Flashcards in High altitude aerodynamics Deck (24):
What is the speed of sound?
the rate at which small pressure disturbances will be propagated through the air and this propagation speed is solely a function of air temperature. (ripple effect)
The speed of sound changes with altitude since temperature changes with altitude.
- increases with altitude increase (temp decrease)
- decrease with alt decrease (temp increase)
What is mach number?
ratio of true airspeed to local speed of sound
= true airspeed / speed of sound
What is critical mach?
it is the "free stream mach number at which a local mach number of 1.0 is attained at any point on the body under consideration
- the transition of subsonic to supersonic inducing a shockwave
- shockwave produces a large increase in static pressure behind it causing possible airflow separation
What is Drag Divergence?
Drag rise: happens 5-10% above Mcrit.
- (Mcrd) Drag divergence Mach number
- Drag sharply increases causing a decrease in range and economy
- Lifting decreases, a change in pitching moment occurs
- buffet, trim and stability changes causing a decrease in control surface effectiveness
- "buzz" surface vibration occurs
What is Mach buffet?
airflow over wing reaches speed of sound creating a shockwave. This shockwave disturbs airflow causing airflow separate, reducing lifting ability of the wing
- acceleration = center of pressure movement aft
- airflow movement = rearward and outward
- Nose will want to drop requiring greater elevator and trim pressure to maintain level flight = mach tuck
What is Mach Tuck?
Nose-down pitching movement from shifting center of pressure (towards aft)
1. Flow Separation: byproduct of shockwave
2.Center of Pressure Movement rearward: CG farther ahead of aircrafts aerodynamic center at slower flight.
What is Control "Buzz" or Flutter
Term used to describe a shock-induced flow separation of the boundary layer air before reaching the control surface. Results in very rapid oscillations of the control surfaces
- below Vmo,Mmo,Mcrit oscillations not large enough to be dangerous, but flight above these speeds could result in a inability to control the aircraft due to lack of control surface effectiveness.
Explain how speed affects induced drag:
- High lift condition
- maneuvering flight will create high induced angles of attack
- high induced drag
- take-off, approach, landing, and braking
- low lift condition
- very small induced angle of attack
- low induced drag
- climb, cruise, initial descent
What is total drag?
Induced + parasite drag
What is the Boundary Layer?
The layer at which the air is markedly slow due to viscosity.
1. Laminar: over the wing
2. Turbulent: aft of the wing
What are 2 ways to cause airflow separation?
1. Exceed Critical Angle of attack
2. Exceed Critical Mach (Shockwave)
Where do sweptback wings stall first?
- outboard sections of the wing trail the inboard sections, causing outboard suction pressure to draw the boundary layers towards the wingtip
- low energy boundary layer at the tip easily separated
What are the advantages of the swept wing?
1. Increase Critical Mach number
2. Delayed onset of compressibility effects
3. Reduced changes in force due to compressibility
What is a Dutch Roll? How do you prevent it?
- coupled lateral-directional oscillation
- combined rolling-yawing.
- Rolling motion precedes yawing
Counter with aileron control. DO NOT use Rudder
What controls more than half of the output of thrust?
Top 20% of RPM control
What is the effects of altitude if flying below the Tropopause?
- An increase in altitude produces a decrease in density, pressure, and temperature
- An increase in altitude causes the engine air mass flow to decrease in a manner nearly identical to altitude density ratio
What are the benefits power plant performance with an increase in altitude?
1. Below the Tropopause and increase in altitude will provide lower inlet air temperatures.
2. Increased RPM required to sustain cruise at that altitude. Meaning an increase in engine speed reduces the specific fuel consumption
What is the most important restriction to the operation of Turbofan engines?
Exhaust Gas Temperatures EGT
example: Compressor Stall
What is a Compressor Stall or "Surge"? How do you prevent it?
Separation or Lack of airflow through the engine producing excessive damaging engine temperatures
1. Malfunctioning fuel control: identified during engine acceleration (power-up)
2. Poor inlet airflow: high angles of attack, sideslip (Over controlling)
3. High Altitude Flight: reduced stall margins (exceeding performance limitations)
4. Malfunction of variable geometry inlet control: changing AoA of inlet compressor components (Mechanical installation)
What do you do if you experience a Compressor Stall?
1. Reduce thrust setting (fuel control)
2. Lower airplane angle of attack
3. increase speed (restore airflow)
What is the effect of steep bank angles?
Increased Load Factor = increase Stall Speed
- induced drag predominates at slow speeds
- Turns at slow speeds require greater thrust or pitch to maintain altitude
- Steep turns must be avoided after Takeoff, approach, and Single Engine
What are some contributors to High Altitude stalls?
1. Pilot error:
- lack of monitoring,
- mistake difference btw high/low speed buffet
- delayed corrective action
- icing: reduced lift, increased drag
- turbulence: CAT, Convective, Wake, Mountain Wave
3. Aircraft Performance:
- insufficient Thrust to maintain altitude/airspeed
- Changes in Angle of Attack
- increased Load Factor
How do you recover from a High Altitude Stall?
1. Change AoA (reduce)
2. Trade Altitude for Performance (descend)
- regain sufficient airspeed for recovery