High Advance Ratios Research Articles

Slowed Hingeless Rotor Wind Tunnel Tests and Validation at High Advance Ratios

In forward flight, slowing down a rotor alleviates compressibility effects on the advancing side blade tip, extending the cruise speed limit, and inducing high-advance-ratio flight regime. Previous wind tunnel tests have shown that an articulated rotor trimmed to zero hub moment generates limited thrust at high advance ratios, because the advancing side needs to be trimmed against the retreating side with significant reverse flow, where the rotor is ineffective in generating thrust. A rigid hingeless rotor with lift offset may help overcoming this problem. A series of wind tunnel tests were conducted to investigate the behavior of slowed hingeless rotors at high advance ratios. Performance, control angles, hub vibratory loads, and blade structural loads were measured and compared with comprehensive analysis predictions. The results demonstrate that a hingeless rotor with lift offset is more efficient in generating thrust and exhibits higher lift-to-drag ratio at high advance ratios. The blade structural load level is significantly higher compared with an articulated rotor, especially its flap bending moment, which can pose a critical structural constraint on the rotor.

Blade Structural Load/Airload Correlation on a Slowed Mach-Scaled Rotor at High Advance Ratios

To expand the cruise speed of a compound helicopter, alleviating the compressibility effects on the advancing side with reduced rotor RPM is proved to be an effective design feature, which results in high advance ratio flight regime. To investigate the aerodynamic phenomena at high advance ratios and provide data for the validation of analytical tools, a series of wind tunnel tests were conducted progressively in the Glenn L. Martin Wind Tunnel with a 33.5-inch radius fourbladed articulated rotor. In a recent wind tunnel test, the rotor blades were instrumented with pressure sensors and strain gauges at 30% radius, and pressure data were acquired to calculate the sectional airloads by surface integration up to an advance ratio of 0.8. The experimental results of rotor performance, control angles, blade airloads, and structural loads were compared with the predictions of comprehensive analysis and computational fluid dynamics (CFD) analysis coupled with computational structural dynamics (CSD) structural model. The paper focuses on the data correlation between experimental pressure, airload, and structural load data and the CFD/CSD predicted results at various collective and shaft tilt angles. Overall, the data correlation was found satisfactory, and the study provided some insights into the aerodynamic mechanisms that affect the rotor airload and performance, in particular the mechanisms of backward shaft tilt, the effect of hub/shaft wake, and the formation of dynamic stall in the reverse flow region.

Refined Performance Results on a Slowed Mach-Scaled Rotor at High Advance Ratios

Owing to its ability to alleviate the compressibility effect on the advancing side, the slowed rotor operating at high advance ratios is a key feature in high-speed compound rotorcraft. A series of wind tunnel tests were conducted in the Glenn L. Martin Wind Tunnel with a four-bladed Mach-scaled articulated rotor. The objective of the tests was to gain a basic understanding of unique features of high-advance-ratio aerodynamic phenomena, such as thrust reversal and dynamic stall in the reverse flow region. In this study, high-advance-ratio tests were carried out with highly similar, noninstrumented blades and on-hub control angle measurements, to minimize possible error due to blade structural dissimilarity and pitch angle discrepancy. The tests were conducted at 900 and 1200 RPM, advance ratios of 0.3–0.9, and a shaft tilt study was conducted at±4°. Pitch and flap motion at the blade roots, rotor performance, and vibratory hub loads were investigated during the test. The test data were then compared with those of previous tests and with predictions from comprehensive analysis. The airload results were investigated using comprehensive analysis to gain insights into the influences of advance ratio and shaft tilt angle on rotor performance and hub vibratory loads. Results indicate that the thrust benefit from backward shaft tilt is dependent on the change in the inflow condition and the induced angle of attack increment, and the reverse flow region at high advance ratios is the major contributor to changes in shaft torque and horizontal force.

Nonlinear Blade Stability for a Scaled Autogyro Rotor at High Advance Ratios

Despite current research advances in aircraft dynamics and increased interest in the slowed rotor concept for high-speed compound helicopters, the stability of autogyro rotors remains partially understood, particularly at lightly loaded conditions and high advance ratios. In autorotation, the periodic behavior of a rotor blade is a complex nonlinear phenomenon, further complicated by the fact that the rotor speed is not held constant. The aim of the analysis presented in this article is to investigate the underlying mechanisms that can lead to rotation-flap blade instability at high advance ratios for a teetering autorotating rotor. The stability analysis was conducted via wind tunnel tests of a scaled autogyro model combined with numerical continuation and bifurcation analysis. The investigation assessed the effect of varying the flow speed, blade pitch angle, and rotor shaft tilt relative to the flow on the rotor performance and blade stability. The results revealed that rotor instability in autorotation is associated with the existence of fold bifurcations, which bound the control-input and design parameter space within which the rotor can autorotate. This instability occurs at a lightly loaded condition and at advance ratios close to 1 for the scaled model. Finally, it was also revealed that the rotor inability to autorotate was driven by blade stall.

Numerical Investigation of Rotor/Wing Aerodynamic Interactions at High Advance Ratios

Rotor/wing aerodynamic interactions in forward flight up to high advance ratios are investigated through numerical simulations using a rotorcraft computational fluid dynamics solver, rFlow3D, developed at Japan Aerospace Exploration Agency (JAXA). Rotor/wing aerodynamic interaction is one of the key technology issues in developing an efficient compound helicopter design. A validation based on Leishman’s previous experimental study is performed to study the prediction accuracy of the numerical methods of computational fluid dynamics. Then, rotor/wing aerodynamic interaction at high advance ratios is investigated. A simplified computational model with a rotor and a wing is assumed. A rectangular wing model is considered for the UH-60A helicopter with the wing size specified for the original maximum takeoff weight. The UH-60A rotor blade is used, assuming it to be rigid. The isolated rotor and the isolated wing configurations are first simulated, and the results are then compared with the results of the rotor/wing combined configuration so that the effect of interaction can be identified. Two flight conditions, one at cruising speed and the other at maximum speed, are considered. The rotor speed is reduced to 75% RPM at the high-speed flight conditions. The results show that the total effective drag of the combined rotor/wing configuration is approximately 20% higher than the simple summation of the effective drags of the isolated models. The wing lift decreases because of rotor/wing aerodynamic interaction, and the required rotor thrust increases to maintain the same total lift. The sectional normal force on the wing has an asymmetric distribution. The sectional lift significantly reduces, and the sectional drag increases on the wing under the rotor advancing side. On the wing under the rotor retreating side, the rotor has little effect on the sectional lift and drag of the wing. It is observed that the rotor/wing aerodynamic interaction at high advance ratios has a significant influence on the aerodynamic performance of the winged compound helicopter.

고속 전진비 조건에서의 로터 진동하중 특성 연구

본 연구에서는 고 전진비 조건의 로터에 대해 전진비 변화에 따른 허브 진동하중의 변화를 예측하고, 블레이드에서 발생하는 구조하중 예측 및 조화 분석을 통하여 구조하중 변화를 고찰하였다. 로터 전진비는 0.40부터 0.71까지 범위를 가지며, NASA에서 수행한 풍동시험 결과에 대해 수치묘사 연구를 수행하였다. 검증한 결과를 토대로 허브 진동하중 및 블레이드 모멘트를 예측하였다. 허브 진동하중은 최초에 증가하다가 전진비가 0.5 이상의 경우에는 변화가 거의 없음을 보여주었다. 블레이드 구조하중은 전진비가 증가할수록 진폭의 크기가 증가하며, 블레이드 모멘트의 조화 분석을 수행한 결과 플랩 모멘트는 3/rev, 래그모멘트는 4/rev의 영향이 매우 크다는 점을 확인하였다. 이는 전진비가 증가할수록 2차 플랩과 2차 래그 모드의 고유진동수가 각각 3/rev와 4/rev에 근접하기 때문인 것으로 파악되었다.

Validation of Loads Analysis for a Slowed Rotor at High Advance Ratios

Validation of Loads Analysis for a Slowed Rotor at High Advance Ratios

UH-60A Rotor Performance and Loads Correlation at High Advance Ratios

The UH-60A slowed rotor wind-tunnel test data is a unique and high-quality database for the validation of rotorcraft analysis tools at high advance ratio flight conditions. The U.S. Army’s rotorcraft comprehensive analysis system, known as RCAS, incorporates several analysis options, which are evaluated for their suitability at high advance ratios. RCAS predictions of performance, controls, airloads, and structural loads are compared against high advance ratio test data. RCAS modeling options of dynamic inflow, prescribed vortex wake, yawed flow corrections, and radial drag are evaluated. The strengths and weaknesses of the analysis options are highlighted. Rotor thrust and drag predictions are in good agreement with test data, with the addition of radial drag and yawed flow models. Dynamic inflow and prescribed vortex wake performance and loads predictions provide comparable correlation; however, dynamic inflow provides the most robust convergence without numerical issues in the reverse flow region. Yawed flow corrections improve normal force correlation at inboard stations but do not improve the structural load correlation at midspan.

Performance and Loads Predictions of a Mach-Scaled Rotor at High Advance Ratios

Performance and Loads Predictions of a Mach-Scaled Rotor at High Advance Ratios

Experimental Investigation and Fundamental Understanding of a Full-Scale Slowed Rotor at High Advance Ratios

Experimental Investigation and Fundamental Understanding of a Full-Scale Slowed Rotor at High Advance Ratios

Investigation of UH-60A Rotor Performance and Loads at High Advance Ratios

Wind tunnel measurements of the performance, airloads, and structural loads of a full-scale UH-60A Black Hawk main rotor operating at high advance ratios (up to 1.0) are compared with calculations obtained using the comprehensive rotorcraft analysis Comprehensive Analytical Model of Rotorcraft Aerodynamics and Dynamics II to understand physics and quantify this comprehensive code’s accuracy and reliability in the prediction of rotor performance and loads at high advance ratios. Detailed comparisons are made on rotor thrust, control angles, power, and section loads to illustrate and understand unique aeromechanics phenomena in this regime. The analysis correctly predicts the thrust reversal with collective at high advance ratios. Rotor induced plus profile power is also reasonably well predicted with proper modeling of the shank. Airloads and structural loads correlation is fair. A significant underprediction of 2-per-revolution structural loads is observed.

Computational Investigation and Fundamental Understanding of a Slowed UH-60A Rotor at High Advance Ratios

Computational Investigation and Fundamental Understanding of a Slowed UH-60A Rotor at High Advance Ratios

Dynamics of Rotors Operating at High Advance Ratios

Dynamics of Rotors Operating at High Advance Ratios

Prop‐Rotor Stability at High Advance Ratios

An analytical method for determining the stability of wing‐mounted prop‐rotor pylon systems in fully converted airplane configuration has been developed and is described. Application of this method to an existing tilt‐rotor VTOL configuration, the Bell <emph type="4">XV</emph>‐3, and to subsequently tested scale models, has indicated that the method simulates the instabilities arising from the coupling of prop‐rotor flapping motions to pylon motions. It has been found that the principal destabilizing factor in these pylon motions is the inplane force generated by blade flapping at high advance ratios. The stability of the prop‐rotor pylon system is decreased with decrease in pylon mounting stiffness and damping and with increase in speed and pitch‐flap coupling (delta‐three). Stability of the system may generally be increased by Proper controls coupling, such as lagging swashplate coupled to pylon displacements.

Analysis of the Stability of a Flexible Rotor Blade at High Advance Ratio

Analysis of the Stability of a Flexible Rotor Blade at High Advance Ratio