Characteristics of helicopter engine exhaust through scaled experiments using stereoscopic particle image velocimetry

Abstract

Helicopter engines are often mounted atop the fuselage to keep the aircraft footprint small and optimal for operations. As a result, hot gases produced by the engines may inadvertently impinge upon the tail boom or dissipate inefficiently that compromises on operation safety. In this study, a scaled fuselage model with a hot air blower was used to simulate hot exhaust gases. The velocity field immediately outside the exhaust port was measured through stereoscopic particle image velocimetry to capture the trajectory and flow behaviour of the gases. Two cases were considered: freestream to exhaust velocity ratios of 0 (no freestream velocity) and 0.46 (co-flowing free stream), respectively. The formation of a counter-rotating vortex pair was detected for both cases but were opposite in the rotational sense. For the case without freestream, the plume formed into a small “kidney” shape, before expanding and dissipating downstream. For the case with freestream, the plume formed into a slenderer and more elongated “reversed-C” shape as compared to the case without freestream. It also retained its shape further downstream and maintained its relative position. These observations on the trajectory and shape of plume provide basis to understanding the nature and interaction of the plume with its surroundings.

On the numerical assessment of engine exhaust plumes with a scaled cowling

“…Another area of concern in modern-day helicopter aerodynamics (a term introduced in literature to include the interactions between other parts of the helicopter aside from classical rotor aerodynamics) is the impingement of hot gases onto the helicopter tail boom. 1 For some flight regimes such as hovering, the rotor downwash-fuselage interaction is intense, and the aerodynamic stress contributions from the engine exhausts are both impulsive and periodic in loading. 2 The exhaust plume is also one of the sources of infrared signature on a helicopter, together with their impingement on various parts of the helicopter such as the tail boom.…”