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HELINOVI

Helicopter Noise and Vibration Reduction
vibration and noise are major nuisances to helicopter occupants

Funding: European (5th RTD Framework Programme)
Duration: 04/2002 - 03/2005
Transport themes: Air transport , Security and Safety , Environmental impacts (key theme) , Economic and regional impacts
  • Outline
  • Funding
  • Results
  • Documents
  • Contact

Background & policy context

The conventional helicopter is close to the edge of its performance envelope. In the future emphasis is on making it a more efficient, environmentally friendly mode of transport. Both, vibration and noise, are major nuisance to aircraft occupants. In addition, they increase the pilot's strain during flight and reduce his capability to safely pilot the aircraft.

Noise radiated from aircraft is a general nuisance for the community. Vibrations cause major problems for the operation and maintenance of the aircraft. Induced by the complex interaction between main rotor and airframe, vibrations may cut the fatigue life of helicopter components down to 50% creating major costs for the aircraft operator.

Helicopter noise not only bothers helicopter occupants, but the total flight environment of the aircraft. Recognising that police and health/rescue services helicopters are often operated in populated areas, noise reduction is of major importance and a strong marketing aspect. A quite helicopter is benefit for the community in quest of a quite environment.

Objectives

HeliNOVI's overall research objective is the enhancement of helicopter performance, safety and ride comfort.

In detail the objectives are to:

  • investigate complex configurations which may reduce noise by at least 7EPNdB;
  • double the lifetime of components by decreasing the vibration energy;
  • improve passenger comfort for high fidelity ride esp. in medical service;
  • shorten the time needed for helicopter type certification;
  • expand the maintenance cycles of the helicopter structure and avionics.

Methodology

The methodology within the 'Tail Rotor Noise Reduction Potential' will comprise:

  • Adaptation, upgrade, or refinement of rotor aerodynamic and acoustic simulation codes for the prediction of high resolution unsteady blade surface pressures and the radiated noise. Objectives are to conduct pre-test predictions for code-to-code and later code-to-test comparisons. As regards the noise calculations, in all cases this is done in an a posterior procedure closely related to the aerodynamic calculations which makes them all indispensable.
  • Design of a wind tunnel test employing the existing powered 40%-scaled BO 105 helicopter model featuring pressure instrumented main and tail rotor blades. Measurements will cover simultaneous acquisition of aerodynamic, dynamic, acoustic, and operational data all to be carefully analysed. Objective is the generation of a quality data base for code validation and assessment of tail rotor noise palliatives.
  • Validation of the simulation codes by performing code-to-test comparisons using the prediction, wind tunnel, and flight test results (to be complementary provided by DLR). Analysis of potential discrepancies will lead to improved simulation tools. Objective is to improve the understanding and prediction capability of tail rotor (and main rotor) noise for a more accurate prediction of acoustically relevant design changes.
  • Quantification (numerically and experimentally) of the tail rotor noise reduction potential through variation of blade air load and tip speed, through change of the tail rotor sense of rotation, and by modifying the tail rotor position (to minimise or avoid the interaction with the main rotor wake). Objectives are to assess the acoustic benefit in view of realistic helicopter operation and to eventually establish design guidelines for future less noisy helicopters with conventional tail rotor.

The methodology for the 'Vibration Reduction Potential' consists of the following key topics:

  • Wind tunnel test results of a scaled helicopter model will be analysed in detail for tools validation purposes. Measurements will cover both aerodynamic (PIV, pressure pick-ups) and dynamic issues (blade gauges, hub balance). Objective is the validation of design codes and modelling techniques for the prediction of dynamics affected by interactional aerodynamic effects.
  • The results of the wind tunnel tests and complementary theoretical investigations will be used for the identification and assessment of vibration excitation sources. Objective is an improved physical understanding of rotor induced vibrations in the complex aerodynamic environment of main rotor, fuselage, empennage and tail rotor.
  • Full scale flight test results will be compared to the wind tunnel test data of the scaled model and related theoretical results. Objective is the assessment of wind tunnel test results with respect to full scale and free flight conditions. Focus will be given on the transferability of the vibratory response behaviour from model to full scale under the light of interactional aerodynamic effects.
  • Based on the improved understanding of the aerodynamic and dynamic phenomena, means for vibration reduction with emphasis on interactional aerodynamics will be discussed and evaluated. Objective is the establishment of design guidelines regarding improved interactional aerodynamic behaviour for future low vibration helicopters.

Research Programme

FP5 - GROWTH - KA4 (AERONAUTICS) - New Perspectives in Aeronautics

Leading institution(s)

Public institution:

European Commission, Directorate-General for Research (DG Research)

Type of funding

Public (EU)

Key results

The first major step in the programme was the creation of a data base capable to cover interactional phenomena between main rotor, tail rotor and fuselage. Such data base is necessary firstly to evaluate the noise and vibration reduction potential. Secondly it allows evaluating the simulation capability of the prediction tools with respect to interactional phenomena. Once the tools have been validated, they can be used to locate the main noise and the vibration radiating areas of the aircraft as basis for future research and design work.

There is high interest of the industry to tackle noise and vibration phenomena which have significant impact on type certification, crew and passenger comfort, and the life cycles of helicopter components.

The goal of reducing noise by 60% was achieved by changing the tail rotor sense of rotation from "Advancing Side Down" to "Advancing Side Up". When comparing with tail rotor in "Advancing Side Down" mode, an average noise reduction of between 5 to 8 dBA has been measured depending on the flight condition. There was no performance penalty observed by the reversing tail rotor sense of rotation.

As a result of tip speed reduction, an averaged noise reduction value of more than 2 dBA was observed for all flight conditions. The reduction of main rotor BVI noise, especially in the retreating side area, was even more than 3dBA.

The experimental acoustic set up of the model helicopter (in DNW-LLF 8m by 6m open jet) measures the noise pattern and directivity. . The microphone array (red plane) - with more than 120 microphones - moves with the microphone traverse. The helicopter model is a 40% Mach scaled Bo-105. The diameter of the main rotor is 4 meter. In most of cases the experimental noise reduction benefits were confirmed in numerical simulations.

Tail rotor modifications were also analysed in order to study the vibration reduction potential based on interactional aerodynamics. The investigations and also the theoretical results confirmed that the tail rotor configuration can be optimised without affecting the vibratory behaviour.

In particular the clearance between main rotor and fuselage was studied in some detail. The Bo-105 as small helicopter has a large relative clearance leading to an interference factor of 0.6 compared to transport helicopters with small main rotor shaft lengths. The latter are showing high interference factors between 1 and 3. In a parametric study clearance between main rotor and fuselage was systematically reduced in order to analyse the hereby increasing vibration level of a top fairing of the wind tunnel model. The interference factor of the wind tunnel model was modified to approximately 2 by the top fairing.

The aerodynamic influence of the fuselage on vibratory loads increases with flight speed underlining the high importance of this interaction for operational needs.

The higher harmonic vibratory flap bending loads in the rotating system contribute to the 4/rev roll and pitch moments in the airframe system leading to a difference of at least 33%. Higher differences are expected for increased wind speeds. The theoretical results are consistent to the experimental data.

These HeliNOVI test results confirm the importance of interactional aerodynamics on helicopter vibration characteristics for industrial design considerations. They underpin the importance of the comprehensive data analysis and code validation work performed in this project. In summary the following results are available:

  • A unique database for high resolution air loads on rotor blades and fuselage as well as for the radiated noise has been generated. Such database is presently not available in Europe. The now available test data are providing work for many years to come after HeliNOVI. Each partner has a full set of data (> 60 Gigabyte) available for future analysis work.
  • The prediction tools for tail rotor noise including main/tail rotor interactions have been validated. They are available for future aircraft design and retrofit purposes.
  • Tail rotor noise and vibration reduction potentials have been demonstrated and validated theoretically and experimentally.

Technical implications

See key results

Policy implications

See key results

Partners

  • DLR - Deutsches Zentrum für Luft- und Raumfahrt (D) - Co-Ordinator,
  • Eurocopter SAS (F),
  • EUROCOPTER Deutschland GmbH (D),
  • SENER Ingenieria y Sistemas S.A. (E),
  • VIBRATEC (F),
  • CIRA - Italian Aerospace Research Centre (I),
  • QINETIQ (UK),
  • NLR - Nationaal Lucht- en Ruimtevaartlaboratorium (NL),
  • ONERA - French aerospace research centre (F),
  • NTUA - National Technical University of Athens (GR),
  • UMIST - University of Manchester Institute for Science and Technology (UK).

Contact for further information

Mr Juergen Langer
DLR - Deutsches Forschungzentrum für Luft-und Raumfahrt
Lilienthalplatz 7
38108  Braunschweig
Germany

Tel: +49 531 295 2696
Fax: +49 531 295 2641