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.
Vortex ring state protection flight control law
The current and future trend towards fly-by-wire (FBW) control architectures on helicopters is aimed at bringing improved handling and enhanced safety. By giving a direct input through electrical signals, commands are much more precise enhancing overall safety. Furthermore, the pilot commands are monitored by the FBW control system to ensure the helicopter is kept within the flight protection envelope. As a result, the pilot always can get the maximum performance out of the helicopter without running the risk of exceeding safety margins. Vortex Ring State (VRS) conditions are among adverse flight conditions which mostly occur within a boundary confined to a part of the flight envelope at low forward airspeeds and at steep angles of descent or high rates of descent. The contribution of the present paper is to incorporate VRS boundaries inside a FBW flight control law in order to prevent the helicopter from entering the VRS region. The stability margin given by the heave mode damping is used as a metric to quantify the safety margin. Simulations show the good performance of the VRS-protection function.
The micro-mechanical behaviour of artificially cemented sand particles under compression and shear
The mechanical behaviour of cemented sands at macro-scale has been widely studied in the past, while there is still a lack of laboratory test data for the micro-mechanical response. Therefore, a series of uniaxial compression and tangential shear tests on artificially cemented sand particles have been conducted using a microscope to observe their behaviour. First, the consistency of cemented particles with different cementing agents is discussed. Three breakage modes are proposed according to the images taken using a microscope camera. A new parameter named ‘local roundness at contact’ is introduced to emphasise the effect of contact morphology on sample strength. Moreover, the effect of bond thickness on the mechanical response of cemented particles has been investigated. Finally, the shear strength parameters of artificially cemented sand samples have been determined for two different sample diameters.