DIRECT COMPARISON OF FSI OPTIMIZED THEODORSEN AND LARRABEE PROPELLERS

The goal of this study is the analysis of the design process of aircraft propellers which are coupled to a piston engine, aiming to find the best design approach. The first design step is the calculation of the initial geometry. This phase is particularly critical since it will affect the following optimization. Several theories for blade design have been proposed during the years. The most popular are the Larrabee's procedure and the Theodorsen's theory. The Larrabee theory is the most used in recent years, while the Theodorsen was most popular in the WWII era. This work focuses on the differences on the results of the two approaches for a general aviation propeller for light aircrafts. For this aerial vehicle category both the strength and efficiency should be considered, since the production technology cannot be as refined as for larger propellers. As it will be seen, the subsonic nature of these aerial vehicles makes it possible to use both initial design approaches. In a second phase, the evaluation of the effect of aerodynamics and centrifugal loads requires the union of the results that come from CFD (Computational Fluid Dynamics) and the ones come from the CSM (Computational Structural Mechanics), through the execution of several one way FSI (Fluid Structure Interaction) analyses. However the starting point proved to be critical for the final result. The Larrabee procedure proves to be ideal for high speed aircraft propellers manufactured with up-to-date materials and procedures. The “old” Theodorsen theory leads to a stronger blade that can be easily manufactured with wood or simplified technologies. The Theodorsen blade is superior for the centrifugal load bearing capacity. This geometry leads to lighter blades. The efficiency of the Larrabee blade seems to be superior. However, experience proved that the CFD analysis can be tricky and unreliable for efficiency evaluation. The pressures are better distributed along the Larrabee’s blade with better results at high airspeed. Eventually two geometrically optimized blades have been designed, which have a deformed shape (at cruise conditions) similar to the best aerodynamic geometry and comparable technological characteristics. The Larrabee and Theodorsen designs lead to different optimized blades even after the FSI simulation, demonstrating that the optimization procedure is largely influenced by the initial propeller blade design.