Blade thickness effects on mechanical performance of a high speed propeller

Abstract

Open rotor is a type of airline jet engine with two unducted contra-rotating fan stages intended for cruise speeds similar to a turbofan. Due to the lack of a duct containing the fan stages, noise generated from sources such as tip vorticies and blade interactions can propagate freely into the surroundings. Therefore noise emissions are of special concern when designing open rotor engines today. The boxprop is a high speed propeller blade concept with a design intended to reduce the induced drag and noise generated from tip vorticies. The aerodynamic properties of the boxprop, in the front rotor of an open rotor configuration, are to date relatively well understood. However the mechanical properties are not. In this thesis the structural integrity of the boxprop has been investigated. An initial assessment of the structural stability towards forced excitation were conducted with preliminary margins towards resonance adapted from an open rotor project conducted in the 80’s called GE36. Pre-stressed modal analysis were performed using finite element analysis for a range of different composite laminate material models and thickness distributions. A bilinear parameterization of the thickness distribution was used, in which thickness change in the tip and the root from a nominal distribution could be specified. Cambell diagrams were drawn were eigenfrequencies and possible excitation frequencies were plotted together to evaluate risks for resonance. The penalty in aerodynamic performance for a given change in thickness was later evaluated with the aid of computational fluid dynamics and stochastic optimization. In general a 25% increase in blade thickness resulted in an increase of natural frequencies for the first ten eigenmodes between 10 − 20%. The corresponding aerodynamic efficiency loss mounted to about 0.7%. Suggestions on alterations of the thickness distribution to fulfill the criteria towards resonance for an aerodynamically optimized baseline blade were made. A 85% increase in tip thickness and a 20% thinner root were required which resulted in an estimated loss in efficiency of 1.5%.