Module 4 - Transonic Flight & Wings Flashcards
What is the limiting factor that determines the low speed limit?
- An aerodynamic stall
- Caused by the boundary layer separation form the aerofoil at low speeds and high AoA resulting in the loss of ability to produce lift
What is the limiting factor that determines the high speed limit (transonic)?
- A mach stall
- When the turbulent wake behind a shockwave separates from the surface
- The shockwave formation increases the local pressure suddenly, causing BL separation and increasing drag significantly causing loss of lift
What does the low speed stall TAS change with altitude? What is the nature of this change?
- Low speed stall TAS increases with increased altitude due to a decrease in density
- It changes non linearly due to the non linear change of density with altitude
How does the high speed mach stall TAS change with increased altitude? What about in the Tropopause? Why?
- Mach stall TAS will decrease linearly, and remain constant in the tropopause
- Assuming a constant mach stall number, the TAS will change as the speed of sound changes due only to the change in temperature of the air
What is coffin corner? What will happen to an aircraft on coffin corner?
- The altitude where the low speed stall TAS and mach stall TAS intercept
- The aircraft will stall (mach or low speed) if its airspeed deviates from this intercept speed at all
What is the buffet boundary?
- The speed at which the boundary layer starts leaving the surface causing buffet.
- The wake vortices will separate at a certain frequency causing vibration and oscillation
- Serves as a warning that the aircraft is approaching a stall
What is the buffet margin?
- The speed range between the lower buffet boundary and the upper buffet boundary for a given altitude
- The margins at different altitudes from the normal operating envelope
Can the buffet boundary ‘coffin corner’ be escaped?
Yes, using limited manoeuvres
What things will affect the buffet boundary and buffet margins?
- Altitude
- Weight of aircraft
- Load factor/ Angle of bank
Why is it important to try and increase M(crit)
-Because shockwave formation will; =Increase drag =Decrease lift =Can cuase a stall =Cause stability and contro issues
How does having a slim aerofoil increase M(crit)?
- An aerofoil forms half of a ocnverging-diverging nozzle
- Speed increase to the thickest point is proportional to the change of area of flow path
- If the ‘area’ decreases at a large rate along the chord, the airspeed will increase greatly along the chord to M=1.
- If the ‘area’ decreases slowly (slim aerofoil) the M(fs) when the first M=1 will be relatively higher
What is a disadvantage of a slim aerofoil with a low thickness-chord ratio?
- Low coefficient of lift
- Lower lift produced for a given airspeed - especially at low airspeeds
What are some advantages of having a slim transonic aerofoil?
- Decreases drag
- Delayed shockwave formation
- Decreases shockwave intensity
- Better stability
How does a flat leading edge increase M(crit)? Why?
Note: often found on a supercritical section
- Reduces the change in ‘area’ of the airflow if the aerofoil is assumed to be half of a con-div nozzle
- As the ‘area’ decreases gently the velocity will increase relatively slowly, and M(crit) will occur at a higher M(fs)
What features of a supercritical section increase M(crit)? What features maintain the C(L) and increase the effectiveness of the aerofoil?
- Upper surface is flattened, so the change in the ‘area’ for the airflow is reduced
- Has a flatter leading edge to reduce the change in area
- Has reflex camber at lower rear surface to maintain lift at the rear part and stabilise trailing edge flow
- Reduces wake separation
- Requies less wing sweep
What are the advantages of have a supercritical transonic aerofoil? Why?
- Reduced sweep allows for a lighter structure
- Increased M(cdr) in cruise: reduced wake separation
- Weaker shockwave: flattened upper surface
- Delayed formation of shockwave: flatter upper surface
- Better lift at low speed: due to thicker (than slim) aerofoil
How does sweepback increase M(crit)?
- A swept wing will have 2 components of airflow, chordwise and spanwise
- There is no change in area for the spanwise component of airflow, so it is not going to chase a shockwave
- The M(fs) when the chordwise component of airflow becomes sonic will be relatively higher than a straight wing
How doe Vortex generators increase M(crit)?
- Produce microscopic energetic vortices over the surface of the aerofoil
- They delay the formation of the shockwave and the separation of turbulent wake behind a normal shocwave if one has formed
- Aircraft can reach a higher M(fs) before the first shockwave forms
How does a low thickness to chord ratio affect the shockwave over a transonic aerofoil? Why?
What are the follow on effects of having a higher M(crit)
Thin aerofoil changes the pressure slowly and by having a gradual decrease in pressure over the wing. It will have a higher M(crit)
- Reduces the intensity of the shockwave
- Reduce shock drag
- Increases the M(cdr)
- Decreases the maximum CD
- Less violent turbulent wake
- Buffet margin can be increased
How does the coefficient of lift change with airspeed for a normal aerofoil versus a low t/c aerofoil?
Normal:
- Large increase up to formation of shockwave
- Larg dip due to shockwave formation/ wake separation
- Slow small increase due to shockwave moving to TE
Low t/c:
- Very gradual increase until shockwave settles on TE
- Very slow, gradual decrease
- Has a higher M(crit)
What are the features of a supercritcal aerofoil? How do they affect transonic flight?
- Flattened surface at LE and moderate t/c ratio: Ensures increase in airflow speed is gentle and smooth, has higher M(crit)
- Moderate thickness: Has a higher CL than a thin aerofoil so can produce more lift, especially at lower speeds
- Flat upper surface: Formation of upper shockwave is delayed, intensity fo normal shock is reduced, increases in shock drag delayed
- Reflex camber at rear of aerofoil: Improves lift production at rear of aerofoil, delayes the formation of lower shockwave, reduces wake separation, increases M(cdr)
- Increases efficiency: span and sweep angle can be reduced since CL and CD are increased and decreased respecitvely
What are some limitations to a transonic sweepback wing design? How do they occur? (5 points to cover in depth)
-Airflow separates from the freestream direction around the wingtip
=Air is a viscous fluid so changes direction aroudn thw wingtip
=Deflects towards the fuselage
=Effective sweep angle at the wingtip is less than that of the wing and so effective M(crit) is reduced from theoretical M(crit)
-Thickness is not uniform along the span
=Airlfow over the wing is compressed towardss the wing root
=A series of compression waves can be formed near the wing root, potentially causing a shockwave
=Drag will be increased due to compression waves
-CL is reduced compared to that of a straight wing
=Due to the spanwise componet of airlfow not contributing to lift producetion
-Ram horn effect/vortex
=Spanwise flow creates a thicker BL towards the wingtip
=Encourage large wingtip vortex, that can start to form on the LE
=Induces more downwash and increases induced drag
=Can cause a tip stall resulting in CoP change due to stalled wingtips
-Deep stall
=When turbulent wake shadows the tailplane, reducing effectiveness
=T-tail design can be used to avoid the deep stall
=Deep stal occurs when the mainplane is stalled and the tailplane cannot produce any nose pitch down moment to recover`
What are the advantages of a sweepback transonic wing? Briefly explain each advantage? (6)
- Positive stability in any direction
- Increased M(crit) - larger sweep angle = higher M(crit)
- Stall AoA is higher than that of a straight wing
- Increased M(dd):due to delays in shockwave formation and turbulent wake separation
- Relatively low CD: due to low shock drag caused by later shockwave formation
- Can have relatively high economical cruise speeds since CD is low
What devices can be used to delay shock stall? How do they work?
- Wing fences: prevent development of ram horn and can interrup rear shockwave on swept wings
- Vortex generators: Increase BL kinetic energy and delay separation, delay formation of normal shockwave and thus delay shock stall
- Anti-shock body: Interrupt rearward movement of shockwave, Mach buffet reduced and shock stall delayed
What is an anti-shock body and how does it work?
- Streamline pod shaped bodies on the wing
- Start form most cambered or thickest part of the wing and extend behind the TE
=Reduce flow stream interference from different parts of the body
=Improves buffet behaviour and reduces interference drag
=Interrupts the movement of the shockwave as it moves rearwards, Mach buffet will be reduced and shock stall delayed
=Delaying the shockwave and shock stall formation will also reduce the wave drag experienced
What is the area rule?
“To achieve the minimum transonic drag rise, the cross sectional area of the whole aircraft should increase and decrease smoothly from nose to tail”
What is crossover altitude? How is it used?
The altitude at which a specified IAS/CAS and Mach value represent the same TAS
- Above this altitude Mach number is used for speed reference, below CAS is used for speed reference
- Crossover altitude determined using a graph with different climb profiles
