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Aviation History
1910
1910 - 0979.PDF
NOVEMBER 26, 1910. I/SH 1 1 • DIAGRAM I. Graph, showing the thrust in lbs per rP(E) obtained from different . values of Slip Stream velocity (\0;from the formula £ " Mrf — N.B The horse power is thai actually expended in the slip stream itself . V SLIP STREAM VELOCITY Ft.perSec. at any rate the case with the designer oi a helicopter, he >vill naturally desire that the entire disc area should be effective, and that the velocity should be uniform. Any sacrifice of area is detrimental on grounds that were explained at the beginning of this article, and any variation from uniformity of velocity likewise results in ISO h rV s. ' t,„ a ,. 5 " -L So « , ' ! f 1 1 jl \ \ I \ \ ' ! 1 • 1 \ \ welocitv is equivalent to ar» effective blade ai^»le of 5 situated 0-6 of the radius from the ax.i&of rotation — N.p. The horse power is thai actually expended in the— flip stream ^j | j 1 ; ' i- 900 TJ~ 1 TIP sptire Ft per S-c inefficiency, because those currents in the main stream that flow at a more rapid rate than others waste power proportionately to V:l while they only add to the thrust in proportion to V-. The net result must, therefore, be a loss. Whichever way the process is regarded, however, the fact remains that the practical values obtained from any screw can always be resolved to the basis that forms the founda tion of the foregoing analysis, and for the reasons stated above this method of comparison is eminently helpful. D.. OF SCRE W F t 5 C 20 5 10 -/ \l / / \ \ / / \ /A / ^ *fs $> J* .^^^ ^^--^^\^i r^ \ i 1 1 "" •^x^-- toe :oo Me ••00 soo iM 700 SOU DIAGRAM 4- THRUST IBS Graphs snowtng the Ihrusls pF«tuced by screvrs of different dla»W*rs D operating M different tip sp^ds V under the conOilnt. Mated m diatom 3 FormutA " Tk«Bt " 53oo SLIP »TRE* Thus, for example, suppose we consider a case represented by the fourth problem above. The data there given somewhat approxi mates to actual values that have been obtained. It will be noticed that the efficiency works out at 33 per cent., and if we suppose that the screw was direct driven the entire loss must be accounted for in the screw itself. This brings into consideration the vexed question of skin friction, and in view of the fact that a propeller of 10 ft. diameter running at, say, 1,200 r.p.m. has a lip velocity of some 600 ft. per sec. or 400 m.p.h., it is really a little difficult to know what view to take of the situation, seeing that no actual skin friction experiments have been made at anything approaching these speeds. It is, however, not necessary to confine ourselves solely to the question of skin friction when seeking for sources of inefficiency in a static thrust screw. The slip stream is certainly not uniform, and it is questionable whether the cross sectional area is correctly defined by the disc area. The turbulence in the slip stream, including the spiral character of its main current, must necessarily be a cause of lost power, but the numerical values of these factors await the results of practical experiments. For the time being the nearest approximation that can be made is to measure the brake- horse-power of the engine and the thrust obtained, and to work out an all-round efficiency on the basis of the example already discussed. The examples given above are useful in the design of a screw, as preliminary calculations for the estimate of the minimum revolu tions and pitch required. In the absence of more exact practical data it must be left to the personal experience of individual con structors to make such compensations as they may deem necessary to create the effective slip stream that is undoubtedly required to produce the given thrust. As it may be helpful to some in obtaining a general understanding of the problem, we have prepared a few charts and tables so as to illustrate the above conclusions in a graphic manner. Diagram I shows the helicopter efficiency throughout a range of slip steam velocities, and it must be under stood that the power referred to in the value E is the power actually manifested in the slip stream itself, so that an allowance must be made for screw inefficiency. Thus if the transmission and screw losses are 33 per cent, the overall helicopter efficiency will be one- thkd of the value obtained from the graphs. Thus suppose the graph indicates an efficiency of 60 lbs. per h.p. this will be the 1000 2000 woe «w *xx> ww 1000 sooo 977
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