break their necks. More usually they get into a high-speed spiral dive whilst still fairly low in the cloud, and emerge in a Miltonian descent with the A.S.I, needle twice round the clock, the variometer in hysterics and the other instruments in knots. After a number of spins and spiral dives under the hood one's reflex actions seem to be quickly subdued and, whilst some pilots still experience the vertigo and nausea, it be comes possible to control one's muscular reactions inde- (Concluded on page h.)
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