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Aviation History
1914
1914 - 0988.PDF
every case, without fatigue, and of running without seizing even in descent. In gliding it would facilitate a slow landing. It would enable the minimum vertical speed of fall to be attained. Let us suppose that the second speed is, by experience, realisable and stable, and let us consider an experimental curve of M. Eiffel as a function of the incidence (Fig. II). We see that Ky maximum corresponds to an angle bordering on 20°. This angle would make the machine alight too obliquely. We will take as the maximum *y Fig. 11. angle of attack for the glide an angle slightly smaller ; for example, 15°, or the angle corresponding to the minimum speed of fall. This angle approaches a0 with the increase of the surface and camber. We will select the camber for whichjlthe Ky corresponding to that angle is as large as possible. We shall find ourselves then, with the second speed, in the best possible conditions for landing. We know the quarrel that has raged around the second speed, between its partisans and its detractors. Some of them have shown theoretically that it was not practicably realisable. It is important to apply calculations to concrete cases in order to obtain from experience their confirmation or their denial, experience pro nouncing the final judgment in everything appertaining to applied science. Whatever it be, the accentuation of the longitudinal V, resulting from the increase of the angle of attack with the second speed, will have a favourable effect on the automatic stability. The flight path of the machine will also be more fixed. Note I.—If one wishes to make a rapid descent, it is evident that the conditions previously established must be reversed, and the surface of the wings must be flattened and reduced to the strictest minimum possible. We have seen that the optimum angle of the wings, taken separately, allows of the most rapid horizontal flight. For this reason, as well as to prevent a dive, it would be logical to take such an angle as the lower limiting angle to descend rapidly with the reduced surface. Nevertheless, immediately before landing, the wings must be extended and present the maximum surface which characterises slow landing. They will then support their highest pressure, and the drag component of this pressure will act as a powerful air brake. It is important to know their safe load. When the machine lands, after a rapid descent, with reduced surface, at the optimum angle of the wings, it will have a speed V given by the relation : W-yx — A"xl s V'1, yj being the gliding angle, A'Xi the horizontal unit component corresponding to the optimum angle of the wings, maximum surface. The pilot will then tilt up his elevator in order to give to the machine the maximum angle of attack and the maximum surface. (It is certain that the machine will not ascend again, since A'y will have attained its maximum, and can only decrease.) At this moment the machine will still have, by virtue of its inertia, the speed V; the pressure on the wings will then be equal to K2 being the total resistance, K^, A^j the unit components corresponding to the maximum angle of attack. In order that the safety of the machine may not be jeopardised it is necessary that the pressure be inferior to the load Z, the safe load for the wings. So that C TXT C IXf Q > c Jp ^K2^+K-n and Q > - . -jp- ^MV The safe load must then be taken at as much larger as is the angle of attack at landing ; surface, camber and weight will be larger also. If Q remains constant one can increase the weight only on con dition that one diminishes the range of the values which the angle of attack can take, i.e., in diminishing the facility of manipulating the machine. Nevertheless, whatever may be the safe load, the surfaces must, at the moment of landing, be such that the load per square metre of aerofoil does not exceed 25 kilogs. This condition seems to characterise the correct landing. It does not exclude the possibility of very great speeds in horizontal flight, if one bears in mind the employment of a surface that can be halved, and of a variable camber. Note II.—It must be noted that for a fixed type of variable wing V the ratio - of the two speeds lower and higher corresponding to the two limiting angles of the glide (optimum angle of the wings in horizontal flight, and maximum angle of attack during gliding) is in proportion •y/f-f [S and s being the maximum and minimum surfaces of the wings) and consequently constant. We have, then, = constant = C, and V - v be as much larger as V the speed of horizontal flight is higher. This speed depends on the power utilized, as also on the shape and size of the wings. If we suppose a surface capable of varying to the extent of half S we have = 2. If we suppose, further, that the greatest lifting force Ky can become equal to eighteen times the smallest, which is not impossible by the variation of the camber and the utilization of the second speed, we have further Ky k = I Ky V and = v K: = -/18 X2 = 6. This ratio of flight speeds is twice as great as any obtained at the present day. It defines, then, the lowest and highest limiting speed of the glide which can be realised by a machine with a variable surface, camber and incidence. V Under the same conditions, we may presume the ratio y — 5 to define the lowest and highest limiting speeds of the horizontal flight with a motor sufficiently flexible, or with two motors of unequal power, working in conjunction. Stimming up, the greatest reserves of sustentation, safety and speed belong to the aeroplane that is endowed with a variable surface or camber, with a system of reversed controls, or with these three factors together. If the ascent has to be rapid, the surface will be as large as possible, not exceeding the limiting surface, and rationally curved for each angle of attack. This ascent must be carried out at the angle a„t defined above, and which corresponds to the maximum vertical speed of ascent. If the descent must be slow, the maximum surface, rationally curved, will be taken at the optimum angle of the machine, or, better, at the maximum gliding angle within the second speed. For rapid descent the surface must be flattened and reduced, in the same way as in horizontal flight, at normal speed, to the lowest possible mini mum, at the optimum angle of the wings. The optimum angle of the wings will be, in every case, the lower limiting angle. For reconnaissance purposes, one will take the angle corresponding to the second branch of the curve of inclination, maximum surface with an arched profile, if one possesses a motor flexible enough and has the means of using the second speed. Such are the advantages of the aeroplane with variable surface and camber, and some of the methods of obtaining them. Among the qualities that one can look for in the Aeroplane of To morrow, variable surface and camber are certainly the most desirable, in order that it shall be able to enter into everyday use, as regards transport. That is why, without making pretension of here solving the problem of variable speed we have, at least, foreseen the direction in which the researches should be made.
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