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Part 4 Aerodynamics > Module 4 - Transonic Flight & Wings > Flashcards

Module 4 - Transonic Flight & Wings Flashcards

1
Q

What is the limiting factor that determines the low speed limit?

A
  • 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
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2
Q

What is the limiting factor that determines the high speed limit (transonic)?

A
  • 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
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3
Q

What does the low speed stall TAS change with altitude? What is the nature of this change?

A
  • 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
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4
Q

How does the high speed mach stall TAS change with increased altitude? What about in the Tropopause? Why?

A
  • 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
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5
Q

What is coffin corner? What will happen to an aircraft on coffin corner?

A
  • 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
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6
Q

What is the buffet boundary?

A
  • 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
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7
Q

What is the buffet margin?

A
  • 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
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8
Q

Can the buffet boundary ‘coffin corner’ be escaped?

A

Yes, using limited manoeuvres

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9
Q

What things will affect the buffet boundary and buffet margins?

A
  • Altitude
  • Weight of aircraft
  • Load factor/ Angle of bank
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10
Q

Why is it important to try and increase M(crit)

A 
-Because shockwave formation will;
=Increase drag
=Decrease lift
=Can cuase a stall
=Cause stability and contro issues
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11
Q

How does having a slim aerofoil increase M(crit)?

A
  • 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
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12
Q

What is a disadvantage of a slim aerofoil with a low thickness-chord ratio?

A
  • Low coefficient of lift

- Lower lift produced for a given airspeed - especially at low airspeeds

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13
Q

What are some advantages of having a slim transonic aerofoil?

A
  • Decreases drag
  • Delayed shockwave formation
  • Decreases shockwave intensity
  • Better stability
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14
Q

How does a flat leading edge increase M(crit)? Why?

Note: often found on a supercritical section

A
  • 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)
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15
Q

What features of a supercritical section increase M(crit)? What features maintain the C(L) and increase the effectiveness of the aerofoil?

A
  • 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
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16
Q

What are the advantages of have a supercritical transonic aerofoil? Why?

A
  • 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
17
Q

How does sweepback increase M(crit)?

A
  • 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
18
Q

How doe Vortex generators increase M(crit)?

A
  • 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
19
Q

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)

A

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
20
Q

How does the coefficient of lift change with airspeed for a normal aerofoil versus a low t/c aerofoil?

A

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)
21
Q

What are the features of a supercritcal aerofoil? How do they affect transonic flight?

A
  • 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
22
Q

What are some limitations to a transonic sweepback wing design? How do they occur? (5 points to cover in depth)

A

-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`

23
Q

What are the advantages of a sweepback transonic wing? Briefly explain each advantage? (6)

A
  • 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
24
Q

What devices can be used to delay shock stall? How do they work?

A
  • 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
25
Q

What is an anti-shock body and how does it work?

A
  • 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

26
Q

What is the area rule?

A

“To achieve the minimum transonic drag rise, the cross sectional area of the whole aircraft should increase and decrease smoothly from nose to tail”

27
Q

What is crossover altitude? How is it used?

A

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

Part 4 Aerodynamics (6 decks)

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