EXPERIMENTAL REVIEW OF TRANSONIC SPILLAGE DRAG OF RECTANGULAR INLETS

Inlets sized for supersonic aircraft operation are oversized at transonic speeds. Spilling excess air around the inlet creates spillage drag which can seriously penalize the low altitude penetration range of mixed mission aircraft. Spilling, also, creates inlet cowl lip suction forces which can cancel a portion of this drag, but available data on spillage drag and its partial recovery on the cowl lip were not sufficient for necessary design and performance studies. In 1964, NAA/LAD designed and built a workhorse model for in-house tests of pitot inlet spillage drag. Under contract AF33(615)-2496, the workhorse portion of this model was fitted with rectangular supersonic inlets. Wind tunnel tests were conducted. Testing was done in the 0.7 to1.4 Mach number range. The model had four interchangeable ramps, four sets of side plates and ten interchangeable cowls. Low drag flow spillage requires decreasing the inlet flow area by (1) increasing the external ramp angle or (2) rotating the cowl inward. Test data show that ramp spillage creates lower total drag. The minimum spillage drag configuration would use minor deflections of both ramp and cowl. However, the cowl actuation weight penalty must be considered. Experimental transonic ramp pressure drags were normalized and compared with transonic similarity work on wedge airfoils. These ramp drag data, together with cowl drag and spillage drag correction (KADD) factors developed in this report, are valuable tools for inlet design and performance studies.