CFD Calculation of Stability and Control Derivatives For Ram-Air Parachutes

Abstract

Generation of aerodynamic models for ram-air parachutes is currently the subject of active research. These parachutes resemble a rectangular wing of low aspect ratio. The aerodynamic characteristics of these unswept wings can be very different from those predicted by lifting-line theory due to openings in the leading edge for the admission of ram air. This research specifically investigates the aerodynamics of ram-air parachutes with open and closed round inlets. All wings are assumed to be rigid and have an aspect ratio of two. Aerodynamic predictions are made with flow solvers of Cobalt and Kestrel and are compared with available wind-tunnel experimental data. Simulations and measurements are carried out at a Mach number of 0.25 and Reynolds number of 1.4 million. The aerodynamic changes are predicted due to pulling the left trailing edge down. Aerodynamic stability derivatives are calculated from simulations of forced periodic motions in directions of pitch, yaw, and roll. The effects of motion reduced frequency are studied as well. Two different estimation methods are used, namely linear regression method and a method based on points of maximum and minimum angular velocity. The experimental data of wings considered here match the computational predictions quite well. For the wings with a left-side bending, the lift and drag will increase, the pitch moment at the quarter chord point will decreases and wing will produce a positive roll and a negative yaw moment. The open wings stall earlier than the closed wings, have higher pressure-drag values, and the pitch moment slope becomes more negative. The calculated derivatives are similar for both methods and show only a small change with reduced frequencies less than 0.1. The results show that damping derivatives of closed wings remain fairly constant up to ten degrees angle of attack. However, the open wings show a very sensitive behavior in damping derivatives with respect to angles of attack. Finally, the models are evaluated for the closed and open wings undergoing a chirp motion. The results of the comparison show that the aerodynamic models of the closed wing match time-marching full CFD calculations well, but some discrepancies can be seen in the open wing plots. The lift values from model and full CFD do not match everywhere and there is a time lag between pitch moment predictions and time-marching solution, suggesting substantial unsteady effects on the numerical simulations of open wings during the motion.