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International Science Index

Commenced in January 1999 Frequency: Monthly Edition: International Abstract Count: 48522

Wind Tunnel Study on Efficiency Improvements of a Wing Embedded Lifting Fan Remaining Open in Cruise Flight
The ability to generate additional lift in hovering flight by a lifting fan is of great interest for aircrafts with vertical take-off and landing capabilities. So far, in most cases, the duct containing the fan has been closed by a fairing during cruise flight. Keeping the fan ducts open for cruise flight supersedes complex mechanics for closing the openings properly and reduces the number of parts and overall weight while enhancing robustness and reliability. The investigated aircraft concept includes/incorporates two large wing-embedded lifting fans for vertical take-off and landing. The overall aerodynamic performance of the airframe is negatively affected by the disturbed, uneven flow around the fan ducts that reduces lift and increases drag. The result of an explorative study conducted by the authors is that any rotation of the fans or installation of grids across the fan duct inlet plane have significant negative impact on the aerodynamic efficiency. A positive impact, however, has been predicted for optimized inlet and outlet geometries of the fan duct. Published in 2016 by the authors, a step at the front end of the fan duct's outlet was used as element for improving aerodynamic performance for the first time. A rounded inlet lip on the upper side of the wing appeared to be advantageous for the cruise performance. Despite these findings, it is still unknown whether an increase of the inlet lip radius will have further advantage or if the inlet lip radius can be optimized regarding inlet lip suction during hovering, without compromising cruise flight performance. A wind tunnel study was conducted in the Seitenwindversuchsanlage Göttingen at the German Aerospace Center in Göttingen to quantify the impact of the duct inlet radius and step. Different model configurations (30 mm and 60 mm inlet lip radius; no step, 9 mm, 11 mm, and 13 mm step height) were investigated at 16.5 m/s and 33 m/s flow velocity and between -4° and 10° angle of attack using force, surface pressure and stereoscopic particle image velocimetry. The diameter of the fixed rotor was 360 mm, and the local chord length was 600 mm. The results show that a variation of the inlet radius on the upper side of the wing only has marginal impact on the achievable lift-to-drag ratio. It can be concluded that the inlet radius can be optimized for maximum fan efficiency during hovering flight. The impact of the step on the lift-to-drag ratio is significant. All steps increase the aerodynamic efficiency between +2° and +10° angle of attack for both inlet lip radii. The measurement data indicate that this is caused by a combination of increased pressure on the pressure side of the wing, rearward movement of the stagnation point, increased suction on the suction side of the wing and delayed detachment on the fan duct inlet lip. The measurement results serve as a reference database for the validation of future numerical flow simulations on the optimization of the height and step inclination as functions of the angle of attack and Reynolds number.
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