Abstract
Differing from the maximum lift coefficient for takeoff/landing, the cruise lift coefficient must have high aerodynamic efficiency and sufficient stall margin. Conventional 2D subsonic airfoil has the typical cruise lift coefficient C-L in the range of 0.4 to 0.6. This study introduces a 2D flapped CoFlow jet(FCFJ) airfoil to achieve a cruise lift coefficient about one order of magnitude higher to C-L of 4 with a constraint that the 2D aerodynamic efficiency should be about 50, similar to the C-L/C-D level of the baseline airfoil with no flow control. The regular CFJ airfoil applies the injection very close to leading edge at about 2-4%Chord location. The FCFJ airfoil has a long flap(60%C) with the CFJ applied inside the flap as a part of the airfoil. The research is based on validated CFD simulation, which employs a 2D RANS solver with Shear-Stress-Transport(SST) turbulence model, a third-order WENO scheme for the inviscid fluxes, second-order central differencing for the viscous terms. The regular CFJ airfoil and FCFJ airfoil are created from the baseline NACA 6421 airfoil. For a CFJ airfoil to be used at cruise conditions with high aerodynamic efficiency of C-L/(C-D + Pc), both the drag and power coefficient, D-C and Pc, must be low. Applying CFJ on the flap appears to be the desired configuration to substantially increase lift coefficient and maintain very low C-D and Pc. This is benefited from the feature that applying CFJ at the region of adverse pressure gradient is the most effective and efficient. Increasing cruise lift coefficient to such a high level would bring many advantages such as reduced aircraft size/weight, increased payload, high transportation productivity, high altitude flight, and possible fixed-wing VTOL air vehicles in thin Martian atmosphere. In this study, we are able to achieve C-L of 4.17, C-L/C-D of 263.5, and (C-L/C-D)(c) of 48, through parametric studies on flap deflection angle, slot size, and jet momentum coefficient. The final configuration has a flap deflection angle beta of 35 degrees, the injection slot size of 0.4%C, and the jet momentum C-mu of 0.1. This 2D numerical study indicates that a cruise lift coefficient about one order of magnitude higher than conventional level is possible.