Aerodynamic modelling of fin stabilised underslung loads

Abstract

Box-like slung loads exhibit periodic yaw response instabilities, while carried externally by a helicopter. When coupled with the slung load longitudinal and lateral pendulum motions, these instabilities result in significant pendulum oscillations of the load. High amplitude oscillations lead in many cases to the limiting of a load’s flight envelope. Using wind tunnel and flight tests, rear mounted fins were previously demonstrated as efficient means for stabilisation of a problematic load. However, the lack of a proper analytical model of the stabilised load’s aerodynamic characteristics, led to a trial and error development process, without an appropriate physical understanding of the stabilisation problem. The present paper describes a method for the aerodynamic modeling of fins stabilised slung loads based on a limited number of simple static wind-tunnel tests. The resulting database is incorporated in a dynamical slung load simulation that shows good agreement with dynamic wind-tunnel tests. The applicability of the proposed method is demonstrated, by the calculation of stabilised loads aerodynamic databases for interim fin inclination angles not covered by tests.

Computational and Experimental Static Stability Analysis of Rectangular Prisms

“…The results of these earlier studies were sufficient to support investigations to minimize the effect of the HSL instabilities such as attaching two vertical stabilizers to the top sides of a box and reported in a flight test experiment performed by Enciu and Rosen [9]. While this addition to the box showed promising stability at slow speeds, between 24.6 knots and 30.8 knots, the container still reached an increasingly unsteady state as velocity increased.…”