Initiative promises more performance and less noise for general aviation, say researchers at Miami University

A University of Miami research team has successfully completed initial studies of a forward-swept propeller blade and is seeking industry support to continue full-scale tests for applications on general aviation, utility and other propeller driven aircraft.

While aft-swept blades are in use, forward-swept blades have proved more challenging to develop, despite promising better performance.

Although General Electric halted work on a NASA contra-rotating propfan project in 1994 after the blades suffered instability, GE later proved the concept's viability. In 1998 it developed a forward-swept, two-stage fan with improved efficiency and greater stall margin.

Theoretical benefits of forward- swept blades are that the tip encounters undisturbed air first, which generates greater mass flow with higher kinetic energy, and lower noise due to reduced tip turbulence intensity.

The Miami team designed the blades by leaning the leading edge forwards rather than by adding axial sweep. This approach is believed to be advantageous as it does not create additional axial space compared to a conventional radial blade.

Eight 155mm (6.1in)-diameter aluminium propellers were made for the tests: a baseline straight blade and seven with different degrees of sweep.

The swept designs include four with an aft-swept inner region and a forward-swept outer section, one with an unswept inner region and a 15° forward-swept outer section and two examples forward swept by 10° and 15°, respectively, along their entire lengths.

The four aft forward-swept blades are designated 5-10, 10-15, 15-20 and 20-30, the first number denoting the aft sweep angle of the inner section in degrees and the second denoting the forward sweep of the outer section.

Windtunnel tests and computational fluid dynamics (CFD) simulations show the swept blades are more efficient and have higher stall margins than straight blades.

The 20° forward swept blade gave the best results, but the team is uncertain why. "It is possibly because this configuration has the greatest aspect ratio, which may help reduce induced drag and downwash, or the profile drag may be cut because the velocity component and chord length normal to the leading edge are reduced," it says. "Another possible cause is the greater mass flow in the tip region."

The team acknowledges that the forward-swept designs have greater stress at the blade root, but noted no structural problems during tests. Further tests are planned on larger blades and variable pitch versions.

Source: Flight International