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Flow Control on Helicopter Rotor Blade Airfoils

Flow control on a rotor blade airfoil using jets

Leading Edge Vortex Generators installed on a wind tunnel model

The rotor blades of a helicopter need to perform well over the entire range of flow conditions expected for the helicopter. Each blade section is optimised for a number of operation points, but can experience large variations in local Mach number, Reynolds number, angle of attack and sweep angle. Due to the extremely wide Mach number and Reynolds number ranges in the flight envelope, modern helicopter airfoils are often used outside the range for which they are optimised, leading to a performance penalty for parts of the blade. Flow control has the aim of significantly extending the range of conditions in which an airfoil can be optimally operated, and significantly improving the performance of the airfoils when operating outside of the optimal range of the base airfoil, leading to improvements in helicopter performance and an extension of the helicopter operating envelope.

In the department of helicopters, a range of active and passive flow control mechanisms are being investigated using experiments and numerical methods. In order to reduce the drag on the advancing blade, camber control devices are being investigated, including Miniature Trailing Edge Devices (Mini-TEDs) and trailing edge flaps. To reduce the effects of flow separation on the retreating blade, the DLR/ONERA project SIMCOS is investigating separation control with air jets and with “Leading Edge Vortex Generators” (LEVoGs). Both of these methods reduce the pitching moment peak caused by the onset of Dynamic Stall, and are investigated using both numerical simulations with the DLR-TAU code and wind tunnel experiments.

Flow control with air jets is an active method of improving the flow using constant blowing or air pulses with a frequency of up to 500Hz from holes or slots in the top of the airfoil. As an active control measure, these jets can simply be turned off when no flow control is necessary. LEVoGs are a passive flow control method where rubber discs are glued to the leading edge of the rotor blade, causing a measurable improvement in the dynamic flow behaviour at high angles of attack. In addition to the standard measurement techniques for unsteady flow (pressure and force measurements), a range of new optical methods are used, including tomographic Particle Image Velocimetry (PIV) to map the flow field and optimise the flow control devices.

Literature

• A.D. Gardner, K. Richter, H. Rosemann, "Numerical Investigation of Air Jets for Dynamic Stall Control on the OA209 Airfoil", ERF2010, Paris 7-9 September 2010
• Heine, B., Mulleners, K., Gardner, A., Mai, H., On the effects of leading edge vortex generators on an OA209 airfoil, ODAS2009, 2009