HIGH SPEED ICE ACCRETION ON ROTORCRAFT AIRFOILS

A high-speed icing wind tunnel test has been conducted to measure ice growth and the associated performance degradation for two-dimensional helicopter airfoils over a wide range of angle of attack and Mach number. Prior to this test, conducted in the Canadian National Research Council's High Speed Icing Wind Tunnel under NASA sponsorship in September-November 1982, airfoil icing data had either been acquired in low speed wind tunnels or during flight testing. Information from a low speed wind tunnel is useful, but its applicability to the high speed domain was open to question. Flight testing can provide a good means of determining capabilities and power penalties of a particular design, but conditions cannot be well controlled, do not cover a wide range, and are difficult to observe. High speed icing information, as reported in this paper, is necessary to predict the effect of icing on rotor performance and to aid in the design of more efficient ice protection systems. Ice was accreted on seven rotorcraft airfoil models for a range of liquid water content, temperature, and Mach number at several steady and unsteady angles of attack. The test explored the temperature boundary for the onset of ice accretion, as well as the boundary between wet and dry ice growth. Ice shapes were documented by measurement, photographic and silicone rubber molding techniques and aerodynamic lift, drag and pitching moment were measured. In moderate icing conditions, the airfoil lift at angles of attack is reduced slightly and this loss in lift increases to about 20% at high angles. The drag coefficients of these airfoils increased to 0.0400 in moderate icing at low angles of attack, a factor of 3 to 4 above the drag of a clean airfoil. The drag increment increases for angles of attack above six degrees and higher liquid water contents. The effect of Mach number varies for each airfoil configuration. Airfoil oscillation increases the lift loss but reduces the drag penalty.