Transonic Flight & Wings Flashcards
What is coffin corner? Draw out coffin corner for IAS & TAS
The lower speed limit can be defined by the high AoA/low IAS stall speed (due BLS), while the upper speed limit is where the Mach stall occurs (due turbulent wake separation). Both will cause a decrease in lift and increase in drag so unable to maintain level flight. As altitude increases, the range of speed between these two limits decreases. The intercept of the two is known as coffin corner and is where only one speed is able to maintain level light… any faster or slower and the a/c will enter a form of stall.
What is a buffet boundary? What about buffet margin?
Buffet boundaries mark the beginning of a margin between the start of buffeting and the stall. It is a speed above high AoA/low IAS Vs but below shock stall Vs. The max speed is either the high speed Vs or VMO/Vne.
A buffet margin is the speed range between the upper and lower limit of the buffet boundary ie) the safe operating speed range.
The buffet margin will decrease with altitude.
What factors affect buffet boundary?
Altitude: will decrease the margin as altitude increases.
Weight: as weight decreases, Vs (high AoA/low IAS) will decrease therefore margins are increased for a given altitude.
This also means you can fly higher before reaching coffin corner.
Load factor: (manoeuvres/turbulence), increasing load factor increases the stall speed so the margins are decreased.
These mainly affect low speed Vs, however will effect high speed Vs a small amount
Which is more practical for flying… buffet boundary or Vs speeds?
Buffet boundary
In theory the Vs boundary is the limit but it is not sensible to fly beyond the safe margin in a buffeting state, so when the corner of the buffet margin is reached you have reached coffin corner in practice.
What is crossover altitude?
What is special about this altitude?
The altitude at which a specified IAS/CAS value and Mach value represent the same TAS.
Above the crossover altitude M is used as reference and below kt is used.
Load factor, weight and altitude do not affect crossover altitude.
What is the main longitudinal transonic control issue? Why does it occur?
Mach tuck, a nose down pitching moment/nose heavy feeling.
Is caused by shockwaves forming over the aerofoils. As Mfs increases and the shockwaves move toward the TE, so will the CoP. A more rear CoP increases the arm between the CoP & CoG resulting in a larger arm hence a larger nose down pitching moment.
Downwash from the main planes is also reduced, due to SW wake, so this reduces the downwash onto the tailplane and hence the nose up restoring moment. The a/c as a result will remain nose heavy, causing the airspeed to increase so the shockwaves will move further rear making the whole situation worse.
What other control difficulties arise longitudinally in transonic flight?
1) When a shockwave forms it will initially form at the hinge of the control surface. The control surface will therefore sit in the wake of the SW, so it is ineffective at controlling the airflow ahead of the SW which can provide control difficulty. As the SW moves over the control surface the pressure increase will make the controls feel heavy.
2) Adverse stick force is when a pull force is required for a push or visa versa. Eg) we want a pull force (nose up)… assuming the elevator deflection causes the SW, the pressure increase aft of the SW will result in more upwards lift being produced which will cause a nose down
What designs can be used to prevent longitudinal transonic control issues?
- Thin tailplane/sharp LE to increase Mcrit by delaying shockwave formation.
- Mach trim can be used to correct adverse stick force.
- Slabs which operate different parts of the elevator a slab can operate a section less affected by shockwaves.
- A fully adjustable power operated tailplane to relieve the extra forces so the control surface does not have to deflect so much/continue to respond to control inputs.
What lateral control difficulties arise from transonic flight?
Why?
Because ailerons are located behind the shockwaves, hence lying in the turbulent wake, this is what causes the control issues laterally. The turbulent wake can cause aileron flutter, varying the amount of lift produced by each wing which creates roll disturbances.
Also, due to high speeds during transonic flight, ailerons have the ability to create a strong force, which can cause ailerons to twist about the lateral axis causing roll reversal.
How can lateral transonic control issues be fixed?
Vortex generators are placed upwind of the ailerons to delay the formation of shockwaves and reenergise the BL so wake separation is delayed.
Spoilers can be placed mid chord/span to disrupt airflow over downgoing wing which helps reduce aileron reversal.
Lastly, small outboard ailerons can be placed in areas known to be less affected by wake separation.
What transonic control issues arise directionally?
Similar to other control surfaces, a shockwave will initially form at the hinge of the control surface, meaning the control surface lies in the turbulent wake and unable to control airflow ahead of the shockwave.
The rudder will feel more heavy due to the pressure increase over the rudder.
The wake oscillation will cause small rudder deflections and therefore yaw disturbances which can induce Dutch roll.
What can be done to fix transonic directional control issues?
Yaw dampers can be installed to reduce/eliminate effects of Dutch roll.
Control surfaces can be split into different independently operating slabs so that one that is less affected by wake separation can still operate.
A fully adjustable power operated vertical stabiliser to reduce input by pilot to overcome the extra forces.
Name the ways to increase Mcrit
Low t/c ratio
Flat LE/super critical section
Vortex generators
Sweptback wings
How does a small t/c ratio (slim aerofoil) reduce Mcrit?
Upper half can be treated as a C/D nozzle. The equation tells us that change in area will be proportional to change in speed. So the small change in area results in a small change in speed… so the a/c can fly faster before Mcrit (ML=1) is reached at the most cambered section.
A disadvantage is they often have a small Cl value.
How does a flat leading edge aerofoil and a supercritical section help increase Mcrit?
Describe the supercritical aerofoil in more detail
For the exact same reasons as a slim aerofoil!
They can however have a greater thickness than a thin aerofoil so a greater Cl, and therefore more useful.
A supercritical section has a flattened top to allow the smooth increase in speed. There is reflex camber at the rear underside of the aerofoil to help increase lift at the rear and stabilise TE flow. The higher Mcrit (delaying shock stall/separation) means a reduced wake separation which increases Mdd allowing for a reduced swing sweep therefore a lighter structure.
