Speed Flashcards
What is speed stability?
Speed stability is the behaviour of the speed after a disturbance at a fixed power setting.
Speed is stable if an increase in speed causes and increase in drag, thus returning to original speed. And vice versa.
What is the critical Mach Number?
Is the speed at which the airflow over a wing becomes sonic.
It is usually less than the aircraft’s. Usually at speeds around 0.72 Mach experiences sonic Mach 1 over the wing. This causes a rapid increase in drag.
What are the characteristics of Mach Number?
1 - Initial Mach Buffet, causes by shock waves on upper surface
2 - An increase in drag because of the breakdown of airflow
3 - Nose Down change in attitude (Mack Tuck) occurs at Mcrit
4 - A possible loss of control
What is Mach Tuck?
Nose down pitching moment an aircraft experiences as it passes Mcrit
Due to the aft movement of Centre of Pressure. This is because shock waves move towards leading edge, this disrupts lift distribution.
How do you mitigate Mach Tuck?
Mach Trimmer
Artificially correct for Mach Tuck by sensing aircraft’s speed and signalling a proportional upward movement of the elevator.
Explain Speed margins?
VMO/MMO Max operating speed in knots or Mach Number
VDF/MDF Max demonstrated flight diving speed
Margin between the top and bottom ones is relatively small
What is coffin corner?
Aircraft’s absolute ceiling. Where speed of Mach Buffet and Pre-stall buffet occur are coincident.
For constant Mach Number the IAS decreases with altitude due to decreasing LSS. To prevent decreasing to it’s stall speed Mach Number must be increased. Which results in increased IAS.
