The intake stream tube at cruise operates in a subcritical condition (see Section 10.8), which is complex and makes the intake-drag estimation difficult. There is spillage during the subcritical operation due to the stream tube being smaller than the cross-sectional area at the nacelle highlight diameter, where external flow turns around the lip creating suction (i. e., thrust). This can be considered precompression, ahead of the intake, when the intake velocity is slower compared to the free-stream velocity expressed in the fraction (Vintake/VTO). At (Vintake/VXJ) < 0.8, the excess air flow spills over the nacelle lip. The intake lip acts as the LE of a circular aerofoil. The subcritical air-flow diffusion ahead of the inlet results in preentry drag called additive drag. The net effect results in spillage drag, as described herein. The spillage drag added to the friction drag at the lip results in the intake drag, which is a form of parasite drag. (For the military aircraft intake, see Section 9.17 and Chapter 10.)
• spillage drag = additive drag + lip suction (thrust sign changes to -ve)
• intake drag = spillage drag + friction drag at the lip (supervelocity effect)
Figure 9.6 shows intake-drag variations with the mass flow rate for both subsonic and supersonic (i. e., sharp LE) intake.