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Aviation History
1913
1913 - 1202.PDF
seems that there should be some check or limit prescribed to the turning downward of the tail members. In the case of the simple ballasted plane the natural velocity in its two alternative positions is essentially the same, but in the case of the aerofoil of pterygoid form the flight velocity of a model in the inverted position is commonly higher than when the right way up; in the latter, consequently, if reversal should take place, the downfall is greater. A rough estimate indicates that in the case of a flying- machine of ordinary proportions and flight speed, the extent of the drop in the event of reversal, if not corrected by the pilot, might amount to some 600 ft. or 700 ft. Having discussed in a general way the character of the type of instability under consideration, we will proceed to examine more closely the conditions and controlling factors on which it depends. We may assume that the result brought out as the general result of experiment to date, that the absence of any discontinuity in the pres sure reaction holds good in all cases, and that the phenomenon under 5. A flight model or machine designed with the mass centre forward of the centre of pressure of its aerofoil, so that the tail- plane carries a negative-pressure reaction, will be catastrophically stable, for its change of attitude consequent on the reversal of the pressure reaction is the reverse to that laid down in (2). If it is capable of flight one way up, it cannot possibly be capable of flight when reversed. 6. A flight model or machine designed with the mass centre aft of the centre of pressure of the aerofoil, so that the tail-plane carries positive-pressure reaction, behaves in accordance with condition (2) and may be catastrophically unstable. 7. The conditions that define the limit of catastrophic stability are closely akin to those that define directional stability in the vertical plane. If both aerofoil and tail-plane follow the law that reaction varies as angle, and if the influence of centre-of-pressure changes'.be ignored, and if the tail-plane be arranged clear of the wake stream, the limiting conditions may be regarded as identical, and it would Pig. 6 THE PHUGOID CHART. TUB FLIGHT PATH PLOTTED PBOH THE EQUATION ,0 » «o *t> so <w ictFeet M x1s .<a fit Angles Fu,.4 \ ^AftCKMS *-Angle$ fes- 1 -a ^ ^ f AM a-i/^r 0 •01 V discussion therefore is not due intrinsically to any properties or individuality of the aerofoil. The following may consequently be laid down :— 1. There mu,t be a change of attitude of the aerofoil simultane ously and corresponding to the inversion of its flight path. 2. The change of attitude must take place in a sense that will cause the after or tail portion to move towards the pressure side— that is to say, when the pressure side of the aerofoil changes, the tail must swing in the corresponding direction. Conditions I and 2 are illustrated by the behaviour of the ballasted plane in Fig. 8. 3. The model being by hypothesis rigid, the change in the attitude of the tail or directive member will be of like sense and equal in degree (equal angle) to that of the aerofoil itself. 4. A flight model or machine whose mass centre coincides approximately with the centre of pressure of its aerofoil, fitted with a tail-plane of ample area, will be catastrophically stable (example, author's 1894 model, Fig. 9), for the directive member (the tail- plane), carrying neither positive nor negative load, cannot change its attitude as necessitated by the reversal of the pressure reaction. 1 to 9 reproduced by courtesy of the Editor of " Engineering." be virtually impossible to design a model to be directionally stable and show catastrophic instability. 8. A model or machine whose tail plane or member is loaded'up to a point approaching the limit of directional stability (in a vertical plane) may be catastrophically unstable as the result of one or all of the following causes :— (a) The aspect ratio of the tail-plane being relatively low when compared to the main aerofoil.* (6) The movements of the centres of pressure of both the main * The importance of the relative aspect rati > of the aerofoil and tail member is due largely to the different character of the pressure law and the changing values of the constants where the aspect ratio differs in any considerable degree. It is established that for aeroplanes of high aspect ratio the reaction for small angles is approximately directly as the angle, whereas for aeroplanes of extremely low aspect ratio it more closely follows the sin2 law of Newton (see " Aerial Flight," vol. i, sections 150 and 151). It is consequently possible by designing a tail like that of a pheasant, of low aspect ratio, to reach a condition of catastrophic instability without destroying the directional stability in the vertical plane. A good illustration of the effect of aspect ratio in the present connection is given by the author's experiments of a falling "T," as shown in his lectures before the Royal Society of Arts and elsewhere. 1228
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