Coffin corner (aerodynamics)
Coffin corner (also known as the aerodynamic ceiling or Q corner) is the region of flight where a fast fixed-wing aircraft's stall speed is near the critical Mach number, at a given gross weight and G-force loading. In this region of flight, it is very difficult to keep the airplane in stable flight. Because the stall speed is the minimum speed required to maintain level flight, any reduction in speed will cause the airplane to stall and lose altitude. Because the critical Mach number is the maximum speed at which air can travel over the wings without losing lift due to flow separation and shock waves, any increase in speed will cause the airplane to lose lift, or to pitch heavily nose-down, and lose altitude. The "corner" refers to the triangular shape at the top of a flight envelope chart where the stall speed and critical Mach number are within a few knots of each other. The speed where they meet is the ceiling of the aircraft.
Consideration of statics shows that when a fixed-wing aircraft is in straight, level flight at constant-airspeed the lift on the main wing plus the force (in the negative sense if downward) on the horizontal stabilizer is equal to the aircraft's weight; and its thrust is equal to its drag. In most circumstances this equilibrium can occur at a range of airspeeds. The minimum such speed is the stall speed, or VSO. The indicated airspeed at which a fixed-wing aircraft stalls varies with the weight of the aircraft but does not vary significantly with altitude. At speeds close to the stall speed the aircraft's wings are at a high angle of attack.
At higher altitudes, the air density is lower than at sea level. Because of the progressive reduction in air density, as the aircraft’s altitude increases its true airspeed is progressively greater than its indicated airspeed. For example, the indicated airspeed at which an aircraft stalls can be considered constant, but the true airspeed at which it stalls increases with altitude.
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