Monday, July 29, 2019

Movable Stabilizer

Swept-wing airplanes inevitably suffer from pitch-up near stall, due to the tendency for their wing tips to stall before the wing root. This same effect is compounded by compressibility effects when nearing stall at high altitude (which brings high Mach number into play).

The fuselage itself can contribute to pitch-up at high angles of attack. An example of that can be related to the 737MAX engine nacelles (or pods).

Gen. Chuck Yeager credited the trimmable stabilizer as the key technology for dealing with Mach Tuck, or shifting the lift aft with supersonic speeds causing a pitch down. While Mach Tuck is managed separately, the principal objective with a trimmable stabilizer is to minimize drag along with maximizing the elevator for whatever flight condition or center of gravity.

The following discussion is based largely on the writing of D. P. Davies, Handling the Big Jets, third edition. I have copied in some of his figures in the discussion. Anyone interested in these matters is highly encouraged to obtain and read his book, it is amazing.

Various regulatory factors are listed at the end.