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Aviation History
1914
1914 - 0111.PDF
JANUARY 31, 1914. "A WARNING A REPLY TO MR. BUSK By HARRIS BOOTH, A WHEN I wrote my article, " A Warning to Pilots," I never supposed that a serious defender of the gravity-type of air speed indicator would arise. Since, however,'Mr. Busk has published an article intended to prove that the speedometer is wrong and the velometer is right, it is incumbent upon me to deal with his ideas rather fully. Mr. Busk is really only actually wrong in one point; but when this is corrected, the consequent modifications in his proof bring it entirely into line with mine. This point is as follows :— After correctly pointing out that the lift is equal to the apparent weight under all conditions, stalling and falling included, he goes on to say that the " maximum lift is equal to (maximum lift coefficient) x (area of planes) x p x (speed)3 " : that is correct also. Then he says that it follows that "for safety it is necessary that (apparent weight of aeroplane) < (a constant) x p x (speed)2" : this is wrong ; he should have said " for safety it is necessary that (weight of aeroplane) < (a constant) x p x (speed)2." From this one error Mr. Busk naturally goes on by steps which are right in themselves, to a false conclusion. If Mr. Busk will try the effect of correcting this one slip in his proof, and the errors which follow from it, he will find that his calculations lead him exactly where mine led me. Of course, I do not know for certain whether Mr. Busk's error proceeds from a slip or from an incorrect opinion. If he really is of opinion (which I do not for a moment believe) that the safety condition is, as he says, " (apparent weight) < (a constant) x p x (speed)2," the following reductio ad absurdum will immediately convince him. Substitute in the alleged safety condition the equation " (lift) = (apparent weight)" and you arrive at the statement "for safety the condition is (lift) < (a constant) x p x (speed)2." Now, looking back to see exactly what is meant by this "constant," we see that this statement is equivalent to " for safety the [/HCNf] TO PILOTS." AND MR. BAIRSTOW. M.Inst.C.E., A.F.Ae.S. condition is (lift) < (maximum lift)"—a condition which is always fulfilled. Are aeroplanes then always safe ? There is one other point in Mr. Busk's article on the velometer about which I want to make some remarks. It appears that he takes more interest in the critical angle of attack than in the critical speed. Now, as a matter of fact, the reaching of the critical angle causes failure of control, while the passing of the critical speed causes actual falling. As a personal preference, being a passenger, instead of a pilot like Mr. Busk, I would rather be out of control than falling—but I don't want to start a discussion on the relative demerits of the Devil and the Deep Sea. Mr. Bairstow has entirely misunderstood the question. What I said in my article, "A Warning to Pilots," applied, of course, to machines which are unstable longitudinally. Such machines require a speed indicator, and the attention of the pilot to form what Mr. Bairstow has aptly called a "stable combination." Mr. Bairstow has apparently considered the case of a stable aeroplane, and, of course, there is no need to fit a speed indicator in this class at all. If Mr. Bairstow's letter really applies, as I imagine, to a stable machine, let him work out the corresponding case for a machine which is just neutral—I suppose that is possible—in the longitudinal sense. Mr. Bairstow further says that my calculations refer only to the instantaneous motion. This is incorrect. The next number of the Aeronautical Journal will con tain an article in which I give a complete proof of the theory of the gravity-controlled air-speed indicator. When Mr. Bairstow has seen, this he will, I think, admit that my statement of the condition of affairs appliesyfrr the whole motion, and not only for the instantaneous or initial motion. ® ® » (V? THREE HISTORIC MACHINES IN LINE AT HELIOPOLIS.— In the centre is Marc Pourpe's 60 h.p. Gnome-Morane-Saulnier, which he flew from Cairo to Khartoum and is now flying back—this is the identical machine on which Garros crossed the Mediterranean ij on the left is Vedrines 80 h.p. Gnome Bleriot which was flown from Paris to Cairo ; while on the right is Bonnier's 80 h.p. Gnome-Nieuport, which has also been flown from Paris to Cairo. Ill C 2
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