Aerodynamics of Transonic Aerofoils Flashcards

1
Q

When will a normal shockwave form on an aerofoil? (3)

A
  • When free stream Mach number reaches the critical Mach number, the local Mach number over the aerofoil at the most cambered location will reach one (ie the airflow becomes sonic and Mach waves will occur at this location).
  • When the local airspeed over the aerofoil is greater than the speed of sound, a normal shockwave will be formed due to the pressure disturbance, which could be caused as a result of the pressure difference between the free-stream pressure and local pressure.
  • An initial normal shockwave will occur at the upper surface of an aerofoil
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2
Q

What happens to pressure, density and temperature following a shockwave? Why?

A
  • Air pressure, density and temperature all will increase.
  • This is because the airflow behind a shockwave becomes subsonic as the freestream airspeed is lower than the speed of sound.
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3
Q

Describe what a lambda foot is?

A
  • The root of a shockwave on an aerofoil is much thicker than the top part of the shockwave.
  • One side of the lambda foot is a part of the normal shockwave, and the other side of the lambda foot is a small oblique shockwave.
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4
Q

What is lambda foot caused by? Explain the pressure in the lambda foot? (3)

A
  • Partially caused by “pressure leak”.
  • Pressure change is not sudden through the boundary layer, where the local speed can be lower than speed of sound.
  • Thus pressure in the lambda foot is greater than the pressure in front of the normal shockwave, but smaller than the pressure behind the normal shockwave.
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5
Q

What happens to the turbulent wake when the Mach number continues to increase?

A

Higher the Mach number is before the normal shockwave, the stronger the shockwave, the stronger the turbulent wake’s oscillation will be.

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6
Q

What happens to the separation point if the shockwave is relatively weak? What happens to the turbulent wake and separation point if the shockwave is strong? (4)

A
  • Turbulent wake tends to separate from the surface.
  • Separation can occur at the rear part of the aerofoil if the shockwave is relatively weak.
  • When the shockwave gets stronger, separation point moves forward - gets closer to the shockwave.
  • The turbulent wake can separate immediately if the shockwave is strong.
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7
Q

Explain in detail, the movement of a shockwave along the aerofoil when the free-stream Mach number increases from Mcrit (8)

A
  1. ) When Mfs reaches Mcrit, the local airflow becomes sonic (local Mach number ML = 1) and Mach waves occur at this location.
  2. ) When Mfs increases after reaching Mcrit, a normal shockwave forms on the upper surface of the aerofoil.
  3. ) As Mfs increases further, after reaching Mcrit, the normal shockwave moves rearwards along the upper surface. The intensity of the shockwave is greater than that in (2). Turbulent wake behind the shockwave oscillates more, and separation point moves forward.
  4. ) As Mfs continues to increase, a normal shockwave forms on the lower surface of the aerofoil. The upper shockwave gets stronger, and its turbulent wake separtes just behind the shockwave.
  5. ) Both the upper and lower shockwave move rearwards as Mfs increases. The normal shockwave on the lower surface moves faster than the upper shockwave and settles at the T.E. of the aerofoil first.
  6. ) As Mfs increases further close to 1, the upper shockwave moves to the T.E, thus both shockwaves settle at the T.E. Both upper and lower surfaces are covered in supersonic airflow, no separations on both surfaces of the aerofoil
  7. ) When Mfs becomes greater than 1, a bow shockwave forms at the leading edge, then detaches from the leading edge.
  8. ) For a symmetrical aerofoil at 0 AoA, shockwave formation and movement on upper and lower surfaces occurs simultaneously when the Mfs changes from Mcrit to Mdet.
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8
Q

What is Detachment Mach Number? What happens to airflow at this stage? (i.e before and after shockwave and behind an oblique shockwave). (3)

A
  • The free-stream Mach number when the bow shockwave detaches from the L.E.
  • At this stage, the airflow in the freestream is supersonic. - The Mach number of the airflow behind the bow shockwave in front of the leading edge is subsonic (behind the normal part of the bow shockwave).
  • Behind the oblique parts of the bow shockwave, the airflow is still supersonic, meanwhile supersonic flow is both surfaces of the aerofoil.
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9
Q

What is Mach buffet and shock stall? (4)

A
  • Vortices in the turbulent wake behind a normal shockwave detach from airflow, and produce airflow oscillation, and the shockwave oscillates.
  • When the shockwave intensifies with the increase of Mfs, the detachment of turbulent wake vortices from the aerofoil surface vibrate the structure of the aerofoil with noise, known as Mach buffet.
  • The pilot experiences it like in a low speed/high AoA buffeting, when the aircraft is in Mach buffet.
  • Following the Mach buffet, the aerofoil will lose lift due to turbulent wake separation and get into a stall, known as shock stall.
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10
Q

What are the two types of shock drag?

A

Wave drag and boundary separation drag.

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11
Q

What is wave drag? (4)

A
  • When a shockwave is formed, air pressure, temperature and density all increase.
  • It requires energy to form the shockwave, and the energy is provided by the airflow.
  • Higher the Mach number, more kinetic energy the airflow would lose to form the shockwave i.e. to overcome more “resistance” to flow past the shockwave.
  • This “resistance” is known as Wave Drag, also called Energy drag.
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12
Q

What is boundary separation drag? (3)

A
  • Turbulent wake detaches from the surface of an aerofoil behind the shockwave, when the Mfs increases, it affects the aerodynamic forces just like it is in subsonic boundary layer separation.
  • The vortices in the separating turbulent wake causes fore-aft pressure differences.
  • This pressure difference exerts on the aircraft to form a drag, known as boundary separation drag.
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13
Q

Explain what happens to Cp, CL and CD when travelling from Mcrit to Mdet in the first stage? (5)

A
  • The Mfs reaches Mcrit (0.75), a shockwave is formed on the upper surface of a wing, and gets stronger with the increase of Mfs, but slowly moves rearwards.
  • The shockwave is at the most cambered position, the pressure in front of the shockwave decreases due to the increase in local airspeed.
  • CP moves forward.
  • CL increases due to the lower pressure before the shockwave.
  • CD increases, because the formation of the shockwave, and potential of the turbulent wake behind the shockwave.
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14
Q

Explain what happens to Cp, CL and CD when travelling from Mcrit to Mdet in the second stage? (4)

A
  • Upper shockwave moves rearward, the turbulent wake starts separating, and as Mfs increases further, a shockwave forms on the lower surface of the aerofoil.
  • After CoP moves to the front most position, CoP will move rearward.
  • CL starts decreasing from its peak when the upper shockwave get stronger, then suddenly decreases sharply, due to the formation of lower shockwave and the wake separation.
  • Cd increases significantly during this period due to the same reasons.
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15
Q

What is the drag-divergence mach number (Mdd)? What are its other names?

A
  • The free-stream Mach number at which the Cd increases significantly is called drag-divergence Mach number.
  • Drag rise Mach number Mdr, or critical drag rise Mach number Mcdr.
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16
Q

Explain what happens to Cp, CL and CD when travelling from Mcrit to Mdet in the third stage? (7)

A
  • Both shockwaves move rearward and settle at the rear end of the aerofoil, but lower one moves faster.
  • The separation of turbulent wakes behind the shockwaves will move closer to shockwaves, when Mfs increases further from Mdd, so CD still increases just after the Mfs reaches Mdd.
  • CoP moves to the middle of the aerofoil, and settles at 50% of the chord.
  • CL gradually recovers from the sharp fall due to the separation point moving towards the rear end.
  • Eventually there is no separation on both sides of the aerofoil.
  • CD reaches its peak because there are two shockwaves and both turbulent wakes separate.
  • CL and CD start decreasing when the shockwave and the separations move out of the surfaces.
17
Q

Explain what happens to Cp, CL an CD when traveling from Mcrit to Mdet in the fourth stage? (5)

A
  • Both shockwaves settle at the T.E. and both surfaces are covered with supersonic airflow.
  • A bow shockwave is formed in front of the leading edge when the Mfs increases further and reaches Mdet.
  • CoP stays at the middle of the aerofoil.
  • CL continues to decrease, because the pressure distributions on both surfaces are similar.
  • CD changes a little due to no shockwaves on the surfaces and no separations.
18
Q

What happens to CL and CD during a shock stall?

A
  • CL lowest at shock stall.

- CD highest at shock stall.

19
Q

Describe the dangers of an aircraft in Mach buffet?

A

The vibration caused by the intensive Mach buffet will lead to structure damage of the aircraft and cause the failure of control of the flight.

20
Q

Where do shockwaves first form on a control surface? (2)

A
  • Shockwave would first form at the hinge area of a control surface first, e.g. hinge of elevator, or hinge of rudder.
  • The shockwave will then move rearwards on the control surface as Mfs increases.
21
Q

What are the effects of a shockwave forming on a control surface? (3)

A
  • Pressure will increase behind the shockwave, making the control feel heavy.
  • The aerodynamic force exerted on the control surface due to the shockwave could be too great for the pilot to move the control surface.
  • Ineffectiveness of the control surface, even producing opposite effects from the initial control intention.
22
Q

What happens to the turbulent wake behind a shockwave that has formed on a control surface?

A

The turbulent wake separation causes the vibration of the control surface, and, in turn, the vibration of the control surface can cause instability.

23
Q

What is Mach tuck?

A
  • When an aircraft is in transonic flight, it feels nose heavy, i.e. experiences nose down pitch due to the shockwave over it wing.
  • This nose-heavy phenomena is called Mach tuck.
24
Q

What causes Mach tuck? What are the effects of Mach tuck? (6)

A
  • When Mfs increases, the shockwaves over the wings of a transonic aircraft moves rearward, the centre of pressure over the wing moves rearward generally.
  • Therefore, the nose-down pitch moment increases due to the increase of the distance between the centre of gravity and centre of pressure.
  • When the turbulent wake behind the shockwave separates earlier with the increase of Mfs, the downwash from the wing onto the tailplane reduces, thus, the tailplane produces less recovering nose-up pitch moment to overcome the pitch-down moment caused by the movement of the shockwave on the wing.
  • Thus, the transonic aircraft experiences nose-heavy.
  • This nose-down is adverse to the aircraft as it causes the aircraft to accelerate.
  • This acceleration leads to the increase of Mfs, making the control difficulties worse.
25
Q

What will happen when a shockwave forms on the elevator? (3)

A
  • Feels heavy to move the elevator, because the shockwave sets on the elevator, and the sudden increase of pressure on the surface of the elevator;
  • The aircraft does not respond to the elevator movement’s effectively, because the pressure distribution has changed over the elevator now, it cannot produce efficient “lift” as it is designed for;
  • The elevator vibrates and “buzzes” noisily, or buffets, due to the wake separation behind the shockwave.
26
Q

What is adverse “stick force”? (4)

What causes adverse stick force?

A
  • The push force on control column at higher Mfs in the transonic regions results in “pull” action instead of “push” action. This is known as adverse stick force.
  • This is caused by the shockwave formed on the elevator.
  • When the control column is pushed, the elevator deflects in order to produce lift - nose-down pitch.
  • However, pressure behind the shockwave on the elevator increases significantly, especially if there is turbulent separation, so the “lift” produced by the elevator is now a negative lift - nose-up pitch “pull”.
27
Q

What are some designs used to deal with longitudinal control issues? (4)

A
  • Using a thin tail plane, or relatively sharp leading edge design to increase the Mcrit of the elevator, delaying the formation of a shockwave on the elevator.
  • Using a Mach trim system allows to correct the adverse “stick force”, and eliminates confusion.
  • All movable slabs which can be operated at different parts of an elevator can be constructed, so the slab in the place where it is not affected by the shockwave is operational, while others carry the shockwave.
  • Adjustable and power-operated tailplane can be used to overcome the extra forces exerted on the elevator due to the movement of shockwave on the elevator, so it can continue to respond to the demand from the control column.
28
Q

What could happen to ailerons when an aircraft travels at transonic speed and shockwaves form on the upper surface of the wings (Lateral issues)? Also explain aileron reversal. (6)

A
  • Ailerons cannot operate effectively, since aileros are normally located behind shockwaves;
  • Ailerons flutter and vibrate when the turbulent wake separate behind the shockwave;
  • Aircraft will be in a roll disturbance, because the fast vibration of ailerons varies the lift on the aerofoil randomly;
  • A greater force can be produced by a deflecting aileron in a high-speed flight.
  • This force can cause twist of the wing about its lateral axis, changing the angle of attack in the opposite direction.
  • Thus if the aileron was applied to roll left, it would roll to the right, called AILERON REVERSAL. Boundary layer separation can cause aileron reversal.
29
Q

What design features are used to deal with the control difficulties associated with a wing and its ailerons? (4)

A
  • Vortex generators installed upstream of ailerons delay formation of shockwaves, and re-energise the airflow over the control surface to delay boundary layer separation;
  • Small outboard ailerons, which operate in less turbulent wake affected area;
  • Inboard (mid chord/mid span) spoilers disrupt airflow to reduce the lift on the roll-downward wing (instead of an aileron to increase lift on the roll-upward wing) to avoid aileron reversal;
  • Power generated controls located inboard to ensure stiffness of the wing.
30
Q

What happens when a shock forms on the hinge of a rudder in transonic flight? (3)

A

The directional control experiences similar difficulties as the control in other directions such as:

  • Ineffective rudder function caused by the shockwave. The rudder is behind the shockwave, so it affects the aerodynamic forces on the tail/fin to produce yaw moments;
  • Feels heavy to move the rudder, as the shockwave moves rearward on the rudder as Mfs increases;
  • Turbulent wake and the increase of pressure behind the shockwave on rudder causes oscillation, making the aircraft yaw in the opposite direction from the intended rudder movement. This oscillation leads to Dutch Roll.
31
Q

Describe ways to assist the directional stability control in transonic flight? (3)

A
  • Installing yaw dampers to reduce directional oscillation and to weaken/eliminate Dutch Roll;
  • Conventional fin and rudder combination with powered control surface to increase the effectiveness of rudder;
  • All-movable slab fin/rudder, e.g. vertical stabiliser can be operated separately to avoid the effects of the shockwave on the rudder.
32
Q

What is a sonic boom? (5)

A
  • When a flight reaches sonic speed and breaks the sound barrier, a shockwave forms attached to the aircraft.
  • The shockwave propagates in the air at the speed of sounds, there are sudden changes in air properties, specifically, the air pressure.
  • The sudden change of pressure strikes every object along its path including the ears of observers.
  • This sudden strike is Sonic Boom.
  • The sudden pressure increase caused by the shockwave is received by ears as a high intensity sound like an explosion.