The performance of an optical system mounted in a hemispherical turret can be seriously limited at certain viewing angles due to aero-optical aberrations on the outgoing beam. The strength of these aberrations is related to the density fluctuations in the air immediately surrounding the turret and is proportional to the Mach number squared; helicopters are therefore an attractive platform for optical systems due to their low flight speeds. In this case, aero-optic aberrations can still arise, however, from the wake and especially the tip vortices from the helicopter blades. A simplified model of a rotor tip-vortex system is used to gain insight into the potential aero-optical environment of a helicopter in forward flight. The simplified model allows for the rapid calculation of aero-optic effects at different forward-flight speeds. The results indicate that the largest aero-optic aberration originates from "super vortices" that appear along the lateral edge of the helicopter wake. Estimated aberration amplitudes are presented, as well as an estimate of the undisturbed viewing angle of the optical system as a function of forward-flight speed.
NomenclatureZ tip = vertical location of tip vortex R = gas constant; Blade radius C p = specific heat (at constant pressure) n = index of refraction OPD = optical path difference V = velocity vector t = time P = pressure P s = static pressure T Ad = adiabatic temperature T o = total temperature T = temperature ρ = density r c = vortex core radius C T = thrust coefficient σ = rotor solidity θ tw = linear twist rate of blade ψ w = wake age N b = number of blades r tip = radial location of tip vortex h = perpendicular distance μ = advance ratio γ = ratio of specific heats Γ = circulation
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.