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Coffin corner (also known as the aerodynamic ceiling [1] or Q corner) is the region of flight where a fast but subsonic 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 an airplane in stable flight.
This accelerated air can, and does, reach supersonic speeds, even though the airplane itself may be flying at a subsonic airspeed (Mach number < 1.0). At some extreme angles of attack, in some airplanes, the speed of the air over the top surface of the wing may be double the airplane's airspeed. It is, therefore, entirely possible to have both ...
English: Graph of Speed vs. Altitude for U-2 high-altitude airplane, region depicting Coffin Corner. Stall and Mach limits for one particular gross weight clarified in color. Note that the operational envelope gets narrower if the plane is more heavily loaded.
At the top of the B-47's envelope, about 35,000 feet (11,000 m), it was in "coffin corner". [49] That means that at this level, which produced the most range at most weights due to fuel consumption, there was an envelope of 5 kn (9.3 km/h) between maximum mach and stall speed. For the B-47 to cross the Atlantic Ocean, it had to be flown this high.
Transonic flow patterns on an aircraft wing, showing the effects at and above the critical Mach number. In aerodynamics, the critical Mach number (Mcr or M*) of an aircraft is the lowest Mach number at which the airflow over some point of the aircraft reaches the speed of sound, but does not exceed it. [1]
This narrow window is called the "coffin corner", [42] [43] because breaching either limit was likely to cause airflow separation at the wings or tail. [44] For most of the time on a typical mission the U-2 was flying less than five knots (6 mph; 9 km/h) above stall speed.
On a Monday morning in late March, the confronted was a reticent 44-year-old man. He sat in the far corner of a second-floor room at the Grateful Life Center, dressed in jean shorts and a T-shirt, looking isolated and forlorn. Around him sat a few dozen fellow addicts–a jury of much younger peers–keen to let him have it.
Also, since the mach limit in the example is well above the stall speed at the 1G flight altitude limit, the described peril of the coffin corner is not very apparent. Many aircraft are able to climb well into their coffin corner region where stall and max become very close, but this aircraft has hundreds of knots of separation between the two.