To meet the future requirements for aircraft engines in terms of weight reduction at ever increasing efficiencies, a test vehicle with a counter-rotating rotor in a modern construction was planned as part of the CRISP2 project, using carbon fibre reinforced polymer (CFRP) materials based on the CRISP test vehicle from the 1980s.
The focus of the development of the test vehicle was on the use of computational fluid dynamics (CFD) and other optimisation methods that have long been developed and established by DLR, as well as on the development of new manufacturing processes for the production of CFRP blades. Manufacturing a test version with slimmer blades made of lighter and more pliable materials, as well as the aerodynamic interactions in the counter-rotating rotor, were interesting from an aeroelastic perspective. The use of anisotropic materials enabled us to cross check the flutter freeness and vibration response resulting from the aerodynamic excitation of the rotors. As an added benefit, these materials also provided the possibility to use their anisotropy specifically to either improve the flutter resistance or reduce the forced response. The manufacturing process, provided by the Institute of Structures and Design, in the carbon fibre/polyether ether ketone (CF/PEEK) design, left little room for practical studies into the aeroelastic use of anisotropy (Aeroelastic Tailoring). In this process, carbon reinforced polymer sheets, consisting of prefabricated straightened individual layers, are machine-finished to the blade contour, and the anisotropy is almost completely utilised to cushion the stationary aerodynamic load. However, structural dynamic analyses have shown that, when using CFPR constructions, large deformations of lightweight structures can be expected, so the assumption of a linear elastic material behaviour can be increasingly questioned. The figure on the bottom right shows an example of the blade vibration amplitude distribution in the first modular form of the blades of the rear rotor.