Open jet windtunnel tests at Australia’s University of New South Wales have found that vibrating a wing’s upper surface at 400hz can increase lift coefficient by 22% and stall angle by up to 5˚.
The discovery suggests that aircraft could fly slower than their accepted stall speeds because the vibration delays the airflow separation that occurs just before an aircraft stalls.
The vibration adds energy to the boundary layer to stop the reduction of airflow velocity on the upper surface and delay stall as the aircraft moves to a higher angle of attack. The 400hz signal was generated using two vibrating piezo-electric films beginning at quarter chord and extending towards the trailing edge and covering about 75% of the wing’s upper surface.
“The size and location of the [piezo-electric] panel was not optimised and it was done at slow speeds, at Reynolds numbers of 105,” says Noore Alam Ahmed, associate professor of aerospace engineering at the University of New South Wales. “It needs to be improved, but this could be useful for small unmanned air vehicles,” he says.
The researchers achieved a 0.68 co-efficient of lift from the wing section at an angle of attack of 17˚, and with vibration the wing is achieving a lift co-efficient of 0.83 at 22˚.
Once flow separation has occurred, however, the effect was less pronounced and reattachment of flow could only be achieved at angles up to 2˚ beyond the normal stalling angle of attack. Ahmed says he expects lift coefficient improvements of at least 16%.
ROB COPPINGER/LONDON
Source: Flight International
