Designing an UAV propulsion system for dedicated acceleration and deceleration requirements

The FLEXOP Project aims to develop new methods and tools to assist the design of aircraft with highly flexible wing structures. Flight and especially flutter testing with an unmanned flying demonstrator shall help to validate and verify these approaches. Though, regulations for the operation of UAV in German airspace introduce additional challenges to the aircraft design: Since maneuvers have to be performed well below the airspeed of flutter-onset, the delimitation of the flight testing to visual line of sight increases significantly the demands on acceleration and deceleration of the vehicle. Following, the requirements for the propulsion and brake system of this demonstrator differ noticeably from classic aircraft design problems. Additionally, for budget and schedule reasons, only off-the-shelf solutions should be implemented for the propulsion system. In the following, an alternative evaluation and optimization approach was implemented by a variation of propulsion and deceleration principles, incorporating a dynamic simulation of the test flight mission. The optimization focused on sufficient acceleration performance while minimizing total system costs, as well as system mass (including mission fuel weight). Evaluating the results, a single micro turbojet engine in combination with fuselage mounted airbrakes is proven to be the lightest, low-cost solution, despite the characteristic slow throttle response and low fuel efficiency. Subsequently, propulsion system components are designed in detail, integrated and tested. The results are used to update the mission simulation model to verify and validate the propulsion and deceleration system selection.