This research is currently studying the performance of forward swept propeller blades by comparing them to a baseline straight blade. The forward sweep of all propellers was created by leaning the leading edge in the direction of rotation while remaining in the rotating plane. The projection of the leading edge on the axial-radial plane is a straight line with a constant axial position across the entire blade span. Hence, the blade has no sweep in the axial direction as described in the aerodynamic design method of this paper. Two forward swept blades, 10-15 backward-forward and 20-Forward, have been studied in detail.
The preliminary wind tunnel tests show that the swept blades have higher efficiencies and larger stall margins than those of the straight blades. CFD simulations of the 3D propeller blades also confirm the same trend. The 20-Forward swept blade has the highest efficiencies and stall margin of all swept configurations studied. The CFD results show that the forward swept blades slow down the decreasing angle of attack and therefore have a larger range of the favorable angle of attack. At the same freestream velocities, the swept blade has a higher angle of attack and yields higher thrust and efficiency.
There are three suspected reasons why the swept blades have higher efficiencies and larger stall margins. First, the effective aspect ratio, as defined in this paper, is slightly larger. The AR of the 20-Forward is 1.7% larger than that of the straight blade. This slightly larger AR may help to reduce the induced drag and downwash. Secondly, according to the swept wing theory, the velocity component and chord length normal to the leading edge are reduced in the swept blades. These two factors contribute to reduce the profile drag of the airfoil. Finally, it is speculated that an important factor is the 3D effect created by the forward sweep, which pulls more mass flow in the tip region. This causes the tip flow to have more kinetic energy and suppresses the tip vortex. The weakened tip vortex will reduce the downwash and drag, thus maintaining an angle of attack closer to that desired over a wider range of freestream velocities.
Further wind tunnel tests will be conducted to exclude uncertainties related to the omission of the nacelle drag. Additional analysis of the CFD results will be conducted to fully understand the mechanism that causes the 3D effect of forward swept blades to improve efficiency and stall margin. An effort will be made, through CFD and wind tunnel tests, to determine the optimum sweep angle and configuration. Finally, new wind tunnel tests and CFD simulations will be carried out on full-size propellers to determine the effect of scaling.