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2FTS Aerodynamics > Transonic Flight > Flashcards

Transonic Flight Flashcards

1
Q

What is the transonic flight regime?

A
  • Speed range in which the transition between subsonic and supersonic flow occurs.
  • Characterized by the development and movement of local shock waves.
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2
Q

Between what Mach numbers is the transonic regime?

A

Mcrit at the lower end and Mdet at the upper end

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3
Q

When does positive pressure occur on the lower surface of the wing?

A

When there is excessive Angle of Attack (AoA)

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4
Q

When does separation of the boundary layer occur for transonic flow?

A

Immediately behind the shock

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5
Q

What are characteristics of laminar boundary layer?

A
  • Thin
  • Slippery
  • Low energy
  • Cannot negotiate an adverse pressure gradient
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6
Q

What are the characteristics of a turbulent boundary layer?

A
  • Thick
  • Draggy
  • High energy
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7
Q

What are the characteristics of the separation point?

A
  • Boundary layer detaches
  • Wing lift coefficient reduces
  • Drag increases
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8
Q

What is the formula for lift?

A

L = 0.5ClrhoV^2S

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9
Q

Is this formula accurate for subsonic, high subsonic and supersonic airspeeds?

A

Subsonic

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10
Q

What is the formula not accurate for high subsonic and supersonic airspeeds?

A
  • At high subsonic airspeeds, compressibility begins to take effect and the amount of lift produces is no longer proportional to V^2
  • At at supersonic speeds the amount of lift is influenced by upwash and formation and movement of shockwaves.
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11
Q

What occurs to wing upwash as speed increases?

A

Pressure waves do not propogate as far upwind, so incoming airflow has less warning of the approaching wing and upwash is less.

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12
Q

At low subsonic speeds, describe the upwash on the wing?

A
  • At low subsonic speeds, upwash ahead of LE gives an angle of attack AoA and related Cl
  • As flow becomes compressible, upwash occurs closer to the LE which creates a higher effective AoA and subsquent higher Cl
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13
Q

What occurs to upwash at high subsonic speeds?

A
  • At high subsonic compressible flow, inertia of the airflow begins to have an effect by reducing the warning and upwash.
  • Airflow deviates less, creates less effective AoA and reducing Cl
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14
Q

What occurs to upwash at supersonic flow?

A
  • At speeds above Mdet(superosnic flow), free stream airflow experiences no warning of the approaching wing and is not deviated
  • AoA and Cl are low
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15
Q

What occurs on the wing at Mfs = 0.75 or Mcrit? What occurs on top surface and lower surface? What happens to movement of Cp? What happens to value of Cl?

A
  • Some airflow over the wing approaches M = 1.0, and Mach whiskers begin to form on to surfaces
  • No shockwave is formed yet as Mach whiskers hold it off
  • Airflow below the wing is also accelerated but not to sonic speeds
  • Cp moves forward to around 25% of chord
  • Cl increases to about 160% of low speed value for same AoA
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16
Q

What occurs on the wing as Mfs increases to 0.81? Is there a shockwave on bottom surface of wing? How does the lamda foot form? What happens to Cp movement? What happens to Cl compared to low speed value for same AoA?

A
  • A shockwave first begins to form around 70% of chord on upper surface
  • Flow not yet sufficient to form shock on bottom surface
  • Pressure drop ahead of shockwave is larger
  • Adverse pressure gradient behind the shock thickens the boundary layer and forms turbulent flow
  • Lamba foot is formed as pressure waves move further forward in boundary layer
  • Cp moves rearward to about 30% of chord - nose down pitching moment is created
  • Cl nearly doubles and is at its highest point of low speed value for same AoA
17
Q

What occurs on wing at Mach = 0.89 on upper and lower surfaces? Where is lower edge shock positioned? What happens to Cl value? What happens to Cp movement?

A
  • Upper surface shockwaves remains in place
  • Flow on bottom surface has created shockwave on lower surface, which positions at trailing edge due to weaker adverse pressure gradient
  • Cl reduces to around 70% of low speed value due to lower pressure on bottom of wing
  • Cp moves forward to around 15% chord
18
Q

What occurs at M = 0.98 on upper and lower surfaces? What is pressure differential like now? What occurs to Cp movement? What occurs to Cl movement?

A
  • Speed of flow is sufficient that both shocks on upper and lower surfaces are at the trailing edge.
  • Turbulent flow is significantly reduces and uniform pressure differential is established between upper and lower surface
  • Cp is around 45% of chord rearwards
  • Cl is around 110% of lower speed value
19
Q

What occurs at speed greater than Mdet to upper and lower shocks? Pressure distribution over the wing? Cp position? Cl compared to low speed value?

A
  • Bow shockwave has formed at is attached to leading edge
  • Upper and lower shockwaves are more oblique and are established on trailing edge
  • Uniform pressure distribution is maintained and Cp is around 50% of chord
  • Cl stabilizes to 70% of low speed value
20
Q

For SUBSONIC flow, what are the two types of drag?

A

1) Zero lift drag

2) Lift Dependent drag (induced drag)

21
Q

What is zero lift drag and what are the different types?

A
  • Drag due to geometry and structure of aircraft

- Surface Friction, Form and interference drag

22
Q

What is Lift Dependent drag (induced drag)? What are two types?

A
  • By product of creating lift

- Induces and Trim drag

23
Q

What are the two types of drag at transonic/supersonic speeds?

A

Wave Drag which is broken down into Energy Drag and Boundary Layer Separation Drag

24
Q

What is energy drag and how is it formed? How does energy drag increase with Mfs?

A
  • Energy drag results from the non adiabatic temperature rise that occurs as air passes through a shockwave
  • The energy that is added for this temp change comes from aircraft’s kinetic energy which results in drag on the aircraft
  • Increases with Mfs
25
Q

What is boundary layer separation drag and when does it occur? How does it change with increases in Mfs?

A
  • Boundary layer separation drag occurs with the movement of the shockwave
  • The thickened boundary layer caused by lambda foot separates from the base of the shockwave and causes a thick turbulent wake
  • This turbulence results in increase in drag
  • As Mfs increases, the shockwaves move to the TE and lamba foot is reduced which reduces boundary layer separation drag
26
Q

What occurs to Cd as Mfs increases? Draw the graph of Cd vs Mfs?

A
  • Increases past Mcrit to peak where boundary layer separation drag is greatest
  • Reduces to steady state value past Mach 1 as boundary layer drag is no more because of energy drag
27
Q

What is Mach tuck and what are the two main causes?

A
  • Mach tuck is the nose down pitch experienced by aircraft as they increase in the transonic region.
  • Caused by rearward movement of Cp during Transonic regime
  • Rearward of Cp reduces downwash on the tail plane which decreases the balancing force and causes the nose to pitch down.
28
Q

What is wing drop and when can it occur?

A
  • Can occur at Mcrit
  • Shockwaves occur at different point on each wing due to minor differences in construction, surface and variation in lift and drag between wings can cause wind drop.
29
Q

What is transonic/supersonic buffet?

A
  • Turbulent shock induced separation can contact rear control surfaces and cause buffet throughout the airframe.
30
Q

What can occur to control effectiveness when shockwaves form?

A

Can be reduced due to:

  • Operating in area of turbulent flow and lift is reduced
  • Any change that occurs in supersonic flow cannot propagate forward
  • Aircraft controls can form their shockwaves which reduce lift and drag over them
31
Q

What special control surfaces are used for high speed aircraft?

A

1) All moving tail planes (Stabilators)
2) Combined ailerons and elevators
3) Multiple fins and rudders

32
Q

What can be fitted to overcome Mach tuck?

A

Mach trimmers

33
Q

What occurs to longitudinal stability with the rearward movement of Cp? What happens to maneuverability ?

A
  • Lift Weight couple is increased which increases longitudinal stability
  • Maneuverability is reduced and can be fixed by using larger control surcfaces
34
Q

What is often used to counteract wing drop?

A
  • More complex flight control computers
35
Q

What occurs to lateral stability at supersonic speeds?

A
  • Less laterally stable due to reduce in Cl on the wing and thus correcting forces are less.
  • Mach cones form on the wingtips which define an area of reduced effectiveness.
  • Advanced flight control systems can be used to counteract this effect
36
Q

What occurs to directional stability at supersonic airspeeds? What is used to counteract this?

A
  • Differential shockwaves can create a yawing moment on the aircraft.
  • Yaw dampers can be used to counteract this
37
Q

What is Dutch Roll? What can overcome this?

A

Airplane is yawing and rolling at the same time when lateral stability exceeds directional stability
- Can be counteracted by increasing directional stability or with a yaw damper

2FTS Aerodynamics (8 decks)

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