Multi-element Wings Flashcards
Describe four methods of increasing lift with LE and TE devices.
Increased chord and wing area.
Increased camber.
Increased AoA before stall.
Improve state of BL.
How do LE devices affect aerofoil characteristics?
+ve/-ve
They keep flow attached at higher AoA.
Slightly decrease lift for the same AoA.
How do TE devices affect aerofoil characteristics?
+ve/-ve
They increase camber, and thus lift for the same AoA.
Can cause earlier flow separation.
How do multi-element wings affect the flow field to improve lift?
Circulation effect
Dumping effect
Slat effect
Fresh boundary layers
Off-surface pressure recovery
Describe the slat effect.
The velocities due to circulation on the two elements at their TE/LE run counter. Thus, the pressure peaks on the downstream element are decreased, so the p grad is less steep.
Describe the circulation effect.
The flow around the upstream element is modified by the circulation of the downstream element, increasing the velocity at the TE of the upstream element, inducing higher circulation on the upstream element. Thus, the AoA of the upstream element increases.
Describe the dumping effect.
The downstream element will create a lower pressure at the base of the upstream element, meaning that less pressure recovery is required, so the gradient is less steep on the upstream TE.
Describe the effect of a fresh boundary layer.
BLs are thinner, so can withstand stronger adverse p grads.
Describe how off-surface pressure recovery occurs.
For all but the final element, the BL leaves the TE faster than the freestream and becomes a wake. Wakes withstand more adverse pressure gradients than BLs, making recovery back to freestream more efficient. A well-designed multi-element wing will keep BLs and wakes separate.
Why can the camber of the ME increase after adding a flap.
The TE of the ME can be pushed harder due to the dumping effect.
What happens to circulation on the ME and a flap when a flap is added to a single element?
Increasing the circulation on the flap will also increase the circulation on the ME, but this has to be managed because the flap has to recover to freestream.
What will happen if a flap is directly behind the ME?
The positive pressure of the stagnation point of the flap will increase the pressure at the TE of the ME.
What will happen if a flap is aligned at an angle to the ME?
It can improve the TE condition, since its suction surface is closest to the TE of the ME.
How should slot gaps between aerofoil elements be designed, considering boundary layers?
Large enough that wakes and BLs do not merge, causing early separation.
How should slot gaps between aerofoil elements be designed, considering area ratios?
The slot gap should be at its minimum exactly at the TE of the ME. This means the flow is maximally accelerated at the TE, improving the condition as much as possible, after which the flow expands again.
What would having no overlap between ME and flap do?
A lot of the dumping and slat effects are lost.
What would having too much overlap between ME and flap do?
The flow is maximally accelerated before the TE, and then it will be decelerating at the TE.
How should the AoA of the flap be aligned?
The streamlines of the elements need to be aligned. The flap should be at an AoA such that it allows the streamlines along the ME to continue their paths once they reach the flap. For example, if the flap is at too low an AoA, then the streamlines will inflect, resulting in a loss of lift.
