Airflow Over Transonic Plane Flashcards
What is the Mach range for transonic flight?
Mcrit
Where does the shockwave initially form on an aerofoil?
Mfs does what to intensity
M before after SW
Come back if req
List the designs used to increase Mcrit.
Why might this be useful?
Slim aerofoil Sweptback wings Flat leading edge/supercritical aerofoil Vortex generators Aircraft flying fast enough will encounter shockwaves, which cause an increase in drag, decrease in lift, shock stall and stability/control issues. If Mcrit can be increased, then a/c are able to fly faster before encountering these issues.
How do slim aerofoils increase Mcrit?
Any disadvantages?
Above the aerofoil can be treated as a C/D nozzle, between the free stream and aerofoil. dA/A= (M^2-1) dV/V shows that with a converging area in subsonic flight, the speed increases proportionally to the decrease in area. In thick aerofoils, there will be a large change in area hence a large change in speed. This means you can have a relatively small Vfs that will reach M=a=1 at the most cambered section. A thin aerofoil will have a small change in area hence a small change in velocity so the Vfs can be much greater before reaching M=1 and Mcrit is reached.
A disadvantage of the small t/c ratio is a low Cl.
How do sweptback wings increase Mcrit?
Flow across the chord (perpendicular to wingspan) will cause the formation of shockwaves. Chordwise flow over a straight aerofoil will equal the Vfs. A sweptback wing with sweep back angle (theta) will have a reduced chordwise flow and have a spanwise component. The chordwise flow is given as Vcos(theta). The reduced chordwise component allows for a greater Vfs before M=1 over the aerofoil is reached and hence Mcrit is reached.
How does a flat leading edge/supercritical aerofoil allow for a greater Mcrit?
For the exacts same reasons as a slim, small t/c aerofoil!
Expect these aerofoils are able to have a greater t/c ratio so have the advantage of increasing Mcrit, without the disadvantage of a low Cl.
A supercritical section is a flattened upper section from the leading edge of an aerofoil.
How do vortex generators increase Mcrit?
Technically they don’t, but they effectively do.
As airflow passed VG they produce microscopic energetic vortices which disrupt and delay the formation of shockwaves and add Ek to the BL, which helps delay the separation of the turbulent wake behind a normal shock. They can however produce additional drag.
As VG delay shockwave formation once Ml=1, Mcrit is not technically increased but as they allow the aircraft to fly faster before shockwave formation, they essentially have the same effect as increasing Mcrit.
Describe the movement and structure of a shockwave from Mcrit-Mdet
Refer aeronasssignment
Refer “shockwaves on aerofoils” too
Important: when Mfs=1 both SW on TE
What factors can cause Ml to reach 1 earlier?
AoA increasing and certain manoeuvres can cause Ml=1 earlier
What is a lambda foot? What causes it?
The root of a shockwave is thicker than the end, due to the lambda foot. It is made up of a normal and oblique shockwave.
It is caused by a partial pressure leak. This is as the pressure change in the BL is gradual, where the local speed is often less than the speed of sound. This means the pressure in the lambda foot is greater than the pressure ahead of the shockwave but less than the pressure aft of the shockwave.
How does a higher Mach number change the characteristics of a shockwave and the shockwaves effects?
A higher M before a shockwave means a lower M after.
A higher M before a shockwave means a higher wake oscillation and strength. The stronger a shockwave becomes, the turbulent wake separation point will begin to move closer to the shockwave.
Shockwaves affect the pressure distribution around an aerofoil which is constantly changing due to the shockwaves movement and intensity.
Describe the movement of the CoP on a transonic aerofoil… as a general rule of thumb
(For a symmetrical aerofoil at a small AoA, with increasing Mfs).
- When Mfs reaches Mcrit, a shockwave (relatively weak)is formed on the upper surface of the wing at the most cambered section. Pressure before the shockwave decreases (due increasing local V) while pressure aft of the shockwave will increase. This causes the CoP to move toward the LE, up until the fore most position it can reach.
- Now as the shockwave strengthens as Mfs increases, it continues to move rearwards so now less of the wing is aft of the shockwave so the CoP follows the shockwave rearwards. A lower shockwave has also nor formed and moving rearwards.
- As both shockwaves settle near the trailing edge, the CoP will settle about 50% of the chord and will remain there regardless of any futhur Mfs increase as the whole aerofoil is covered in supersonic flow.
Movement of CoP will result in pitching moment changes and stability/control.
Describe how Cl changes on a transonic aerofoil with increasing Mfs
- As the Mfs reaches Mcrit and a shockwave forms, pressure in front of the shockwave decreases as the local airspeed increases, increasing the pressure differential between the upper and lower surface, increasing Cl.
- As Mfs increases further, the shockwave becomes stronger and the wake separation occurs closer to the shockwave, so the pressure rises aft of the shockwave, reducing the lifting capacity of the wing.
- As the 2nd shockwave forms under the aerofoil, Cl drops drastically as a second wake separation reduces the lifting capacity of the wing.
- However, as both shockwaves begin to settle near the trailing edge, Cl will begin to recover as the separation points move toward the trailing edge and more of the wing is before the shockwave and able to produce lift.
- As Mfs approximates Mdet, and both shockwaves are settled on the trailing edge, and separation occurs entirely beyond the TE, so supersonic airflow covers the upper and lower surface. Because of the small difference in pressure, as a result Cl will begin to decrease again.
Describe how Cd changes on a transonic aerofoil as Mfs increases
- As Mfs reaches Mcrit and a shockwave is formed, the formation of the shockwave/turbulent wake separation will cause an increase in Cd.
- As Mfs increases, the upper shockwave will strengthen and so does the turbulent wake separation, so Cd will continue to increase. When the second shockwave forms under the aerofoil, Cd will drastically rise due to the secondary formation/wake separation. This point in known as Mdd (drag rise)/Mcdr (critical drag rise).
- Cd will still rise after Mdd as Mfs increases due to the turbulent wakes moving forwards and will reach peak value. However once both shockwaves reach the trailing edge, and separation occurs beyond the TE, Cd will decrease.
- Once supersonic airflow covers the whole upper and lower surface, Cd will decrease but only change a little.
Can D be substituted for Cd in the previous flash cards?
No.
For example, when Mfs is close to Mdet, although Cd decreases, D is proportional to V^2 and V is very high at this point so increasing.
