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Robinson Helicopter Company has introduced a new empennage for the R66 Turbine helicopter intended to reduce the likelihood of mast bumping, one of the most vexing causes of fatal crashes in the model.
The existing design of the R66, like that of the R44 and R22 before it, features an asymmetrical horizontal stabilizer on the right side of the tail, directly across from the tail rotor. The new empennage, recently certified by the Federal Aviation Administration and officially revealed on Sept. 5, replaces this surface with a fixed, symmetrical horizontal stabilizer located forward of the tail rotor.
Robinson Helicopter President Kurt Robinson said the shift to a symmetrical stabilizer reduces rolling moments, providing enhanced roll stability during high-speed flight. The stabilizer and accompanying new tail cone also have extended times between overhauls (TBOs) of 4,000 hours, compared to 2,000 hours for the existing versions.
The updated empennage will be standard on all new production, FAA-registered R66 helicopters commencing with serial number 1279, which is scheduled for delivery from the factory in the next few weeks. Robinson is also pursuing validation of the design change with other civil airworthiness authorities around the world, and will deliver foreign-registered R66s with the new empennage as regulatory approvals are obtained.
Kurt Robinson told Vertical that the modification is the culmination of extensive data analysis and flight testing and a two-year certification process. “The areas that we’re concentrating on are the edges of the flight envelope . . . to enhance the stability, and what we’ve come up with we believe provides that,” he said.
Mast bumping is a phenomenon that can occur in any helicopter with a two-bladed main rotor system, although it is often associated with Robinson models. In two-bladed rotor systems, when one blade flaps up, the other blade flaps down, much like a seesaw. If the flapping motion is excessive, the root of the down-flapping blade can contact the rotor mast, while the rest of the blade may impact the tail boom or cabin, resulting in the catastrophic break-up of the helicopter in flight.
Mast bumping can be induced through low-G maneuvers, when the weight of the helicopter’s fuselage is momentarily unloaded from the rotor disc. The most common example of such a maneuver is a cyclic pushover — an abrupt nose-over following a climb.