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.
Small-Scale Propellers Operating in the Vortex Ring State
The behavior of small-scale propellers operating in vertical descent and through the vortex ring state (VRS) was measured. In particular, a total of 26 propellers with diameters ranging from 9 to 11 in. (0.229 to 0.279 m) were tested in the University of Illinois at Urbana-Champaign (UIUC) 2.8×4 ft (0.853×1.219 m) low-turbulence wind tunnel at low Reynolds numbers to measure the propeller thrust over a range of advance ratios from −0.8 to 0 (rapid descent to hover). Also, time histories of the measured thrust for different propellers were recorded to characterize the unsteady thrust fluctuations for advance ratios through the vortex ring state. The effects of propeller geometry, e.g. pitch and planform effects, on the thrust characteristics of these propellers in the vortex ring state is discussed.
Effects of reservoir volume on performance of pulse tube cooler.
A pulse tube cooler has the advantages of long-life and low-vibration over conventional cryocoolers such as G-M and Stirling coolers because of the absence of moving parts at low temperature. On the other hand, the combination of a reservoir and orifice is indispensable to optimize the performance of pulse tube coolers. In order to make the pulse tube cooler compact for practical applications, the volume of reservoir should be minimized. This paper analyzes the effects of the reservoir volume on the thermodynamic performance of various components in a simple orifice and a double-inlet pulse tube cooler by combining a linearized model with a thermodynamic analysis. Expressions of entropy production for those components are presented. The results show that the reservoir volume has a significant influence on the entropy production in the various components when the reservoir to pulse tube volume ratio is smaller than about 5. The ratio is important to determine the minimum reservoir volume for a pulse tube cooler. Optimum settings for a double-inlet pulse tube cooler are also discussed.