Aerodynamic and Aeroacoustic Interaction Effects for Tip-Mounted Propellers

Aerodynamic and Aeroacoustic Interaction Effects for Tip-Mounted Propellers: An Experimental Study

Sinnige, T. (TU Delft Flight Performance and Propulsion)

Veldhuis, L.L.M. (promotor)
Eitelberg, G. (promotor)

Delft University of Technology

2018-09-25

Propellers can enable a significant reduction in energy use of future aircraft by offering a higher propulsive efficiency than turbofan engines. This is especially relevant for a new generation of (hybrid-)electric aircraft. However, the integration of propellers with the airframe remains a challenge, and leads to performance and noise penalties. Yet, by optimally integrating the propellers with the airframe, these penalties can be minimized or even converted into significant performance benefits. A key example of a potentially beneficial integration approach is the tip-mounted propeller. This thesis provides an experimental analysis of the aerodynamic and aeroacoustic interactions and potential performance-enhancement strategies for such propellers. The unique experimental results highlight that tip-mounted propellers provide a significant efficiency benefit due to tip-vortex attenuation and swirl recovery. For the tractor-propeller configuration, this led to a measured 15% reduction in drag at typical cruise conditions when compared to a conventional propeller–wing configuration. For a vehicle with co-rotating propellers, i.e. propellers with equal rotation direction on both sides of the aircraft, the tip-vortex interaction would cause asymmetric aerodynamic loading. This was alleviated by installing swirl-recovery vanes, which reduce the swirl in the propeller slipstream before its interaction with the downstream aerodynamic surface. Besides the time-averaged effects, unfavorable unsteady loads occur on the downstream surface immersed in the propeller slipstream, possibly leading to structure-borne noise. These unsteady loads were shown to be dominated by the periodic impingement of the propeller-blade tip vortices, and were reduced by installing a flow-permeable leading edge. For pusher-propeller configurations, the inflow to the propeller is nonuniform due to the momentum deficit in the wake of the support pylon or wing positioned upstream of the propeller. The resulting wake encounter causes unsteady propeller-blade loads, which resulted in a noise penalty of up to 24 dB. The deficit in the wake was reduced by using a blowing system, installed in the trailing edge of a pylon model. Measurements showed that this alleviated the effects due to the wake encounter, resulting in noise levels comparable to those emitted by the isolated propeller. The results presented in this thesis emphasize the sensitivity of the aerodynamic and aeroacoustic performance of installed tip-mounted propeller propulsion systems to interactions between the propeller and the airframe. It is shown that significant integration benefits can be obtained by exploiting the beneficial interactions, while both active and passive control techniques are available to mitigate the adverse interactions. The knowledge gained from the research study discussed in this thesis can be used to advantage in the design of future highly efficient aircraft.

Propeller–airframe interactions
propeller aeroacoustics
propeller aerodynamics
propeller slipstream characteristics
tip-mounted propellers
wind-tunnel testing

https://doi.org/10.4233/uuid:214e1e9a-c53e-47c7-a12c-b1eb3ec8293b

978-94-9301-446-6

Institutional Repository

doctoral thesis

© 2018 T. Sinnige