Aerodynamic Study on Efficiency Improvement of a Wing Embedded Lifting Fan Remaining Open in Cruise Flight

Kurzfassung

An aerodynamic study on efficiency improvement of a wing embedded lifting fan with a diameter of 0.36 m remaining open and not closed by doors in cruise flight has been conducted. Two of these fans are used to enable an unconventional unmanned transport aircraft of 16 kg maximum take-off mass and almost 2 m wing span to take-off and land vertically. A preliminary study showed that a step for flow deflection on the lower side in front of the lifting fan duct opening increases the lift-to-drag ratio while even a large change of the duct inlet lip curvature radius has only a negligible impact on cruise flight lift-to-drag ratio. For validation and quantification of these effects, an experimental wind tunnel investigation was completed in the German Aerospace Center (DLR) Seitenwindversuchsanlage Göttingen (SWG) using surface pressure, force, and stereo particle image velocimetry measurement techniques. Wind tunnel data showed an increase of the lift-to-drag ratio of 20% of the overall model by the step for all positive angle of attack. Only minor improvement could be achieved due to a larger inlet lip curvature. The wind tunnel measurement data were used for validation of numerical fluid simulation parameters. Using these validated simulation settings, the step height and inclination was optimized for 2°, 4° and 6° angle of attack, the original model size and model sizes scaled by 1:5 and 5:1 to investigate and quantify the influence of a change of the Reynolds number. At a 4° angle of attack of the original size, the lift-to-drag ratio was increased from 4.7 without a step to 6.6 with a step, compared to a lift-to-drag ratio of 20.75 with smoothly closed lifting-fan-ducts.