Wind tunnel tests have successfully demonstrated the superior performance of the co-flow jet (CFJ) airfoil concept to dramatically increase the lift coefficient, stall margin, and drag reduction. In the CFJ airfoil, a high energy jet is injected tangentially on the suction surface near the leading edge and the same amount of mass flow is sucked in near the trailing edge. The turbulent shear layer between the main flow and the jet causes a strong turbulence diffusion and mixing, which enhances the lateral transport of energy and allows the main flow to overcome the severe adverse pressure gradient and stay attached at high angles (AOA). The airfoil always achieves a significantly higher lift due to the augmented circulation. The operating range of AOA, hence the stall margin, is significantly increased. The energized main flow fills the wake deficit and dramatically reduce the airfoil drag, or generates thrust (negative drag). CFJ airfoil is to achieve 3 goals: high effectiveness, energy efficient, easy implementation.
In the wind tunnel tests, two airfoils with different injection slot size were tested to study the effect of geometry. The airfoil with smaller injection slot size (0.65% chord length) performs better. With the momentum coefficient varying from 0.1 to 0.29, compared with the baseline airfoil, the maximum lift is increased by 113% to 220%, the angle of attack operating range (stall margin) is increased by 100% and 153%. The minimum drag coefficient is reduced by 30% to 127% with the momentum coefficient varying from 0.053 to 0.183. Large negative drag (thrust) is produced when the momentum coefficient is high. The L/D increase of CFJ airfoil is difficult to give because when the drag is zero, L/D=infintiy. To achieve the same lift coefficient of 4.42, the power required (fuel consumption) for the larger slot size airfoil is 3.9 times of that of the CFJ airfoil with smaller size. The two airfoils tested is far from optimum. It is hence believed that there is great potential to further improve the airfoil performance after the optimum configuration study.
In the experiment, it is observed that there is a limit of the jet mass flow rate to maintain the stability of the flow. Below the limit, increasing the jet mass flow (momentum coefficient) will make the flow attached and increase the lift at a high AOA. However, if the jet mass flow rate exceeds the limit, the whole flow field breakdown and a large separation occurs as the jet does not exist. It is speculated that the instability may be attributed to the large dissimilarity of the jet and the main flow, and large centrifugal force of the jet when the jet velocity is high. The exact mechanism is not clear yet and will be studied in future.
A concept study conducted by using CFD simulation indicates that it is possible for the CFJ airfoil to exceed the inviscid limit of maximum lift coefficient due to the high jet velocity inducing high suction velocity of the airfoil. For a cambered CFJ airfoil modified from NACA0025, a lift coefficient of 9.7 is obtained by the CFD simulation, which is far greater than the inviscid maximum lift coefficient limit of 7.8. The concept is named Super-Circulation Airfoil.
Based on the wind tunnel test results and a conservative estimate, for a subsonic area intercept mission analysis of the military aircraft F-5E assuming using CFJ airfoil, the fuel consumption is reduced by 9%, the endurance and range is increased by 38% and 41%, the Vstall is reduced by 44%, and the take off and landing distance is reduced by 68%.,
CFJ airfoil is energy efficient by being able to recirculate the jet to avoid large penalty to the propulsion system. The mission analysis indicates that the penalty due to CFJ pumping work is small and is easily offset by the benefit gain. CFJ airfoil does not require large leading and trailing edge, and does not require moving parts. CFJ airfoil can be considered as a system that can effectively enhance the airframe performance using propulsion system. CFJ airfoil can be applied to aircraft to require extremely short take off and landing distance, personal aircraft to have compact wingsize for easy storage and short take off/landing distance, long range cruiser to save fuel, aircraft on aircraft carrier to have short take off/landing distance, combat aircraft for fast acceleration and high maneuverability, aircraft to have low noise, supersonic aircraft with high subsonic performance. CFJ airfoil can be used for full flying mission instead of take off and landing only to avoid weight penalty during cruise.
Above research results are published in AIAA Paper 2005-1260 and AIAA Paper 2004-2208.