Showing papers in "Journal of The American Helicopter Society in 2009"

Hover Performance Correlation for Full-Scale and Model-Scale Coaxial Rotors

Abstract: The hover performance data of full-scale and model-scale coaxial rotors have been compared with CAMRAD II predictions having a free vortex wake analysis. Performance correlations of a coaxial rotor were made with a variation of key parameters including the rotor spacing and height. To understand aerodynamic behavior of the U.S. Army Aeroflightdynamics Directorate(AFDD)coaxialrotoroperatingoverarangeofReynoldsnumbersfrom36,000to180,000,theReynoldsnumber scaling effect was explored using an airfoil design code, MSES. It was found that the coaxial rotor spacing effect on hover performance was minimal for the rotor spacing larger than 20% of the rotor diameter. The measured performance data showed that more thrust was lost from the lower rotor of a coaxial than the upper rotor due to a larger rotor-to-rotor wake interference effect, and the lower rotor kept only an 81% of the single rotor OGE (out-of-ground effect) thrust whereas the upper rotor maintained a 90%. The lower rotor IGE (in-ground effect) thrust increased quickly by 26% as the rotor approached to the ground from the position of an 80% of the rotor diameter to 10%, and the corresponding IGE power increased by 17%. These thrust and power characteristics were well predicted. Overall, the performance prediction for the coaxial rotor was satisfactory when compared with the measured data.

How Dynamic Inflow Survives in the Competitive World of Rotorcraft Aerodynamics

Abstract: Dynamic wake models have found a firm place in rotary-wing analysis since their inception in the 1950s, proliferation in the 1970s, and maturation in the 1990s. They have maintained their usefulness, despite the appearance of new and more powerful tools—such as prescribed-wake models, free-wake models, and computational fluid dynamics—due to the following five fundamental reasons: (1) the various models and improvements have always come in response to important, yet unexplained experimental results; (2) the response to those results was invariably based on sound physical intuition as to the nature of the discrepancies; (3) the model improvements at each step were based on engineering physics rather than heuristic fit of data; (4) the models included only enough physics to explain the phenomenon—and no more; and (5) each model was hierarchical to earlier models so that no model was ever replaced—i.e., each new improvement included earlier models as special cases. Because of this, dynamic wake models have maintained a strength in the domains of real-time flight simulation, stability computations, and flight mechanics and control. This paper looks in detail at how these developments have transpired and how they relate to the importance of simplified tools, in general.

Performance Analysis of the Slowed-Rotor Compound Helicopter Configuration

Abstract: : The calculated performance of a slowed-rotor compound aircraft, particularly at high flight speeds, is examined. Correlation of calculated and measured performance is presented for a NASA Langley high advance ratio test and the McDonnell XV-1 demonstrator to establish the capability to model rotors in such flight conditions. The predicted performance of a slowed-rotor vehicle model based on the CarterCopter Technology Demonstrator is examined in detail. An isolated rotor model and a model of a rotor and wing are considered. Three tip speeds and a range of collective pitch settings are investigated. A tip Mach number of 0.2 and zero collective pitch are found to be the optimum condition to minimize rotor drag. Performance is examined for both sea level and cruise altitude conditions.

Wake dynamics and rotor-fuselage aerodynamic interactions

Abstract: The unsteady loads experienced by a helicopter are known to be strongly influenced by aerodynamic interactions between the rotor and fuselage; these unsteady loads can lead to deficiencies in handling qualities and unacceptable vibratory characteristics of the rotorcraft. This work uses a vorticity-based computational model to study the governing processes that underpin this aerodynamic interaction and aims to provide greater understanding of the wake dynamics in the presence of a fuselage, as well as an appreciation of how the geometry of the wake affects the loading on the fuselage. The well-known experiments using NASA's ROBIN fuselage are used to assess the accuracy of the computations. Comparisons of calculations against results from smoke visualization experiments are used to demonstrate the ability of the model to reproduce accurately the geometry of the rotor wake, and comparisons with inflow data from the experiments show the method to capture well the velocity field near to the rotor. The fuselage model is able to predict accurately the unsteady fuselage loading that is induced by blade passage and also by the inviscid interaction between the main rotor wake and fuselage.

Flight Test and Simulation of a Cargo Container Slung Load in Forward Flight

Abstract: This article presents some recent results from a cooperative effort by the U.S. Army Aeroflightdynamics Directorate, the Technion Israel Institute of Technology, and the Northern Arizona University. The objective of the article was to study and simulate the behavior of the 6 x 6x 8 ft CONEX cargo container in forward flight suspended beneath a UH-60 Black Hawk helicopter. This load, like other cargo containers, is subject to massively separated unsteady flow and is limited by stability to operational airspeeds well below the power-limited speed of the configuration. The article makes use of aerodynamic data from wind tunnels, flight tests, and computational fluid dynamics. The objective is a simulation of the helicopter-slung load system validated over the complete flight envelope. The principal remaining technical challenge is a mode of the unsteady load aerodynamics that is capable of predicting the critical unstable speed and some progress has been made in meeting this challenge.

Wake structure and kinematics in the vortex ring state

Abstract: High-resolution computational simulations of the vortical wake of a rotor operating both near to and within the vortex ring state have been conducted using Brown's vorticity transport model. The nonlinear vortex kinematics of the wake is exposed using three-dimensional visualizations of the simulated flow field. To reveal the vortex dynamics that underpin the highly unsteady flow within the vortex ring state, a rotor with just one blade was modeled. This blade was decoupled aerodynamically from the surrounding velocity field so that it acted merely as a source of trailed vorticity. The investigation identified a significant change in the dominant dynamics of the wake as it swapped fromthe tubular form that is characteristic of hover or very lowspeed descent into the toroidal geometry of the vortex ring state. Initial vortex 'pairing' leads to rotation of vortex filaments away from their original attitude. This phenomenon plays an important role in regulating the downwash that the rotor can produce and thus in precipitating the onset of the vortex ring state. The considerable and persistent coherence of the vortical structure of the wake when in the vortex ring state is revealed, despite these disturbances, as are themechanisms that lead to both small-scale and large-scale wake breakdown events. Simulations show the balance between the vortex pairing and short-wave instability modes to be different in the vortex ring state at high descent speed, where the wake lies above the rotor, compared to in the vortex ring state at low descent speed when the wake lies predominantly below the rotor. This yields subtle differences to the kinematics and structure of the wake in the two cases.

Toward a Real-Time Measurement-Based System for Estimation of Helicopter Engine Degradation Due to Compressor Erosion

Abstract: This paper presents a preliminary demonstration of an automated health assessment tool, capable of real-time on-board operation using existing engine control hardware. The tool allows operators to discern how rapidly individual turboshaft engines are degrading. As the compressor erodes, performance is lost, and with it the ability to generate power. Thus, such a tool would provide an instant assessment of the engine s fitness to perform a mission, and would help to pinpoint any abnormal wear or performance anomalies before they became serious, thereby decreasing uncertainty and enabling improved maintenance scheduling. The research described in the paper utilized test stand data from a T700-GE-401 turboshaft engine that underwent sand-ingestion testing to scale a model-based compressor efficiency degradation estimation algorithm. This algorithm was then applied to real-time Health Usage and Monitoring System (HUMS) data from a T700-GE-701C to track compressor efficiency on-line. The approach uses an optimal estimator called a Kalman filter. The filter is designed to estimate the compressor efficiency using only data from the engine s sensors as input.

Coupled thrust and vorticity dynamics during vortex ring state

Abstract: This paper focuses on the vortex ring state (VRS) that is observed in the flow around a rotor during rapid descent. In VRS, the vortex filaments trailed off the blades coalesce around the rotor disk. This vortex ring detaches into the wake periodically, causing extreme oscillations in thrust, with periods on the order of several tens of rotor revolutions. The phase relation between the thrust cycle and vorticity distribution at the rotor disk is discussed. Maxima of the VRS thrust oscillations correlate well with the maxima of circulation, enstrophy, and minima of enstrophy dispersion radius observed in the vicinity of the rotor disk.

Influence of Dynamic Stall and Dynamic Wake Effects on Helicopter Trim and Rotor Loads

Abstract: Flight test data of helicopters indicate that vibratory levels in the fuselage exhibit a wide spectrum of frequencies including the dominant blade passage frequency and its integer multiples. The present work attempts to understand the reason for the existence of several frequencies in the response of the fuselage and possible cause for this observed phenomenon by formulating a computational aeroelastic model. In this theoretical study, a systematic approach has been undertaken to identify the effects of inflow modeling and sectional aerodynamic load evaluation, on helicopter trim, rotor blade response, and hub loads. Five different combinations of aerodynamic models of increasing complexity, representing rotor inflow and sectional aerodynamic loads, have been proposed. The differential equations of motion are solved in time domain in a sequential manner to obtain the response of all the blades in the rotor system, the inflow variables, and the sectional loads at every time step. The results of the present study show that the aerodynamic model incorporating dynamic wake and dynamic stall effects introduces a wide spectrum of harmonics in the hub loads including blade passage frequency and its integer multiples. The influence of aerodynamic modeling on the variation of trim parameters with forward speed has also been brought out. It is observed that the aerodynamic model incorporating dynamic wake and dynamic stall effects predicts the trim parameters whose variation with forward speed resemble qualitatively similar to those obtained in flight test. A comparison of the variation of blade sectional lift for various aerodynamic models indicates that in the advancing side of the rotor, a dynamic stall model introduces a shift in the azimuth angle at which the minimum lift occurs. The effect of structural flap—lag coupling due to blade pretwist on trim and rotor loads has been studied, and these results are compared with those pertaining to a straight blade configuration. ©2009 American Helicopter Society

An Effective Crashworthiness Design Optimization Methodology to Improve Helicopter Landing Gear Energy Absorption

Abstract: Crashworthiness design optimization problems are continuously challenged by inherent complexity of long simulation time, numerical instability, and unavailability of sensitivity analysis. In recent years, surrogate-model-based design optimization using design of experiments and response surface methodology has proven to be a promising way to achieve the improved design for crashworthiness problems. However, the method is still hampered by a large number of function evaluations for a large number of design variables or low accuracy for small sample size. This paper presents an effective simulation-based optimization algorithm for optimal design of large-scale, computationally expensive crashworthiness problems. The proposed optimization algorithm is called the sequential regularized multiquadric regression with output space mapping (SRMQ/OSM). The proposed method is demonstrated on two simple numerical problems and is subsequently extended to a generic helicopter skid landing gear optimization problem to improve the energy absorption efficiency with prescribed design constraints. The results show that the proposed approach converged quickly to a feasible design with no constraint violation. The improved skid gear design obtained by the proposed SRMQ/OSM method exhibits better impact performance in terms of energy absorption, peak acceleration, onset rate, and pulse duration. ©2009 American Helicopter Society.