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A theoretical ice accretion model has been established applicable to both aircraft propellers and helicopter rotors to determine the effect of rime ice on the thrust, power, and efficiency as a function of exposure time in a natural icing condition. Comparisons have been made of theoretical performance levels with previously published experimentally determined propeller thrust and efficiency for five natural icing conditions. Agreement between test and theory was acceptable. Nomenclatureefficiency h,t = blade thickness / = drag constant, Eq. (1) J = advance ratio = V/nD k/c = roughness height, Eq. (1) LWC = liquid water content n = revolutions per second q^ = freestream dynamic pressure = l/zp^ V 2r = local radius R -radius Vi = induced velocity V^tU = freestream velocity x = airfoil chord location y = airfoil thickness location a = angle of attack (3 = blade twist angle; droplet impingement efficiency i^max = maximum droplet impingement efficiency At = icing time r/ = propeller efficiency = (C T / C p ) // p ice =• ice density p^ = freestream density