Transonic Flight and Aerofoils

Question 1

What is the aerodynamic limitation in transonic flight mean?

Answer 1

Means the limit in airspeed, at which the aircraft can no longer produce the aerodynamic forces effectively to maintain the flight.

Question 2

What are the two transonic speed limits? Explain the two transonic speed limits

Answer 2

• The low-speed limit of a transonic flight is the airspeed, at which the aircraft is at a low speed-high AoA stall caused by the boundary layer separation over the aerofoil and the aerodynamic feature of the airflow field to produce lift over the aerofoil is destroyed.
• The high-speed limit is the airspeed at which the aircraft starts Mach stall caused by the turbulent wake behind the shockwave on the aerofoil separating from the surface. The formation of a shockwave over an aerofoil increases the local pressure suddenly, and the boundary layer separation of the turbulent wake causes the complete loss in lift and the significant increase in drag, therefore, level flight cannot be maintained.

Question 3

What happens to the low speed limit when altitude increases?

Answer 3

Increases with the increase of altitude as the air density decreases with the increase of altitude assuming the same level of lift is maintained ( L = CL1/2pv^2s).

Question 4

What happens to the high speed limit as altitude increases?

Answer 4

The TAS of high speed limit remains constant when the altitude is beyond the Troposphere; IAS is constant when ALT increases, while IAS decreases with altitudes.

Question 5

What is the coffin corner?

Answer 5

• At each altitude there is an airspeed range from a low to a high speed limit.
• The two limits get closer as altitude increases and will intercept at an altitude called COFFIN CORNER.

Question 6

What happens when aircraft reaches the coffin corner?

Answer 6

It will be in a stall if its airspeed decreases; and it will be in shock stall if its air speed increases. The aircraft at the coffin corner is in an unflyable situation.

Question 7

What is the buffet boundary?

Answer 7

• An aircraft experience buffeting before it reaches stall.

- The airspeed at the start of the buffet is known as BUFFET BOUNDARY.

Question 8

What is the buffet margin?

Answer 8

The speed range between the lower speed buffet boundary and the high speed buffet boundary at each altitude is called BUFFET MARGIN.

Question 9

What does the buffet margin change with? What happens to the buffet margin when it reduces to 0?

Answer 9

• BUFFET MARGIN CHANGES WITH altitude.

- When the margin reduces to “0”, the aircraft is up at a certain altitude, then the aircraft is at the coffin corner.

Question 10

What are the factors that affect the buffet boundary and buffet margin?

Answer 10

• Altitude
• Weight
• Load factor

Question 11

What effect does altitude have on the buffet boundary and buffet margin? (2)

Answer 11

• As altitude increases, the buffet boundary get closer to each other, and it means that the boundary margin gets smaller.
• If the altitude increases to a certain level, e.g. 23000 ft, the margin reduces to “0” so the aircraft in practice is in “coffin-corner”.

Question 12

What effect does weight have on the buffet boundary and buffet margin? Give an example of it?

Answer 12

• Buffet margin with a high weight is smaller than that with a low weight, assuming the altitude is the same.
• e.g. The altitude of 30,000 ft, if the weight of the aircraft is 60 tons, the margin is 180 kts between 160 kts and 340 kts; while the margin reduces to 155 kts between 180 and 335 kts, if the weight becomes 70 tons (see FIG 9-3).

Question 13

What effect does load factor have on the buffet boundary and buffet margin?

Answer 13

Buffet margin of the aircraft decreases, when the load factor increases.

Question 14

What is the crossover altitude? (2)

Answer 14

• The altitude at which a specified IAS or CAS and
  Mach value represent the same TAS (True airspeed) value.
• At this altitude, Mach number is used to reference airspeeds.
  SEE ASSESSMENT

Question 15

What does crossover altitude depend on?

Answer 15

• Climb target
• Aircraft type
• Speed target cruise speed

Question 16

Explain how a flat leading edge affect Mcrit? (4)

What is the advantage of using a flat leading edge? (2)

Answer 16

• A laminar aerofoil with a slim leading edge, the aerofoil with a supercritical section have a high Mcrit.
• A supercritical section has a flattened top upper surface from the leading edge of the aerofoil.
• Both of the laminar flow aerofoil and the supercritical section have relatively small dA, thus dV of airflow on both is gentle and small.
• Their t/c/ ratio can be greater than that of slim design.
• CL could be greater than that of slim aerofoil.
• Thus flat leading edge and supercritical section can have a high value of Mcrit without the disadvantage of a low CL.

Question 17

Explain how a sweepback wing affects Mcrit? (4)

Answer 17

• As air flows over an aerofoil along its chord, it increases sonic and the aircraft’s Mach number reaches Mcrit.
• The chord wise speed of a a straight wing is greater than a sweepback wing if their free stream speeds are the same..
• When their chord wise air speed reaches sonic, the Vfs of straight wing is smaller than that of a sweepback wing.
• Thus a sweepback wing can increase its Mcrit.

Question 18

How do vortex generators affect Mcrit? (4)

Answer 18

• When airflow passes vortex generators, microscopic energetic vortices are produced over the surface of the aerofoil.
• These vortices delay the formation of a shockwave, and delay the separation of the turbulent wake behind the normal shockwave if a normal shockwave has formed.
• So the aerodynamic difficulties caused by the normal shockwave on the aerofoil would be weakened and delayed.
• Thus, vortex generators on the upper surface can result in the aircraft travelling faster, therefore INCREASING MCRIT.

Question 19

What are the main difficulties caused by a shockwave forming on an aerofoil travelling at transonic speed?

Answer 19

• Significant increase in drag.
• Decrease in lift by affected aerofoils.
• Shock stall.

Question 20

Describe the characteristics low thickness to chord ratio transonic aerofoil? (5)

Answer 20

• Thin design of wings used for high speed flight.
• Cross sectional area of airflow path over an aerofoil increases gently if its t/c ratio is low.
• Thin aerofoil = high Mcrit.
• For the same design, the cross-section area of airflow path over an aerofoil decreases gently as well after it passes the most cambered location.
• Therefore, the airflow speed over a thin aerofoil increases gently and smoothly, while the pressure over the aerofoil decreases slowly, thus delaying the formation of shockwaves.

Question 21

What are the effects of a strong shockwave with regards to a low t/c ratio? (4) What advantage does a low t/c ratio provide?

Answer 21

• Produces high shock drag; the turbulent wake behind the shockwave is more unstable;
• Mach buffet and shock stall can also occur easily with a strong shockwave.
• Therefore, a wing with a low t/c ratio experiences low shock drag and a higher Mcdr.
• Since the Mach number over the thin wing doesn’t increase quickly, the turbulent wake behind the shockwaves will be less violent, so the aircraft travels at high speed with better stability.

Question 22

What happens to the buffet margin regarding t/c ratio?

Answer 22

It is possible that the buffet margin of an aircraft with thin wings greater than that with thick wings.

Question 23

What happens to the lift and lift coefficient of an aircraft with a low t/c ratio?

Answer 23

• Pressure decreases slowly over a thin wing, therefore less lift compared to a thick wing, especially at low air speeds, i.e. less lift at low airspeeds (lack of ‘circulation’ when using circulation theory of lift)
• CL of thin aerofoil is low.

Question 24

What is the supercritical aerofoil designed for?

Answer 24

Supercritical section to reduce shock drag, and to delay shock stall.

Question 25

What are the features of a supercritical section aerofoil? (8)

Answer 25

• Flatted up surface at leading edge with a modest t/c. ratio.
• Ensures increase of airflow speed over the aerofoil is gentle, and smooth like thin aerofoil, thus Mcrit is higher compared to an ordinary shaped aerofoil.
• Higher CL because has a greater thickness.
• Increases Mcdr because airflow speed does not increase significantly (increases Mcrit).
• Formation of shockwave on top of the aerofoil will be delayed, the intensity of the normal shockwave reduces.
• The increase in shock drag is delayed, thus the aircraft can cruise at higher subsonic speeds.
• Reflex camber. Camber line reflected upwards at the rear part of the aerofoil. Improves lift production at the rear part of the aerofoil, and delays lower shockwave formation.
• Makes aircraft operate more efficiently, sweep angle and the wing span can be reduced if the sweepback wing uses the supercritical setion design.

Question 26

Describe the characteristics of a sweepback wing of a transonic aerofoil (only up to airflow around sweepback wing). (6)

Answer 26

• Has positive stability in any direction. E.g. recovering pitching, rolling, or yawing moment can be produced by a sweepback wing if aircraft is in a pitching, rolling or yawing disturbance.
• Higher Mcrit. Higher sweepback angle = greater Mcrit.
• Stall AoA higher compared to a straight wing.
• Mdd higher for sweepback wings. Means shockwave formation can be delayed and the turublent wake separation behind the shockwave. Thus, low shock drag for sweepback wings.
• Transonic aircraft with sweptback wings can operate at relatively high economical cruising speeds, since CD is low.
• Airflow around sweepback wing departs from the free-stream direction: Since air is a viscous fluid, airflow at the wingtip changes its direction when it flows around the sweep wingtip. Effective sweep angle at wingtip smaller than sweep angle of the wing, so effective Mcrit reduced from the theoretic Mcrit.

Question 27

Describe how thickness affects a sweepback wing (with regards to the characteristics of a sweepback wing). (4)

Answer 27

• Thickness of a sweep wing not uniform along its wingspan.
• Thickness gradually reduces from wing root to wingtip. - This makes the airflow over the wing compressed toward to the wing root.
• Series of compression waves form close to the wing root, even a shockwave can be built from the compression waves.
• Drag increases due to the compression waves.

Question 28

Describe how spanwise flow affects a sweepback wing (with regards to the characteristics of a sweepback wing). (4)

Answer 28

• Spanwise flow over a sweep wing means the boundary layer over the sweep wing gets thicker toward the wingtip.
• This encourages the formation of a large wingtip vortex.
• Wingtip vortex is large enough to start from the leading edge of the wing, known as RAM HORN vortex.
• Larger vortex leads to more induced downwash, therefore higher induced drag.

Question 29

Describe what happens when a shockwave forms over a transonic sweepback wing. Describe what a deep stall is as well. What can be used to prevent a deep stall? (4)

Answer 29

• Turbulent wake behind the shockwave shadows the T-tail (if A/C has T-tail configuration).
• Separation of the turbulent wake causes loss of lift, and introduces instability in pitching, requiring the tailplane to stabilise the plane.
• Tailplane cannot work effectively when the turbulent wake shadows the tail plane. Restoring function of the tailplane is not available, which is called DEEP STALL
• High T-tail design used to set tail plane away from the turbulent wake and to avoid deep stall.

Question 30

List the three devices used to delay a shock stall.

Answer 30

• Wing fence
• Vortex generator
• Anti-shock body

Question 31

How does a wing fence delay a shock stall? (2)

Answer 31

• Featured on a wing close to the wingtip area to prevent a large wingtip vortex developing into a “ram horn” vortex, which cause a high induced drag and wingtip stall.
• They can also disrupt rear shockwave over a sweepback wing developing widely, then reducing the turbulent wake separation behind the shockwave.

Question 32

Where are vortex generators installed on a transonic wing and how do they delay a shock stall? (5)

Answer 32

• Installed near the leading edge of a wing.
• Vortex generators produce streets of micro vortices over the wing within the boundary layer, to transfer more K.E. into the boundary layer.
• Greater K.E. results in boundary layer separation delay, thus stall caused by the boundary layer separation is delayed or prevented.
• When the Mfs becomes greater than Mcrit, vortex generators could de-stabilise the formation of the normal shockwave over the wing.
• It delays formation of shockwave, delays turbulent wake separation behind the shockwave, then delays shock stall.

Question 33

What is a design of an anti-shock body called? Where are they usually installed? (2)

Answer 33

• Kuchemann carrots, or Whitcomb bums which are are streamlined pod shaped bodies.
• Added to the leading edge or the trailing edge of an aerodynamic surface, starting from the near point of maximum thickness, or most camber, and extending beyond the T.E. of a wing.

Question 34

How do anti-shock bodies delay shock stall? (4)

Answer 34

• Can reduce interference of flow streams from different parts of the aircraft, improving the buffet behaviour of the wing in the transonic range and reducing interference drag.
• Local airflow caused by anti-shock bodies interrupt shockwave when it moves toward T.E. when Mfs increases.
• Therefore, Mach buffet will be reduced, and shock stall delayed.
• Delays shockwave and shock stall, therefore transonic wave drag is also reduced.

Question 35

What is the purpose of the area rule? How is the area rule incorporated on an aircraft?

Answer 35

• Achieve minimum transonic drag rise.

- The cross-sectional area of the whole aircraft should increase and decrease smoothly from nose to tail.