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88 PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL We have m h v* a) Eqruvalent air speed v b) True air speed Fig.2.12 Levelftighterrvelope. T-D 1 = ~p V2S(CDO +kC~.) 1 = ~p V2SCDO + (jk,_WS Equation (2.85) can bc written as follows: A/r4 _ Tlr2+ B = O Where = ~pSCDO . B = (2kpVg.) (2.85) (2.86) (2.87) (2.88) AIRCRAFT PERFORMANCE SoMng Eq, (2.86), we obtain Vmax = Vmin = ~] (2.89) (2.90) Hcywever, a more elegant form of analytical solution can be obtained using the nondimensional. form ofEqs. (2.27) and (2.28). With y = 0, Eqs. (2.27)'and (2.28) take the following form, Then, U4 _ 2zu2 +1 = 0 n:1 ILmax = ~ Umin = V Vmax - llmax VR Vmin -. ILmin VR (2.91) (2.92) (2.93) (2.94) (2.95) (2.96) (2.97) Given the variation of thrust available with altitude, using this approach, we can construct the level fiight envelope for a jet aircraft whose thrust is independent of flight velocity. At the absolute ceiling, Unun - Umax, which gives z = 1. Physically, when z - 1, T - Dmn. As said before, at absolute ceiling, level fiight is possible only at only one speed where the drag is minimum. Whether the flight is possible at Vmin depends on the magnitude of the level flight stalling velocity, which is given by Vstall - (2.98) If Vmm < VstaU, then the airplane cannot fly at Vmin in steady level flight because it asks for a lift coefficient in excess of CL,max. For such cases, the level flight solution Vnun has no physical significance, and VstaU effectively 'oecomes the low- speed solution. To flyinlevelflight at any speed at aspeed V such that V V < 'V~x, the pilot has to throttle the engine so that the drag at this speed equals the thrust available as schematically shown in Fig. 2.13. . AIAA EDUCATION SERIES ., o..
40 --. o. . ..~ ' . ; \- TL671,4 .P28 Performance, Stability, Dynamics9 and Control of Airplanes Performance, Stability, Dynam.ics, and Control of Airplanes Bandu N. Pamadi NASA Langley Researcb Center Hampton, Mrginia EDUCATION SERIES J. S. Przemieniecki Series Editor-in-Chief Air Force Institute of Technology Wright-Patterson Air Force Base, Ohio Published by American Institute of Aeronautics and Astronautics, Inc. www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：动力机械和机身手册1
