Engineering Solutions for Circulation Control Applications 2005-01-3193
Circulation Control (CC) by blowing over a rounded Coanda surface is capable of generating large lift forces while entraining the surrounding fluid. Its concept originates with Davidson in his 1960 British patent application. Within the sixties, CC was actively investigated for application on a high lift helicopter with elliptic rotors. The appeal of Circulation Control for high lift generation is its low blowing power required. However, supplying blowing air efficiently to the CC slot has proven to be an aerodynamic and structural engineering challenge. This delayed the first flight application of Circulation Control until a CC Technology Demonstrator STOL aircraft was completed in 1974. Elliptic airfoil based wings will not fly if blowing fails, so the airplane was designed with conventional wings. The engineering challenge was to provide in-flight transition from a rounded CC blown trailing edge to a conventional sharp trailing edge for low-drag cruise. The most difficult problem arose in ducting the hot compressor bleed air efficiently to the CC blowing slot without over heating the fiberglass wing. Test flights began in April 1974 and demonstrated the potential of Circulation Control for take-off and landing at high lift coefficients and at level attitude with good pilot visibility. Use of Direct Lift Control (DLC) on approach by dumping CC air was also very successful. In 1979 a second STOL CC Technology Demonstrator took to the air. CC conversion of this Grumman A6-A was limited to modifying the wing by adding a fixed CC trailing edge. With CC blowing, the A6-A take-off and landing performance was significantly improved, but the drag on its fixed rounded trailing edge reduced its maximum speed to 125 knots. Soon after, the first CC blown rotor helicopter performed its first flight test and another helicopter was flown with its tail rotor replaced by a CC blown circular tail boom. This was followed by a CC blown stoppable rotor, capable of acting like wings. Currently a host of CC applications have been developed such as: upper surface blowing augmented by CC, noise reduction schemes and automotive drag reduction techniques. Still in the research stage are new NASA designs using CC to eliminate noise and moving control surfaces. This paper presents some of the engineering solutions found to render these CC applications practical.