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Principles Of Flight > 81 02 Effects Of High Speed Flight > Flashcards

81 02 Effects Of High Speed Flight Flashcards

1
Q

Define Mcrit

A

Mcrit is the fastest possible Free Stream Mach Number where all Local Mach Speeds are <= 1.0 (I.e. Subsonic/Sonic) and there are no shockwaves

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2
Q

What happens to CP as Normal Shockwaves develop?

A

CP moves aft

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3
Q

What happens to CL as an AC progresses through the Transonic region for a constant AOA?

A
  • Overall CL decreases across the Transonic region
  • Initially CL increases due to the greater upwash of the free stream flow as it approaches the leading edge
  • CL then rapidly drops due to the separation of the flow behind the Normal shockwave (adverse pressure gradient)
  • CL falls further due to the development of a Normal shockwave below
  • CL recovers slightly once the lower shockwave reaches the trailing edge
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4
Q

What is the effect on CL, CLmax and Crit AOA with increasing Mach Number?

A
  • For any given AOA CL is increased due to the increased upwash ahead of the leading edge
  • CLmax and Crit AOA are both reduced due to the adverse pressure gradient created by the shockwaves
  • CLmax slowly decreases with increasing Mach until Mdrag-divergence
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5
Q

What is Shock Stall and how does it differ from low speed stall?

A
  • The Mach at which CLmax is reached
  • Occurs at a higher speed then Mcrit
  • Similar to low speed stall as it is caused by flow separation due to an adverse pressure gradient
  • Differs from low speed stall as it can occur at any AOA and only at high speed
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6
Q

What is Mdrag-divergence, where does it occur in relation to Mcrit and Shock Stall and what causes it?

A
  • The point at which CD rises rapidly
  • At a point after than Mcrit and Shock Stall
  • Caused by the separation of flow by adverse pressure gradient, which generates Wave Drag
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7
Q

What is the effect of increasing Mach on drag and CL/CD ratio?

A
  • Drag increases significantly past Mdrag-divergence
  • Due to a greater increase in Parasite drag and Wave drag
  • As a result the CL/CD ratio drops considerably after Mdrag-divergence
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8
Q

What is the effect on CP across the Transonic range?

A
  • As the Mach number increases the CP moves forward and aft eventually settling on about 50% MAC
  • Generally the moment arm between the CG and CP increases with increasing Mach
  • Therefore the AC will generally become more longitudinally stable
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9
Q

Describe what Mach Tuck is?

A
  • When Shock Stall occurs close to the wing root, this decreases the downwash in this area
  • This reduces the AOA on the tail plane meaning that it losses downforce
  • Also caused by a forward movement of the CP
  • This results in a sudden nose down pitching
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10
Q

What is a Mach Trimmer and can you fly without one?

A
  • Mach trimmer automatically adjusts the variable incidence tail plane to counter the pitch changes due to movements of the CP with increasing Mach
  • It is possible to fly without a Mach Trimmer but the max Mach number allowed will be limited to well below Mcrit
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11
Q

What is Stick Force Gradient and how can it change when unsuitable AC start to fly at high Mach numbers?

A
  • Stick Force Gradient: As an AC increases in speed, it is necessary that the pilot pushes forward on the control stick to remain straight and level
  • At increasing Mach numbers it is possible that an AC’s Stick Force Gradient may become neutral and then negative (i.e. Pull back on stick rather than push forward)
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12
Q

What is the effect on control surfaces of high Mach speeds?

A
  • As Mach numbers increase, shock waves can form just in front of control surfaces
  • These cause the air flow to separate making the control surface ineffective
  • This can be especially serious if it occurs to the elevator due to the marked trim changes through out the transonic region
  • At high Mach numbers ailerons are locked out and spoilers are used for roll inputs
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13
Q

What is Control Buzz?

A
  • Occurs when a shockwave forms directly on a control surface
  • Due to the rapid pressure changes this causes high frequency oscillations of the control surface
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14
Q

What is High Speed Buffet?

A
  • Separation of the air flow due to shockwaves causes tumbling air to be formed
  • This high speed tumbling wake can strike the airframe and the horizontal stabiliser causing severe buffeting
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15
Q

What is the Aerodynamic Ceiling?

A
  • It is the altitude where the Mach number for the low speed and high speed stall are incident
  • It is not possible for an AC to fly higher than this as it will stall
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16
Q

What is the Buffet Margin and how is it effected by altitude?

A
  • Buffet Margin describes the available Mach speeds between the low speed stall and the high speed stall
  • As altitude increases this margin reduces until you reach the Aerodynamic Ceiling at which point there is only one available Mach speed
17
Q

What are the factors effecting the Buffet Margin other than altitude?

A
  • Buffet Margin is mainly effected by the amount of lift the wings need to generate
  • As this increases the Mach No for low speed stall increases and the Mach No for high speed stall decreases
  • Mass of the AC: increased mass, smaller buffet margin
  • Load Factor: increased LF, smaller buffet margin
  • AOB: increased AOB, smaller buffet margin
  • CG Position: fwd CG increases the downforce needed from the tail plane, smaller buffet margin
18
Q

What can be implemented to delay the formation of shockwaves and therefore increase Mcrit?

A
  • Reducing the thickness to chord ratio
  • Increase the sweep back of the wings
  • Anything which reduces the air flow speed over the wing
  • Area Ruling
19
Q

What are the advantages and disadvantages of thinner wings with regards to high speed and low speed flight?

A

Advantages:

  • Reduces the flow speed over the wing, increasing Mcrit
  • AC can fly at higher Mach numbers for the same CAS drag

Disadvantages:

  • Less room for control surfaces, fuel and landing gear
  • Less camber and thickness means less lift
  • Longer takeoff and landing distances
  • Faster stall speeds
20
Q

Describe a Super Critical aerofoil and its advantages and disadvantages

A
  • Delays the formation of wave drag whilst still retaining a larger thickness to chord ratio
  • Flatter upper surface to reduce the speed of the flow over the wing
  • A more rounded leading edge helps recover some of the lost lift
  • The lower surface is negatively cambered, also to increase lift

Advantages:

  • Thicker means it can be made stronger
  • More space for control surfaces, fuel, landing gear

Disadvantages:

  • More susceptible to contamination
  • Produces a rapid and unpredictable movement of the separation point
21
Q

What is the effect of sweepback on high speed flight?

A
  • Only the flow perpendicular to the leading edge of the wing effects the pressure distribution, the effective flow
  • Increasing the sweep back of the wings reduces the effective flow therefore increasing Mcrit
  • Also reduces the effective camber of the wing so the flow has a gentler acceleration over the upper surface
  • However, this means the wings must work harder thus it increases the stall speed
  • Short haul: 25% sweepback
  • Long haul: 37% sweepback
22
Q

What effect do Vortex Generators have with regards to high speed flight?

A
  • Mix high energy free flow into the low energy boundary layer flow
  • Re-energising of the boundary layer flow delays separation due to shockwaves and can even reattach separated flow
  • Can be placed in front of control surfaces to improve their effectiveness
23
Q

Describe what Area Ruling is and what is its effect on high speed flight

A
  • Area ruling states that to reduce form drag, the cross sectional area of an AC should increase and decrease gradually
  • Biggest cause of rapid increase and decrease in cross sectional area is the wings
  • By ‘Wasting’ the fuselage this can be reduced
  • However, wasting reduces the capacity of the AC
24
Q

What can be determined from a BOB (Buffet Onset Boundary) chart?

A
  • Mach numbers at which low speed and Mach buffet occur and the aerodynamic ceiling for different masses and altitudes

Principles Of Flight (23 decks)

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