High Speed Flight Flashcards
Local Speed Of Sound
LLS = 39 x square root (K)
Mach Number calculation
Mach number = TAS/LSS
What is Mcrit
Fastest speed at which there is no supersonic flow
Or
Slowest speed at which there is no supersonic flow
First signs of a shockwave
Appear on the upper surface just after Mcrit half way back
What does transonic mean
Some Mach numbers lower than 1, but some Mach numbers above 1
Flying faster in transonic speed does what to shockwaves
Shockwaves intensify and move backwards a shockwave can also then form on the bottom on the aerofoil leading more drag
Wave drag
As shockwave intensify more separation aft of shockwave
Airflow in front of shockwave properties
Static pressure
Velocity
Temperature
LSS
Density
Total pressure
Static Pressure - Less
Velocity - Higher
Temp - Lower
LSS - Lower
Density - Lower
Total pressure - higher
Airflow properties behind a shock wave
Pressure
Temperature
LSS
Density
Total pressure
Pressure - higher
Temperature - higher
LSS - Higher
Density - higher
Total pressure - lower
What changes Mcrit
Factors which influence the acceleration of airflow over the upper surface
Factors influencing Mcrit
Forward CG - Lower
Thicker wing - lower
Increased Mass - lower
Turning - lower
Anything that is producing additional lift on upper surface to increase the airflow
Mach Drag Diversion
Mach number at which CD rapidly rises due to flow seperation
Mach buffet
High Mach number will do what to lift and drag
Reduces lift and increases drag in supersonic flight
Advantage of flying close to Mcrit
Increase in cruise speed for not so much drag
Increasing Mcrit - Sweepback Wings + consequences
Speed of airflow at 90 degrees to leading edge is going to be slower than in relation to airplane
Mcrit greater for sweep angle
- reduction in clmax higher stall speed
- increased t/o l/d distances
- poor stall characteristics
Practical design for supercritical wing
Flat Top
Negative Camber on lower surface
Extreme positive camber at trailing edge
Mach Buffet is
Produced by the turbulent flow behind a strong shock wave
Can lead to structural fatigue
Vortex generators and Mach buffet
Used to reattach airflow behind normal shockwave located inboard and further back
CP movement in transonic flight
Due to rearward movement overall L/W couple gets bigger so more nose down
Mach Trimmer
To improve stick force stability in transonic flight due to L/W couple movement at Mcrit
Mno is limited if inoperative
Why can we fly at a higher TAS/Mach at altitude
Reduces temp and density therefore allows a greater TAS and a greater MACH as LSS is reduced and TAS increases
Chicken Tika Masala
CAS
True Airspeed
Mach
Inversion effect on CAS/TAS/MACH
As temperature is getting warmer Mach number decreases
C M T
Isothermal layer on CAS TAS MACH
TAS and Mach will stay the same
Climb Schedule
Switching between TAS and Mach from low altitude to high altitude
What speed limits you in a climb
Mach number above climb schedule
CAS is decreasing at constant Mach
AOA increase to maintain lift reducing stall margin
What happens to climb angle in the climb up
Climb angle decreases as less thrust
Speed of sound is affected by
Temperature of the air
Critical Mach number is when there is evidence of
Local sonic flow
Dutch Roll and shockwaves
Shockwaves have different intensity’s on each wing therefore Dutch roll gets worst
Yaw damper must be operative
Why do we need a Mach trimmer
To adjust incidence of horizontal stabiliser to counter aft movement in cp due give us a positive stick force stability
Mach Tuck is
Shock induced separation near wing roots which decreases downwash on tailplane
Nose down pitching moment
High speed buffet
Constant MACH speed CAS decreases (low speed stall)
Altitude decrease Mach buffet margin to high speed buffet
Reduction in lift and increase in drag
Aerodynamic ceiling
Altitude at which high speed stall and low speed stall coincident
Factors on the low/high buffet margin
Mass
Load Factor
Altitude
Anything that requires more lift from the wing
Aircraft in climb at constant Mach what happens to pitch angle/CL and climb angle
Climb angle decrease
Pitch angle decreases
CL increases
Constant Mach in a descent on:
AOA
CL
CAS
Pitch Angle
Descent Angle
AOA decrease
CL decrease
CAS increases
Pitch angle decreases
Descent angle increases
Buffet Onset Boundary Chart
Shows us variables for altitude/weight/load factor on the low and high speed buffer margin
Mach numbers at which low speed and Mach buffet occur at different mass and altitudes
Supercritical Wing
Allows for no noticeable shockwaves when flying just above MCRIT
Flying from warm to cold air mass will do what to Mach speed
Mach Speed will remain constant
TAS will decrease as density will decrease
LSS will rise due to temperature decrease
Advantages of flying near mcrit
Flying just past MNO gives a CL increase for a slight CD increase up until the mdrag divergence point
What happens to stall speed with altitude when compressibility is concerned
Stall speed is constant with altitude and then increases due to compressibility
What happens to the aerodynamic centre in the trans/supersonic speed region
CP moves from 25% to 50% chord
Deflection of a aileron upwards when the wing is generating a shock wave will
Move the shock wave slightly forward in front of the upward deflected aileron
Decrease in camber reduces the aft movement of the shockwave- moving slightly forward as increase in Mcrit
Exceeding MCRIT without a Mach trimmer the stick force stability will
Reduce as the nose will want to move down due to reduction in tailplane effectiveness
Mach Trim
Used to trim ac at high Mach numbers - prevents instability and tuck under
Does one of the following to counteract the pitch down:
- adjusting stabiliser
- adjusting elevator
- trim tank
+ stick force gradient maintained
+ longitudinal stability increases
+ tuck under avoided
Compared to a conventional airfoil the supercritical wing has the advantage of
Larger leading edge radius/flatter upper surface/both positive and negative camber and enables thick wing to be used for the same cruise Mach number
Moving the aileron downwards does what to shock wave
Increased camber therefore shockwave will move rearward
What happens to controls surfaces (ailerons) effectiveness in subsonic flight
Shock wave separates the wing into two parts so aileron only affects the back part therefore less effective
Aileron deflection only partly affects the pressure distribution around the wing
How does swept back wings allow for a higher mcrit value
Increases the effective chord
