For its new U.S. Army attack helicopter offering, AVX and L3 have borrowed a page from Russian rotorcraft manufacturer Kamov.
The so-called Compound Coaxial Helicopter, or CCH, is topped by two sets of four counter-rotating flexible rotors, similar to the Ka-50. Using decades of data gathered by Kamov, AVX was able to reduce the separation between the rotors and offer a lighter attack aircraft than others with rigid rotors, according to chief executive Troy Gaffey.
“The Russians have been flying coaxials for years,” Gaffey told Vertical Oct. 15 at the Association of the U.S. Army’s annual conference outside Washington, D.C. “Surprisingly, they’ve been very open with their technology. They have presented technical papers at various meetings and one of the subject matters they have investigated is the clearance between the rotors. They have developed a criteria for clearance that works.”
The AVX/L3 team is one of five competing to build the Army’s Future Attack Reconnaissance Aircraft (FARA). Unique to its design, aside from the coaxial flexible rotors, are a pair of fixed ducted fans that provide forward thrust. A pair of lift-sharing wings allow the rotors to be “lazy” in fast forward flight, Gaffey said. The companies brought a full-size mockup of the CCH aircraft to AUSA, making its public debut.
The team has tested a 15-percent scale model in a wind tunnel and is comfortable that at cruise, the aircraft should easily meet the Army’s 180-knot threshold speed requirement, Gaffey said. At max-power level flight, the CCH will be capable of 200 knots and possibly more, he said.
From the GE T901 engine, a shaft runs aft to a T-box and then runs laterally out to the fans, which have 90-degree gearboxes just like a tail rotor gearbox. The fans are always engaged during flight, though AVX debated adding a clutch, Gaffey said.
Flexible rotors also make for a lighter aircraft because the airframe does not have to be reenforced to withstand the vibration a rigid rotor causes.
Built into the AVX design is a 50 percent margin of clearance for ultimate loads “never to be reached,” he said. The “never, ever” region of flapping is about one third the total separation of the rotors. Some restrictions are placed on roll rate to avoid the rotors contacting in high-speed maneuvers.
Pitch-flap coupling in the main rotor hubs also reduce flapping. That technology was developed by Bell for the V-22, on which Gaffey worked for many years prior to launching AVX.
“So those things we’ve studied and we’ve simulated it, piloted simulation, to convince ourselves that you can do helicopter maneuvering … without danger of colliding,” Gaffey said. “In contrast to the very stiff approach, [you] probably end up with a little more spacing with our configuration. We’ve also looked at a smart control system that would allow us to move the rotors a little closer together, just to keep the height down.”
