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Helicopter engine exhaust rotor downwash effects on laser beams

: Henriksson, Markus; Sjöqvist, Lars; Seiffer, Dirk


Ackermann, H. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Technologies for optical countermeasures XII and High power lasers 2015 : Technology and systems; 21, 23 - 24 September 2015, Toulouse, France
Bellingham, WA: SPIE, 2015 (Proceedings of SPIE 9650)
ISBN: 978-1-62841-860-6
Paper 96500M, 9 pp.
Conference "Technologies for Optical Countermeasures" <12, 2015, Toulouse>
Conference "High Power Lasers - Technology and Systems" <2015, Toulouse>
Conference Paper
Fraunhofer IOSB ()
laser countermeasures; laser beam propagation; turbulence; rotor downwash; helicopter platform effects; engine exhaust; beam wander

The hot exhaust gases from engines on helicopters are pushed down by the rotor in a turbulent flow. When the optical path of a laser beam or optical sensor passes through this region severe aberrations of the optical field may result. These perturbations will lead to beam wander and beam distortions that can limit the performance of optical countermeasure systems. To quantify these effects the Italian Air Force Flight Test Centre hosted a trial for the "Airborne platform effects on lasers and warning sensors" (ALWS) EDA-project. Laser beams were propagated from the airport control tower to a target screen in a slant path with the helicopter hovering over this path. Collimated laser beams at 1.55-, 2-and 4.6-mu m wavelength were imaged with high speed cameras. Large increases in beam wander and beam divergence were found, with beam wander up to 200 mu rad root-mean-square and increases in beam divergence up to 1 mrad. To allow scaling to other laser beam parameters and geometries formulas for propagation in atmospheric turbulence were used even though the turbulence may not follow Kolmogorov statistics. By assuming that the plume is short compared to the total propagation distance the integrated structure parameter through the plume could be calculated. Values in the range 10(-10) to 10(-8) m(1/3) were found when the laser beams passed through the exhaust gases below the helicopter tail. The integrated structure parameter values calculated from beam wander were consistently lower than those calculated from long term spot size, indicating that the method is not perfect but provides information about order of magnitudes. The measured results show that the engine exhaust for worst case beam directions will dominate over atmospheric turbulence even for kilometer path lengths from a helicopter at low altitude. How severe the effect is on system performance will depend on beam and target parameters.