Nonlinear Large Angle Solutions of the Blade Element Momentum Theory Propeller Equations

Propeller blade element momentum theory is a first-order method commonly used to analyze propeller performance. Blade element theory discretizes the rotor, analyzes aerodynamic forces acting on each element, and requires only a rudimentary description of the blade geometry. Blade element theory alone lacks the ability of predicting the propeller-induced inflow velocity needed to complete the flowfield description. The flow model is completed using concepts from momentum theory, which assumes a single continuous axisymmetric flow-through rotor disk. The traditional method used to solve the blade element momentum equations assumes a small local angle of attack at all sections along the blade and that local induced drag negligibly reduces the local propeller thrust coefficient. These assumptions, while allowing a closed form solution to be obtained, are known to be inaccurate at high advance ratios and along the inner half-span of the blade. An alternative nonlinear, numerical solution method thatavoidstheseinaccuratesimplifyingassumptionsispresented.Solutionmethodsarecomparedformultiplepitch anglesandadvanceratios.Solutionsarecomparedwiththrustandpowercoefficientdatacollectedfromwind-tunnel testsofsmallradio-controlaircraftpropellers.Thenonlineartheorycorrectionsbetterrepresentmeasuredpropeller performance, especially at high advance ratios.