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Comprehensive Aerodynamic-Aeroacoustic Analysis of a Trimmed Compound Helicopter

Periodic Reporting for period 3 - CA3TCH (Comprehensive Aerodynamic-Aeroacoustic Analysis of a Trimmed Compound Helicopter)

Okres sprawozdawczy: 2018-12-01 do 2020-05-31

Within the Fast Rotorcraft program of CleanSky 2 Airbus Helicopters develops a compound rotorcraft configuration in succession of the X³ demonstrator having reached a record speed of 255 kts in 2012. The goal of the LifeRCraft (Low Impact, Fast & Efficient RotorCraft) ITD, recently renamed Racer (Rapid and Cost-Effective Rotorcraft), is a rotorcraft being capable of flight speeds not reachable for classic helicopter configurations by means of auxiliary wings and thrust generating propellers instead of a tail rotor, keeping the VTOL (vertical take-off and landing) capability, in constrast to a conventional plane. This enables new application areas for such a rotorcraft, where high speed is as important as the flexibility of VTOL without specific infrastructure, as rescue operations in remote areas, for example. At the same time, efficiency in terms of power consumption is a challenge, as is the target of minimal community noise.
For the aerodynamic, aeromechanic and aeroacoustic simulation and analysis of the complete configuration IAG has been selected as the sole partner having shown unrivaled competence in rotorcraft simulation capability and capacity in Europe. Primary objective of the CA³TCH (Coupled Aerodynamic-Aeroacoustic Analysis of a Trimmed Compound Helicopter) project in general is the de-risking of the LifeRCraft configuration in the development process, identifying potential problems in aeromechanics, aerodynamics and aeroacoustics before first flight, in order to initiate appropriate countermeasures as early as possible.
The project in total was a great success technically, continuously supporting the topic leader with new insights and valuable information regarding the significance of impeding aeromechanic risks.
At first, the simulation framework had to be adapted for the compound configuration, including an extension of the coupling interface to the Airbus Helicopter’s tool GHOST. Within less than half a year, the simulation was up and running, such that production simulations of the full configuration could be started and results analysed afterwards, in order to support further development of the compound demonstrator as quickly as possible. During several geometry updates, flight mechanical analyses were accomplished and transferred to the topic leader for further consideration in the development process. Within the first period, of primary interest were download factors on the wings, especially in hover, and flight stability, with focus on autorotation and other critical flight states, like slow lateral flight. Later on, further interference phenomena were more of a target.
In another work package acoustic analysis of the rotorcraft started with focus on interference phenomena between the main rotor and propellers and the interaction of the sound generated there with the airframe. The tool chain has shown the capability to represent proper directivities due to shading and reflection effects, and the post-processing of aerodynamic results is quite straightforward.
In total more than a hundred million core hours of computing time have been utilised for over 50 flight states covering the most interesting parts of the full flight envelope. The second reporting period has seen some tool chain enhancements, improving computational efficiency by utilizing automatic mesh refinement and more conservative load evaluations. In addition, several simulations on the X³ demonstrator validated the computational framework for compound helicopters. For the RACER configuration itself, after another loft update many new flight states were run and analyzed aerodynamically as well as acoustically, pinpointing the topic leader at areas requiring more attention and eventual optimization opportunities.
For dissemination, three presentations were held at AHS conferences and another one at the European Rotorcraft Forum in Delft, The Netherlands. The latter one was recognized with the Gareth Padfield Best Young Paper Award for its outstanding contribution to the field of rotorcraft aeromechanics. Another contribution for this year's AHS conference has been accepted, but shifted to October due to the Corona pandemia. Three articles have been published in reviewed journals (2*AHS Journal, 1*CEAS Aeronautical Journal), another one accepted for publication in the AHS Journal. This year's AHS conference paper is intended to be published as an article in the AHS Journal as well, and the tree PhD candidates having worked on the project are currently busy finalizing their dissertations on the findings and insight from the CA³TCH project. The extended tool chain has been implemented at and licensed to Airbus Helicopters, the Topic Leader.
"The aerodynamic and aeroacoustic simulation of a complete compound configuration in trimmed free flight with CFD is itself already beyond state of the art, and no previous results of this detail and quality level have been known so far. The existing simulation framework developed over the last decades at IAG had to be adapted only slightly for the new comprehensive tool HOST provided by Airbus Helicopters, and the larger numbers of degrees of freedom for control with according trim laws taken into account. The IAG tool chain has reached an unprecedented capability level by now for the aerodynamic, aeromechanic and aeroacoustic analysis of any rotorcraft, based on first principles and thus being able to deliver accurate and reliable results and thus answer relevant engineering questions very early in the development process, long before hardware exists. This allows an ""right on first attempt"" approach and thus much faster time-to-market for rotorcraft development, and, in the long run, more advanced products in terms of capability, efficiency and society compliance.
Looking back, the simulations run and analyzed within CA³TCH have given interesting insight into the peculiarities of this novel compound configuration and provided the topic leader ample opportunity to add this knowledge to engineering judgement on several levels and thus complement other approaches. The capabilities of the extended framework allow for detailed investigations early on, which would otherwise sometimes necessitate flight tests and thus delay information backflow into the development process."
Flow field visualisation of Racer in fast forward flight