Eurocopter studies speedy compound helicopter concept

 - March 30, 2009, 6:21 AM

Eurocopter is working on a compound helicopter with a single main rotor, a fixed wing with two propellers in puller configuration and no tail rotor, probably in response to Sikorsky’s X2 and Bell/Agusta Aerospace’s BA609 Tiltrotor, which are attempts to create a faster rotorcraft. The Marignane, France-based helicopter manufacturer in May last year filed a patent application at the U.S. Patent and Trademark Office. Eurocopter has yet to confirm if this concept is the “new aircraft predevelopment” that CEO Lutz Bertling has mentioned a few times since January.

The patent is for “a rotorcraft [able to] perform cruising flights at high speed and also to enable long distances to be traveled.” The document uses as an example a rotorcraft for 16 passengers flown at 220 knots. Range would be 400 nm and cruise altitude 5,000 feet. The aircraft would weigh about eight metric tons (17,600 pounds). For the sake of comparison, an AS 332L1 Super Puma weighs 8.6 metric tons, can carry 19 passengers at 141 knots and has a maximum range of 529 nm.

On Eurocopter’s proposed compound, the total span of the wing is said to be “substantially equal to the radius of the rotor,” which is 26 feet. The chord is set at five feet, resulting in an aspect ratio of about 5.3. The wing can be fitted with ailerons.

The preferred location for the two propellers is near the tips of the wing. In the example, each propeller has a diameter of 8.5 feet. One propeller can offset the torque effect of the main rotor, eliminating the need for a tail rotor.

The principle behind a compound is to spread the lift between the main rotor and the wing. This reduces the rotor’s load, which is the limiting factor when it comes to increasing speed. Therefore, a compound can fly faster.

In Eurocopter’s project, the rotation speed of the rotor is reduced above 125 knots forward speed. The airspeed of the rotor blade tips would be kept at or below Mach 0.85. At 220 knots, the tip speed would be 560 feet per second. The wing would then account for 31 percent of the total lift.