[49APU] 冷知识：为什么737的APU进气口会有一个涡流发生器？

In April, 1977 a vortex generator was added to production airpianes, mounted on the APU inlet door, to improve inlet ram recovery and thereby improve the pressure difference across the APU during flight. 

The vortex generator successfully reversed the former negative pressure difference across the APU so that aerodynamic cranking could assist the starter motor during in-flight starts. Service experience showed that the resulting improvement in starting reliability fell short of expectations. 

Consequently, modification to the vortex generator, consisting of the addition of a trailing edge articulated flap, was introduced. When the APU is not running the flap is faired with the vortex generator and so imposes no significant drag penalty. When the APU inlet door is opened the flap articulates inboard through an angle of 25 degrees. 

The articulated flap provides a substantial increase in pressure difference across the APU. The resulting aerodynamic cranking assist, in conjunction with an acceleration limiter crack pressure of 60 PSIG and P/N 899981-1 fuel atomizer, provides reliable starts at 35,000 feet altitude even after three hours of cold soak. The articulated flap vortex generator was incorporated in production by PRR 32612-11, Line Position 526, and is covered for retrofit by Boeing Service Bulletin 737-49-1037.