The Aero-Optical Environment of a Helicopter in Hover

Porter, Chris; Rennie, R. Mark; Jumper, Eric J.

doi:10.2514/6.2011-1328

Cited by 5 publications

(21 citation statements)

References 13 publications

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“…In a previous investigation into the aero-optic environment around a helicopter in hover, the required spatial discretization of the vortex filaments and number of rotations required to obtain a converged velocity field was investigated [11]. Figure 5 shows the convergence of the calculated velocity for a typical field point.…”

Section: A Numerical Setupmentioning

confidence: 99%

“…The problem is complicated even further by the potential for the exhaust to pass through the line-of-sight of the optical turret, as well as the wake associated with the helicopter fuselage. Investigations into the dynamics of rotor wakes for the purpose of predicting nearfield aero-optic aberrations and farfield irradiance patterns are limited [3,11]. On the other hand, density gradient measurements in the wake have been used to determine the trajectories of the tip vortices, blade vortex sheets, and root vortices [2,12]; these results have consistently shown that density variations in the rotor wake appear primarily in the blade tip vortices.…”

Section: Introductionmentioning

confidence: 97%

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The Aero-Optical Environment of a Helicopter in Forward Flight

Porter

Rennie

Jumper

2011

42nd AIAA Plasmadynamics and Lasers Conference

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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

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Section: A Numerical Setupmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 97%

The Aero-Optical Environment of a Helicopter in Forward Flight

Porter

Rennie

Jumper

2011

42nd AIAA Plasmadynamics and Lasers Conference

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“…Finally, the vortex was given a solid-body core with radius of 0.05 m, which is typical for the helicopter rotor tip vortices as described in [6][7][8].…”

Section: B Implementation and Evaluationmentioning

confidence: 99%

“…For helicopter-mounted optical systems, where the aircraft travels at incompressible speeds, the flow field around the hemispherical turret is of less importance to the aero-optic problem. For this type of application the aerooptic aberrations are expected to primarily originate from the helicopter rotor blades [6][7][8]. Specifically, the tip vortices shed from the helicopter blades rotating at high transonic Mach numbers possess significant compressible flow features and propagate downward into the field-ofregard of the optical system, as illustrated in Fig.1.…”

Section: Introductionmentioning

confidence: 99%

“…In our previous investigations into helicopter aero optics [6][7][8], we used prescribe-wake models to parametrically define the locations of the helicopter tip vortices for the cases of hover and forward flight. The resulting tip-vortex geometries were then used to determine the velocity field using the Biot-Savart law, after which an experimentally-validated computational model was used to determine the associated density and index-of-refraction fields.…”

Section: Introductionmentioning

confidence: 99%

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The Aero-Optical Environment Around a Helicopter Computed using the Compressible Vorticity Confinement Method

Kelly

Jemcov

Rennie

et al. 2013

44th AIAA Plasmadynamics and Lasers Conference

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The methodology and results of a computational investigation into the nearfield aerooptic effects on a helicopter-borne optical system are presented. The approach investigated in the study was to incorporate a vorticity-confinement method into a commerciallyavailable CFD code, thereby allowing the rotor wake to be modeled in a less computationally-expensive manner with a higher degree of accuracy. This method is expected to produce more-accurate vortex dynamics, including vortex interactions and other secondary effects, thus leading to improved information regarding the aero-optic environment encountered by an optical system mounted on a helicopter.

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