Trade Study of Regular and Flapped CoFlow Jet Airfoils, Part I: Injection Slot Size Effect

This paper conducts a trade study of injection sizes for regular CoFlow Jet (CFJ) (RCFJ) airfoil and flapped CoFlow Jet (FCFJ) airfoil. Three injection slot sizes, 0.4%C, 0.65%C, and 1.15%C, are studied under Martian low Reynolds number condition of Re = 5.6 × 10^4 and Earth high Reynolds number condition of Re = 1.0 × 10^6 at M∞ = 0.17. The FCFJ airfoil has the injection positioned at the flap shoulder, whereas the RCFJ has the injection located at about 2% chord from the leading edge. For all the slot sizes and Reynolds numbers, the RCFJ airfoil demonstrates better aerodynamic efficiency over most of the lift coefficient range. For CL below about 5–7, the RCFJ airfoil exhibits lower drag coefficient CD, and a lower coefficient of power required Pc, and higher corrected aerodynamic efficiency (CL/CD)c and (C^2_L/CD)c than the FCFJ airfoil. For CL ≥ 7, the FCFJ airfoil has better aerodynamic efficiency and can achieve comparable or lower Pc at higher lift. When the injection slot is choked with the injection Mach number of 1 or greater, the CFJ power coefficient increases exponentially with CL due to the rapidly increased jet total pressure ratio to overcome the high energy loss at high injection speed. For corrected aerodynamic efficiency (CL/CD)c, all three injection slot sizes have a similar peak value for both configurations. However, the injection slot size has a stronger effect on the productivity efficiency (C^2_L/CD)c on the FCFJ airfoil than on the RCFJ airfoil. For the FCFJ airfoil, the 0.4%C slot yields the highest peak productivity efficiency of (C^2_L/CD)c = 50.5 at low Reynolds number, whereas the (C^2_L/CD)c for 1.15%C slot decreases 25.9%. In contrast, the RCFJ shows less sensitivity on productivity efficiency to slot size, with peak (C^2_L/CD)c decreases only 15.0% from 0.65% slot size to 0.4% slot size. Increasing Reynolds number substantially improves the corrected aerodynamic efficiency and productivity efficiency for both the configurations, with peak (CL/CD)c and (C^2_L/CD)c values both increase about 100%. Increasing the Reynolds number also reduces both airfoils’ sensitivity on the injection slot sizes. These results suggest that RCFJ airfoils are better suited for efficient cruise and moderate-lift coefficient (e.g., CL < 7) operation, whereas FCFJ airfoils provide a more efficient option for ultra-high-lift applications such as short or vertical takeoff and landing.