Originally Posted by
Dougdrvr
Where does low speed Mach buffet fit into this ?
Doug:
I've never heard of
low-speed Mach buffet, but there is such a thing as
high-speed Mach buffet.
High-speed Mach buffet happens on aircraft that typically have relatively thick airfoils (like airliners). I've felt it in both the 727 and 747-400. If you get close to redline speed, the air that goes over the top of the wing, following Bernoulli's equations, speeds up....to the point that it (locally) goes supersonic. This happens over the thickest part of the wing, or canopy, or cockpit windows (which is supposedly why the 727 cockpit is so loud). It then slows down as it matches-up with the flow at the trailing edge.
You may have seen this at airshows when a fighter comes ripping by at about 550 knots in humid conditions: you see momentary "cloud" or "wave" forms over the thickest parts of the wing, intakes, fuselage, or canopy. If he pulls some g as he does it, it becomes more pronounced, as the air going over the top of the curved surface must now speed up even more (for more lift coefficient).
During the time the air was supersonic, though, it usually detaches the boundary layer, so that when it slows down again, it is turbulent. A traditional stall produces buffet because of detached laminar flow and the resulting turbulence. So, even though the mechanism was different (One is extreme angle of attack, the other is almost no angle of attack), the symptom is the same: you feel buffet.
In airliners, this buffeting air usually is felt as aileron "buzz," as swept-wings have a natural tendency to stall wingtips-first. The turbulence is right in front of the ailerons so it gets transmitted through them.