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Aviation History
1912
1912 - 0327.PDF
HEAD RESISTANCE AND WING STRESSES. THIS week we publish below further views upon the important report made by M. Bleriot to the French Government. Mr. Archibald R. Low:— Monsieur Bleriot's conclusion that the upper stay wires of mono plane wings should have nearly as large a factor of safety as the lower stay wires is probably correct. From the reasoning by which it is attained I must dissent. There are two main propositions in the argument. First, that a pressure may be exerted on the wires equal to or greater than the normal sustentation, but in the opposite direction, during the ordinary manoeuvre of a changing from horizontal flight to a glide. Second, that it is needless to carry the factors of safety higher than five or six, because when greater stresses than five or six times the normal are imposed the resultant forces of inertia on the pilot's organs will be as dangerous as the collapse of the machine and the subsequent shock of meeting the earth. To the first proposition it may be objected that an instantaneous change of direction, such as is shown in M. Bleriot's diagram, requires an impulsive couple to be applied to the machine. But such a couple cannot be produced either by an eddy in the air or by the elevator, as from these sources we can only look for couples of strictly finite value. If, however, the pilot, or a gust of wind, did produce an angular velocity of such magnitude, and a curvature in the path of the centre of gravity so great that the centrifugal force along the radius of curvature was greater than the force of gravity, then a downward pressure would be exercised on the wings. The front part of the machine would iaXXjaster than a stone, and the whole apparatus would probably tumble head over heels like a tumbler pigeon, while the pilot, who would fall only as fast as a stone, would be left behind in the air, unless held down by a safety belt. If the pilot (or the weather) is so bad as to make this result probable he had much better stay on the ground. If such an accident does happen, then once the machine is upside down, the full stresses will come on the wings in the reverse direction, and it depends on the ability of the upper stay-wires to resist these stresses whether the machine collapses in the air, or holds together and reaches the ground by a series of irregular glides, possibly righting itself in the process. There is actually the case on record of Reynold's biplane, which was upset by a violent eddy, and which did bring its pilot to the ground safely by a series of irregular glides. Finally, it must be observed that the lower and upper stay-wires are strained pretty taut against each other. If the lower wires are much heavier than the upper ones, the stresses required to keep the heavy lower stays taut are quite sufficient to stretch the light upper wires. Theresultant sloppiness is probably cured, by the average mechanic, by tightening up the upper wires which may thus be drawn out and crystalised to the point of collapse before the machine leaves the ground. A hard landing, and much more a serious accident, might readily snap off the wires thus weakened, without the breaking of the wires being in any way directly responsible for the accident. With regard to the second proposition will you permit me to amend slightly your translation of the last paragraphs of M. Bleriot's report. These should read (translated somewhat freely) :— " There is therefore occasion for avoiding exaggerated factors of safety. " Their scale of magnitude should be fixed with reference to the resisting powers of the human frame, for the pilots are subject to impulsive forces of inertia which are transmitted through their whole bodies, and whose maximum possible intensity is proportional to the numerical value of the factors of safety, at least in the case of rigid machines with which alone we are concerned in actual practice.'1 This implies that a safety factor of five is required to meet actual working stresses of five times the normal stresses of steady flying ; and, as already stated, that after such a point is reached, the inertia stresses are so high that the pilot receives fatal injuries internally, and is therefore indifferent to the collapse of the machine. But in the practise of aviation general stresses of double the normal stresses must be considered extremely rare, although local increases of much more than double may reasonably be anticipated. And a factor of safety is usually defined as the ratio of the ultimate breaking stress of the member considered to its maximum normal load. Such a factor of safety has to account for the stresses arising from internal strains, for possible defects in the material, for vibration and its weakening effect on materials, as well as for general increases of stresses in gusts of wind. We have not yet heard of internal injuries sustained by pilots excepting the passing troubles of mal dc mer. And all machines, and the eddies of air which buffet them, »rc highly elastic, so that even if the machine meets a gust of air travelling at its own speed (a very severe condition) the stresses are increased but four times even according to the very limited theory that the force is applied instantaneously to all parts of the machine. And actually they are increased much less than four times, on account of the give of the machine and the elasticity of the air. M. Bleriot may not really expose his true views on construction in his interesting re|K>rt, but those he does express are certainly exaggerated and misleading. Mr. A. V. Roe:— I certainly think it is a wise precaution to pay more attention to the upper bracing than has been usual in many cases, but it seems hard in spite of M. Bleriot's explanation to conceive a pilot making a sufficiently sudden descent to cause excessive down thrust on wings, and yet it is possible. The pilot can easily feel when he makes a loo sudden descent, for if the air acts as a brake on the upper side of wings, there will l>e a tendency to throw the pilot forward from his seat, owing to the centrifugal action and checking of the speed. However, in future, pilots will be more careful when starling their vol plants, or when they require to make a sudden descent. This knowledge, together with a larger factor of safety on the up]>er bracing, should render any more accidents of this nature unlikely. Mr W. O. Manning: I am in agreement with M. Bleriot as to the possibility <>i getting a pressure on the top-side of monoplane wings, and have no doubt that this occasionally occurs in practice, especially if the monoplane is fast, and is fitted with controls so powerful as to enable the machine to be very suddenly dived. It is also certain that this top pressure would be considerably increased if a gust of wind struck the machine full in front at the critical moment. But at the same time, it is not easy to see how this pressure can amount to very much, as it is known that when the ordinary curved plane is placed at such a great negative angle that one gets the usual direction of the lift reversed, that this lift is very small indeed, and certainly as a maximum could hardly amount to as much as half the weight of the machine. I have discussed this matter with several well-known monoplane pilots, and they all assure me that they have never found them-elves being violently forced upwards out of their seats when diying, which effect would certainly occur if the top pressure were considerable. Generally speaking, it seems to me quite impossible that a letiOU pressure can ever occur on the to]) side of monoplane wings, and I therefore consider that this theory is insufficient to explain the too numerous cases of failure of these wings during flight. Mr. Frederic Strickland :— It has always appeared tome that, judging from purely theoretical grounds, monoplane wings were weak in certain directions, especially horizontally. With the usual arrangement of trussing there is provision for taking the vertical load, but very little for taking the horizontal one. True in most cases the truss wires lead forward somewhat, and if it is assumed that the only stresses on the wing of an aeroplane are those due to air pressure at right angles to the surface, this may be sufficient. This assumption does not, however, seem justified. The head resistance and the weight bourne by the planes are not necessarily proportionate to each other. Hence, if the wings are tem|wrarily relieved of the weight, there is no trussing to prevent the wings folding back, and they may do so, especially if the speed is at the same time high, as at the end of a steep vol plant. The possibility of failure in this direction was pointed out in Engineering some two years ago, and repeated several times since. Whether the various failures are due ito this it is impossible to tell accurately, but there seems some reason to think so. In the first place high speed rather than excessive loading appears to have generally been the cause of breakage ; in the second, the only photo I have seen of a wing failing gives the impression of its folding back more than up ; and in the third, there is a record in FLIGHT of a wing failing from horizontal weakness when so near the ground that a safe landing was effected. In this case there was, therefore, definite proof that the wing was too weak horizontally, though strong enough vertically, and had the failure occurred when far from the ground the aviator would have been killed. With regard to M. Bleriot's letter, it is, of course, quite possible that when making a sudden dive the pressure is momentarily above the 327
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