AV

Description

In aviation, coffin corner (or Q corner) refers to the point at which the Flight Envelope boundary defined by a high incidence stall intersects with that defined by the critical Mach number. In other words, coffin corner occurs when, for a specific gross weight and G-force loading, the aircraft has climbed to an altitude where the speed differential between the onset of low speed stall buffet and the onset of high speed Mach buffet approaches zero.

As an aircraft climbs towards the altitude that defines its coffin corner, the margin between stall speed and critical Mach number becomes smaller and smaller until the Flight Envelope boundaries intersect. At this point, any change in speed would result in exceeding one or the other of the limits. In the most critical case, simply turning the aircraft could result in exceeding both limits simultaneously as, in a turn, the inside wing slows down whereas the outside wing increases speed. Likewise, encountering turbulence could result in a "beyond limits" change in airspeed.

Solution: Consider the flow of air over an airfoil. As the gas expands around the top surface near the Ieading edge, the velocity and hence the Mach number will increase rapidly. Indeed there are regions on the airfoil surface where the local Mach number can be greater than the free-stream Mach number.

By definition, the free-stream Mach number at which sonic flow is first obtained somewhere on the airfoil surface is called the critical Mach number of the airfoil.

The drag divergence Mach number (not to be confused with the critical Mach number) is the Mach number at which the drag coecient of an airfoil, a wing or an aircraft begins to increase rapidly.

The drag divergence Mach number is usually close to, and always greater than, the critical Mach number. Generally, the drag coecient reaches its maximum value at Mach 1.0 and begins to decrease again after the transition into the supersonic regime.

The large increase in drag is caused by the formation of shock waves on the upper (or lower) surface of the airfoil, which can induce flow separation and adverse pressure gradients on the aft portion of the wing.

For an airfoil, both the critical and drag divergence Mach numbers are influenced by the airfoil thickness, camber, and angle of attack. In general, it can be said that the critical and drag divergence Mach number are reduced by

- increasing the airfoil thickness; - increasing the airfoil camber;
- increasing the angle of attack.

Solution: The phugoid, or long period oscillation, involves noticeable variations in pitch attitude, altitude, and airspeed but nearly constant angle of attack. Such an oscillation of the airplane could be considered as a gradual interchange of potential and kinetic energy about some equilibrium airspeed and altitude.

The period of oscillation in the phugoid is quite large, typical values being from 20 to 100 seconds. Since the pitching rate is quite low and only negligible changes in angle of attack take place, damping of the phugoid is weak and possibly neg- ative. However, such weak or negative damping does not necessarily have any great consequence. Since the period of oscillation is so great, the pilot is easily able to counteract the oscillatory tendency by very slight and unnoticed control movements. In most cases, the necessary corrections are so slight that the pilot may be completely unaware of the oscillatory tendency.

The short period oscillation can be assumed to take place with negligible changes in velocity and altitude. It consists of a pitching oscillation during which the airplane is being restored to equilibrium by the static stability and the amplitude of oscillation is decreased by pitch damping.

The typical motion is of relatively high frequency with a period of oscillation on the order of 0.5 to 5 seconds. For a conventional subsonic airplane, this mode is characterized by heavy damping with a time to damp to half amplitude of approx- imately 0.5 seconds or less. Usually, if the airplane has static stability stick fixed, the pitch damping contributed by the horizontal tail will provide sucient dynamic stability for the short period oscillation.

In general, the period of the phugoid increases with increasing airspeed. For high altitude flying aircraft, density has a considerable influence on longitudinal static stability. At high altitude, the period of the phugoid oscillation is increased, and the damping is reduced.

An important aerodynamic parameter, the eciency (lift/drag ratio, L/D) has a negative influence on the phugoid damping. The more ecient the aircraft, the less damped is the phugoid.