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Aviation History
1913
1913 - 0579.PDF
Edited by V. E. Tractors v. Propellers. MR. G. P. BRAGG-SMITH sends us the following interesting com munication >v the above : " With reference to the question, tractors v. propellers, I have paid a considerable amount of attention to the relative efficiency of these two types of models, and from numerous experiments I have made I am fully convinced that that type in which the propeller is situated in the extreme rear is the most efficient. It is not necessary to look far to see the reason why the tractor type is less efficient. It is clearly obvious that a consider able percentage of the air displaced by the propeller in this type is thrown back on to the machine, thereby tending to check its progress by setting up abnormal head resistance. This, of course, also applies to some extent to machines of the Farman type, in which, although the main planes are no longer affected by the slip stream of the propeller, the tail and certain parts of the fuselage are still so affected. The most efficient position of the propeller, so far as all my experiments go, is in the rear of the machine, where the air displaced by the propeller does not come in contact with any part of the machine. " The foregoing not only applies to models but full-sized machines as well, and if experiments with models (of sufficient sue) were resorted to for finding relative values both in this instance and others as well, constructors of full-sized machines would, in my opinion, save themselves a considerable amount of trouble and expense. " 1 may add that the position of the propeller plays a very im portant part in contributing to the stability or instability of a machine, and there is a great deal to be said as regards the efficiency of the tail type and small surface forward type of machine, but to go fully into this matter would occupy considerable time and space." Mr. R. R. Drake (South Chingfotd, Essex), writing re the same matter, says : " I note your invitation to experts and others to give their conclusions upon the subject of tractors v, propellers, and would like, as one of the others, to submit mine along with them. From experiments I contend there is no real difference in the efficiency of similar sciews, whether tractive or propulsive, but that any difference which apparently exists arises from a difference in the efficiency of the two systems. " In the case of a propeller-driven model, one may set the planes to give the finest possible gliding angle, and, in consequence, flight is possible upon a minimum of rubber ; whereas, in the case of a tractor-driven model, if this angle be too fine, the additional lift obtained by the main planes from the slip stream of the propeller JOHNSON, M.A. results in over elevation, this rendering necessary a coarser gliding angle and, in consequence, more rubber. Actually, in order to get the same results from a tractor as from a propeller, the tractor would need to be more efficient ; for not only has it to contend with the inferior gliding angle, but it must drag the model through its own slip stream. By careful design it is possible to minimise this evil, but a tractor-driven model driven by a stated amount of rubber can never do the distance of a propeller-driven one, although it may approach it in duration of flight," The fact referred to by our correspondents, that in the ca-e of the tractor-type model, it has to be dragged through its own slip stream, is undoubtedly an important one, the effect being that the tractor pulls the machine forward, and the slip stream or reaction drives it backwards. If we keep the angle constant, the power required is proportional to the cube of the velocity; the necessary power to drive all the non-lifiing parts of the machine is also proportional to the cube of the velocity. Let us suppose we keep the weight constant, but double the speed, then the angle of inclination of the planes can obviously be reduced. The necessary power then, so far as mere support is concerned, varies at a lesser rate than the cube of the velocity, but the necessary power to drive the non-lifting parts through the air does not do so. Instead of lessening the angle and keeping the area constant, we can lessen the area of the supporting surfaces and keep the angle constant. Although the tractor-type is undoubtedly less efficient than the Canard, it does enable the span to be reduced, and this type, also, allows the designing of a body of true streamline form offering minimum resistance and possessing minimum weight, which, in the case of the Canard type, is very difficult of attainment, our streamline body, if deigned for minimum weight, now being " the wrong way about " to offer minimum resistance. There is, undoubtedly, great scope for both ingenuity and originality in the design of covered-in bodies or fuselages in the case of Canard-type machines. The whole subject being one of especial interest from the scient ifir point of view, we shall be pleased to receive further correspondence on the matter. The K.. and M.A.A.'s Programme for 1913. The programme of the Kite and Model Aviation Meetings o>" the present season is now ready, and can be obtained from the lion- Sec, Mr. W. H. Akehurst, 27, Victory Road, Wimbledon, S.W. MR. N. V. BRASNETT'S ORIGINAL OLYMPIA MODEL.—Note that the planes are staggered In the opposite direction to the usual custom, 601
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