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Aviation History
1913
1913 - 0995.PDF
SEPTEMBER 13, 1913. [/ycjiT] THE THEORY OF THE DUNNE AEROPLANE. (Concluded Now opposed to this we have at the front of the machine a very slight positive dihedral effect, due to the coning of the wings making it easier for the air to get away outwards than inwards, and also to the fact that the windward wing opposes a deeper camber to the current than does the leeward wing. Each of these opposing devices being dihedral, and so—even if unopposed by the other—tending to take up a position of equilibrium, it is not of very great import whether they exactly balance each other or only nearly so. But what is of importance is that since each is strongly resisting the action of the other it effectually damps out any oscillations that other might tend to produce. We have, however, found no difficulty in so balancing the opposing couples that ordinary side gusts, such as one encounters when flying in moderate winds, produce absolutely no perceptible lateral disturbance. But side gusts unfortunately are not always steady. We have to +rom page 994.) we have seen, increases the amount of the windward-rolling negative surface, and decreases the amount of the leeward-rolling positive surface. Flying this machine in very "bumpy" air, or side-on to a thoroughly bad wind, one notices little sudden movements, gently checking themselves, about axes which run, roughly speaking, parallel to the backward-sloping wings. And so we come to our final device. Part of this I have explained so often that I think everyone here must understand it pretty thoroughly ; so I will merely point out once more, as briefly as possible, that when the wind strikes this machine somewhat on the side as shown by the arrow in Fig. 4b, the windward wing meets that wind with its broadest side towards it, and the leeward wing meets it in the end-on position. Now everyone knows that Langley proved experimentally that if you expose two long narrow planes at a small angle of incidence to the wind, the one broadside-on with its longer edge forward and Fig. 19. Fig. 20. Fig. 21. deal with fierce gusts, irregular gusts, gusts with an upward trend or a downward trend, local side-puffs striking one portion of the wing and not the other, and narrow-diametered retnous. Now we noticed while dealing with the longitudinal stability, that the amount of negative surface exposed depends upon the angle of incidence. If you look again at Figs. 4a, 10 and 11, you will note that there are two points in the span where the leading edges cross the back edges. Inside these points the wings are positive, and would lift the windward side. Outside these points they art negative, and would depress the windward side on account of their negative dihedral effect. You will see that as the mean angle of incidence of the machine grows smaller, these points, where the edges cross, move inwards, increasing the negative dihedral surface and decreasing the positive surface. You will also note that at normal flight angles the negative dihedral surface is very small. To carry a lot of unnecessary- negative surface until you actually needed it would be a source of inefficiency. Our negative dihedral surface develops as it is required. Thus, if, after viewing the machine from in front, you were to step to the right or left, you would notice that as you move round, so does the point on the wing nearest you, where the leading and trailing edges appear to cross, travel more forward towards the bow. For example see Fig. 5, which shows the aspect presented to an unusually strong and sudden side gust, and note that a very large portion of the near wing has become negative, the far wing having practically "vanished." Now look again at Fig. 4a. If the side gust affects the distribu tion of pressure on the wings in such a way as to render the negative dihedral couple [i.e., the windward-rocking couple) too powerful : which is equivalent to saying that the negative pressures, in the region of the negative tail tips, are augmented more than those on the positive surfaces : the effect will obviously be not only to roll the machine to windward, but also to elevate it. But elevating the front has, as we have seen, the effect of reducing the amount of the negative (windward-rolling) surface and increasing the amount of the positive (leeward-rolling) surface. Thus the windward roll started by the unevenly distributed gust is almost instantly checked. Conversely, if the gust accentuates the effect of the leeward-rolling couple, which is equivalent to saying that the positive pressures are augmented, the effect will be not only to roll the machine to leeward, but also to depress the bow—for the centre of positive pressure is behind the centre of gravity. Depressing the bow, as the other end-on with its shorter side forward, the former will experience the greater pressure. Let us refer to this superiority of the broadside-on plane as "Aspect-Ratio Effect." Now, curiously enough, hardly anyone seems to have noticed as anything worth remarking, that the experiments showed further that as the angles of incidence are gradually increased, this disproportion in pressure rapidly diminishes, till, when the angles are thirty degrees, the pressures are equal, while beyond that angle the conditions are actually reversed and the pressure on the end-on surface becomes the greater. So in such a condition of affairs as is depicted in Fig. 4b, the pressures, negative as well as positive, all over the right wing are greater than those on the left wing. Now, if the machine rotates to leeward, the angle of incidence of the positive (leeward-rolling) portion is increased, and so the aspect ratio effect of that part is diminished. But the negative angle of incidence of the negative (windward-rolling) portion is decreased, and so the aspect ratio effect on that portion is augmented. Exactly the converse occurs if the machine is rotated to windward by an over-powerful negative dihedral couple. This damping and stabilising action has to be regarded as super posed upon the various lateral stabilising effects we have already considered, which is, perhaps, a rather complicated idea to absorb. Under ordinary conditions of flight its action is but slieht, and is in general a damping one. It renders the effect of the alteration in the relative proportions of positive and negative surface due to change of mean angle of incidence less sudden in its action, while it adds to the damping effect the opposing dihedrals have on one another. But its great value lies in this : that apparently no matter how sudden, violent, and unevenly applied be the gust, you cannot possibly be blown over either way to much beyond 30 degrees. I say " apparently," because I have not yet flown the machine in the hurricane which would be necessary to put this quality to actual test, and indeed I find that in this case I have quite a strong conviction that theory is preferable to practice. To sum up. We have :— FOR LATERAL SAFETY.— (a) The fact that no matter how the machine be banked, it will automatically supply sufficient centrifugal force to keep up the support against side-slip. (b) The fact that, owing to its tendency to level up from any bank and widen out any turn, it cannot spiral dive. I02I
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