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Aviation History
1913
1913 - 0685.PDF
JUNE 28, 1913. K Edited by V. E. Some Remarks on Stability, THE question of stability is one of the most important in the whole of aeronautics ; naturally it is one of the most difficult In the first place, what do we mean by the term " stability " ? At first sight the answer to this question is not apparently a very difficult one, and one would naturally expect to find a common agreement as to what stability was or should be, although there might and probably would be considerable diversity as to ihe best means to obtain the same. But the actual personal experience of the writer is that such is by no means the case. Not only do no two people seem to hold the same views with regard to how stability should be obtained, but they do not even hold the same views or mean the same thing by the term. Is it not curious that in mcie than one well-known work on stability no actual definition of the word is given, i.e., the author's definition, his personal idea of stability? Apparently there is an assumption that there is a generally accepted idea as to the meaning of the word stability. The writer's experience is against this (so far, at any rate, as aero- modellists are concerned), or that if there is a general concensus of opinion, it by no means follows that it is the correct one. The subject is such an important one that I propose to go into it a little in detail. Let us first take the ordinary dictionary definition of the word. Nuttall gives the following :—stableness, firmness, steadiness. The last term should be especially noted. In " Principles of Flight," p. 36, we find under the heading " Stiff and Rolling Stability" ; stability is, of course, a relative term. Perfect natural stability would presumably mean that the machine is absolutely stiff in the air ; that the righting and dis turbing forces synchronise under all possible conditions. As at present understood, a naturally stable system is expected to pitch or roll to a certain extent in order to recover equilibrium. A machine that is stiff in the air would never lose its equilibrium. The writer's experience is that stability and steadiness are generally accepted as the same thing. Certainly most judges in model competitions appear to take this view. Personally it appears to be most desirable to separate the two and admit with the author of the " Principles of Flight " the existence of two forms or types of stability, stiff or (say) steady stability and rolling stability. It is quite an error to suppose that an aeroplane is necessarily unstable because it rolls. Ships roll but are not unstable in consequence. Ships can be steadied by means of gyro scopes (Schlick method) ; in the case of one vessel experimented on the "See-Bar," the rolling was actually reduced from some 250 to 300 to about l° ; in other words, the vessel was rendered perfectly steady. But what is the practical result of so steadying vessels in JOHNSON, M.A. the long run, nothing more nor less than the causing of enormous strains and stresses on the vessels and their ultimate breaking Up under the action of the waves. The chief factor in the steadying of an aeroplane is speed, and the same is true of any other form of locomotion. A rather remarkable practical instance of this question of " Flight" Copyright. Tall and propellers on the Brags-Smith biplane (Olympia model). steadiness and stability was afforded at the recent hydro-aeroplane competition at the Welsh Harp. The steadiness exhibited by the models generally in actual flight was very much better than that exhibited at previous competitions ; generally speaking, the models were more or less quick-flying ones. The model flown by Mr. Bragg-Smith was, however, an exception—being perceptibly slower in flight, during one flight — the only one actually witnessed by the writei—the model rolled considerably, never, however, beyond a certain angle. More than one competitor observed to another : " Well ! I don't think much of Bragg-Smith's stability to-day, at any rate.'1 But was the model less stable because it rolled, because it exhibited the rolling stability of slow flight rather than the " stiff or Mr. Cyril M. Wright's Olymp'a model- 711 "Flight" Copyright.
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