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Aviation History
1909
1909 - 0332.PDF
JUNE 5, 1909. bottom members pass above and below the spars, which are thus entirely enclosed, so that both surfaces of the deck have a perfectly smooth contour. Area Required for a Given Weight. The extent of the surface required for a given weight depends upon the speed of flight, and some idea of the figures which might be realised, if the practical conditions agree with Mr. Lanchester's theoretical hypothesis, will be found in a table which appeared in FLIGHT, page 297. Merely as a practical figure to begin with, however, a lift of 2 lbs. per square foot should not over-estimate the lifting capabilities of a machine travelling through the air at a speed in the order of, say, 35 to 40 miles an hour. Better effects ought to result from decks of relatively greater span than from those in which the " aspect ratio " (i.e., the ratio between the span and the chord or fore-and-aft dimension) of the surface is small. As in most practical machines, however, the absolute dimension itself is an important limiting factor, and apart from the unwieldiness of large spans, there is apt to be constructional difficul- ties in the way exceeding values such as 40 feet which are now employed. In practice a reasonable aspect ratio seems to be about 5, although by special design and under special circumstances it might be possible to exceed this value. In any case the requisite area seems to involve the biplane system of construction except very high speeds are indulged in so as to make a monoplane possible. The Area for "a Glider. That which has already been said applies in the main to the glider as well as to the flying machine proper, except that an allowance of half a pound per square foot of supporting surface would be more in the nature of a proper estimate, since the speed through the air would hardly exceed 20 miles an hour. This speed is made up by a head-wind of, say, 15 miles an hour, which is perhaps the strongest that it would be safe to experiment in, and a velocity relative to the earth of 5 miles an hour, which is about as fast as a couple of men running could succeed in launching a machine. Tails and Elevators. By itself an arched aerofoil is quite unstable in flight, and needs some sort of device to ensure safety. Lan- chester has shown that, assuming the conditions of a certain hypothesis as defining the conditions of the atmosphere, it is possible to convert an arched aerofoil into an automatically stable flying machine by the addition of a suitable tail member. The practical exemplification of this system may be seen in the Voisin flyer. Wright, on the other hand, disregards the artifice of a tail and relies upon hand manipulation of an elevator for the mastery of his machine. A similar member also, it should be noted, exists in the Voisin type. The " elevator," although conveniently so called, has not a great deal to do with ascent. In fact it has nothing at all to do with continuous ascent, because that alone is the outcome of an increased development of power beyond that necessary to sustain horizontal flight. The elevator is initially a controlling device for damping out oscillations, but conversely, it can be used to produce them, and thus serves a useful purpose as a means of making the flyer "jump " an obstacle. Its manipulation disturbs the distribution of pressure, and it is thus a means of performing a number of useful operations which come within its scope. The correct size for an elevator is doubtless susceptible of theoretical solution on the basis of a suitable hypothesis, which may or may not define the practical conditions, but it is obvious that practice is the only reliable guide in this matter at the present time, and it is on such details as these that the early experience of pioneers becomes invaluable. An area in the order of one-seventh or one-eighth of the area of the main surfaces would seem to approximately define the size in use on practical machines to-day, but the best size for any particular model could only be determined by experiment, and the same applies to the distance at which it should be mounted in front of the machine. Use Strong Outriggers. One point on which a warning may be given in this connection is to make the outrigger framework strong enough. It is very difficult to say what is the magnitude of the severest shock that the elevator has to withstand, but there is not the least doubt that the pilot's safety depends more on that member than any other, and a liberal allowance of strength should be made. An out- rigger is the kind of member which is probably only just strong enough when it begins to look too strong. On the Short flyer, where the elevator stands out about 8 ft., spars 2^ ins. by 1^ ins. have been introduced into the out- rigger framework. One reason for making this part of the machine as strong as possible is because it is very liable to suffer through the shocks of landing. As to the dimensions and position of tail, if it is decided to use one, the details can only be established by experience. It is unquestionably a very tricky member to get quite right. Under certain conditions it may be inclined to interfere with starting, and possibly also with manoeuvring in the air. It increases the moment of inertia of the machine about the transverse axis, and conversely it affords a leverage through which a locally adverse gust might conceivably have an un- pleasant effect upon the machine as a whole. Power Required for Flight. There remains in this cursory, although not very brief, review of the building of a flyer, the question of motive power and propulsion. In the case of a glider the motive power is gravity, and the course of flight always inclined towards the earth. The designer's object will therefore be to construct a machine with the least possible resistance, so that the energy available, represented by his launching altitude, will carry him as far as possible before he touches the ground. On a motor-driven flyer the object is to overcome the gravity attraction and to sustain flight indefinitely in a horizontal path. Once again, however, the machine having the least resistance (which as a glider would have travelled furthest) will require least power, and that is why gliding experiments are so useful and instructive, as they provide so much direct light on this somewhat abstruse problem. The Three Load Units. In any case, however, whatever the resistance may be, it is always permissible to express it as a fraction of the total weight. The weight is made up in the main of three parts, (1) framework, (2) power-plant, (3) pilot. Of these, nothing but the weight of the pilot is initially fixed, for the weight of the framework depends upon the weight of the engine, which in turn is governed by the total load itself. The only way to commence is to start 334
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