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Aviation History
1927
1927 - 0049.PDF
JANUARY 27, 1927 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT H.P SPEED CURVE & POWER BALAMCE SHEET HIGH PERFORMANCE SINGLE ENGINED TRACTOR BIPLANE(LAND%) WITH DIRECT DRIVE WATER COOLED ENGINE 1-4 18 20 2-2 2-4 SPEED/STALLING SPEED 2-6 Fig. 3. Dropln.Revs,52 W r«|-urn«d as useful work in actual climb 32% (*lirv H.P. required ho Sustain M/c 18-1% 100 60 60 40 20 of course, partly due to the smaller speed range, and any possible engine temperature effect can only be disentangled from propeller phenomena if a large number of tests are available. An examination of a great number of tests of de Havilland aeroplanes, extending over 11 years, shows that on the whole there is a much greater drop in power on the climb in air-cooled than in water-cooled engines. Figure 3.—This figure relates to a water-cooled, ungeared, outfit of fairly high speed range, and shows the inevitably bad propeller and slip conditions, which are. however, com- pensated to some extent by the small drop in revolutions. As the speed range of aircraft is increased, the need for the variable pitch propeller becomes more insistent, but if per- formance at great heights only is required, where the speed range is much contracted, this is not so much the case. Never- theless it may be found that when operating from temporary war aerodromes exceptionally good " take-off " and " climb " qualities will always be useful. A word is necessary, perhaps, as to how the " horse-power "' required, shown in these curves, is arrived at. There are, of course, many different ways of doing this, but the following seems to be more free from uncertainties than others. The level .speed and rate of climb are carefully ascertained for ground level, and it is then assumed that the propeller efficiency at speed and drag of the wing surface alone is known (there is much full scale, theoretical and other evidence on this point). The intercept between the wing and total horse- power is then considered to vary as the cube of the speed. The horse-power available curve is put in in the usual way. It will then be found that the measured climb is less than would be indicated by the intercept between these curves and the difference is debited to the propeller as '" slip loss " or rather additional losses from obstructions over and above those incurred at speed. This somewhat crude method of dis- playing what is measured in routine tests has certain advantages when comparing many different results. There has always been some discussion as to whether engine power varies more nearly as the pressure or the density. 100 80 H.P. SPEED CURVES & POWER BALANCE SHEET SINGLE ENGINED TRACTOR BIPLANE (LAND M/c) WITH VEE AIRCOOLED GEARED ENGINE EFFECT OF VARIABLE PITCH PROPELLER. wirH B. CONSTANT PITCH P OPfLLCRWITH VARIABLE PITCH PROPEL LER Rg. 4. li-70* WITH CONSTANT PITCH PROPELLER100 20 Min. H.P. required ha sustain M/c 29-7 % WITH VARIABLE. PITCH PfWtGLLEfl H.P. nefunwtd as useful work in acHio' c'innb 335% VProptlkr > 1-6 1-6 20 SPEED/STALLING SPEED 2-2 2-4 4fic r> 2
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