Compounding a helicopter with additional wings and propulsion units allows to increase the maximum speed from 250 km/h to 400 km/h. To explore this potential Airbus Helicopters is developing the RACER compound helicopter demonstrator as part of the European CleanSky2 research initiative. The DLR-Institute of Aerodynamics and Flow Technology and the french research establishment ONERA have joined their skills in the NACOR core partner project to support Airbus Helicopters in the aerodynamic and aeroacoustic design of RACER. The design responsibilities of DLR included the box-wings and the horizontal stabilizer, while ONERA designed the propellers and the vertical fins. Both research organizations, DLR and ONERA, contribute also to the noise assessment of RACER.
Little knowledge exists world-wide from previous compound helicopter designs. Therefore, the flow field about the initial configuration provided by Airbus Helicopters was analyzed in detail by means of CFD. The study revealed strong interference effects between the different components and provided an understanding of their impact on flight characteristics and shape design. In parallel the researchers from DLR-Institute of Aerodynamics and Flow Technology designed the wings. The studies aimed at optimizing planform and twist of the upper and lower wings. New airfoils with better lift characteristics were designed. The fairings at the junctions between wings, fuselage and nacelles were optimized to reduce drag. For the horizontal stabilizer a new airfoil and junction fairings were designed. In all design studies tight constraints imposed by the flight characteristics and the wing's and stabilizer’s internal structural layout had to be respected.
Aeroacoustic studies by DLR-Institute of Aerodynamics and Flow Technology aimed at identifying the dominant noise sources of RACER and to predict the noise emission for certification flight conditions. For each flight condition various helicopter trims need to be studied studied as RACER can fly in helicopter mode or airplane mode or combinations of both.
DLR’s research activities are currently concluded by simulations for the complete configuration to verify the final design and to study the flight behavior during initial flight testing. As a large number of different flights states are studied, the resulting performance data are also fed into the flight simulators at Airbus Helicopters in order to train the pilots ahead of flight testing.