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Unformatted text preview: 2.5. BRAYTON 51 Solving for the thrust force, neglecting the small differences in pressure, one gets, F = ˙ m ( v 5 − v 1 ) . (2.294) The work done by the thrust force is the product of this force and the air speed, v 1 . This gives  ˙ W p  =  F · v 1  =  ˙ m ( v 5 − v 1 ) · v 1  . (2.295) Now the efficiency of the cycle is a bit different. The net work of the turbine and compressor is zero. Instead, the propulsive efficiency is defined as η p = Propulsive Power Energy Input Rate =  ˙ W p   ˙ Q H  , (2.296) =  ˙ m v 1 · ( v 5 − v 1 )  ˙ m  ( h 3 − h 2 )  . (2.297) Note the following unusual behavior for flight in an ideal inviscid atmosphere in which the flow always remains attached. In such a flow D’Alembert’s paradox holds: there is no aerodynamic drag. Consequently there is no need for thrust generation in steady state operation. Thrust would only be needed to accelerate to a particular velocity. For such an engine then the exit velocity would equal the entrance velocity:...
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 Fall '11
 ParkSou
 Dynamics, Thermodynamics, Energy, lbm, J. M. Powers, propulsive efficiency

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