Circulation Controlled Airfoils
Circulation controlled (CC) airfoils rely upon the Coanda effect to generate high lift independently of AoA. The Coanda effect has been introduced previously in Section 6.10.2 and is the tendency of a fluid issuing from a tangentially ejected jet to travel close to a
surface contour, even if the surface curvature diverges from the jet axis – see Newman
(1961) . Usually CC airfoils are elliptical or quasi-elliptical in shape, with well-rounded, almost blunt trailing edges. A balance of centrifugal force and reduced static pressure causes the thin jet to adhere to the blunt trailing edge. The jet is usually obtained by pressurizing a plenum inside the airfoil and ejecting flow out of a thin slot. Because of the high lift coefficients that can be attained with CC airfoils (often in excess of 2) they have been envisioned or applied for use on both fixed-wing and rotating-wing aircraft – see Wood & Neilsen (1986) and Reader et al. (1978).
Representative results for a CC airfoil at several free-stream Mach numbers are shown in Fig. 7.53 in terms of the jet momentum coefficient defined by
where m is the mass flow through the slot and V) is the jet velocity. At lower values of blowing the jet initially acts as an effective form of boundary layer control through flow entrainment on the upper surface of the airfoil. At higher blowing coefficients boundary layer control yields to super-circulation. At some point downstream of the slot the reduced static pressure and centrifugal force balance is ultimately lost (usually near where the jet velocity becomes sonic) and the jet detaches from the Coanda surface. This defines the maximum lift of the CC airfoil. The location of the jet detachment depends mainly on the strength of the jet, the slot geometry, the curvature of the Coanda surface, as well as the characteristics of the boundary layer prior to the slot.
The ideas of circulation control for application to a rotor were first investigated in depth by Cheeseman & Seed (1967). In the early 1980s, Kaman built and flight tested a CC rotor system on a HH-2D Seasprite, although only in hover and in low speed forward flight. Other applications of CC technology to rotorcraft are the ill-fated X-wing (see Linden & Biggers (1985) and Section 6.11.3) as well as the very successful NOTAR anti-torque system (see Section 6.10.2). However, while offering considerable possibilities when applied to rotor (either for either direct lift production or secondary lift control using higher harmonic
lift), the behavior of CC airfoils operating in a rotor environment is not well understood. In forward flight, a CC rotor encounters a time-varying flow environment in which both the AoA and local onset velocity vary periodically with azimuth position. Because the geometric pitch of the blade is generally held constant, jet blowing must be cyclically adjusted to for trim and control. This necessitates the modulation of the blade plenum pressure, and therefore, the jet blowing as a function of azimuth position. This is a highly unsteady aerodynamics problem.
The first theoretical work of CC airfoil aerodynamics was made by Kind (1968), Kind & Maull (1968), Dunham (1968), Dvorak & Kind (1979), and Soliman (1984). Dvorak & Kind (1979) incorporated integral and finite-difference boundary layer/jet mixing models into an external potential flow (surface singularity) model. Shrewsbury (1986) has used Navier-Stokes methods in the analysis of circulation controlled airfoils. Englar (1975) has shown results for pulsed blowing on a circulation controlled elliptical airfoil. Schmidt (1978) carried out experiments to examine the unsteady aerodynamic effects on an airfoil with a Coanda surface, reporting a transportation lag effect between the duct pressure and the blowing, as well as a decrease in lift augmentation ratio with increasing blowing frequency. A detailed bibliography of CC aerodynamics research through 1980 is given by Englar & Applegate (1984). The results of CC airfoil research as it applies to rotorcraft has been reviewed by Abramson et al. (1985).
Lorber et al. (1989) have investigated the unsteady aerodynamic behavior of an oscillating jet flap at a constant AoA and over a range of Mach numbers, albeit at limited blowing levels and at very low reduced frequencies. Ghee & Leishman (1990, 1992) and Zandieh & Leishman (1993) have investigated the behavior of a CC circular cylinder with periodic jet blowing operating in a steady free-stream. This work has also shown significant phase lags exist between the application of blowing and the buildup of the airloads. Furthermore, under some conditions unsteady blowing may cause sudden jet detachment, whereas this may not occur with steady blowing under the same free-stream conditions.
Computational work on modeling the behavior of CC airfoils under unsteady conditions is relatively scarce. Raghavan et al. (1988) and Sun & Wang (1989) used a surface singularity-boundary layer method for CC airfoils operating in an unsteady free-stream flow and with unsteady blowing, showing that CC airfoils will exhibit both a lift attenuation and a phase lag with respect to the blowing. While CC is clearly attractive because of the potentially high lift capability, the behavior of CC airfoils in an unsteady rotor environment is still not well understood. Nevertheless, CC airfoil concepts continue to be suggested for use on rotorcraft, but none have yet proved successful or practical.