T. Alan Egolf

An unsteady helicopter rotor-fuselage aerodynamic interaction analysis

A computational method has been developed to treat the unsteady aerodynamic interaction between a helicopter rotor, wake, and fuselage. Two existing codes, a lifting line-prescribed wake rotor analysis and a source panel fuselage analysis, were modified and coupled to allow prediction of unsteady fuselage pressures and airloads. A prescribed displacement technique was developed to position the rotor wake about the fuselage. Also coupled into the method were optional blade dynamics or rigid blade performance analyses to set the rotor operating conditions. Sensitivity studies were performed to determine the influence of the wake and fuselage geometry on the computational results. Solutions were computed for an ellipsoidal fuselage and a four bladed rotor at several advance ratios, using both the classical helix and the generalized distorted wake model. Results are presented that describe the induced velocities, pressures, and airloads on the fuselage and the induced velocities and bound circulation at the rotor. The ability to treat arbitrary geometries was demonstrated using a simulated helicopter fuselage. Initial computations were made to simulate the geometry of an experimental rotor-fuselage interaction study performed at the Georgia Institute of Technology.

Implementation of a rotary-wing Navier-Stokes solver on a massively parallel computer

An unsteady, compressible, three-dimensional, implicit Navier-Stokes solver (NSR3D) for helicopter and propeller applications has implemented using FORTRAN with 8X array extensions on the massively parallel connection machine (CM-2). The modifications to the original algorithm necessary to overcome communication bottlenecks and achieve reasonable computational efficiency on the CM-2 are described.

Analysis of a Rotor in Hover using Hybrid Methodology

The AIAA Applied Aerodynamics Technical Committee’s Rotorcraft Simulation Working Group put together standardized evaluation of the S-76 planform in hover. The objective of this paper was to construct a hybrid analysis framework for the baseline S-76 planform and compute performance predictions for various collective settings and compare with experiments. The hybrid method consists of a near-body rotor-blade Navier-Stokes solver coupled with a farfield Lagrangian wake evolution model. Apart from thrust, torque coefficients and rotor figure of merit, the computed wake structure and surface pressure distributions are also obtained. The paper concludes with some observations on the suitability of hybrid methods for hover computation. Nomenclature μ advance ratio Cp power coefficient CT rotor thrust coefficient FM figure of merit αs shaft angle of attack σ solidity CnM 2 sectional normal loads CmM 2 sectional pitching moments

Physics-Based Modeling of Maneuver Loads for Rotor and Hub Design

A hybrid Navier–Stokes free-wake computational-fluid-dynamics methodology, coupled with a multibody dynamics analysis code, has been applied to the UH-60A rotor to study the loads developed during a severe diving turn maneuver (flight counter 11680) characterized by a mean advance ratio of 0.388 and a mean load factor of 1.48. The helicopter undergoing flight counter 11680 experiences the most severe pitch-link loads and root torsional moments recorded in the NASA/Army UH-60A Airloads Program. The feasibility of a quasi-steady loosely coupled approach for predicting severe maneuver loads was explored for predicting critical aerodynamic and aeroelastic phenomena. The structural model is first validated through measured airloads, which helps in decoupling the physics of structural dynamics and aerodynamics. The blade aeromechanical loads, push-rod loads, and harmonic content of blade structural loads for a rotor revolution characterized by peak load factor (revolution 12) have been examined in detail. The p...

Assessment of Planform Effects on Rotor Hover Performance

A hybrid Navier-Stokes/Free Wake methodology is applied to helicopter rotors in hover. Three tip planforms were analyzed: the S-76 rotor with a swept tip, a rectangular planform, and a swept tip with anhedral. The solidity of the three rotors was matched. A pitch sweep was conducted. Computed thrust, power, and figure of merit values have been compared with test data. Where available, comparisons with other calculations for these quantities and the near wake tip vortex trajectory have been done. The simulations are in reasonable agreement with published data for the thrust, power, and figure of merit. The simulations differ from full Navier-Stokes calculations in the predictions for tip vortex descent rate and contraction rate. The simulations correctly predicted the trends. The swept tapered anhedral tip had the highest figure of merit, followed by the swept tapered tip. The rectangular planform had the lowest figure of merit.

Multiscale Modeling of Active Flow Control for Fuselage Drag Reduction

Multi-scale modeling of synthetic jet devices is em ployed to investigate the effect of active flow control (AFC) on fuselage drag reduction. The flow field in the vicinity of the synthetic jet devices was modeled using a Lattice Boltzmann equation (LBE) approach, while the external flow over the fuselage was modeled using a Reynolds-averaged Navier-Stokes (RANS) methodology. Computations with and without flow control also reported for flowfield around the isolated fuselage of ROBIN-mod7 model. The computed results, in the absence of flow control, have been compared with the measured data for ROBIN-mod7 model and show good agreement. Results from the CFD simulation with AFC using LB/NS coupled methodology indicate in an estimated 18% drag reduction.

Study of Dynamic Tip Vortex Tracking in Rotorcraft CFD

This paper describes recent work on the application of vortex tracking grids (VTG) in a dynamic manner for a UH-60A rotor in hover to predict the blade airloads and wake. It was found that with appropriate time step size and limiters combined with an automated adaptation process using implicit hole-cutting that allows the nested vortex tracking grids to follow the tip vortices, reasonable computations of the blade section loading and wake geometry characteristics were obtained. While the results are currently inviscid, they indicate the potential of the VTG approach for wake capturing in an affordable manner, but more work is still needed to mature the approach for design application.