In a hybrid-electric aircraft, energy management plays a crucial role in aircraft performance. All mission stages (from take-off, climb, cruise and even the descent) have to be oriented towards energy consumption minimization. Therefore, the propeller design should not be focused on a single operating point, but on combination of all operating points through the mission, i.e. maximizing take-off thrust, minimizing cruise power, maximizing recuperation power at the descent. The latter comes as an advantage in the electric driven aircraft with a battery storage system since some aircraft potential energy at cruise altitude can be transformed to electrical energy during the descent.
The propeller design is therefore subjected to a multi-objective optimization that yields design candidates from the Pareto front, from where user can choose a propeller with suitable characteristics. The propeller blade is defined with chord and twist distribution with a predefined set of airfoils at discrete locations along the propeller blade. During the optimization, requirements for a local airfoil (performance at specific lifting coefficient, Reynolds number) may change. Therefore, an optimization sub-process may be activated to derive an airfoil complying to new requirements.
A custom propeller was designed for a prototype hybrid-electric aircraft developed in the MAHEPA project. The optimization of the propeller was performed in three stages. First, the propeller was designed and optimized with airfoils selected from a database. Multiple propeller properties, like twist and chord distribution, were varied to maximize take-off thrust and recuperation power, keeping climb and cruise thrust values at required values. Then, the airfoil requirements were identified and used in the airfoil optimization. It starts with a large DOE compiled from the selection of industry-standard airfoils and continues with hybrid GA-gradient optimization. At last, the optimum airfoils were used in the propeller optimization again, to obtain the final propeller design. In the first and third stage a 10 % and 18 % increase in take-off thrust was observed, respectively.