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Aviation History
1919
1919 - 1554.PDF
!£'IS DECEMBER 4, 1919 But in connection with supercharging the factor causing the above phenomena is of extreme importance. This is that if a machine be driven through the air at a constant L/D (equivalent to a constant angle of attack) the speed of level flight increases with the altitude inversely proportionally to the square roots of the varying air densities. To show this increase the writer has superimposed on Fig. 1 a series of curves of the form -£= .. / — ^max- where p — JP V Ky air density at any height. A scale of heights of flight has been marked on the left- hand side of the diagram, and from each of the speed range numbers a curve has been drawn proportional to the increase of speed with height when flying at the L/D immediately above the speed range number. To make the above clear it has been assumed that the machine is to be flown at an L/D of 0/0, approximately 90 per cent, of its maximum efficiency. Follow the dotted hue from the L/D scale to the point where the L/D curve is cut for the second time, the first cut can be neglected, it merely concerns " getting off," the speed range number immediately below this is 1.5. From 1 • 5 a dotted curve is interpolated between the speed increase curves running from 1 -4 and 1 -6 and the curve has been run up to 14,000 ft., corresponding to a speed range of 1 -965—see vertical dotted line. What this means is that we can maintain an efficiency of 90 per cent, with a speed range of 1 -96 by going to 14,000 ft., but if we try to keep this efficiency at sea-level our speed range is only 1.5, or, conversely, if we are to maintain a speed range of 1-96 and work at sea-level we have to be content with an L/D of 6-3, corresponding-to an efficiency of 64 per cent. Any test pilot knows that the top speed of an aeroplane falls off very little up to some few thousand feet below her ceiling, the air speed indicated falls more rapidly, but that is another matter. The curve shows the reason for this. Take the normal engine and machine made to fly at 1 «g6 speed range at ground level, her L/D is 6-3, but if she is still flying at the same speed by the time she has reached 14,000 ft. her L/D has become 9-0, the engine loss of power having been balanced by the gain in efficiency of the machine. In the above respect a supercharger-engined machine is in a very much happier position. If she is flown at a constant L/D her resistance remains the same at any altitude, but her speed increases, and the increase in power required to drive her is only proportional to this increase of speed. No other vehicle than the aeroplane has this property of being able to increase its speed with an only proportional increase of power. It is true t^at as the horse-power provided by a super charger motor only remains constant or is subject to a slight decrease, the actual gain in top speed at high altitudes will not be so great as the curves appear to indicate, but it will be very considerable. The folio-wing figures are calculated from the curves of Fig. 1 :— Ordinary Super- engine, charger. Landing speed (sea-level) 50 m.p.h. .. 50 m.p.h. Top speed, 1 -96 .. 98 m.p.h. .. 98 m.p.h. Height of flight .. .. Sea-level .. 14,000 ft. L/D at top speed .. 6-3 .. 9*0 Resistance per 10,000 lbs. 1,590 lbs. .. 1,110 lbs. Propeller h.p. required .. 416 .. 291 Engine h.p. at 70 per cent, propeller efficiency .. Summarising the evidence that :— (a) Owing to the low landing speed imposed on aircraft by natural limits, high speed at low altitudes involves a poor L/D ratio ; only some 60 to 65 per cent, of the possibilities of the machine being utilised. (b) When flying at a given L/D, high altitudes give a most valuable increase in flight speed with no increase in resistance and an increase of power only proportional to the increase of speed. (c) It explains the curious fact that the modern aeroplane, in spite of greatly reduced horse-power, is able to maintain a practically constant top speed up to a high altitude. To enable a quantitative comparison of the gain from supercharging to be made, the curve of Fig. 2 has been drawn. It is based on Fig. 1 and calculated from it, and shows directly the connection between speed in m.p.h. and loading in pounds per b.h.p. The lower curve is drawn for a normal-engined machine working at 2,000 ft., the upper for the same machine having a supercharged engine and working at 14,000 ft., in each case the basis of comparison is shaft horse-power and the same propeller efficiency, 74 per cent., has been assumed. 59S of the Fig. 415 1, we see 160 J 150 140 130 - 120 110 ( 1 i. 100 £ QO >L £ do T ^^ r Ft r • ' 6.2. . 7 9 % 1 i 1 1 1 0* • 1 ^ ^ 1 13 15 J ' \ COMPARATIVE PERFORMANCE Landing Speed 4J 8 m.p.h. TropeUer Zfficienaj 74% ^V __ > V • • . I""" • - - - - - 1 J 19 21 23 2S Zt /fo/B.HP 1556
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