Use of Experimental Separation Limits in the Theoretical Design of V/STOL Inlets

Experimental data from several model inlets have been used to generate two parameters which are related to the limit of operation for inlet flow separation. One parameter, called the diffusion ratio, is the ratio of the peak velocity on the inlet surface to the velocity at the diffuser exit and is related to the boundary-layer separation at low throat Mach numbers. The other parameter, the peak Mach number on the inlet surface, is related to the separation at high throat Mach numbers. These parameters are easily calculated from potential flow solutions and thus can be used as a design tool in screening proposed inlet geometries. Any of the geometric design variables can be analyzed by this technique; but, this paper is restricted to the consideration of the internal lip contraction ratio. An illustrative example of an application to an inlet design study for a tilt nacelle VTOL airplane is presented. The study will show what value of contraction ratio is required to meet the operating requirements yet allow the inlet to remain free of separation as indicated by the two separation parameters.