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Aviation History
1912
1912 - 0865.PDF
SEPTEMBER 21, 1912. [/OCHT) CORRESPONDENCE •»• Tie name and address of the writet (net necessarily for tmblication) MOST in ail casts accompany letters intended for insertion. or containing outfits. Correspondents communicating with regard to letters which have appeared in FLIGHT, would much facilitate ready reference by quoting the number of each letter. The Dunne Monoplane. [1627] In reply to "Dunnite's" questions I must own to one mishap other than when landing. I was piling leaden weights on to what corresponds to the tail, in order to verify the critical loading at which the uncontrolled machine would pancake, and eventually failed to assume control quickly enough to save my skidwork. The longest straight I have done without touching any control is a little over a mile, on a " bumpy " day in a ten mile average wind. At the end of that stretch the machine was still travelling smoothly and evenly straight ahead, and there was nothing to indicate that it would not continue to do so until the petrol was exhausted. Having satisfied myself on that point, I turned the machine, because it was getting over mud-flats and shoal-water. Variation in engine torque is the most common cause of departure from the true compass course, so the use of the engine throttle would greatly simplify matters. Therefore I am inclined to think that "Dunnite's" first test could be effected without much difficulty. The second test is, I believe, far more simple. Here again the most that I have done or attempted is two consecutive circles without moving any control. The circles were very narrow and steeply banked and appeared to be traced uniformly. The termi nation of the second was similar in all respects to the commence ment of the first, so I see no reason why the machine should not have completed another four if necessary. I would not myself be inclined to regard tests such as these as in any way sat^factory, for it is obvious that a machine might be able to do all these things and still be distinctly dangerous. Circling over raiwus with very steep banking ; rough-weather flying with sharp turns ; sudden stoppages of the engine ; all to be done "hands off" and "feet off" constitute the only practical tests of stability. And the measure of that estimable quality is inversely as the distance the machine drops below its proper air-path before it has completely recovered itself from the effects of some given atmospheric disturbance. United Service Club. J. W. DUNNE. The Danger of the Vertical Rudder. [1628] It is the present fashion to steer aircraft by means of two separate rudders working at right angles to each other, one normally vertical for direction, the other normally horizontal for elevation—in other words a turner and a lifter. This system of steering is easy, simple and effective, and each rudder performs its own duty without interfering with the function of the other, provided the aircraft is travelling on an even keel. But the moment the machine heels therefore naturally banks, quite apart from the fact that the inner wing, by slowing down, has lost some of its resisting power against gravity alone. In a very sharp turn at high speed, the centrifugal force may be so strong, that gravity for the time being can be ignored, as in the case of looping the loop. This banking, however steep, is not only quite natural but perfectly safe in itself, provided the wings are strong enough to resist the extra strain. Of course, there is a certain amount of slip, hut it is not a side slip, nor is it dangerous. If a man whirls a cannon ball round and round by a chain, the weight may pull him outwards, thus increasing the circle, but even if he suddenly lets the chain go, the ball will not at once fall to the ground, but fly off at a tangent to the circle it was describing. The danger of a sharp turn arises, not from the natural banking, but from the action of the turning rudder, which is now swinging the tail upwards as well as outwards (Fig. 3). If this upward swing of the tail is not at once checked by' raising the elevator (Kig. 3), the machine will make a dangerous dive earthwards. M. Bleriot has pointed out that, at the commencement of the dive, the pressure has been transferred from the under to the upper surface of the wings. This might very well cause the machine to turn over, to enable the wings to meet the pressure in the way they were designed to meet it. But at any rate it would appear to seriously hamper any attempt to get the tail down. My own opinion is—but I do not wish to press it—-that the pilot would have the best chance (because the complications would be fewer) if he trusted to the normal elevator alone, and look his feet at once off the helm control. But (unfortunntely, it seems to me) he has come to regard this control as a foot-rest, whose only power of action is to turn the machine to the right or left when required. Of course, the turn can be made without banking nt all by counteracting the natural inclination to bank. But this necessitates a certain amount of room which is not always available. Sept. nth. R. BROCKI.RUURST. (iyroacopic Force of the Revolving Motor. [1629]. In the copy of American Aero of June 15th, appeared an article under the above heading that told of " actual results " obtained by "direct experiment" of Louis and Laurent Seguin, builders of the Gnome Motor, which, they claim, show that the gyroscopic force generated in a revolving motor on a turn is not sufficient to be dangerous. Let us examine closely the report of the experiment in question. A 50-h.p. Gnome was mounted on a platform in such a manner that while running it could be turned either in a vertical or a horizontal direction. When the platform made a horizontal turn in 45 sees, the tendency to turn in a vertical plane (gyroscopic force) was over to the right or left, the turner begins to interfere with the lifting, and the lifter has something to say to the turning. If the list amounts to 45°, then each rudder is equally a turner and a lifter (Fig. 3). The system is no longer simple and easy, but complicated, obscure, and full of danger. It is now recognised—but it has taken a sorry list of tragedies to bring the fact home—that it is dangerous to turn sharply in the air. People talk loosely of "side slips" (apparently inwards), just as they talk of " holes in the air," neither having any existence in fact. A "side slip" inwards is impossible. The term belongs to cycling, and the slip is caused by the wheels getting an insufficient grip of the ground to resist the centrifugal force of a sharp turn. The wheels slip outwards, and man and machine come to the ground. With aircraft the conditions are different. The machine is not propped up but suspended in the air. In turning, the wings resist the centrifugal force in addition to that of gravity. The machine balanced by a weight of 117 lbs. at a distance of 39 ins. from the axis. When the platform made a turn in 24 and 12 sees, the balance weight increased to 23-3 and 57*6 lbs. respectively. Now first and most important notice that they admit the presence of gyroscopic force in all these experiments. The claim is that it is not sufficient to be dangerous. Let us see. In a 12 sees, turn, the quickest one which they report, there is 57 lbs. of the force to be overcome. Now M. Bouchard Praceig, the French engineer, in /.<? Nature of March 4th, 1911, has shown that the gyroscopic force increases in the same ratio that the speed 01 the turn increases. A 12 sees, turn is a slow tum, and the slow turns never have been and never will be dangerous. The turns that have been fatal, such as those of Chavez, Blanchard, Hoxsey, Moisant, Johnstone and many others, have all happened in the fraction of a second, so quick that the eye of an observer could hardly see what happened. If a 12 sees, turn generates a 57-lb. blow, then a one 865
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