Model for Vortex Ring State Influence on Rotorcraft Flight Dynamics
The influence of vortex ring state (VRS) on rotorcraft flight dynamics is investigated, specifically the vertical velocity drop of helicopters and the roll-off of tiltrotors encountering VRS. The available wind tunnel and flight test data for rotors in vortex ring state are reviewed. Test data for axial flow, nonaxial flow, two rotors, unsteadiness, and vortex ring state boundaries are described and discussed. Based on the available measured data, a VRS model is developed. The VRS model is a parametric extension of momentum theory for calculation of the mean inflow of a rotor, hence suitable for simple calculations and real-time simulations. This inflow model is primarily defined in terms of the stability boundary of the aircraft motion. Calculations of helicopter response during VRS encounter were performed, and good correlation is shown with the vertical velocity drop measured in flight tests. Calculations of tiltrotor response during VRS encounter were performed, showing the roll-off behavior characteristic of tiltrotors. Hence it is possible, using a model of the mean inflow of an isolated rotor, to explain the basic behavior of both helicopters and tiltrotors in vortex ring state.
Flight measurements of the vibration experienced by a tandem helicopter in transition, vortex-ring state, landing, approach, and yawed flight
Note presenting an investigation involving several flight maneuvers that are critical with respect to vibration. The maneuvers include transition near the ground, landing approaches, vortex-ring state, and yawed forward flight. Some methods of reducing vibration during these maneuvers are described.
Performance and Pressure Data from a Small Model Tilt-Rotor in Hover.
Abstract : A test of a small three-bladed model rotor, with geometry typical of that used on tilt-rotor aircraft, was conducted in the U.S. Army Aeroflightdynamics Directorate's anechoic hover chamber. The objectives of the test were to determine the hover performance of the rotor and investigate the pressure distributions on a blade operating at various collective pitch angles and tip speeds. This report presents the rotor performance data and blade surface pressures.