Ducted-Fan Force and Moment Control via Steady and Synthetic Jets
27th AIAA Applied Aerodynamics Conference
The authors have explored novel applications of synthetic jet actuators for leading and trailing edge flow control on ducted fan vehicles. The synthetic jets on the duct are actuated asymmetrically around the circumference to produce control forces and moments. These forces and moments could be utilized as flight control effectors for combating wind gusts or reducing control surface allocation required for trimmed flight. Synthetic jet component design, vehicle integration, CFD modeling, and
... FD modeling, and wind tunnel experimental results are presented with a comparison to steady blowing. The flow control concepts demonstrated production of aerodynamic forces and moments on a ducted fan, although some cases required high flow rate steady blowing to create significant responses. Attaining high blowing momentum coefficients from synthetic jets is challenging since the time-averaged velocity is only a function of the outstroke: from bench test experiments it was seen that the time-averaged velocity was roughly one fourth of the peak velocity observed during the outstroke. The synthetic jets operated at lower blowing momentum coefficients than the steady jets tested, and in general the ducted fan application required more flow control authority than the synthetic jets could impart. However, synthetic jets were able to produce leading edge separation comparable to that obtained from steady jets with much higher blowing coefficients. A. Ducted Fan Background A ducted fan produces more thrust than a fan (propeller) of the same diameter in isolation . This is due to the thrust/lift produced by the duct lip. In general, the pressures on the duct surface created by the flow induced by the fan are a large contribution to the overall forces and moments on the ducted fan unit. In particular, the high-speed flow into the duct induced by the fan causes a low-pressure region on lip. This phenomenon results in a net force in the thrust direction during hover and can produce lift and pitching moment in forward flight  . Under certain conditions the flow over the duct lip can separate, affecting the thrust, lift, and pitching moment. It is a complex problem that depends on lip geometry, angle of attack, free stream velocity, and fan rpm ,  . In general, ducted fans experience large nose-up pitching moments during transition from hover to cruise (low speed and high angle of attack), and the objective of the concepts investigated is to reduce the pitching moment of the vehicle under these conditions in a controlled manner. This reduction in pitching moment would allow for lower control surface allocation during transition to forward flight and would improve wind gust rejection performance.