Aspects of yaw control design of an aircraft with distributed electric propulsion

Distributed electric propulsion (DEP) offers new options in aircraft design. Besides the optimization of the wing, another area of optimization is the vertical tail plane (VTP) and yaw control. The large number of engines significantly relaxes the one-engine-inoperative (OEI) case during take-off, which is mostly the sizing case for the VTP. This offers the possibility to reduce the VTP and rudder size to a certain amount. Also, the dynamics of electric motors offer the possibility to use differential thrust for yaw control. This can compensate at least some of the reduced rudder effectiveness coming from the smaller VTP size. In the framework of the German nationally funded project SynergIE, different aircraft designs of a hybrid-electric regional aircraft were investigated. Three aircraft concepts with 2, 6 and 12 propellers were designed in the project, for which reasonable minimum VTP sizes were investigated. For the 12-propeller aircraft, the investigations showed that the VTP could be reduced by 50%, still allowing the compensation of OEI during take-off and the generation of sideslip angle during crosswind operations. This reduction in VTP size results in a reduction of the block fuel by about 4%. For the 12-engine aircraft, a 6-degrees-of-freedom simulation model was developed including flight control laws for yaw control using the rudder and differential thrust. Virtual flight tests were performed in a full-flight simulator. The tests generally showed a good agreement with the theoretical results from the handling quality analysis but also outlined deficiencies in aircraft handling at low speed with full flaps. The use of a flight simulator at this early stage of aircraft design has proven to be a useful tool to investigate such unconventional designs.