Propeller Slip-Stream Model in Subsonic Linearized Potential Flow

Abstract

A model has been developed for computation of the time-averaged subsonic flowfield over a nacelle and a wing induced by a propeller. The slip-stream model is based on a classical propeller theory and is included in an existing panel program. The geometry of the slip stream is determined by the nacelle. The influence of the propeller is given by a combined momentum-blade element theory. No experimental data is necessary. The computed pressures and velocities are compared to wind-tunnel data for two angles of attack and two geometries: 1) an axisymmetric nacelle and wing, and 2) a nonaxisymmetric nacelle and wing. HE prediction of the influence of propeller slip stream on the flow is important in the design phase of a new propeller-driven aircraft. The flow pattern over the nacelle and wing is affected considerably by a tractor propeller mounted on the wing, particularly at take-off conditions. Panel methods are a standard tool for aerodynamical computations around three-dimensional configurations at subsonic speeds, see Ref. 1. Usually panel programs are written to solve potential flow problems but they can be amended to also handle the vortical flow behind a rotating propeller. The advantage of panel methods is that they are easy to use and inexpensive in terms of CPU time. When a propeller slip-stream capability is added to a panel program the extension should also have these two properties. The flow in the slip stream is much more complicated than ordinary freestream flow and simplifications in the computational model are necessary. Therefore, we cannot expect to obtain the same accuracy in the predictions with the slip stream as we are used to without the slip stream. This is also true for wind-tunnel experiments. In this article we describe a propeller slip-stream model which has been incorporated in an existing panel program.2 The time-averaged flow behind the propeller is generated by a system of vortices following a classical propeller theory.3 The strength of the vortices is determined by a combined momentum-blade element theory. The propeller data needed in the simulation are the number of blades, the geometry of the blades, the speed of revolution, etc. No supporting windtunnel experiment is necessary. The geometry of the slip stream is approximated taking the surface of the nacelle into account. This is important in order to obtain realistic pressures on the nacelle. The velocities in the slip stream determined by the model are introduced together with the freestream as onset flow in the panel method. The computed pressures and velocities are compared to wind-tunnel measurements from the FFA.4'5 The configuration geometries are an axisymmetric nacelle with a wing5 and a nonaxisymmetric nacelle with a wing.4 The nonaxisymmetric nacelle is such that the shape of the innermost sections of the slip stream is changed by the presence of the nacelle. The Mach number is 0.15 and the angle-of-attack a is 0 or 5 deg.