Engineering models of aircraft propellers at incidence

QA Mathematics;QC Physics;TA Engineering (General). Civil engineering (General);TL Motor vehicles. Aeronautics. Astronautics

Smith, Harry Redgrave ; Gillies, E.A.

University:University of Glasgow

Department:School of Engineering

关键词: aircraft propellers incidence installation induced velocity blade element model normal force;   

Others  :  http://theses.gla.ac.uk/6799/1/2015smithhrPhD.pdf

来源: University of Glasgow

【 摘 要 】

Aircraft propellers in any flight condition other than pure axial flight are subject to an incident flowfield that gives rise to time-varying forces. Means of modelling these time-dependent forces have been presented in the literature, to varying degrees of success but a review of the different models is missing, and there is a need for an instructive means of simulation using physically realistic but computationally light methodologies. This dissertation provides a comprehensive overview of the relevant work to date, in addition to providing a logical framework in which the problem of propeller blade cyclic load variation may be assessed. Through this framework, the importance of different aerodynamic features pertinent to this problem are compared, and a new solution methodology based on adaptations of existing models is presented. This research project was commissioned by Dowty Propellers (DP), who chose Glasgow University and the supervisors for their rotorcraft simulation experience. Prediction of the propeller induced flowfield is shown to be of importance for the calculation of blade cyclic loads. Momentum models are fit for purpose owing to relative computational simplicity - this dissertation suggests a new radially-weighted implementation of momentum theory that provides better correlation with wind tunnel data than existing models.Swept propeller blades are discussed and the inherent problems faced by a designer or performance engineer are highlighted. An Euler transform to resolve velocities and forces between disc and blade element axes is presented, along with the assertion that ‘simple’ sweep correction methods can be deleterious to propeller aerodynamic simulation if used naïvely. Fundamentally, representation of a swept propeller blade by a blade element model is described as wholly more problematic than a straight propeller blade owing to the displacement of blade elements with respect to the blade pitch change axis - and that flow information will always be lost with such a representation.Installation effects are simulated and installed load fluctuations are predicted to a reasonable degree of accuracy compared to what little data is available. Different means of resolving installation velocities to disc and, subsequently, blade element axes are compared, and it is shown that representing installation effects by an effective incidence angle as is ‘standard practice’ will most likely underpredict installed load fluctuation. In addition to a varying blade root bending load caused directly by load fluctuation on a propeller at an angle of incidence, the reacted net loads at a propeller hub may include a constant yawing moment and in-plane force. This in-plane force has been well documented in the literature, but the equations for its calculation may miss a component of force due to a tilting of the blade tangential force. New equations for this additional force term are presented that validate well to legacy experimental data.