Propeller Performance at Low Advance Ratio

Abstract

At low advance ratios that are much smaller than the advance ratio where the maximum efficiency of the propeller is obtained, large portions of the blades' cross sections operate at stall conditions. Using two-dimensional nonrotating airfoil data to calculate the propeller's aerodynamic performance at low advance ratios results in large differences between the calculated and measured performance. It turns out that because of rotation the stall characteristics of the airfoil are changed because Coriolis effects delay the boundary-layer separation. Based on previous investigations associated with wind turbines that have been reported in the literature, a simple correction model is presented. It is a straightforward matter to implement this model in existing strip models. The agreement between the calculated and measured results, at low advance ratios, is significantly improved after the introduction of the new model. ROPELLERS are usually designed to operate optimally at a cer- tain design point, for example, the aircraft's cruise conditions. It is clear that propellers also operate at off-design flight conditions that can include takeoff, steep climb, etc. At off-design flight condi- tions, the efficiency of the propeller drops significantly. To avoid an operation at low efficiencies, propellers are often equipped with vari- able pitch mechanisms. Yet, there are many aircraft that do not have av ariable pitch mechanism because of price, weight, or reliability considerations. These included unmanned aerial vehicles (UAVs), ultralight, and low-priced general aviation aircraft. A fixed-pitch propeller that operates at flight speeds that are much lower than the cruise speeds, namely low advance ratios, suffers from a sharp drop in its efficiency. At low advance ratios, large portions of the blades are operating at stall conditions. Most of the literature that deals with propellers' performance does not include results for low advance ratios. Evans and Liner 1 present experimental results for propellers at low advance ratios, but they do not present calculated results for these cases. Similar results are also presented in Yaggy and Rogallo. 2 An attempt to use classical methods to calculate the performance of propellers at low advance ratios (advance ratios that are much lower than the advance ratio where maximum efficiency is obtained) exhibits large differ- ences between the calculated and measured thrust, while the same methods exhibit excellent agreement at higher advance ratios. The differences between the calculations and measurements increase as larger portions of the blades experience high cross-sectional an- gles of attack, beyond the stall limit. Thus, it becomes clear that there are problems in modeling the stall characteristics of cross sec- tions of propellers' blades. It turns out that the measured propeller's thrust is higher (sometimes much higher) than the thrust predicted by calculations where the stall characteristics of a two-dimensional nonrotating airfoil are used. Himmelskamp 3 was probably the first to investigate the influence of rotation on the stall characteristics of a rotating airfoil. He ob- served lift coefficients as high as three near the hub of a rotating fan blade. Although it seems to the authors that Himmelskamp's results were not applied in previous analyses of propellers, they were widely used, especially during the last 15 years, in the aerodynamic analysis