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Aviation History
1914
1914 - 0987.PDF
SEPTEMBER 25, 1914. QFJJGHT) "THE AEROPLANE OF TO-MORROW." VARIABLE SURFACE, CAMBER AND INCIDENCE, AND THEIR EFFECT SPEED AND SAFETY. By L. DE BAZILLAC, Engineer, Ecole Superieure d'Aeronautique de Paris. (Concludedfrom page 968.) Gliding.—If we pass now to the descent with the motor shut off, there is no doubt that the smaller the minimum gliding angle 7, the greater the distance that the machine will be able to travel, the height at the commencement of glide being equal; consequently the greater will be the pilot's facility of choice of a landing place. 7 is given to us by the equation found above for ascending flight: where H= o; hence: tan -' IVzos, i We have tan » = ON SUSTENTATION, us examine again the curve of inclination, which represents tan 7. The larger f-S— •'-') is, the quicker this curve becomes straight again. If one wishes to give security, it is necessary to carry back, as far as possible, the limiting angle corresponding to the steep fall, and consequently reduce A'v = tan 7 = S whether one increase* 5 or A', with X = \ + AjA'S. Hence, tan 7 = „ Ay A", = (A'xS + A) V' A, = -VT7 h\Sx x S -_ Kx + 5 The (Fig. 9) graphically representing this formula or curve of inclination becomes straight again less quickly, in order to give A* "0 *^* ^i*-*l • — 00 as ,, is smaller. It is more inclined in its lower part, as the lift is the greater for each angle of attack, the resistance to motion being equal. In order to diminish 7, as A„ remains approximately constant, and KXSX increases much more slowly than S, it is necessary to give to the surface the greatest possible value, and to increase the curvature with the angle of attack (the minimum of the curve, j,' corresponds to the optimum angle of the machine ; this angle is about 7° with actual machines). The velocity of translation is, in every case, inversely proportional to the square root of the lifting surface, and the larger the surface the smaller will be the speed of landing. The advantage of a larger surface is, therefore, very obvious in gliding. It becomes a primary condition for the angles near to the angle of zero incidence. We know, in fact, that in order to pass from descent at 45 , which is done by certain aviators (motor shut off) to a steep fall, it is sufficient to produce with the actual machines a range of incidence angles in the neighbourhood of l°, sometimes even lower than that. This raDge of incidence angles is much more considerable for machines of large surface than for those of reduced surface. Let diminishes A0. Now A<, arrives rapidly at a limit below which it cannot be reduced, and A\S\ increases more slowly than S, there fore it will be necessary to increase S. A large aerofoil, rationally curved for each angle of attack during descent with motor shut off, will, as a result, diminish the chances of diving, reduce the speed of landing, and diminish the gliding angle, whatever the angle of attack may be. But it is evident that the larger the angle of attack, the slower will be the speed of landing. One has utilised, until now, only the angles corresponding to the first speed of flight, or the first branch of the curve of inclination, that is to say angles included between o° and the optimum angle of the machine %. There exist, however, angles larger than the latter, and corresponding to the second branch (Fig. 10). These angles characterise a second speed which experience would, as a last resort, be able to decide. Why do aviators seldom make use of these? The reason is simple. **n Fig;. 10. Suppose we had a machine having an angle of attack smaller than on; if one desires to ascend, it will be necessary, in every case, to increase this angle. If one wishes to descend, it will be necessary to diminish it. Now, if the angle of attack is larger than a,, the contrary will happen. If one increases the angle of attack, the machine descends ; if one decreases the angle of attack, the machine ascends ; so that in order to manoeuvre when a is greater than a„ the pilot would have to make manoeuvres the reverse of those he makes when o < a0. Then it is necessary, in order to make use of the second speed, without fear of accidents, to reverse the controls from the moment when the gradient of the trajectory changes sense— that is to say, from the moment when an increase of the angle of attack corresponds to an increase of the ratio * • A, In horizontal flight the reversal of the controls, or the utilisation of the second speed, would permit one to obtain a much greater range of variations of speed. It would render flight possible with the minimum of power ; the certainty for the motor of working in 987
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