Module 3 Aerodynamics of transonic aerofoils

Question 1

What is the structure of a shockwave on a transonic aerofoil?

Answer 1

Airflow is supersonic just before the shockwave, however, free stream airspeed is lower than the speed of sound, the airflow behind the shockwave becomes subsonic, where a P, p and T all increase after the shockwave.

Question 2

What is the lambda foot?

Answer 2

This indicates the root part of the shockwave is not a simple plan thin wavefront, but it is a wider structure.
Caused by a pressure leak.
Pressure in the lamda foot is lower than the pressure behind the shockwave but it is higher than the pressure in front of the oblique part of the shockwave.

Question 3

Explain what happens with the turbulent wake and separation point if the higher number before the shockwave occurs.

Answer 3

The higher the mach number stronger the turbulent wake, the turbulent wake oscillates behind the shockwave. Separation can occur at the rear part of the aerofoil if the shockwave is relatively weak and this separation point will move forward when the shockwave is stronger. (separation point will get closer to the shockwave)

Question 4

When do normal shockwaves move along the surface of an aerofoil?

Answer 4

When the Mfs mach number changes

Question 5

What happens to the shockwave on an aerofoil when the Mfs increases from Mcrit to Mdet

FINISHHHHHHH

Answer 5

A shockwave is formed on the upper surface of the aero foil and starts to move rearward along the upper surface. The intensity of the shockwave increases and the lambda foot of the shockwave is greater, when the separation moves forward the turbulent wake behind the shockwave increases.

Question 6

What is shock drag?

Answer 6

The formation of shockwaves and the separation of the turbulent wakes behind the shockwaves on an aero foil contribute a sudden increase to the total drag.This additional drag caused by the shockwave is shock drag.
There are two types:
Wave drag and boundary layer separation drag.

Question 7

What is Mdetachment?

Answer 7

The free stream Mach number when bow shockwaves forms and detaches from the leading edge.

Question 8

Describe the bow shockwave?

Answer 8

Consists of one piece of normal shockwave, and two pieces of oblique shockwave. M behind the normal shockwave is subsonic and M behind the oblique parts of the shockwave are supersonic, aswell as the rest of the airflow around the aero foil.

Question 9

What is the mach number range range for transonic aircraft?

Answer 9

Between Mcrit and Mdet

Question 10

What effect does AOA and maneuvers have on Mcrit?

Answer 10

Decreasing Mcrit

Question 11

What is the effect on CL after a normal shockwave?

Answer 11

Ability to produce lift is weakened, so the CL of the aerofoil decreases, the magnitude it decreases depends on the strength of the shockwave, as stronger the shockwave higher the pressure after the shock therefore lower the CL.

Question 12

What is the effect on CP after a shockwave?

Answer 12

CP will change because the pressure distribution has changed. CP will initially move FWD due to the position of formation, and then will move rearward with shockwave.

Question 13

What is wave/energy drag?

Answer 13

When a shockwave forms air pressure, temperature and density all increase. Energy is required to form and maintain the shockwave. The higher the mach number the more kinetic energy the airflow would lose to overcome the resistance to pass through a shockwave, this resistance is wave drag.

Question 14

What is boundary separation drag?

Answer 14

Turbulent wake detaches from the surface of the aerofoil behind the shockwave, when Mfs increases. This separation affects the aerodynamic forces just like the subsonic boundary layer separation causing a type of form drag. due to a pressure difference exerted on the aircraft. This is boundary separation drag.

Question 15

With regarded to a symmetrical aero foil sitting at 0 aoa, what occurs when the Mfs changes from Mcrit to Mdet?

Answer 15

Shockwave formation and movement on upper and lower surface occurs simultaneously

Question 16

When is the CP at it most forward point during shockwave formation?

Answer 16

When the lower surface shockwave starts to form.

Question 17

What is Mcritcal drag rise number?

Answer 17

Above a certain Mfs, due to the formation of shockwaves there is a significant rise in drag.
Usually 5-10% above Mcrit.

Question 18

What causes a CL increases after the Mcdr?

Answer 18

The supersonic region in front of the shockwaves, where pressure distribution establishes and produce aerodynamic forces, is extended towards the trailing edge. CL will continue to increase as the proportion of the aerofoil surface behind the shockwaves is getting smaller

Question 19

Where does Cp settle when both upper and lower shockwaves settle at the TE?

Answer 19

Sits approximately 50% of the chord.

Question 20

What is mach buffet and how does it occur?

Answer 20

Turbulent wake behind a shockwave separate from the surface of the aero foil with the separating point moving close to the shockwaves when Mfs increases.
Vorticies produced from the turbulent wake detach from the wake alternatively with a certain frequency. This causes a noise and a oscillation of the shockwave, eventually the detachment becomes very intense and happens immediately after the shockwave. The aircraft will experience vibrations and loud noise which is called the mach buffet/sonic buffet.

Question 21

How will a shock stall occur?

Answer 21

Aero foil fails to produce lift if the intensity of the shockwaves continue to increase after the mach buffet, the aircraft will then lead into a stall. This is shock stall

Question 22

What are ways to prevent a shock stall?

Answer 22

Get into regions of high temperature
Stop going so fast
Shock stall can be reduced by good design