Prediction and Measurement of Blade - Vortex Interaction

Abstract

An extensive quantity of airload measurements was obtained for a pressure-instrumented model of the BO-105 main rotor for a large number of higher harmonic control settings at Duits-Nederlandse Wind Tunnel. The wake geometry, vortex strength, and vortex core size were also measured through a laser light sheet technique and laser Doppler velocimetry. These results are used to verify the blade-vortex interaction (BVI) airload prediction methodologies developed by Army Aero ightdynamics Directorate, Deutsche Forschungsanstalt fur Luft-und Raumfahrt, NASA Langley Research Center, and the Of ce National d'Etudes et de Recherches Aerospatiales. The comparisons show that an accurate prediction of the blade motion and the wake geometry is the most important aspect of the BVI airload predictions. IntroductionA LTHOUGH the current civil helicopter has been used extensively because of its excellent hover and low-speed forward-ight capability, the civil helicopter eet continues to be too noisy for community acceptance, preventing it from reaching its full potential of utilization. One of the major noise sources comes from the rotor blade cutting through its own wake. This phenomenon is known as blade-vortex interaction (BVI). It occurs mostly during descent ight for landing.Over the last decade many experimental studies from fullscale ight tests 1,2 to model-scale tests 3-5 have been performed to investigate this BVI phenomenon. It was found that the BVI noise can be scaled 6,7 and expensive ight tests can be simulated by model rotor tests. The development of BVI prediction methodology lagged behind the experimental studies because no comprehensive rotor code had enough azimuthal (or time) resolution to predict the BVI event. However, several analytical methods 8,9 were developed with limited success as compared with measured blade surface pressure distributions and acoustic signatures. References 8 and 9 indicate that the wake geometry and vortex strength are two major parameters affecting the BVI noise signatures. The information of the wake geometry and vortex strength was not measured in most tests.An international cooperative test, called the higher-harmonic-control aeroacoustic rotor test (HART) was conducted by the Army Aero ightdynamics Directorate (AFDD), Deutsche Forschungsanstalt fur Luft-und Raumfahrt (DLR), Duits-Nederlandse Wind Tunnel (DNW), and the Of ce National d'Etudes et de Recherches Aerospatiales at DNW to measure not only the blade surface pressure distributions, the Presented as Paper 95-021 at the AIAA/CEAS 1st Joint blade deformation, and the acoustic signatures, but also the wake geometry using the laser light sheet (LLS) technique 10 and the vortex strength using the laser Doppler velocimetry (LDV ) method. 11,12 The measured data will be used to validate the prediction methodology and to set the future direction for code improvement.Analytical predictions of BVI events by AFDD, DLR, NASA, and ONERA were performed before the HART test. The predicted wake geometry served as a guide for the LLS and...

Rotor Wake Vortex Definition-Evaluation of 3-C PIV Results of the HART-II Study

“…As ψ approaches 180°(y TUN = 0, x TUN is negative), they weaken. This is anticipated from predictions 13,14 and previous measurements for this rotor system 9 . The blades have high tip loading near ψ ≈ 90°but reduce almost linearly to ψ = 180°.…”

“…Significant advances in aeroacoustic analysis and their validations were achieved, substantially due to improved free-wake and prescribed wake models (Refs. [12][13][14][15][16][17]. It was determined that HHC reduced noise and vibration by increasing blade-vortex-miss-distance and altering the wake geometry and strength (Refs.…”

Key Results From a Higher Harmonic Control Aeroacoustic Rotor Test (HART)

“…Correlation of some of these experimental results with HART pre-test and post-test predictions are presented in Refs. (26,27).…”

Evaluation of Rotor Structural and Aerodynamic Loads Using Measured Blade Properties

“…Despite the fact that HART I test was completed much earlier than HART II and valuable sources of high precision data were provided, the published efforts associated with HART I are significantly less in volume, as compared with HART II [4]. The representative ones of HART I are Tung et al [5], Beaumier and Spiegel [6], Brooks et al [7], Lim et al [8][9], and Yeo and Johnson [10][11]. Most of the research focused on correlating the airloads and vortex wakes against the measured data.…”

“…This suggests uncertainty in the DEI blade property set and there is a need to supply reliable HART I blade properties data which can only be achieved by a careful measurement process. Furthermore, it has been known that up to four different sets of blade property data for HART I are in use [5][6][7][8][9][10][11] and there is a clear need for unified version of the blade properties.…”