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Mach (Critical Drag Rise)
appreciable increase in drag due to unstable formation of shockwaves turning energy into heat.
Usually about 110-115% of MCRIT
What happen to the C of P with increasing Mach Number
CP moves back as the upper surface shockwaves appear
But it then suddenly moves forward as the lower surface shockwaves appear
It then moves back steadily again
How is CD affect during transonic flight
Coefficient of Drag increases from MCRIT to MCDR then jumps massively to Mach 1, then decreases again
How is CL affect during transonic flight
After MCRIT CL decreases as upper and lower shockwaves form.
As speed increases and shockwaves move towards the rear of the aerofoil and CL comes back up
It decreases again near Mach 1 when the bow ave attaches to the leading edge.
What happens to the shape of shockwaves as mach number increases?
They remain attached at the bow and the trailing edge but bend backwards further from the contact points
Describe Air going into and coming out of an oblique (corner) shockwave
- Velocity behind is reduced but remain supersonic
- pressure and Density behind the wave is increased (it's squashed)
- Some of the energy is turned into heat (wasted)
Describe Air going into and coming out of an Normal shockwave
- Marks the change from supersonic to subsonic flow
- Temp, Density, and Pressure increase
- Airstream is slowed to subsonic (because of the temp increase)
- Energy of the airstream is greatly reduced (why lots of thrust is needed to push through it)
Describe Air going into and coming out of an expansion (corner) shockwave
- Airstream is accelerated
- Flow direction is changed gradually to follow surface
- Pressure, Density, Temp are decreased
- No shock so no loss of energy
What is Sonic/Mach buffet, and how is it related to Shock Stall?
Mach buffet is the separation of the air behind the shockwave running over the control surfaces and tailplane
This separation may cause a "shock stall" - the C of P to move backwards and the nose to pitch down like a normal low speed stall.
How is MCRIT affected by wing thickness and sweep back
Thinner wing and more Sweepback give a higher MCRIT
Characteristics of Transonic aerofoils?
- Thin wings (low T/C ratio)
- Low T/C ratio smooths out the CL curves during the transition phase
- Sharp leading edge Radius
Explain Aileron reversal
Twist in flexible wing causes actual opposite lift force to be created
Advantage of Vortex Generators in high speed flight
Vortex generators pull down air from above the boundary layer, re-energising it, and allowing it to better cope with the adverse pressure gradient
Meaning Of buffet margin
Margin between between high IAS mach buffet - and low IAS normal stall buffet
Define Coffin Corner
When an aircraft has reached it's absolute or aerodynamic ceiling
- Cant go faster without getting shock stall
- Cant slow down without getting a stick shaker.
How do these effect Buffet margin?
Angle of attack has the greatest effect of inducing mach buffet so;
All of these things require a higher AoA so will reduce the buffet margin.
What causes Mach Tuck during transonic range
1) centre of pressure moves aft
2) Shockwave flow separation (at the sing root first) causes a decrease in downwash over stabiliser
Speed unstable as the pitch down speed up the A/C and makes things worse.
What is area rule in airframe design?
to minimise drag at transonic speeds, Cross sectional area should increase gradually to a max about half way back and then decrease again
(Shock)wave drag has 2 parts, what are they
1) Energy drag - cause by temp rise across the shockwave
2) Boundary Layer Separation -rise in pressure behind the shockwave causes separation and turbulence, hence drag.
What happens to the C of P when tip stall occurs in swept wing designs?
- It moves inwards as the stall moves from tip to root, and because of the sweep it moves forward of course.
- Causes a nose pitch up which sux
When does Dutch Roll occur as opposed to spiral instability
Dutch roll When lateral stability is too strong for the directional stability and it picks up a wing too quick.
Spiral instability when a large fin or rudder will turn it into a slip (keel effect) before the wing gets picked up, making it worse.
What is the diff between Dutch Roll and Snaking?
Both Oscillatory lateral instability but in the snake the yaw predominates
Define MAC (Mean AeroDynamic Chord)
The average distance from the leading edge to the trailing edge of a section of wing which has the same aerodynamic characteristics ans the actual wing
Consequence of too forward CofG
- Increased Longitudinal Stability
- Dangerous Spin Characteristics
- Nose will drop on power reduction
- decreased range
Consequence of aft CofG
- Decreased Longitudinal Stability
- Dangerous Spin behavior
- increased risk of stall and difficult recovery
- Decreased Range
Change or CofG Formula
CofG Change =
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