Aero surfaces geometry Flashcards
Four key aerofoil geometry parameters.
- Chord length
- Thickness distribution
- Mean camber line
- Leading edge radius
Explain the principles behind how an aerofoil creates lift.
Aerofoild change the velocity of the air passing over and under it. Its camber causes air over the top to travel faster which causes lower pressures. Higher pressure air on the bottom pushes up on the aerofoil to create lift.
Why do aerofoils have camber?
- To enable the airflow to remain attached over the wing (stop flow separation), which increases lift and reduces drag.
- Camber allows a zero-AOA aerofoil to generate lift.
What is M_crit and why do commercial airliners cruise below it?
Speed at which supersonic flow appears. Leads to sudden drag increases, loss of lift and change in pitching moment.
What are some key wing geometry parameters?
Aspect ratio, taper ratio, sweep
What wing geometry aspect is used to position the wing to attain desired stability?
MAC (Mean Aerodynamic Chord)
Why do lower aspect ratio wings stall at a higher AoA?
Air wants to escape from the bottom to the top of the wing which reduces the effective angle of attack and increases drag particularly for low A.
What are some advantages and disadvantages of increasing sweep?
- Reduces adverse affects of trans and supersonic flow.
- Increases wing weight and at lower speeds reduces lift and effectiveness of control surfaces.
What is the most effective way of distributing lift for an unswept wing? Why is this impractical.
Elliptical wings (like Me109 or Spitfire) but are difficult and expensive to manufacture.
What is taper ratio and give a typical value
tip chord / root chord (around 0.3 for most airliners)
What is dihedral angle and why is a positive angle beneficial?
Vertical sweep of wings, tends to roll the aircraft level when banked.
Describe wing location arrangements (high or low)
High - allow placing fuselage closer to ground which can be better for payload loading / unloading (C130). Fuselage needs to be stronger.
Low - Good for landing gear stowage and efficient landing gear load transfer to wing box. Requires large landing gear and specialist equipment for disembarking.
Discuss a conventional vs T-tail arrangement.
Conventional - provides adequate stability and control at lightest weight.
T-Tail - allows for smaller vertical tail and lifts horizontal tail into cleaner air. Also allows for aft mounted engine placement.
What is tail placement important w.r.t stall characteristics?
If tail enters wing wake it will stall and pitching authority will be lost.
Why is the horizontal tail LE sweep usually greater than the wing’s?
Promotes tail stall after wing and provides a higher M_crit to maintain elevator effectiveness.
