Eurocopter has flown an experimental BK 117 fitted with a prototype electrical blade-flap rotor control system. The system, consisting of three flap “modules” containing piezo-ceramic actuators positioned along the trailing edge of each blade, is designed to reduce the noise (“blade slap”) and vibration during a helicopter’s descent.

Program manager Valentin Kloppel told AIN that the flaps typically move between 15 and 40 times a second. “Only two of the modules are active; we do not have the power supply channels to activate all three. If we decide we need more capacity, we can power up the third at a later date.”

The AC power to control the flaps is transmitted from electronic amplifiers acting like compensators, fitted inside a small drum on top of the rotor head.

Actuation is carried out automatically by means of an optical waveguide and a controller. It neutralizes the blade slap noise associated with descending helicopters and lowers the overall noise level by up to 6 dBA (which corresponds to an impression of 50 percent less noise).

The asymmetrical impinging flow of a rotor generates vibration that can be reduced by as much as 90 percent by generating counter-vibration, in turn increasing passenger comfort and extending the service life of important components.

During the September first flight the helicopter flew in a hover; by early last month further tests to explore the critical descent mode were imminent.

“We expect the system to have a noticeable effect, and the principle should work even on models with a traditional blade-slap problem, such as two-blade Bells.

“The effect there is a little different, as the ‘slap’ is a continuous phenomenon caused by sonic shocks experienced at the blade tips, and the flaps would find it difficult to [isolate] and ‘lock on’ to a neutral state. An answer there might be to destabilize the shock. So far, we only have laboratory figures to back up this theory but they look promising.”