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Coffin corner (also known as the aerodynamic ceiling or Q corner) is the altitude at or near which a fast fixed-wing aircraft's stall speed is equal to the critical Mach number, at a given gross weight and G-force loading. At this altitude 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 right of a flight envelope chart where the stall speed and critical Mach number lines come together. ==Aerodynamic basis== 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. Air conducts sound at a certain speed, the "speed of sound". This becomes slower as the air becomes cooler. Because the temperature of the atmosphere generally decreases with altitude (until the tropopause), the speed of sound also decreases with altitude. (See the International Standard Atmosphere for more on temperature as a function of altitude.) A given airspeed, divided by the speed of sound in that air, gives a ratio known as the Mach number. A Mach number of 1.0 indicates an airspeed equal to the speed of sound in that air. Because the speed of sound increases with air temperature, and air temperature generally decreases with altitude, the true airspeed for a given Mach number generally decreases with altitude.〔Clancy, L.J. (1975), ''Aerodynamics'', Section 1.2, Pitman Publishing Limited, London, ISBN 0-273-01120-0〕 As an airplane moves through the air faster, the airflow over parts of the wing will reach speeds that approach Mach 1.0. At such speeds, shock waves form in the air passing over the wings, drastically increasing the drag due to drag divergence, causing Mach buffet, or drastically changing the center of pressure, resulting in a nose-down moment called "mach tuck". The aircraft Mach number at which these effects appear is known as its critical Mach number, or Mach CRIT. The true airspeed corresponding to the critical Mach number generally decreases with altitude. The flight envelope is a plot of various curves representing the limits of the aircraft's true airspeed and altitude. Generally, the top-left boundary of the envelope is the curve representing stall speed, which increases as altitude increases. The top-right boundary of the envelope is the curve representing critical Mach number in true airspeed terms, which decreases as altitude increases. These curves typically intersect at some altitude. This intersection is the ''coffin corner'', or more formally the ''Q corner''. The above explanation is based on level, constant speed, flight with a given gross weight and load factor of 1.0 G. The specific altitudes and speeds of the coffin corner will differ depending on weight, and the load factor increases caused by banking and pitching maneuvers. Similarly, the specific altitudes at which the stall speed meets the critical Mach number will differ depending on the actual atmospheric temperature. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Coffin corner (aerodynamics)」の詳細全文を読む スポンサード リンク
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