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Aviation History
1927
1927 - 0725.PDF
SEPTEMBER 22, 1927 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT Fig. 3. - Formula suggested by Mr. Walker last February, which has been brought more up-to-date by adding some speeds estimated from the King's Cup performances. It is suggested that this formula would form a reasonable basis for discussion and modification. The machine used for fixing the constant K is the Curtiss R3.C1., and the reference line is given by the equation K B.H.P. where K 261-3 and S - Si - 0 125 S,, and Sj = span of larger plane, S2 — span of smaller plane. retioally every cliiTiTrnt amount of inequality in the two win<.'tf will require a different factor, although having the same (iap Span ratio. Fi,Lf. 2 shows the whole picture, which can be worked out from first principles. The general scale connecting (Jap Span ratio of equal wing biplanes with the equivalent monoplane is taken from X.A.C.A. Report No. 151 by Max Munk. It is probably unnecessary to go for great refinement in this factor so long as it is reasonably representative. S, — 0 • 12 S2 looks about rieht. or perhaps on the large side if it is to embrace all cases. Neither monoplanes nor biplanes will then be particularly favoured. In Fig. 2 two rather extreme cases of Gap Span ratio have been taken. The Actual Formula This year's King's (up Race has demonstrated that the speed which the formula credits the entrants with must be reasonably correct for the cleaner machines. Fig. 3 shows a proposal for a formula prepared by the writer, last February, which has been brought more up-to-date by adding some speeds estimated from the King's Cup performances, and which would probably form a reasonable basis for discussion and modification. It will be seen that this is the same as this year's formula, except for the biplane allowance, and the constant which fixes the actual speed of each machine. It is by no means suggested that the constant used in this curve is the best, but it is of some interest to see what machines of various speeds, but all fast in their class, would have to do to tie with the one which has been taken to fix the constant, i.e.. the Curtiss R3.Ci doing 264 m.p.h. The writer has rather limited, and possibly inaccurate, information, except as regards his own firm's machines, and offers this as the explanation of why these are so numerous in the figure. This curve seems to indicate that although the standard set is rather a high one. it is possible to design aircraft to be fast on this formula, whether their top speed is 70 or 2/0 m.p.h. It also seems likely that large and small aircraft could compote equally weil, for it must be remembered that no large aircraft have yet l>cen really cleaned up for a race. A curve bused on area per horse-power would also show a good conforniabllity with the various examples, except perhaps in the case of the monoplanes. The object of a formula, however, should be to get some standard to design to. and the objections mentioned above to the area/ H.P. scheme still stand. The argument may then be summed up as follows : Span is the fundamental dimension of aircraft which determines their capabilities. Chord is a more or less incidental dimension which, to a certain extent, regulates landing speed. There are, however, substitutes for it, such as high camber, slots, flaps, etc. The idea at the back of a formula should be that if every size and speed of aircraft are to be catered for. machines which are slow should be handicapped on the assumption that they possess the virtues attaching to low speed, while low speed, which is due to mere friction and inefficiency, should be discriminated against. Also that it is the speed quality which the formula tests. Engine Power The present arrangement of taxing a machine with the type-test B.H.P., while fairly simple, is unsatisfactory from the progress point of view. It is now immaterial to the designer whether he uses a light or heavy, large or small, engine. If an engine is improved so as to give more horse- power, the new type test figure is taken and the machine taxed with it. so that at present there is no incentive to progress. It would obviously be better to use weight or swept volume as a basis for fixing engine power, and allow the designer to get whatever power he can. so long as the formula race is a fairly long one. Experience has shown that there is not too much reliability in hand now. and designers must be credited with sufficient commonsense to wish, at least, to get round the course. Under these conditions, racing would be more interesting, and there would not necessarily be the present obligation to use full throttle all the time. Another way of assessing the horse-power would be on the actual petrol consumption. This, while giving the greatest technical interest of all methods, must unfortunately be ruled out, as there would be no racing element in it whatever if the winner was not known until calculations had been made. 668c
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