High Speed Flight Flashcards
Define Mach Number
TAS/ LSS
Describe how Mach Number changes as we climb at a constant IAS
When climbing at a fixed IAS
• TAS?
• increases
• Since the air temperature is decreasing, the speed of
sound is also decreasing as we climb
• Remember Mach No = TAS/a
• So, for a given IAS, Mach No increases significantly as
we climb
• We CROSSOVER from climbing at a fixed IAS to a fixed
Mach No when the 2 are equal (typically about 28000’)
• Climbing further at a fixed Mach No leads to a slow
reduction in IAS
Describe how IAS changes as we descend at a constant Mach Number
Initially descent is at a given Mach No
• As we descend into warmer, denser air, that given Mach
No corresponds to faster and faster IAS
• Once again we CROSSOVER from descending at
a Mach No to a given IAS when the two match
• As we descend further at a constant IAS, the Mach No
shown will?
• Decrease further and further
How would crossover altitude changes if our climb profile changed from 255/0.65 to 270/0.65
The crossover would happen at a lower altitude
Describe a shock stall , why does it happen
Shock wave produces a large increase in pressure
downstream
• This is transmitted to the boundary layer
• The adverse pressure gradient causes the
boundary layer to thicken and then separate
• Called shock induced separation and is effectively a
stall, called a”shock stall
What happens to the centre of pressure as a swept wing aircraft accelerates? Is this to a greater or lesser extent than an equivalent straight wing aircraft
the Centre of Pressure moves
continually rearwards as the aircraft accelerates
• This leads to a strong nose down pitching
moment
Swept Back is greater extent
What can this movement of the centre of pressure lead to
This leads to a strong nose down pitching
moment
What causes Mach Tuck
• A nose down pitch with increasing speed (bad)
• Rearward movement of the Centre of Pressure
• Modification of the airflow over the tail
• Most tailplanes work in a region of downwash from the mainplane
• Disturbed flow from the shock induced separation reduces the
downwash and reduces the normal tail down forces
What system does the F100 have to combat Mach Tuck? Under what condition does this system activate
Mach Trimmers
• Automatically deflects the tailplane or elevator slightly more than is
required to counter Mach Tuck
• maintains positive longitudinal stability
• In the F100 the Flight Augmentation Computer provides Mach trim
compensation when the autopilot is disengaged
What effect does a swept wing have on CL Max
Reduces it
What causes Dutch roll
Excessive lateral stability leading to Dutch Roll
What system on the F100 prevents Dutch roll
Yaw Dampers solve this: on the F100 only one Yaw
Damper can be unserviceable
A swept wing aircraft has a tendency to pitch nose up at the stall ? Why
Swept wing stalls first at the tips.
Describe the approach to stall phenomena of the F100
A pre-stall warning is provided at each control column by means of a stick shaker. The shaker provides a clear warning of impending stall in all configurations and most flight conditions. At high altitudes airframe buffet will occur before shaker activation. A stick pusher is installed to limit a dynamic overshoot beyond the stall angle of attack. The stick pusher is activated as a function of angle of attack and angle of attack rate. Inability to arrest the descend rate
Describe the approach to stall recovery actions for the F100
An (approach to) stall can occur at any attitude and at any point of the aircraft’s flight envelope and may be recognized by one or more of the following phenomena: • continuous stick shaker activation • buffeting, which could be heavy at times • stick pusher activation • inability to arrest descent rate.
