Coffin Corner
In the aviation is a coffin corner : (, English literally coffin corner or) Q Corner designated the area in high altitude at which minimum flight speed and maximum speed to approach strong. The aircraft must not fly faster or slower in this area. At even higher altitudes it cannot fly at all, as the theoretical minimum speed is then higher than the theoretical maximum speed, so that there is no longer any flyable speed.
The term "corner" refers to the triangular shape of the envelope of the flight envelope . This is where the curves for the stall speed and the maximum Mach number intersect .
Stall - minimum airspeed
The dynamic lift of an aircraft depends on the air density , the airspeed and the angle of attack . If the speed is reduced, the aircraft must increase the angle of attack in order to generate the same lift and to be able to maintain the altitude. However, too large angle of attack leads to a stall ( stall ). The aircraft must therefore maintain a certain minimum flight speed V s so that the flow does not stop. This minimum speed is called stall (geleg. Stall speed) or stall speed ( Engl. Stall limit or stall speed ). Occasionally the lower limit of the coffin corner is also given as the minimum clean speed , which is higher than V s , but with which the ICAO standardized curve inclination of 25 ° can be maintained - in contrast to V s , which does not exceed a straight flight is possible.
The air density, and thus the flight altitude, also affects the minimum flight speed. The thinner the air, the higher the minimum speed ( True airspeed - TAS) must be set in order to avoid a stall .
A heavily loaded airplane needs more lift than an empty airplane. So at a certain angle of attack it has to fly faster than a lighter aircraft in order to obtain the necessary higher lift. Therefore, the minimum flight speed is higher for a heavier aircraft than for a lighter one (Fig. 2).
Overspeed - Mach limit
Because the air temperature decreases with altitude , the speed of sound also decreases . Even with Mach numbers from around 0.8, the air flows faster than sound in an area above the wing ( critical Mach number ). The compression shock when the flow exits this area is more pronounced in a wing profile developed for the subsonic area and is further forward. Behind it the flow separates, control flaps lose their influence. As the speed increases, the shock waves move backwards and with it the center of lift. The plane lowers its nose and continues to accelerate. This phenomenon, known as mach tuck , has caused several crashes. As part of the approval process, the aviation authorities certify a maximum operational Mach number (M MO ). In modern commercial aircraft, this is beyond the critical Mach number , but below 1.0.
In most smaller aircraft, it is not possible to exceed the maximum speed in level flight due to the power limits of the engine . During the Second World War, however, several types of aircraft came up against this limit, which is why it was researched with further dive flights. Several test pilots were killed when their planes broke apart. The highest Mach number recorded in Britain was 0.9 at altitudes around 40,000 feet in 1943.
hazards
The coffin corner in high-performance aircraft, from which the last reserves are extracted (e.g. Lockheed U-2 ), can be threatening . Here the difference between “too fast” and “too slow” is sometimes only five knots (approx. 9 km / h ). Any increase in the angle of attack, through turning or air turbulence ( Clear Air Turbulence ), leads to a stall. When the aircraft begins to shake ( Buffet ), the pilot must know immediately and precisely whether he is now too fast or too slow, since an incorrect correction would be fatal.
Service ceiling
The service ceiling is always below the Coffin Corner, as it is defined by the fact that a rate of climb of 100 ft per minute for propeller aircraft and 500 ft per minute for jet aircraft must be possible. However, this would no longer be the case near Coffin Corner.
Individual evidence
- ^ Peter E. Davies: Bell X-1 , Bloomsbury Publishing, 2016, ISBN 978-1472814661 , page 7