Rotor blade–vortex interaction noise

Modern helicopter rotor aerodynamics

▪ Abstract Modern helicopter aerodynamics is challenging because the flow field generated by a helicopter is extremely complicated and difficult to measure, model, and predict; moreover, experiments are expensive and difficult to conduct. In this article we discuss the basic principles of modern helicopter aerodynamics. Many sophisticated experimental and computational techniques have been employed in an effort to predict performance parameters. Of particular interest is the structure of the rotor wake, which is highly three-dimensional and unsteady, and the rotor-blade pressure distribution, which is significantly affected by the strength and position of the wake. We describe the various modern methods of computation and experiment which span the range from vortex techniques to full three-dimensional Navier-Stokes computations, and from classical probe methods to laser velocimetry techniques. Typical results for the structure of the wake and the blade pressure distribution in both hover and forward fligh...

Modern helicopter aerodynamics

: In a major cooperative program within the existing US-German and US-French Memoranda of Understanding/Agreements (MOU/MOA), researchers from German DLR, French ONERA, NASA Langley, and the US Army Aeroflightdynamics Directorate (AFDD) conducted a comprehensive experimental program in October 2001 with a 40%-geometrically and aeroelastically scaled model of a BO-105 main rotor in the open-jet anechoic test section of the German-Dutch Windtunnel (DNW). This international cooperative program carries the acronym HART-II (Higher harmonic control Aeroacoustics Rotor Test). The main objective of the program is to improve the basic understanding and the analytical modeling capabilities of rotor blade-vortex interaction noise with and without higher harmonic pitch control (HHC) inputs, particularly the effect of rotor wakes on rotor noise and vibration. Comprehensive acoustic, rotor wakes, aerodynamic, and blade deformation data were obtained with pressure-instrumented blades. The test plan has been concentrated on measuring extensive rotor wakes with a 3-component Particle Image Velocimetry (PIV) technique, along with measurements of acoustics, blade surface pressures, and blade deformations. The prediction team with researchers from DLR, ONERA, NASA-Langley and AFDD was actively involved with the pre-test activities to formulate a test plan and measurement areas of the PIV technique. The prediction team predicted all the test results in advance before performing the wind tunnel test. This was done to obtain the best quality of test data, to improve the speed of measurements, and to determine the necessary measurement information for code validation. In this paper, an overview of the HART-II program and some representative measured and predicted results are presented.

/pdf/the-hart-ii-test-rotor-wakes-and-aeroacoustics-with-higher-142ae1o4pn.pdf

The HART-II Test: Rotor Wakes and Aeroacoustics with Higher-Harmonic Pitch Control (HHC) Inputs - The Joint German/French/Dutch/US Project

This paper reviews the state of the art in helicopter rotor loads prediction using Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) coupling. The application of CFD to rotorcraft problems has evolved, over the period 1990 to 2005, as a viable means to improve the aerodynamic modeling used in rotorcraft comprehensive analyses (CA). CFD/CSD coupling has the potential to meet the long term objective of a coupled rotor-fuselage analysis that can predict accurate loads and vibration at all critical flight conditions without empirical or semi-empirical inputs. This paper reviews the capabilities and limitations of current CFD analyses to capture the key aerodynamic phenomena driving critical rotor loads: wake, dynamic stall and tip transonic effects. Recent initiatives in CFD/CSD coupling, targeted to improve predictions of rotor loads, are discussed. The high speed (155 kts) forward flight condition of the UH-60A BlackHawk is studied as a test case. The flight condition falls in a critical high vibration regime, where the mechanisms of rotor vibratory loads were not clearly understood. Significant improvements in fundamental understanding and prediction of vibratory loads at this flight condition has been recently demonstrated by researchers using CFD/CSD coupling. The physical mechanisms involved, and the role of CFD in improving predictions is explained.

Review of Rotor Loads Prediction with the Emergence of Rotorcraft CFD

Helicopter rotor simulation codes of different organizations are validated against experimental data obtained in the DNW. The comparison addresses rotor blade loading, wake geometry, blade motion and noise radiation. Although specific differences exist the general prediciton of rotor noise is reasonably well.

Aerodynamics and Acoustics of Rotor Blade-Vortex Interactions

A noise study using an aeroelastically scaled BO-105 rotor was conducted in the German-Dutch Wind Tunnel to examine the use of higher harmonic control (HHC) of blade pitch to reduce impulsive blade-vortex interaction (BVI) noise. The noise directivity was measured over a large plane underneath the rotor using a traversing inflow microphone array. Noise and vibration measurements were made for a range of matched rotor operating conditions where prescribed (or open loop) HHC pitch, at various amplitudes and phases, was superimposed on normal (baseline) collective and cyclic trim pitch. Acoustic data are presented for 3, 4, and 5P HHC applied to a typical landing approach rotor operating condition where BVI noise is normally intense. Noise reductions of up to 6 dB were found for the advancing side BVI noise radiating upstream of the rotor, and also for the retreating side BVI noise radiating below and downstream of the rotor. The relative levels between the sides were modified by HHC control phase. To help give insight to the physics of the HHC/BVI noise problem, highresolution loading and noise prediction results are presented for comparison to the data. The predictions are based on a new high-resolution version of the CAMRAD rotor performance program under development at Langley, called HIRES.

Analysis of a higher harmonic control test to reduce blade vortex interaction noise

During the Higher Harmonic Control Aeroacoustic Rotor Test, extensive measurements of the rotor aerodynamics, the far-field acoustics, the wake geometry, and the blade motion for powered, descent, flight conditions were made. These measurements have been used to validate and improve the prediction of blade-vortex interaction (BVI) noise. The improvements made to the BVI modeling after the evaluation of the test data are discussed. The effects of these improvements on the acoustic-pressure predictions are shown. These improvements include restructuring the wake, modifying the core size, incorporating the measured blade motion into the calculations, and attempting to improve the dynamic blade response. A comparison of four different implementations of the Ffowcs Williams and Hawkings equation is presented. A common set of aerodynamic input has been used for this comparison.

https://apps.dtic.mil/dtic/tr/fulltext/u2/a294477.pdf

Effect of wake structure on blade-vortex interaction phenomena: Acoustic prediction and validation

The noise of an aeroelastically scaled helicopter rotor has been studied in the German-Dutch wind tunnel in order to assess the utility of higher-harmonic control (HHC) in reducing blade-vortex interaction (BVI) noise Acoustic data are presented for 3/rev, 4/rev, and 5/rev HHC, as applied to a typical landing approach rotor operating condition; noise reduction of up to 6 dB were found for advancing-blade BVI noise radiating upstream of the rotor, as well as for retreating blade BVI noise radiating below and downstream of the rotor

Klaus J. Schultz

Papers

Aerodynamics and Acoustics of Rotor Blade-Vortex Interactions

Analysis of a higher harmonic control test to reduce blade vortex interaction noise

Effect of wake structure on blade-vortex interaction phenomena: Acoustic prediction and validation

HHC Study in the DNW to Reduce BVI Noise-An Analysis