Speed Flashcards
What is stability?
Stability is a certain behaviour of a constant after it has been disturbed. There is neutral, positive and negative stability. If a constant, be it pitch, bank, speed, flight path or anything else is moved from its original position and returns to this position, it is said to be stable (positive stability). If it increases its deviations from the neutral is is said to be unstable (negative stability) and if it maintains constant deviations it has neutral stability.
What is Mach number?
Mach number is a true airspeed indication, given as a percentage relative to the local speed of sound.
What is Mcrit?
Mcrit is the aircraft’s Mach speed at which the airflow over a wing becomes sonic. The aircraft actual speed will always be lower than the airflow over the wing.
Why is Mcrit so critical?
At Mcrit the speed of the airflow over the upper surface of a wing approaches the speed of sound (sonic). This speed brings an increase in buffet and drag due to the shock wave, a nose down pitch at Mcrit (Mach tuck) and a possible loss of control.
How does the CoP change as an aircraft passes through Mcrit?
The CoP moves rearward due to: - the shock wave at the leading edge changing the pressure distribution due to the greater camber toward the leading edge - swept back wing experience this change at the thicker, inner and further forward part of the wing.
What is Mach tuck?
Mach tuck is the nose down pitching moment that occurs as an aircraft passes through Mcrit.
What is the purpose of a Mach trimmer?
A Mach trimmer automatically compensates for Mach tuck at Mcrit.
How does a Mach trimmer compensate for Mach tuck?
A Mach trimmer senses the speed of an aircraft and the automatically compensates for the nose down pitching moment by adjusting the horizontal stabiliser upwards.
What are the effects of compressibility?
- Compressibility error on dynamic pressure instruments (ASI overread) - Compressed air at leading edge of wing causing increase in shock-wave, drag and temperature
Explain the manoeuvrability margin envelope!
Manoeuvrability margin envelope is drawn on an x-y-axis with the x-axis representing the IAS (kts) of the aircraft and the y-axis the flight load fatcor (g).The envelope is formed by the upper speeds of an aircraft (Vne) and the lower stall speed of an aircraft. The lower end is variable as seen on the graph by the load factor which is a determining factor for stall speed. The higher the LF the higher the stall speed. The top and botton of the envelope is formed by the structural g-limits of the aircraft.
What is coffin corner?
The coffin corner is the speed of an aircraft at high altitude where Vs and Mcrit are identical. This is caused by the stall IAS of an aircraft increasing with altitude and Mcrit decreasing, due to the reduction in the LSS. There comes an altitude at which the aircraft can no longer fly any faster to prevent the stall from occuring due to the limiting Mcrit speed.
