Abstract
This paper conducts a feasibility study of deflected slipstream (DS) airfoil enabled by coflow jet (CFJ) active flow control for VTOL hover based on 2D numerical simulation. Such a DS-CFJ system can potentially provide a fully electric powered Advanced Air Mobility (AAM) platform with distributed propulsors and CFJ actuators. It would have the advantages to eliminate tilting wings, tilting rotors, and separate lift-plus-cruise propulsors, and has the potential to improve cruise efficiency, smooth transition, safety, weight reduction, noise mitigation, reliability, maintainability, and passenger acceptance. The in house high order FASIP CFD code is employed with one equation Spalart-Allmaras turbulence model. The baseline DS airfoil is the double slotted configuration designed by Kuhn and Draper in NACA based on NACA 0015 airfoil. The CFD simulation is validated with the test data of the baseline DS airfoils with several deflection angles. Good agreement with the experiment is obtained for the 2D simulation. The DS-CFJ airfoil is created based on the baseline airfoil. For a single plain flap configuration with a 60% flap chord and 85. deflection angle, applying CFJ on the flap can turn the horizontal slipstream form the propeller vertically downward with fully attached flow. The best performance is to place the propeller a little upward with the airfoil leading edge aligned with the lower 1/4 diameter position of the propeller, named as Case D242 in this paper. The 2D D242 configuration achieves a 90. flow turning and about the same hover efficiency as a vertical rotor facing upward. It obtains 99.3% of the Figure of Merit for DS-CFJ, FMDS. FM DS is defined to compare the total power required of a DS-CFJ system, which includes the CFJ viscous loss and energy consumption, with the power of a vertical rotor disk facing upward for the same amount of total lift. The high efficiency is attributed to the favorable position of the propeller mounted 1/4R upward, the low energy expenditure of CFJ, and the system benefit that absorbs the energy expenditure of CFJ as the system exergy gain. The total lift of D242 is 5.7% higher than the full propeller thrust due to the enhanced momentum of the deflected slipstream by the CFJ. The CFJ power consumption is about 8.7% of the total power. Attributed to the lift contribution of CFJ, the reduced propeller lift and power required would reduce its disk loading and power loading, resulting in potentially increased propeller efficiency and reduced noise. A slotted flap has a lower efficiency than the plain flap. The configuration with the propeller center aligned with the airfoil leading edge, Case D245, also decreases the efficiency to 92.6%. All the injection Mach number is no greater than 0.27 and can be further reduced with optimization. Thus the jet noise is not expected to be a serious concern with the jet Mach number below the noise limit of 0.3 - 0.5. Since a DS-CFJ system avoids the separated flow and large turbulent wakes caused by vertical rotor downwash interaction with the airframe, the broadband noise is expected to be lower. This paper shows that a 2D deflected slipstream enabled by coflow jet active flow control appears to be feasible to achieve a similar hover efficiency to the conventional vertical rotor with the same size and total lift. The performance penalty due to 3D effect needs to be further studied. This study lays a foundation for further 3D study and experimental verification in the next step.