GUY NORRIS / LOS ANGELES
US team recommends further work on technology that could expand operational envelope for rotorcaft
A team of US Army, Navy and NASA researchers has successfully tested a helicopter rotor blade with a variable-droop leading edge (VDLE) that could dramatically expand the operational envelope of helicopters.
The VDLE exhibited better overall performance than a conventional blade, with reductions in drag and pitching moment coefficients of between 50% and 75% while maintaining acceptable lift performance. In addition, positive damping eliminated torsional instability in the rotor.
The team, which comprised members of the US Navy-NASA Joint Institute of Aerospace Sciences and US Army Aeroflight dynamics Directorate, has concluded that there is a "strong case" for future VDLE research.
The 250mm (9.8in)-span model tested in the Fluid Mechanics Laboratory at NASA's Ames Research Center in California indicated that a blade with variable droop reduces retreating-blade dynamic stall, as well as improving the performance of the advancing side of the disc.
The VDLE concept was developed to improve upon previous stall control methods, such as dynamically deforming the leading edge or adding a fixed leading edge slat, by controlling a larger portion of the blade leading edge in a fully dynamic way.
The drooping leading edge makes up 25% of the model's 150mm (6in) chord, and is connected to the main blade section through a hollow hinge which carries instrumentation and drive linkages. Tests were conducted on a basic unmodified aerofoil, blades with 5°, 10°, 15° and 20° fixed droop angles, and the VDLE at freestream speeds of Mach 0.2 to 0.4.
Helicopter blade stall occurs when the retreating blade is pitched to angles near and exceeding the static stall angle.
Traditional helicopters are limited in nap-of-the-earth flying and low-level urban transport because of large pitching moment variations that accompany the onset of compressible dynamic blade stall. Avoiding this part of the envelope means that helicopters have been unable to take advantage of dynamic lift, a side-effect of the strong vortex created by the dynamic stall. Effective stall control of the retreating blade could therefore increase the maximum flight speed by reducing rotor vibrations and power requirements.
Source: Flight International