QUOTE(Sapper @ Dec 27 2009, 05:28 PM)
Isn't that what many call the "coffin corner?"
Ranger is right, naturally, and chillspiller has got a cool post going there with the moving diagrams.
A little word on coffin-corner. dive in anytime, anyone, to refine the follwing but - coffin corner was more in the days of DC8 and 707s and at a very high altitude 41, 45 thou etc. this area wa interesting as the aircraft had climbed up to its operational ceiling probably heavy, where the air is very thin and travelling very fast near mach crit.
The coffin corner problem was: As you were so high and the air was so thin - you were not only near mach crit speedwise but you were also near the stall warner. So, if you approached Mach Crit and got either the buffet or other signs (what are the signs? well just look at the other posts here. . ) then the obvious answer would be to slow down.
The problem was, that to slow down meant that you also had to increase alpha (nose pitch up slightly) to maintain altitude - as - if you slow down then the aircraft will descend, a bit, more on that later. so, if you increase angle of attack (alpha) then you hit the stall warner [which can be 5 knots prior to an actual stall]. So, if you let it go down gently you then have the risk of hitting mach crit as anything that goes downhill, travels faster than anything climbing - nature. I KNOW we can descend without an inherent increase in airspeed but strange things occur at very high alt sometimes - like you raise the nose - stall warner. You lower the nose - stall warner. You raise the nose a tad and let it go and you get stall warner one and stall warner two. . . as it bounces from high nose att - to pitch down and back - all based on a changing, angle of attack. the `bounce` being a slight and gentle occilation of slight nz up to slight nz dn. L1011, leave it alone and it will fly fine = as for other aicraft - dunno.
Assuming you had got yourself boxed into a little corner where, you were so close to mach crit and so close to the stall condition as to render the `natural` descent impossible due to . . .excessive speed - then you were in theory, stuck.
Ranger is right and I will remember to slow down, but if we do that then we MUST accept a descent or, we are demanding an increase in alpha as we have just commanded a lower airspeed - and what happens to lift when you reduce the speed . . . ? see?