Mission and aircraft design of FLEXOP unmanned flying demonstrator to test flutter suppression within visual line of sight

Flexible wing structures hold the potential to enhance the performance of future aircraft. A drawback however is their typically reduced flutter onset speed. The FLEXOP project therefore develops methods for an active suppression of flutter. To perform the risky flutter test flights required for validation, an on-purpose designed unmanned demonstrator aircraft is used. As the test campaign takes place in German airspace, flutter test flights are limited to visual line of sight range of the operator. For this uncommon approach, first a flutter test methodology was identified, a respective mission was designed and airspeeds were assigned to its segments. Looking at the mission, it appears that the aircraft design process has to focus on precise evaluation of all acceleration and deceleration procedures as well as on the achievement of low flutter onset speed, but with high fidelity in its prediction. Hence, an aircraft design toolchain was implemented that includes flutter analysis based on high detail, parameterized aeroelastic models and a mission simulation incorporating also the dynamic behavior of the main relevant aircraft systems. As this design toolchain takes only few fundamental aircraft parameters as input, it was possible to evaluate a number of different aircraft layouts for their mission performance. The resulting aircraft design yields a jet engine powered demonstrator aircraft with seven meters wingspan and 55 kg takeoff mass. The flutter onset for the highly flexible wing is predicted at 48 m/s and 8 Hz. Finally, the identified lessons learned of the applied flexible wing aircraft design process are pointed out and modifications are proposed.