ESDU Engineer

Auxiliary Inlet Aerodynamics

Subsonic drag and pressure recovery of rectangular planform flush auxiliary inlets with ducts at angles up to 90 degrees.

A flush auxiliary inlet is designed to supply air to various aircraft systems or components via a duct. Space restrictions often require ducts to be set at large angles to the onset flow direction and it is important to know the effect on the inlet drag and pressure recovery of turning the flow through large angles.

ESDU 03006 provides an empirically developed graphical method for predicting the pressure recovery (in terms of ram ratio) and drag of auxiliary flush inlets with rectangular planforms. The ram ratio is the total pressure at the throat less static pressure in the undisturbed onset flow divided by the dynamic pressure in the undisturbed onset flow. The method for ram ratio provides the value for inlets with a 7° approach ramp at low Mach numbers and the value for inlets without a ramp at Mach numbers up to 0.9. A graph provides values for a reference inlet geometry; the graph takes account of the effects of mass flow ratio and boundary-layer momentum thickness in the entry streamtube. Factors applied to that ram ratio value treat the influence of different inlet throat width-to-depth ratios and duct setting angles from those of the reference geometry.

To obtain the maximum drag (assuming the loss of all momentum), a theoretical analysis splits it into its component contributions (momentum drag and spillage drag). Graphs enable the momentum drag to be obtained as a function of Mach number up to unity and a factor depending on the momentum thickness of the boundary layer in the entry streamtube. The spillage drag is also given graphically as a function of onset Mach number, mass flow ratio and duct setting angle up to 90° for a sharp lip inlet. It is suggested that such data will represent an upper bound on spillage drag for a rounded lip geometry. Finally, an additional drag contribution determined empirically is required to yield the maximum drag and graphs from which that can be determined are provided taking account of mass flow ratio, duct setting angle up to 90° and Mach number up to 0.9. For inlets with ducts set at small angles, the drag will be less than the maximum as some momentum will be preserved; the reduction in the maximum is calculated theoretically in terms of the pressure coefficient in the throat. The method is illustrated with three practical worked examples which also serve to show the effects of duct setting angle, mass flow ratio and the use of an approach ramp on ram ratio, and the effect of mass flow ratio on drag.

Chris Batt is the Engineer within the Aerodynamics Group responsible for this Item and he can be contacted at cbat@esdu.