Airplanes often have an on-board auxiliary power unit to provide electrical power and compressed air to various systems throughout the airplane. When the airplane is on the ground, the auxiliary power unit is the main source of power to drive the environmental control systems, air driven hydraulic pumps, and the starters for the engines. Auxiliary power units may also provide pneumatic and electric power during flight.
Campbell et al, U.S. Pat. No. 5,655,359 shows a typical arrangement for mounting an auxiliary power unit in the tailcone of an airplane. Referring to FIG. 1, an intake duct 30 extends between an intake opening 32 in the exterior surface 16 of the tailcone and the air inlet of the auxiliary power unit 10. The intake duct 30 defines an airflow passage 34 through which the auxiliary power unit 10 draws in air. An inlet door 33 is pivotally attached to the airplane 14 at the aft end of the opening 32. When the inlet door 33 is open, air flows in the direction of the arrows under the influence of the auxiliary power unit itself (ground operation) or the airstream of the airplane and the auxiliary power unit (flight operation).
These inlet doors also referred to as scoops and typically comprise a closing wall contoured to the fuselage and have channel forming side 5 plates contoured to the side walls of the inlet against which they fit upon closing. An example of such an inlet door is disclosed in Vanderleest et al, U.S. Pat. No. 4,418,879.
It has been found that during ground operation with air entering the inlet from all directions that the air flow around the side walls creates corner vortices or distortions. These vortices or distortions flow through the intake duct 30 and into the compressor inlets of the auxiliary power unit. Because air in the vortices is spinning, the leading edge of the compressor blades rotate through the vortices the incidence of the air on the leading edge flips resulting in losses and a reduction in compressor performance.
Accordingly, there is a need for an inlet door assembly that does not generate corner vortices during ground operation.