Aerodynamic Analysis of Helicopter Rotors Using a Higher-Order, Free-Wake Method

Open Access
Author:
Choephel, Tenzin
Graduate Program:
Aerospace Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 06, 2016
Committee Members:
  • Mark D. Maughmer, Dissertation Advisor
  • Mark D. Maughmer, Committee Chair
  • Kenneth S. Brentner, Committee Member
  • Sven Schmitz, Committee Member
  • Virendra Puri, Outside Member
  • Goetz Bramesfeld, Special Member
Keywords:
  • Free-Wake
  • Inflow
  • Hover
  • Helicopter Rotor
  • Rotor Downwash
Abstract:
The wake has a strong influence on the aerodynamics of a helicopter rotor, and the accurate prediction of its geometry and the resulting induced velocity field is extremely important for rotorcraft aerodynamic analysis. In this thesis, a new higher-order, free-wake method for rotor aerodynamic analysis is presented. The method uses elements of distributed vorticity to model the lifting surfaces and the associated wake. The use of such higher-order spanwise elements ensures higher resolution compared to traditional filament-based free-wake analysis and does not require explicit vortex core modeling with a user-specified core size. Since the method uses a full-span, singularity-free, relaxed wake, it can resolve the effect of on-blade, partial-span devices. The free-wake method is validated in both hover and forward flight against measured data from well-documented experiments. In hover, the blade spanwise lift coefficients predicted by the free-wake analysis correlate well with the measured data from the classic Caradonna-Tung model rotor experiment. The figure of merit, which is a measure of rotor efficiency, predicted by the present method is compared to that from the experiments conducted by Knight and Hefner, and the correlations are found to be quite good given the level of fidelity of this method, which is based on potential flow theory. Rotor downwash, which is one of the most important considerations in rotor aerodynamic analysis, is predicted very well by the free-wake method when compared to measured data from a full-scale rotor test performed by Boatwright. These correlation studies provide a lot of promise as to the ability of the method in predicting the challenging aerodynamics of a helicopter rotor in hover. Validation studies are also performed to assess the accuracy of the free-wake analysis in predicting downwash distribution in forward flight. Comparison of numerical predictions with experimental data requires the rotor to be trimmed to the conditions recorded in the experiment. To ensure this, the free-wake program is coupled with RCAS, a comprehensive helicopter analysis code developed by the iii US Army, in order to take advantage of its robust trim algorithm, among other capabilities. The coupling is achieved through what is called a “loose-coupling methodology”, whereby data is exchanged at the end of each coupling iteration or “converged” rotor revolution. The downwash distributions predicted by the present method are compared to measured data for a model rotor at various advanced ratios and thrust levels taken at the U.S. Army/NASA Langley Research Center (LaRC) facility. The free-wake method not only captures the important phenomena observed in the experiments but the results also correlate well with the measured data both in terms of magnitude and distribution. However, exceptions exist at the highest advance ratio, where other methods also demonstrate poor correlations. The results from this free-wake analysis are also compared to predictions by other existing methods such as the University of Maryland free-wake method (UMDFW) and the vortex transport method (VTM). The present method yields results comparable to the ones obtained using VTM, both of which correlate better with measurements than does UMD-FW. Sensitivity studies show that the blade panel density and azimuthal time-step size do not have a significant influence on the solution fidelity. In addition, free-wake analyses with the azimuthal time-step size of  = 3o demonstrate the robustness of the method even with small time-steps, which is important for certain problems including rotor acoustics. The thesis concludes with a discussion on the capability of the new free-wake method in resolving on-blade, partial-span devices. An analysis of a 2-bladed rotor with a partial-span deflection is performed and the resulting changes in sectional loadings, downwash distribution, wake geometry and aggregate performance parameters are highlighted to demonstrate its potential as a tool for future rotorcraft analysis.