Module 4 Transonic aero foils and increasing Mcrit

Question 1

Why is there a need to increase Mcrit on a transonic aircraft?

Answer 1

Shockwaves form at Mcrit and this will cause a drag increase, lift decrease, stall and stability and control issues

Question 2

What are ways of increasing Mcrit?

Answer 2

Slimness
Flat leading edge
Sweepback
Vortex generators

Question 3

How does slimness increase Mcrit?

Answer 3

Airspeed increases slower over a slim aerofoil then it does over a thick aerofoil. Mfs will be greater, therefore increasing Mcrit

Question 4

How does a flat leading edge design increase Mcrit?

Answer 4

Similar to a slim aerofoil, airspeed increases slow due to its design, allows for a higher Mcrit

Question 5

How does sweepback increase Mcrit?

Answer 5

The chordwise speed of a sweep back wing is smaller than the free stream speed. Greater the sweep angle greater the critical mach number

Question 6

How do vortex generators increase mcrit?

Answer 6

When airflow passes over vortex generators, microscopic energetic vortices are produced over the surface of the aero foil. The vortices can delay the formation of shockwave, and delay the separation of the turbulent wake behind the normal shockwave. So shockwave effects would be delayed, which is a equivalent to increasing Vcrit.

Question 7

What are transonic Aerofoil designs?

Answer 7

low thickness to chord ratio
Supercritical Aerofoil
sweepback wings

Question 8

What are the effects of a low thickness to chord ratio/ (Thin aerofoil)

Answer 8

Airflow speed increases gently and smooth, while the pressure over the aerofoil will decrease slowly over the aerofoil, which will help to delay the formation of the shockwave.
Wing with low t/c ratio will experience a low shock drag, and produce a higher Mcdr. Due to the mach number over the wing not increasing quickly.

Question 9

What is the disadvantage of a thin wing?

Answer 9

Pressure decreases slowly over a thin wing, it will produce less lift than the aerofoil with greater thickness to chord ratio, especially at lower airspeeds.
Cl is relatively low on thin aerofoils.

Question 10

What is the purpose of a supercritical aerofoil?

Answer 10

It has a specially designed cross section for transonic airplanes. The supercritical section is designed to reduce shock drag, and to delay shock stall.

Question 11

What is the design of a supercritical aerofoil?

Answer 11

It has a flattened up surface at the leading edge with a modest t/c ratio. This allows for the gentle increase in speed over the aerofoil to increase Mcrit, with still maintain a relatively high lift coefficient.

Question 12

How does a supercritical aerofoil increase Mcdr?

Answer 12

Formation of shockwaves are delayed and the intensity of the shockwave will be reduced. So the significant increase of shock drag will occur at a higher Mfs, therefore allowing the aircraft to cruise at higher speeds.

Question 13

What effects does the reflex camber feature have on a super critical aerofoil?

Answer 13

The camber line is reflected upwards at the rear of the aerofoil.
it helps to improve lift production at the rear where the shockwaves form and airflow becomes subsonic.
It could help to increase the pressure to maintain a high lift production.

Question 14

What other features does a super critical aero foil have?

Answer 14

Reduce shock drag
improve lift
makes aircraft more efficient, and then can allow for wing span reduction and sweep angle reduction.

Question 15

What are devices to delay shock stall?

Answer 15

Wing fences
Vortex generators
Anti shock body

Question 16

Describe wing fences?

Answer 16

Wing fences feature on a wing close to the wingtip area to prevent large wingtip vortex to develop into a ram horn, which causes high induced drag and wingtip stall.
Interrupt the rear shockwave over a sweep back wing
reduce the turbulent wake behind the shockwave.

Question 17

Explain design anti shock bodies?

Answer 17

Streamlined pod shape bodies added to the leading or trailing edge starting from near the point of max thickness or mast cambered position and extending beyond the trailing edge.

Question 18

What is the purpose of anti shock bodies?

Answer 18

They prevent the local airspeed increasing much further, so the local mach number would not be very high and the intensity shock wave on the aerofoil would not be high, so the mach buffet will be improved, delaying the shock stall and will also reduce wave drag.

Question 19

What is area rule?

Answer 19

The area rule of design of transonic aircraft is that to achieve the minimum transonic drag rise, the cross sectional area of the whole aircraft should increase and decrease smoothly from nose to tail.

Question 20

What are sources of kinetic heating?

Answer 20

Friction with boundary layer
Stagnation points
Shockwave

Question 21

Methods to prevent supersonic aircraft from kinetic heating?

Answer 21

Materials
Insulation
Surface radiation
Surface cooling

Question 22

What is a “Ram horn” vortex? What causes it?

Answer 22

Spanwise flow along a sweepback wing where the boundary layer thickens towards the wingtip. Which encourages the formation of a large wingtip vortex that could be so large that it can start from the leading edge called a ram horn vortex.

Question 23

What effects does a ram horn vortex have?

Answer 23

Induces more downwash
higher induced drag
tip stall can occur which will lead to CoP moving forward and decreasing the restoring moment.

Question 24

What can prevent tip stall?

Answer 24

Washout

Question 25

What is a deep stall and what can prevent it?

Answer 25

When the turbulent wake shadows the tailplane it cannot work effectively and there is no restoring function available, A high T-tail design can be used to set the tailplane away from the turbulent wake.

Question 26

What is a supercritical section of a wing?

Answer 26

Designed to reduce shock drag and delay shock stall by having a flattened up surface at the leading edge to increase the airflow smoothly and slowly allowing for a greater Mcrit. It inputs a reflex camber line to increase Cl so it doesn’t have the disadvantage of the slim aerofoil of minimal lift production.

Question 27

What are the disadvantages of sweep back wing?

Answer 27

Compression waves can be formed close to the wing root and can even form a shockwave, therefore drag will be increased due to the compression waves.