High Altitude Aerodynamics Flashcards
How does stall speed change with altitude?
Stall speed increases with altitude
- Due to thinner air, a greater amount of airflow is needed to generate the same amount of lift.
What is Mach? (Relative Mach)
RM- ratio of the speed of the aircraft vs. the speed of sound.
Ex. 0.8 Mach = the aircraft is flying at 0.8 times the local speed of sound at that ambient temperature.
Mach 1= speed of sound
What is Critical Mach?
When TAS reaches critical Mach.
When any point on airframe the airflow reaches a supersonic speed.
TAS at which air anywhere around the aircraft reaches the speed of sound.
The aircraft’s TAS can be less than the speed of sound, but airflow around the aircraft can accelerate to reach Mach 1 (Speed of Sound)
What is a Mach buffet?
As airflow flows over wings, it accelerates.
Air in front of the wing may be subsonic, but it will accelerate past the speed of sound as it flows over the wings upper surface.
At trailing edge the air slows down again.
Result: a shockwave forms due to the pressure build up on the supersonic reach subsonic flow. Turbulent airflow forms behind the wing, causing a Mach buffet.
What is Mach tuck?
(After reaching Critical Mach) As speed increases- Center of Lift moves aft- losing elevator effectiveness-nose begins to pitch down which increases air flow even more.
How to recover: deploy all drag devices- spoilers, leading edge devices and even landing gear
After reaching Critical Mach- the supersonic air flowing past the leading edge of the wing generates far more lift than the air aft of the shock wave.
What affects the speed of sound?
Temperature of the air
- Speed of sound decreases as air temperature decreases, TAS to reach MMO decreases.
- Cold air increases your chances of reaching Mach 1 sooner.
Describe Coffin Corner
A region of flight where an aircraft is flying at a speed where if they were to slow down- result in a stall— if they were to speed up- result in Mach Buffet.
How to avoid? Lose altitude and go down to where the stall speed is lower.
What are the advantages of Swept Wings?
- The Sweeping wing delays supersonic airflow and raises the critical mach # by tricking the wing to think it’s flying slower then it really is.
HOW? It creates a different component of airflow: 1. Spanwise & 2. Chordwise (air flowing parallel to the chord line)
Chordwise is the only airflow that accelerates, so by reducing the amount of airflow flowing parallel to the chord line, you reduce the amount of acceleration - DELAYING YOUR CRITICAL MACH #
Allows for flight at a higher Mach # before you start to create supersonic airflow over the wing.
What are the disadvantages of Swept Wings?
At slow speeds the performance decreases.
Spanwise flow stacks up making it feel like the aircraft is flying slower than it really is
Stalls occur at the wingtip.
Poor yaw tendencies.
Where does a swept wing stall first?
Wing tip.
WHY?
The Spanwise flow that moves out towards the wingtip builds up near the tip, making it “feel” like it’s actually flying forward through the air slower than it actually is.
When the wing begins to stall, the tip stalls first, reducing aileron effectiveness.
What does an impending stall do to aircraft control?
Reduces control effectiveness, “mushy” controls, buffet.
How does a swept wing react in low vs. high speed flight?
Low speed- less lift, wingtips stall first, reduce control effectiveness
High speed- lower drag, more efficient
What is a Yaw Dampener?
Basically an autopilot for the rudder. It provides immediate rudder input when necessary to aid in cancelling out yaw tendencies, like Dutch Roll.
What is Dutch Roll?
A combination of rolling and yawing oscillations that normally occurs when the dihedral effects are more powerful than the directional stability.
By trying to correct a rolling/yawing motion, you’re keeping it going.
What aerodynamic condition is causing the aircraft to Dutch Roll?
A series of out of phase turns, when the aircraft rolls in one direction and yaws in the other.
