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.
Tiltrotor Descent Aerodynamics: A Small-Scale Experimental Investigation of Vortex Ring State
An experimental investigation was completed in the NASA Ames 80- by 120-Foot Wind Tunnel with the objective of studying the aerodynamic characteristics of a tiltrotor at high descent angles in helicopter mode. A specific objective was to determine whether tiltrotors behave differently from helicopter rotors when operating in the Vortex Ring State (VRS). A single, 4-foot diameter, 3-bladed rotor, with twist and solidity similar to that of rotors on current tiltrotor aircraft, was tested with an image plane to simulate the mean effect of a second rotor. Rotor performance data were obtained over a wide range of simulated descent conditions, both with and without a semispan wing. Results of this study are presented, including changes in both mean and oscillatory rotor thrust, which are reasonably consistent with helicopter rotor VRS characteristics. Large reductions in mean rotor thrust were found in VRS. These were associated with high levels of low-frequency rotor thrust fluctuations. The image plane had a significant effect on the results, while the wing had a relatively small effect. Implications of these results for the safe operation of tiltrotor aircraft are discussed.
Mathematical Time and Physical Time in the Theory Of Relativity
In the Theory of Relativity time is an imaginary quantity that can not be observed; it is a multiplication of a number that indicates duration of material change and number i that is an imaginary number. i on the square is -1. Time i⁄t is a mathematical time that describes the speed and the duration of material change. In the Universe one can observe physical time only as a stream of material change. It is not that change run in physical time, change itself is physical time.. Distinction between imaginary mathematical time and visible physical time opens some new perspectives into interpretation of Theory of Relativity. c