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Aviation History
1950
1950 - 0053.PDF
FLIGHT, 11 January 1950 33 DASED on an American report of considerable merit this article is primarily concerned with the effect on aircraft of gusts likely to be encountered in thunder-clouds. An allied subject—that of the clear-air gust which, though less well- advertised by associated phenomena, may also on occasions be severe—is the subject of a leading article on the previous page. A Northrop P-61 (now designated F-61) Black Widow—the type used by the U.S.A.F. All-weather Flying Division for its thunderstorm-research programme. The key to the puzzle as to why Service pilots and air craft should be more susceptible to thunderstorms than their civilian counterparts is the essence of the information obtained by the American investigators. They have proved that, provided the pilot possesses a high degree of instru- ment-flying ability and has the necessary information as to how to fly his aircraft under such conditions, he can safely negotiate any cu-nim. This is not to say that the Army pilots had a smooth ride on all their flights, and there is no doubt that on some of the worst occasions a great deal of the movable equipment of a passenger aircraft, including the passengers themselves (unless they were securely strapped in) would have been violently displaced, just as in a ship in a storm. But the over-riding point is that during some 1,300 penetrations of thunderstorms of varying intensity and of various origins, no major accidents or fatalities occurred. If one knew no more than this of the report it would be sufficient to change the whole conception of flying in the tropics. As it is, we know much more, and for this we must be ever grateful for the courage of the pilots who faced the imknown in order to obtain this information. Most pilots' approach to turbulence depends on whether it consists of bumps or of violent vertical displacements. In their report, the Americans have also divided the pro- blem in the same way: distinction is made between sharp- edged gusts and draughts, the former being the more im- portant for the structural shock they administer to the air- craft. In all the flights made, the most violent gust encountered was of the order of 43 ft/sec. It was found in a frontal storm over the Middle West of the U.S.A. According to the pilot, the jolt was so severe that he thought that he had collided with another aircraft. Apart from a few skin cracks, however, no structural damage to the machine resulted from this encounter. Comparatively smooth air had previously been noted, but five seconds after entering the storm cloud a sequence of gust velocities amounting to +35 ft/sec, —24, +43 and —29 ft/sec was encountered in five seconds. It is reported that on this occasion the pilot was somewhat unprepared and temporary ^oss of control resulted. Nevertheless, the flight was com- pleted successfully. It_ has been computed by the National Advisory Com- mittee of Aeronautics that such a gust as 43 ft/sec might be encountered once in ten million flight-miles of Air Force flying, or once in every 1,000 traverses of thunderstorms. Coming down the scale, 40 ft/sec might be encountered once in 500-700 traverses, 35 ft/sec once in 125-300 and 30 ft/sec once in 35-100 traverses. Such calculations must involve a fair amount of guess- work, and it is a fact that on only one flight out of 1,300 was a gust of 43 ft/sec noted. But the fact that it was encountered once is a sure sign that such gusts do exist and, indeed, may be of severe proportions. The average gust encountered during the trials was between 25 and 30 ft/sec. It will be obvious that the stresses imposed under such conditions are a direct function of the speed of the gust and the speed and weight of the aircraft. On any flight the first is unpredictable and the last is virtually unalter- able at a moment's notice, so it is the second condition which is the key to successful thunderstorm-flying. The selected speed must strike the necessary mean between the probabilities of stall and structural failure, each being at opposite ends of the speed scale. What this speed should be is of vital importance to all pilots, and one feels that the Air Registration Board syllabi of examinations for pilots might well include a knowledge of the safe turbulence-encountering speed applicable to each type of aircraft on the applicant's licence. At the moment this is not the case. [Strength minima and safety factors for gust velocities m association with various air speeds are, however, laid down in A.R.B. and I.C.A.O. design requirements.—ED.] Modus Operandi If he knows this speed, the pilot can approach the area of turbulence with the confidence that, so far as the risk of structural failure is concerned, he is well prepared. It is, of course, not just a question of settling on the neces- sary combination of r.p.m. and manifold pressure to give the required air speed. This is adequate in calm air, but once in turbulent conditions the decision has to be made whether to maintain air speed or altitude. As is fairly well known, the correct procedure is to ignore the altimeter un- less downdraughts take the aircraft dangerously close to high terrain. The only way to operate an aircraft in severe turbulence is to "fly attitude"—in other words, to maintain the air- craft in the same level attitude by means of the artificial horizon. Such a procedure involves the minimum of elevator movement and will allow the aircraft to ride the waves of turbulence as they come. Although the sharp-edged gusts are of such limited dimensions that they cannot have a great effect on altitude, the draughts can produce far more spectacular results. In contrast to the gusts, which are the sharp up-and-down movements of a sledgehammer, the draughts are of con- siderably greater magnitude and volume: but, although their velocity exceeds those of the gust, their contrast or relative change of direction is much less and they are there- fore of less concern to the pilot. During the trials, draught velocities of up to 100 ft/sec were recorded, and altitude changes when an aircraft was being "flown attitude" ranged from a loss of 2,000ft to a gain of 6,000ft. It is, incidentally, pertinent to note that at no time did a downdraught exceed 2,000ft and at no time did it extend below the limit of the cloud formation. Having by this time established that no conditions exist in a thunderstorm which of themselves can cause an air-
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