This paper addresses the necessity to incorporate an intake-engine matching procedure in any procedure to analyze the off design performance of engines designed to power high-speed civil transport aircraft. Most methods known to the author that are useable in the conceptual design phase of the aircraft (a phase characterized by the fact that very few design variables are known) only analyze the performance of the engine, without taking into account the design and performance of the intake. It will be shown in this paper, that the intake and the design exhibit a strong mutual influence, that cannot be neglected. An alternative method of analysis is presented. Particular reference will be made to the phenomenon of excess air and the resulting spillage drag. It will be shown that spillage air can be quite large in case of an HSCT design, especially in case the engine is designed for a high cruise Mach number. Assuming the presence of a sophisticated intake system generating multiple shocks in order to optimize the intake efficiency at the cruise condition, excess air at lower Mach numbers can amount to more than 60/100.
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