Challenge

Pipistrel’s engineers faced the challenge of designing a propeller driven by a hybrid-electric propulsion system while taking into account the different conditions an aircraft encounters during the four phases of flight: takeoff, climb, cruise, and descent.
Considering that the requirements for speed, power, and thrust change during flight, the objective was to maximize takeoff thrust and power recovery during descent and minimize power use during the climb and cruise phases. The optimization involved three stages: the preliminary optimization of the propeller, optimization of the airfoil, and the final optimization of the propeller.

Solution

For this multi-phase optimization project, Rok Lapuh and David Eržen, aerodynamics engineers at Pipistrel, used modeFRONTIER coupled with CHARM (Comprehensive Hierarchical Aeromechanics Rotorcraft Model) and XFOIL, an interactive program for designing and analyzing subsonic isolated airfoils. By taking advantage of ESTECO’s process automation technology, Pipistrel could automate the simulation workflows, evaluate thousands of designs simultaneously and identify innovative optimized results. This process was conducted completely autonomously leaving Pipistrel’s engineers only with the task of selecting the most appropriate design.
In the first propeller optimization, Pipistrel optimized the chord and torque distribution to achieve maximum thrust and minimum power for a given set of airfoils. These results were then used as the requirements for the airfoil optimization. The design team used modeFRONTIER to design the airfoil based on specific geometric constraints (thickness, curvature, or leading-edge radius), while increasing lift and reducing drag. They initiated a Design-of- Experiments phase and then used the HYBRID genetic algorithm to successfully perform the airfoil optimization and obtain the Pareto front with the optimal designs. Lastly, they used the optimal airfoil for the final propeller optimization. With the ESTECO optimization algorithms, Pipistrel’s engineers were able to evaluate nearly five thousand designs in a limited time and increase thrust by 30% during takeoff.

Benefits

Before using modeFRONTIER, Pipistrel used a manual process to simulate multiple designs and choose the preferred one. After introducing the ESTECO technology, Pipistrel’s engineers were not only able to automate this process but could also evaluate options that would otherwise not have been considered. "modeFRONTIER optimization technology gave me the opportunity to think outside the box," said Rok Lapuh, Aerodynamics Engineer at Pipistrel, “We could find a design that is completely different to what we are used to, but that may work even better.” The company has also dramatically reduced its time to market as it has moved from the simulation directly to production. “We trust the results we get with modeFRONTIER so much that we do not expect to require a prototype,” stated David Eržen, Aerodynamics Engineer at Pipistrel, "We will go straight into production."