Transonic Flight and Aerofoils Flashcards
What is the aerodynamic limitation in transonic flight mean?
Means the limit in airspeed, at which the aircraft can no longer produce the aerodynamic forces effectively to maintain the flight.
What are the two transonic speed limits? Explain the two transonic speed limits
- The low-speed limit of a transonic flight is the airspeed, at which the aircraft is at a low speed-high AoA stall caused by the boundary layer separation over the aerofoil and the aerodynamic feature of the airflow field to produce lift over the aerofoil is destroyed.
- The high-speed limit is the airspeed at which the aircraft starts Mach stall caused by the turbulent wake behind the shockwave on the aerofoil separating from the surface. The formation of a shockwave over an aerofoil increases the local pressure suddenly, and the boundary layer separation of the turbulent wake causes the complete loss in lift and the significant increase in drag, therefore, level flight cannot be maintained.
What happens to the low speed limit when altitude increases?
Increases with the increase of altitude as the air density decreases with the increase of altitude assuming the same level of lift is maintained ( L = CL1/2pv^2s).
What happens to the high speed limit as altitude increases?
The TAS of high speed limit remains constant when the altitude is beyond the Troposphere; IAS is constant when ALT increases, while IAS decreases with altitudes.
What is the coffin corner?
- At each altitude there is an airspeed range from a low to a high speed limit.
- The two limits get closer as altitude increases and will intercept at an altitude called COFFIN CORNER.
What happens when aircraft reaches the coffin corner?
It will be in a stall if its airspeed decreases; and it will be in shock stall if its air speed increases. The aircraft at the coffin corner is in an unflyable situation.
What is the buffet boundary?
- An aircraft experience buffeting before it reaches stall.
- The airspeed at the start of the buffet is known as BUFFET BOUNDARY.
What is the buffet margin?
The speed range between the lower speed buffet boundary and the high speed buffet boundary at each altitude is called BUFFET MARGIN.
What does the buffet margin change with? What happens to the buffet margin when it reduces to 0?
- BUFFET MARGIN CHANGES WITH altitude.
- When the margin reduces to “0”, the aircraft is up at a certain altitude, then the aircraft is at the coffin corner.
What are the factors that affect the buffet boundary and buffet margin?
- Altitude
- Weight
- Load factor
What effect does altitude have on the buffet boundary and buffet margin? (2)
- As altitude increases, the buffet boundary get closer to each other, and it means that the boundary margin gets smaller.
- If the altitude increases to a certain level, e.g. 23000 ft, the margin reduces to “0” so the aircraft in practice is in “coffin-corner”.
What effect does weight have on the buffet boundary and buffet margin? Give an example of it?
- Buffet margin with a high weight is smaller than that with a low weight, assuming the altitude is the same.
- e.g. The altitude of 30,000 ft, if the weight of the aircraft is 60 tons, the margin is 180 kts between 160 kts and 340 kts; while the margin reduces to 155 kts between 180 and 335 kts, if the weight becomes 70 tons (see FIG 9-3).
What effect does load factor have on the buffet boundary and buffet margin?
Buffet margin of the aircraft decreases, when the load factor increases.
What is the crossover altitude? (2)
- The altitude at which a specified IAS or CAS and
Mach value represent the same TAS (True airspeed) value. - At this altitude, Mach number is used to reference airspeeds.
SEE ASSESSMENT
What does crossover altitude depend on?
- Climb target
- Aircraft type
- Speed target cruise speed
Explain how a flat leading edge affect Mcrit? (4)
What is the advantage of using a flat leading edge? (2)
- A laminar aerofoil with a slim leading edge, the aerofoil with a supercritical section have a high Mcrit.
- A supercritical section has a flattened top upper surface from the leading edge of the aerofoil.
- Both of the laminar flow aerofoil and the supercritical section have relatively small dA, thus dV of airflow on both is gentle and small.
- Their t/c/ ratio can be greater than that of slim design.
- CL could be greater than that of slim aerofoil.
- Thus flat leading edge and supercritical section can have a high value of Mcrit without the disadvantage of a low CL.
Explain how a sweepback wing affects Mcrit? (4)
- As air flows over an aerofoil along its chord, it increases sonic and the aircraft’s Mach number reaches Mcrit.
- The chord wise speed of a a straight wing is greater than a sweepback wing if their free stream speeds are the same..
- When their chord wise air speed reaches sonic, the Vfs of straight wing is smaller than that of a sweepback wing.
- Thus a sweepback wing can increase its Mcrit.
How do vortex generators affect Mcrit? (4)
- When airflow passes vortex generators, microscopic energetic vortices are produced over the surface of the aerofoil.
- These vortices delay the formation of a shockwave, and delay the separation of the turbulent wake behind the normal shockwave if a normal shockwave has formed.
- So the aerodynamic difficulties caused by the normal shockwave on the aerofoil would be weakened and delayed.
- Thus, vortex generators on the upper surface can result in the aircraft travelling faster, therefore INCREASING MCRIT.
What are the main difficulties caused by a shockwave forming on an aerofoil travelling at transonic speed?
- Significant increase in drag.
- Decrease in lift by affected aerofoils.
- Shock stall.
Describe the characteristics low thickness to chord ratio transonic aerofoil? (5)
- Thin design of wings used for high speed flight.
- Cross sectional area of airflow path over an aerofoil increases gently if its t/c ratio is low.
- Thin aerofoil = high Mcrit.
- For the same design, the cross-section area of airflow path over an aerofoil decreases gently as well after it passes the most cambered location.
- Therefore, the airflow speed over a thin aerofoil increases gently and smoothly, while the pressure over the aerofoil decreases slowly, thus delaying the formation of shockwaves.
What are the effects of a strong shockwave with regards to a low t/c ratio? (4) What advantage does a low t/c ratio provide?
- Produces high shock drag; the turbulent wake behind the shockwave is more unstable;
- Mach buffet and shock stall can also occur easily with a strong shockwave.
- Therefore, a wing with a low t/c ratio experiences low shock drag and a higher Mcdr.
- Since the Mach number over the thin wing doesn’t increase quickly, the turbulent wake behind the shockwaves will be less violent, so the aircraft travels at high speed with better stability.
What happens to the buffet margin regarding t/c ratio?
It is possible that the buffet margin of an aircraft with thin wings greater than that with thick wings.
What happens to the lift and lift coefficient of an aircraft with a low t/c ratio?
- Pressure decreases slowly over a thin wing, therefore less lift compared to a thick wing, especially at low air speeds, i.e. less lift at low airspeeds (lack of ‘circulation’ when using circulation theory of lift)
- CL of thin aerofoil is low.
What is the supercritical aerofoil designed for?
Supercritical section to reduce shock drag, and to delay shock stall.