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.
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.
How should gaps between aerofoil elements be designed?
Large enough that wakes and BLs do not merge, causing early separation.
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.
What other effect does the circulation effect have on an aerofoil?
It creates it’s own dumping effect as it decreases base pressure of the upstream element.
Draw the effect that Gurney Flaps have on the flowfield.
Circulation region in-front of GF.
Vortex from pressure side rolling around top of GF to base.
Vortex from suction side rolling around TE to base.
