Conventional propellers designed for take-off and cruising flight usually generate significant operating noise when operating in off-design conditions. Examples for such off-design conditions are static operation with advance ratio zero, operation with zero thrust or reverse thrust.
Main objectives of the program were to determine the sources for propeller noise for these operating conditions and to find means (design methods) to avoid or to minimise the contribution of these sources to the overall propeller noise without having a significant negative effect on performance. Noise sources were sought at the propeller itself as well as in the condition of the propeller inflow.
The methods and procedures should then be demonstrated in a full size propeller, tested in a propeller/engine test stand.
Two research paths were followed:
1. Propeller design
2. Propeller inflow and interference effects
For research path 1, a design chain for the design of propellers was set up that consists of a propeller pre-design tool, CAD converter, loads generator, CFD grid generator and CFD solver. This tool chain permitted a fast design and calculation of propellers, including loads and performance calculations and generation of CAD design drawings. Propeller noise and performance was measured at full scale and model propellers. Noise was measured globally at different locations around the propeller as well as locally on the propeller blade with the help of an acoustic reflector mirror to help determine the sources and frequency content of the noise.
Part of the noise of propellers comes from unsteady inflow to the propeller. Therefore, a second research path investigated the interference effects between body/fuselage wake and propeller. The target of this second path was to design a propeller whose noise sensitivity to such unsteady inflow as might be encountered in the wake is reduced.
In the course of the research project, sources for propeller noise for these operating conditions could be determined and means were found to avoid or to minimise the contribution of these sources to the overall propeller noise.
During the research project a significant amount of measured data was collected that would be available for further research.