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Aviation History
1932
1932 - 1234.PDF
u SUPPLEMENT TO FLIGHT DECEMBER 1, 1931! THE AIRCRAFT ENGINEER order to obviate excessive sweep forward (tilt) of the blades the following relationship gives a guide. K = 14.5 0.65 (6) K = Aspect ratio. Pm= Maximum b.h.p. D = Airscrew diameter. This relationship is assumed to hold for these small airscrews. The method of obtaining the airscrew characteristics is a modification of that described in FLIGHT, February 20, 1931—" Calculation of Airscrew Characteristics." It is not known how reliable the methods employed would be when applied to such small airscrews and b.h.p's., but the results obtained will at any rate be relative. losses are 2.9 b.h.p. and in Case 1 the b.h.p. is 92.3, so that the b.h.p. lost due to this cause is 26.2 b.h.p. This is shown more clearly by Fig. 5, where it is seen that any further increase in design r.p.m., although giving an increase in b.h.p., would produce little increase in effective b.h.p. available. If one combines this with the drop in free efficiency shown in Fig. 4, the peak in effective t.h.p. occurs earlier than that of b.h.p. (see Fig. 6). Fig. 7 shows the Thrust h.p. at full throttle at any speed for all five cases, and shows also the t.h.p. avail able when cruising at normal r.p.m. Fig. 8 gives in graphical form the major character istics of the aircraft, where it is observed that rate of climb, maximum speed and cruising speed all peak at about 6,000 r.p.m. It is assumed that the primary object of the really 70 60 '50 <M) 30 FIG.3 PART i 1000 2000 3000 4000 5000 6000 DESIGN NORMAL R.P.M. 7000 •9 Sv<^ N^ FIG.4- PART I. f%«, - S5S2£N ^ ^S^ •y \^< 1000 2000 3000 4000 5000 6000 7000 DESIGN NORMAL R.P.M. Fig. 3 gives the variation of airscrew diameter with design r.p.m., it being assumed that the face pitch of the airscrew (P/) = Vjn at top speed, at which tne maximum permissible r.p.m. of the engine occur. The free air and net airscrew efficiencies, Fig. 4, clearly show the big losses as the airscrew size decreases. In Case 5 the losses due to slipstream effect and inter ference amount to 28.5 per cent, of the total power of the engine; in Case 1 this is reduced to 8.5 per cent. The power available in Case 1 is 34.4, so that the 50 D ft4 a 5 5 30 20 -< X rf tr *&\ ?Sr #£2-~~ F1G.6 PARTI 0 1000 2000 3000 4000 5000 6000 DESIGN NORMAL R.P.M. 1000 2000 3000, 4000 5000 6000 7000 DESIGN NORMAL r.p.m. light aeroplane is a matter of cost, not only initial, but running cost. It is safe to say that the slow-running engine will require no more attention than the fast- running one, so the maintenance cost will be the same, or perhaps in favour of the slow-running engine. The initial cost of the slow-running engine in the series under discussion would certainly be less. The criterion then seems to be the fuel and oil used. There are no statistics on the oil consumed by an engine, so that this will be taken as the same m.p.g. for all five engines. All the five engines run on practically the same throttle when cruising at normal r.p.m., so that unless fuel economy is a function of piston speed as well as the amount of throttling, the specific consumption for each engine cruising is taken as 0.6 pt./b.h.p./hr. The resulting consumption is shown in Fig. 9. An increase in r.p.m. from 1,500 to 6,000 means an increase of 20 m.p.h. (22 per cent.) in cruising speed at the expenditure of twice the fuel for the same distance. The choice of suitable engine r.p.m. eventually re solves into the one giving the minimum permissible rate of climb, since this quantity is the only important one adversely affected by decreasing r.p.m. Before concluding the first part of the article, a word 1152i
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