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Aviation History
1931
1931 - 1345.PDF
91 DECEMBER 25, 1931 THE AIRCRAFT ENGINEER SUPPLEMENT TOFLIGHT The weight of a supercharged engine delivering a con-stant horsepower at a certain altitude would not be practically increased, and the specific fuel consumptionwould not) be much higher, because of the higher com- pression ratio and therefore the greater thermodynamicefficiency of such an engine. As the speed of the plane would be increased and the combined power-plant and fuel weight per horse- power would be constant, the range of the plane would increase with the speed for the same pay load ; and as the good flying qualities, landing speed, take-off, and climb would be maintained, we should have a more efficient, more desirable aeroplane. Fig. 1 shows the performance at altitude of a Pratt <fr Whitney " Wasp " Series SC engine, which has a 5.25: 1 compression ratio, and a 10: 1 impeller gear ratio, and develops 450 h.p. at 6,000 ft. altitude. Fig. 2 shows the altitude performance of Wright " Whirlwind " J.6 engines and Fig. .'5 that of English •' Jupiter " engines. The Diesel engine would seem to offer many advan- tages for altitude flying. As it has a high compression ratio and excess of air and oxygen, the power at altitude decreases slower than with the gasoline engine. This has been verified by different flight tests, where the top speed increased to a maximum value at an altitude of 8,000 ft. Further, the lower specific fuel consumption will, of course, extend the cruising range. The power at altitude of a 225-h.p. Packard Diesel engine is given in Fig. 4. II.—Speed at Altitude Before developing the mathematical equation for high speed at altitude, let us consider the problem for a Fig. 4.—Power at Altitude: 225 h.p. Packhard Diesel engine certain plane, the polar curve of which lias been estab- lished from the test results as shown in Fig. 5. The maximum lift coefficient (Gi — 1.5) and wing loading (16.35 lb. per sq. ft.) determine the stalling speed of 65 m.p.h. Due to the ground effect, the land- ing speed is few miles lower. To any lift coefficient there corresponds a certain speed at sea level—shown by means of the curve at the left in Fig. 5. The efficiency of flying at any speed, or any angle, can be determined by projecting that point corresponding to the desired speed, or angle of attack, on the polar curve from zero point on the vertical line through CD = 0.10, where we find the value of L/l). The efficiency, as deter- mined by the L/'l) ratio, depends very much on the wing loading, and therefore on speed. For planes with low wing loading and with largo wing area, a higher value FIG.5. OO I2O Mi left per Hour 5,000 FIG. 7. 10,000 15,000 M,000 25,000 30,000 Attitude in Feet £«)0 200 300 D——•** w •^200 100 b^5 100 120 140 160 Miles per Hour Fig. 5.—Polar curves of Lockheed "Vega" High-wing Monoplane. Fig. 6.—Thrust-horsepower curves of Lockheed "Vega." Fig. 7.—Increase in top speed at altitude, due to constant power. Fig. 8.—Thrust-horsepower curves. 1264 c
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