High Speed Flight Flashcards
TRANSONIC FLIGHT
● Modern jet aircraft cruise in the speed range of M 0.8 to M 0.92.
● Since the airflow over the wing must travel faster than the flow under the wing, it will become supersonic long before the aircraft as a whole becomes supersonic (M 0.7 to M 0.8)
● Transonic flight is defined as when parts of the airflow over the aircraft and wing have reached supersonic speeds but not all of the airflow has done so.
● For many airplanes, the Critical Mach Number (Mcrit) is approximately M 0.72.
○ This speed is defined as the fastest speed possible without having any supersonic airflow over the wings
● This supersonic airflow over the wing does not bend down like subsonic airflow, and it develops a shock wave above the aft part of the wing.
● The result is increased drag and a tendency for the aircraft to pitch down.
○ This is sometimes called “Mach Tuck!”
● If left uncorrected, it can lead to the airplane getting caught in vicious cycle where it continues to pitch down!!
➢ It can also lead to reduced elevator effectiveness or even to an airframe failure!
● Recovery is simple - Just slow down
● The drag created by a shock wave is known as wave drag.
○ This drag is due to the separation of the air from the wing as the shock wave forms
● Sweeping the wings will also help reduce the drag due to the shock wave and therefore the tendency to have the aircraft pitch down.
● A swept-back wing will also improve directional stability.
● As the aircraft yaws to one direction, one of the leading edges is effectively longer than the other relative to the perpendicular airflow and corrects the yaw
Whitcomb Area Rule
● The Whitcomb Area Rule also helps reduce the transonic drag.
● This rule is creating an inward curve of the fuselage at or just aft of the point of wing attachment
Aerodynamic Instability in Transonic Flight
● Due to this aerodynamic instability in the transonic flight range, commercial aircraft must not only have the yaw damper engaged, but the autopilot must engaged above M.78.
● In a swept wing aircraft any rolling moment can cause yaw.
● The yaw damper uses accelerometers in the tail of the aircraft to measure tail swing (yaw) and to signal the autopilot which then commands the rudder servos to counteract the yaw
