Environmental Research - Rotorcraft Noise

The objectives

To support the full scope of noise monitoring activities required by the European Environmental Noise Directive (END) 2002/49/EC as well as the impact assessment of future aircraft noise policies, an adequate noise modelling capability that encompasses all types of aircraft is required. 

The current international guidance on aircraft noise modelling (e.g. ICAO Doc 9911, ECAC Doc 29, or Directive 2015/996) is limited to fixed-wing aircraft, thus not covering rotorcraft noise although these are within the scope of the END. Helicopters are generating local noise nuisances and, in the near future, drones and new urban mobility aircraft could create additional ones in densely populated areas. Defining a validated noise modelling methodology for these transport modes is therefore a high priority for the EU.

Between 2015 and 2017, the European Commission DG MOVE tendered the contract MOVE-C2-2014-269 to start addressing this modelling gap, with technical support from EASA. Under this contract, a methodology to compute rotorcraft noise was defined. A generic test plan to acquire helicopter data for noise-modelling purposes was designed and subsequently data was acquired for eight small and medium-size helicopter types that are representative of the bulk of the helicopter fleet operating in Europe. Lastly, a software prototype ‘NORAH’ was developed to compute the noise on the ground for different flight conditions for a single event and for multiple helicopter operations.

The purpose of this framework contract is to build on and extend the work of contract MOVE-C2-2014-269 towards developing and validating a full-fledged noise modelling capability for all rotorcraft,   representative of today’s and of future operations. This will include the underpinning modelling methodology, the corresponding modelling software and the experimental datasets required to address the intended coverage. The experimental tests will include large helicopters to complement previous campaigns, and the further development of the NORAH prototype into a tool which Member States, the European Commission and EASA can use to perform noise impact assessments.

The main tasks and deliverables

Within this framework contract, the following tasks are planned:

  • Extend NORAH noise propagation modelling capabilities, e.g. to account for urban environment, for varried terrain and vegetation, and weather effects; 
  • Enhance NORAH source modelling capabilities, covering a wider range of flight conditions than available in the noise database;
  • Prepare for the rotorcraft noise tests, including: optimisation and update of the generic noise test plan to cover additional flight modes (e.g. hover), identification and prioritisation of the rotorcraft for the noise tests (including EVTOL) ensuring a good coverage of European fleet, investigation of the availability and costs for renting rotorcraft and test sites;
  • Expand the helicopter types in the NORAH hemisphere repository by dedicated noise testing;
  • Implement the revised noise modelling methodology into a new software;
  • Validate the NORAH modelling method against benchmark data.

Research Project Report SC01: Rotorcraft noise modelling method

The first part of the project was completed in September 2021. The main deliverable consists of a revised rotorcraft noise modelling method which covers a broader range of flight conditions and sound propagation effects. The method is available in the report.

A prioritised selection of new helicopter types for inclusion in the NORAH model was developed, together with a generic noise test plan for those. This test plan takes into account the lessons learned from the earlier NORAH noise test campaign such that new data can be acquired in a more cost-effective manner.

The next phase of the project will run until the first quarter of 2024. It will include noise tests for the identified high-priority rotorcraft and the development of a new NORAH software prototype in line with the revised rotorcraft noise modelling method.

Research Project Report SC02: Determination of a human dose-response with respect to single events of Urban Air Mobility-type vehicles

This study presents dose-responses relationships for a new class of aircraft vehicles, namely four drones and one air taxi. Also, a small fixed wing aircraft has been included, and two helicopters that can be considered as reference measurements. A-weighted Sound Exposure Level (SEL) has been used to determine the input sound levels (‘dose’) for the stimuli.

Stimuli have been presented using a headphone as a mono sound on both ears, so no binaural representation was used.

Results from the measurements show expected reported higher annoyance rates for increased SEL values for each vehicle. Differences can be seen between the highly annoyance scores of the helicopters on one hand (lower), and the other vehicles on the other hand (higher), especially in SEL values above 70 dB(A).

For more information, please review the study.