Effect of rear-fuselage design on the aerodynamics of a helicopter at side-slip angle

Abstract

The aerodynamic design of a helicopter fuselage encounters many complexities that arise from the large variation in side-slip angle during different flight maneuvers and gusty conditions. A wind tunnel investigation was performed on a scaled-down helicopter model at Reynolds number of Rew=9.6×105 to analyze its aerodynamic performance up to a 40° side-slip angle. Four rear-fuselage configurations were investigated including a removable motor geometry, as well as a cusped and a round enclosure for the motor. A six-axis load cell was used to determine three components of force and moment acting on the fuselage. Planar PIV measurements were performed in the fuselage wake to analyze effect of side-slip angle (β) and rear-fuselage on the wake flow. The drag force coefficients displayed a parabolic increase with rising β. The pitch moment strongly depended on the aft-body design. The cusped rear-body had the largest pitch moment (Cm) at zero-side slip while the round case had the smallest magnitude. For all the rear-fuselage configurations, the slope of the pitching moment (dCm/dβ) changed sign from negative to positive at β=20°. The largest change in dCm/dβ was observed for the baseline case while the round enclose had the smallest change in dCm/dβ. The increase in negative Cm with increase of β from zero to 20° was associated with the displacement of the low-pressure core of the wake toward the rear-fuselage. When β increased from 20° to 40°, the wake became skewed and the low-pressure core moved away from the rear-fuselage, causing reduction of the negative Cm toward zero.