Fastest speed at which there is no supersonic flow Or Slowest speed at which there is no supersonic flow

High Speed Flight Flashcards by Unknown Unknown

Local Speed Of Sound

LLS = 39 x square root (K)

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Mach Number calculation

Mach number = TAS/LSS

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What is Mcrit

Fastest speed at which there is no supersonic flow

Slowest speed at which there is no supersonic flow

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First signs of a shockwave

Appear on the upper surface just after Mcrit half way back

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What does transonic mean

Some Mach numbers lower than 1, but some Mach numbers above 1

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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

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Wave drag

As shockwave intensify more separation aft of shockwave

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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

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Airflow properties behind a shock wave

Pressure
Temperature
LSS
Density
Total pressure

Pressure - higher
Temperature - higher
LSS - Higher
Density - higher
Total pressure - lower

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What changes Mcrit

Factors which influence the acceleration of airflow over the upper surface

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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

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Mach Drag Diversion

Mach number at which CD rapidly rises due to flow seperation

Mach buffet

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High Mach number will do what to lift and drag

Reduces lift and increases drag in supersonic flight

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Advantage of flying close to Mcrit

Increase in cruise speed for not so much drag

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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

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Practical design for supercritical wing

Flat Top

Negative Camber on lower surface

Extreme positive camber at trailing edge

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Mach Buffet is

Produced by the turbulent flow behind a strong shock wave

Can lead to structural fatigue

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Vortex generators and Mach buffet

Used to reattach airflow behind normal shockwave located inboard and further back

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CP movement in transonic flight

Due to rearward movement overall L/W couple gets bigger so more nose down

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Mach Trimmer

To improve stick force stability in transonic flight due to L/W couple movement at Mcrit

Mno is limited if inoperative

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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