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Aviation History
1913
1913 - 1203.PDF
aerofoil and the tail-plane with changes of angle (owing to these movements contributing to the directional stability without contri buting to the catastrophic stability, as in the ballasted plane). (c) The influence of the "wash," or downward wake stream of the main aerofoil, by its influence on the flight attitude. From the point of view of the present paper one is inclined to direct a certain amount of criticism to some of the features commonly met with in actual flying-machines, though it might be rash positively to assert that catastrophic instability has as yet been the cause of disaster.* In the author's opinion it would be wise to avoid the deliberate loading of the tail member, its function should be purely directive. The author has always found that in practice a tail carrying no load or even slight negative load is preferable in model experiments. It would also appear to be desirable to pay attention to the tail member, giving it an aspect ratio comparably equal to the main aerofoil. With due attention to these two points, and with the reasonable curtailment of the fore-and-aft dimension of the aerofoil, giving it a good pterygoid section and aspect ratio (abandoning all idea of obtaining stability from the movement of the centres of pressure on the aerofoil and tail members in the manner of the ballasted plane), and with a " stop " or limit to the elevator or tail control, there can be very little danger in the ^future of any disaster from the cause under discussion. Experiments also might be made with scale models in a wind channel to determine definitely the conditions as to upside-down flight, to prescribe limits of safe-load distribution and safe tail or elevator angles of adjustment, for any given model. Flexibility of the aerofoil has an important bearing on the cata strophic stability (classifying such flexibility as weather-vane flexi bility (as in a bird's wing) and drumskin flexibility ; it may be said in brief that weather-vane stability in the aerofoil tends to stability, and drum-skin flexibility to instability. Catastrophic stability is not definitely related to the other well- known kinds of stability involved in the problem of flight; in order to realise more completely the true position it is of interest to digress and review the different forms of instability known and comprised in the ordinary treatment (mathematical or otherwise) of the problem. There is, firstly, the whole question of longitudinal stability, in volving (a) the stability of the machine relatively to its transient flight path—namely, as in the stability of an arrow, and (b) the stability of the flight path itself in relation to the direction of gravity, as dealt with by the author in his " Phugoid Theory." Both the above (a) and (b) involve complex questions of oscillations about the posi tion of stable equilibrium and the damping of such oscillations as + The recent accident to Major Merrick, which unfortunately cost him his ife, and which took place some few weeks after the reading of the paper, appears to have been clearly due to catastrophic instability. may be set up by disturbing causes. Secondly, there is lateral stability (superficially analogous to the stability of a ship), in which, also, the problem is complicated by considerations of oscillatory motion ; thirdly, there is directional stability, concerned mainly with rotary motion about a vertical axis; and, fourthly, there is a kind of stability known as spiral or rotative stability concerned with the interaction of motions separately considered under the last two headings. All these kinds of stability have been made the subject of previous study, both by the author's method and by the mathe matical method of Dr. Bryan; but the kind of instability dealt with in the present papsr is entirely external to the conditions on which the two lines of treatment mentioned are based ; it is presum ably owing to this fact that the possibility of what in the present paper is termed " catastrophic instability" has been so widely ignored. In the case of a machine or model that will fly either way up there are two entirely different sets of constants, each set giving equations satisfying the conditions of stability, but there is no necessity for relationship between the two ; we may, in fact, look upon the model in its two positions as two different models ; in the case of the ballasted plane and other symmetrical forms the two equivalent models may be regarded as duplicate. Although the existing theories of flight enable us to deal individually with either of the two systems of flight, of which a given model is capable, they take no account of the possibility of a translation from the one condition to the other ; anything of the kind is tacitly ignored. As a concluding illustration of the importance of the present study we may revert to the case of the ballasted plane and compare the magnitude of single gust required to destroy the longitudinal stability with that required to exceed the limit of catastrophic stability. Referring to the chart (Fig. 6) we know that the limit of stability for longitudinal oscillation is reached in the case of the semicircular path—that is to say, ^3 times the natural velocity is the limitjwith the flight path horizontal. In other words, a head gust J 3 - 1, or say, = 07 of the velocity of flight is required to bring about risk of disaster. Against this, in order to induce catas trophic instability, we only require a vertical gust well in excess of the vertical component of the angle of incidence, or, say, one-sixth of the velocity of" flight, so that, in the case of the ballasted plane, in order to change entirely the system of flight and induce catas trophic instability, it only requires a gust of about a quarter the magnitude that it requires to reach the limit of longitudinal stability. It is evident that where there are two systems of flight—that is to say, where a model will fly either way up—in no case can we afford to ignore the considerations raised by the present paper, and, in short any such model must be regarded with the gravest suspicion by the aeronautical designer. A group of the "Savola Italian officers at Pordenone, with M. Chevlllard and Mr. Lawrence Santoni, Managing Dlrectoriof * Co. who are the Farman licensees In Italy, alter the reception of the Farman machines tor the Government. 1229
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