Module 3 - Transonic Aerodynamics

Question 1

Describe the conditions and formation of mach wave? What happens as the Mfs increases beyond the Mcrit? Where does it form?

Answer 1

Forms when the first local mach number = 1 (critical mach number).
It will form at the most cambered point of an aerofoil

As Mfs increases the mach waves get stronger and accumulate to form shockwaves

Question 2

Describe the airflow (speed and nature) at different points around an aerofoil when there is a shockwave formed on the upper surface?

Answer 2

Ahead of Shock: Supersonic
Behind Shock: Subsonic & Turbulent

Stronger shock: more turbulent and more oscillation - can be unstable, will cause separation point earlier

Question 3

Describe the structure and relative dimensions of a shockwave?

Answer 3

• A normal shockwave is about 1mm thick and can form on both the upper and lower surface
• Normal shockwave sits at 90˚to the surface it is formed over
• The base of the shockwave is much thicker and this is called a labda foot (λ)
• One side of the lambda foot is a small oblique shocklwave and the other is the normal shockwave

Question 4

What is the cause of the labda foot? What are the pressure changes that occur ahead, in and behind the lambda foot?

Answer 4

• Caused by a ‘pressure leak’
• The pressure change is not so sudden in the boundary layer and so the local speed can be supsonic

Pressure in the lambda foot is greater then ahead of the oblique shockwave but less than behind the normal shockwave

Question 5

Describe the movement of a shockwave as the Mfs increase from M(crit) to M(det)?

Answer 5

• Shockwave forms at most cambered section on upper surface
• Upper shockwave will move rearwards (intensity increases)
• Shockwave forms on lower surface (less intense than upper)
• Both shockwaves move rearwards, the lower one moves rearwards faster
• Lower shockwave settles open TE before the upper shockwave
• Both shockwaves settle and there is no separation of supersonic airflow
• Bow shockwave forms at LE and will eventually detach

Question 6

How will shockwave movement over a symmetrical aerofoil set at 0˚ AoA vary from the standard model?

Answer 6

The lower and upper shockwave formation and movement will be simultaneous

Question 7

What is the range of transonic flight?

Answer 7

From M(crit) to M(det)

Question 8

What is Mach buffet? What causes it?

Answer 8

• When the aerofoil vibration shakes the aircraft structure and creates intensified noises.
• Caused by detachment of turbulent wake vortices in an oscillation fashion as well as the shockwave oscillation creating variations in lift over each part of the wings?

Question 9

What is Shock Stall? What causes it?

Answer 9

• When the aerofoil loses lift due to the separation of turbulent wake
• Causes by the separation of turbulent wake behind the shockwave and airflow separation from the aerofoil

Question 10

What is shock drag? What specific types of drag is this made up of?

Answer 10

The drag caused by shockwaves

• Wave Drag
• Boundary Separation Drag

Question 11

Describe Wave drag?

Answer 11

• The resistance to the air to flow past the shockwave, it is also called energy drag.
• The airflow draws kinetic energy from the airflow to increase the temperature, pressure and density.
• The higher the mach number the the more kinetic energy the airflow will lose to form the shockwave

Question 12

Describe boundary separation drag?

Answer 12

• The drag due to the fore-at pressure difference that is present over an aerofoil due to the separation of turbulent wake vortices.
• The turbulent wake vortices have a lower pressure than the free stream flow ahead of the wing
• The turbulent wake detaches from the surface behind the shockwave and has much the same impact as in subsonic flow

Question 13

Describe the movement of the centre of pressure as mach number increases from M(crit) to M(det)? Why?

Answer 13

• Moves rearward until incipient shock forms, due to increasing airspeed
• Begins to move forward after shock forms and intensifies due to increasing high pressure turbulent wake behind the shockwave and lower pressure before the shockwave
• After CoP reaches most forward position, it will move slowly rearwards due to shockwave moving rearwards. Lower shockwave also forms
• As both shockwaves move rearward to TE, the CoP will move synchronously and settle at about 50% of chord due to the turbulent wake behind shockwaves not affecting the wing surfaces.

Question 14

Describe the changes to coefficient of lift as the mach number increases from M(crit) to M(det)? Why?

Answer 14

• CL will increase initially due to the lower pressure before the formation of the shockwave
• After shockwave formation CL will slow decrease doe to upper shockwave, and will suddenly decrease when wake separation and lower shockwave occur
• As shockwaves move towards the rear CL starts to recover due to less effect of turbulent wake
• CL will start to decrease again after both shockwaves settle due to similar pressure distribution over both surfaces
• Continues to decrease until M(det)

Question 15

Describe the changes to the coefficient of drag as the mach number increases from M(crit) to M(det)? Why?

Answer 15

• CD will increase as the shockwave forms due to wave drag and potential separation of turbulent wake
• CD increase due to intensification of shockwave and formation of lower shockwave
• M(cdr)/ M(dd) is where the CD begins to significantly increase, usually about when the lower shockwave forms
• As the shockwaves move rearwards the Cd increases to maximum when both shockwaves settle on TE
• CD starts decreasing when the turbulent wake behind the shockwave moves off the surface behind the TE

Question 16

In 4 points summarise the change to CoP, CD, CL as M(fs) increases from M(crit)?

Answer 16

• CoP moves rearward when both shockwaves move towards the TE and settles near the middle of the chord
• CL will decrease soon after initial increase because of the increase in air pressure after the upper normal shockwave. The stronger the shockwave the further forward turbulent wake separates and the lower CL becomes.
• CD increase due to the formation of shockwaves and changes significantly when wake separation takes place
• CL and CD recover from their extreme values when the shockwaves reach the area near the trailing edge

Question 17

How do drag and drag coefficient relate to each other in transonic speed ranges?

Answer 17

May not be intuitive, CD decreases but drag increases

Drag is proportional to v^2.

Question 18

What symptoms can detachment of turbulent wake vortices have? (3) What do these symptoms do?

Answer 18

• Loud noise
• Oscillation of shockwave
• Random airframe vibrations (mach buffet/ sonic buffet)

Increase C(D)

Question 19

What is shock stall?

Answer 19

Loss of lift due to intensity of shockwave increasing due to turbulent wake separation behind the shockwave

Question 20

Why should shock stall be avoided? What will happen to the lift and drag characteristics?

Answer 20

• CL will be at its lowest
• CD will be rapidly increasing
• Intensive mach buffet can lead to structural damage
• Control loss may occur

Question 21

Where is a shockwave on a control surface likely to form? How will it move as M(fs) increases?

Answer 21

Will likely form on the hinge area, the thickest part

It will move rearwards as it does over a mainplane

Question 22

What symptoms might a pilot notice if a shockwave forms over the control surfaces?

Answer 22

• Controls feel heavy
• Controls may be ineffective
• Control reversal
• Vibration o the control surface

Question 23

What longitudinal control issues may be experiences on a transonic aircraft? What things cause these issues?

Answer 23

Mack tuck - feeling of nose heavy

• CoP moving rearwards creates nose down pitching moment as CoG remains in constant position
• Downwash of mainplane is reduced due to turbulent wake separation, this reduces effective AoA on tailplane, thus reducing the nose pitch up moment produced
• Nose heavy effects cause aircraft to accelerate, M(fs) to increase and control issues to worsen

Question 24

What things may be experienced by a pilot if a shockwave forms on the elevator?

Answer 24

• Elevator feels heavy to move - high pressure over elevator
• Aircraft does not respond to elevator movement effectively, shockwave is ahead of elevator
• Elevator buzzes and vibrates noisily
• Force required is too great to move the elevator due to shockwave formation
• Adverse stick force may be required for accurate control

Question 25

What causes adverse stick force?

Answer 25

• Occurs in the high transonic range
• When control column is pulled, it will deflect the elevator control surface upwards
• This will intensify or create a shockwave on the lower surface
• The high pressure behind the shockwave will result in a nose pitch down moment, rather than the low pressure caused normally by such a deflection
• This creates a dangerous situation where controls inputs are reversed.

Question 26

What system or design options are available to fix transonic longitudinal control issues? (5)

Answer 26

• Thin tailplane and relatively sharp leading edge to increase M(crit)
• Use a mach trim system to counter the adverse stick force
• All moveable slab elevator so that part that does not carry the shockwave is still operational
• Adjustable power operated tailplane to take up additional forces so that the elevator can still respond to control column commands
• V(mo) sets safe design limit

Question 27

What lateral control issues may be experienced by a pilot operating in the transonic speed range? Why?

Answer 27

• Ailerons are ineffective: since they are normally located behind the shockwave
• Ailerons flutter and vibrate: due to effect of turbulent wake behind the shockwave
• Roll disturbance: as random vortex separation creates differences in lift on each wing randomly
• Aileron reversal: due to boundary layer separation, or due to aileron deflection causing the wing to twist about its lateral axis, causing a reducing in AoA over that wing

Question 28

What system or design options are available to fix transonic lateral control issues?

Answer 28

• Vortex generators upstream of ailerons: delay shockwave formation and reenergise airflow over ailerons
• Small outboard ailerons: operate in area less affected by turbulent wake
• Large inboard (mid chord/span) spoilers disrupt airflow to reduce lift and cause roll by reduction in lift
• Power generated controls located inboard: to increase the stiffness of the wing

Question 29

What directional control issues may be experienced by pilot operating in the transonic speed range? Why?

Answer 29

• Ineffective rudder: caused by rudder being behind shockwave formed on tail fin
• Rudder is heavy to move: due to shockwave moving further rearward on rudder and increasing arm from hinge
• Rudder reversal: Turbulent wake behind shockwave can cause oscillation and may cause yawn the direction opposite to what is intended
• Dutch roll: due to weak directional control and strong lateral control

Question 30

What systems or design options are available to fix transonic directional control issues?

Answer 30

• Yaw dampers: to reduce directional oscillation and weaken/ eliminate dutch roll
• Conventional fin/rudder with a powered control surface: to increase effectiveness of rudder
• All moveable slab fin/rudder: operated to avoid the shockwave forming on the rudder

Question 31

What is a sonic boom? How is it formed?

Answer 31

• Sudden pressure increase received by ears as a loud high intensity sound, explosion-like noise
• Sonic boom is a result of the sudden pressure increase due to the shockwave propagating through the air and impacting ears and objects.