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Aviation History
1932
1932 - 0845.PDF
FLIGHT, AUGUST 19, 1932 C ORRESP ONDENCE RELATIVE EFFICIENCY [2806] Perhaps it would interest your readers to know that in the vacation we have endeavoured to evolve a formula which will indicate the relative efficiency of various machines. This formula is not intended to be conclusive, since we cannot obtain sufficient data about each machine. It applies to civilian machines fully laden. The data we have taken entirely from your excellent publication. It is interesting to compare certain machines by our formula and we have spent considerable time amusing ourselves in this way. This formula is as follows: — ^ _ .. TT /disposable load + fuel + oil\ R.E. = Everling H.S.F. -f — • - )25 \ tare^weight , / Speed range \ / No. of engines 10 / \pints of fuel/B.H.P./Hr Example: — Relative efficiency of the Percival Gull = 27 + (.75x25) + ra + /lX2 10/ \ -53 = 27 + 18-75+ 10-3 + 3-63 = 59-7 (correct 1 place decs.) We append herewith a list of results for some more machines: — Percival Gull: Hermes IV 59.7 Monospar St. 4: 2 Pobjoy "R" .. .. 59.0 Breda 32: 3 Wasp Junior 57.5 D.H. Fox Moth; Gipsy IHA 56.2 A.W. Atalanta: 4 Double Mongoose .. .. 55.0 Fokker F. XVIII: 3 Wasps 54.7 Spartan Cruiser: 3 Gipsy III ... ... ... 53.9 D.H. Fox Moth: Gipsy III 52.0 Lockheed Orion: Wright Cyclone .. .. 51.1 (Undercarriage not retracted.) D.H. Moth (wooden): Gipsy HIA .. . . 50.4 Curtiss-Reid Courier: Gipsy III ,. .. .. 49.0 Bloudek XV: Cirrus II 47.6 D.H. Tiger Moth: Gipsy III 46.2 D.H. Moth (wooden): Gipsy II 45.3 Ford 8A: Hispano Suiza ... ... ... ... 43.9 Sabca: 3 Renard 120 h.p 41.6 Canadian Cub: Scorpion II .. .. .. 41.6 Lockheed Orion (undercarriage retracted) ... 67.9 Comper Swift: Pobjoy " R " (gross weight 1,060 lb.) 57.2 Doubtless there are small mistakes due to the paucity of our knowledge of the fuel consumption of various engines at full throttle. It should also be noticed that we have taken the wing area to include the ailerons. Whether this is correct or not we do not know, but some of our results for the Everling H.S.F. seem to agree with those published. The constants, as far as we can judge, seem to bring the value of each factor to within a fair degree of its true proportion. We should like to add in this connection that our judgment may be at fault for neither of us is over sixteen years of age and we have relied upon the written opinions of others in this matter. We should very much like to be corrected if anywhere we are found to be grossly inaccurate. We are fully aware that our formula does not take into account such features as view from cockpit, take-off and landing runs, price, and safety devices. Yet our formula includes most of the features usually considered of paramount importance, namely, aerodynamic efficiency, ratio between load carried and structural weight, landing speed and top speed, economy of fuel and immunity from forced landings due to engine failure. From the results obtained it can be seen that machines considered to be outstanding in the category under which they fall are grouped together near the head of the list. Thus, it seems that the efficiency of machines of different sizes and designed for totally different work may be in some way compared. Wimbledon, S.W.19. F. NAISH—F. PAGE. August 15, 1932. RISKS OF CLOUD FLYING [2807] I am extremely interested in Fit. Lt. W. E. P. Johnson's letter in today's issue regarding accidents due to heavy clouds such as in the Meopham and Bossom crashes. It becomes clearer to me almost every day that in this country, which is so variably undulating and has such variations of weather even within small areas on the same day, the ordinary aeroplane has very many disadvantages when away from good aerodromes or the very small areas of flat ground which are only in the eastern districts. Surely, therefore, despite prejudice in many quarters, a development oi the Autogiro principle is going to be the most helpful in the avoidance of such catastrophes from the fact that it is definitely anti-stalling, and even if it should get near the ground when in a cloudbank its angle of incidence will be much nearer to the horizontal than the orthodox type of machine and therefore will not want any violent pulling out. I do not think designers think sufficiently about safety. Incidentally, this principle, provided the price can be brought down, answers the pleas of Mr. A. H. Downes- Shaw and Mr. W. O. Manning for better value, more " blunder proof," and also for small field work. London, W.C.2. H. C. NEWTON. August 12, 1932. COMPARISON OF " GULL " AND " S.6B." [2808] I have read with interest Mr. van Hattum's letter and your comments in FLIGHT of August 12 on the above subject and would like to make one or two observations in this connection. In the first place, it is hardly fair to take 407 m.p.h. as being the top speed of the " S.6B." for comparison with other aircraft. This speed represents the top speed of the aircraft plus some of the speed gained in the initial dive. The official figure for the steady full throttle level speed of the aircraft which broke the speed record is 390 m.p.h. and the corresponding b.h.p. is 2,530. This speed compares directly with the figures of 145 for the Gull " and the corresponding Everling figures are 23.1 and 27 respectively, which looks pretty bad for the " S.6B." However, we can go a step further. From a knowledge of the approximate propeller conditions of the two air craft it is possible to estimate their probable propeller efficiencies at maximum speed. These are 88 per cent, and 82 per cent, for the " S.6B." and " Gull " respectively. Substituting these in the expression 7,- 2KX, we obtain: — for the "S.6B." K> = -0191 „ „ "Gull" KD = -0152 i.e., the drag coefficient of the " S.6B." is 25 per cent, higher than that of the " Gull "; surprising at first sight, but easily explicable, since these drag coefficients are, of course, referred to the wing area which means that, as between any two aircraft having for example the same drag but different wing areas, that which has the smaller wing area will show the larger KD> but it may not be any the less efficient at maximum speed. In other words the Everling high speed figure utilises the wing area as a measure of the size or drag of the aircraft, which is quite all right for conventional types, but cannot possibly be expected to fit in with an exceptional type like the " S.6B." where wing area has been cut down to the utmost limit consistent with safety in the hands of very skilful pilots. The writer is not in any way finding fault with the Everling method of comparing maximum speed, but is only emphasising the great disadvantage inherent in all general methods of comparing top speeds, namely, that of finding some dimension of the aircraft which is a measure of its drag. The writer generally uses the square of Munk's Equivalent Monoplane Span instead of wing area. This, although suffering from the above same disadvan tage, does at least make monoplanes and biplanes directly comparable, which is not the case witri the wing area. The ideal dimension is the total projected front surface of the aircraft, since it gives a measure of the efficiency with which the necessary cross sectional surface of an aircraft has been streamlined; it has been found to give very good results indeed but it is obviously quite imprac ticable for general application. It would, however, be verv interesting to compare the Kns of the " Percival Gull " and " S.6B." when referred to their respective cross sectional surfaces in front elevation. Edgware, Middx. RICHARD M. CLARKSON. August 12, 1932. 789
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