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To fulfill the U.S. Army’s need for a new attack recon rotorcraft, Karem Aircraft is pitching a radically new design that literally transforms from a helicopter at hover to an airplane in speedy forward flight.
Karem, founded by MQ-1 Predator developer Abraham Karem, has long been focused on developing “optimal-speed” tiltrotor technology that improves rotor pitch and angle to improve efficiency.
Much of that work was done in concert with Army programs under the Future Vertical Lift Umbrella, like the Joint Multirole Technology Demonstration (JMR-TD). Now the company is applying that optimal-speed tech to the Future Attack Reconnaissance Aircraft (FARA) program. Karem is teamed with Northrop Grumman and Raytheon on the current $738 million design development contract for initial FARA conceptual work.
Since Karem threw its hat in the FARA ring, it was assumed the company would pitch a tiltrotor, but the Army’s requirement that the aircraft measure no more than 40 feet by 40 feet made that configuration untenable, Karem FARA program manager Thomas Berger told Vertical in an interview.
“We think optimal-speed tiltrotors are the best way to do VTOL [vertical takeoff and landing] with good forward airspeed and efficiency,” Berger said. “That said, the Army has a very particular requirement that FARA fit into a 40-foot space. They [provided] some scenarios that require them to fit into a small space. One big rotor hub of 40 feet is better that two small rotors of sub-20 feet. Long story short, the tiltrotor just doesn’t fit into the 40-foot box.”
Karem’s FARA design, called the AR-40, features three lightweight rigid rotors attached to an aerodynamic rigid rotor hub that provide “favor dynamics in a wide RPM operation range,” Berger said. Karem has extensive experience making rigid rotors of various sizes. Many members of its team, including founder Abe Karem, worked on the Airbus H160, a rigid-rotor conventional helicopter.
“We sort of applied that to the optimal-seed tiltrotor, and now we’re applying it back to another edgewise helicopter.”
Ported over from optimal-speed development work, individual blade control actuation eliminates the swashplate and optimizes each individual blade’s pitch continually during its rotation. The effect is lessened vibration at higher speeds, which in turn allow the aircraft to fly more efficiently and reduce wear and tear.
The truly unique element of the AR-40 is its tail rotor, which is on a swivel that in hover mode orients the hub to one side where it acts like a conventional helicopter tail rotor. When the aircraft transitions to speedy forward flight, the tail rotor points aft and becomes a pusher propeller.
“In the hover configuration, it looks like a conventional helicopter,” he said. “In the airplane, or high-speed configuration, it looks like a turboprop airplane except with a pusher propeller,” Berger said.
The aircraft also features tilting wings that match the main rotor’s 40-foot diameter, by far the largest among FARA competitors. Like fixed wings, rotors generate lift more efficiently the larger and longer they are, Berger said.
“If you want to fly efficiently as an airplane, you want a long wing,” he said. “A stubby wing is not nearly as efficient. . . . We wanted the most efficient wing we could get, but we still had to fit into a 40-foot box. We probably would have gone longer if that requirement wasn’t there.”
“A larger wing gives you a lot of area to provide lift and the more lift you can get on the wing at high speed, or even moderate speed, the less you have to put on the rotor. That buys you dynamics and efficiency. If I don’t carry load on the rotor, that means the blades are de-pitched and are going to move way more efficiently through the air.”
A side-by-side cockpit promotes crew coordination, widens the space behind the crew for internal electronics processing and mission equipment and weapons stores. The Army’s requirement for internal weapon stores drives the fuselage width, Berger said. Once that was determined, it made sense to go with a side-by-side cockpit that allows better crew coordination. The space made available by the side-by-side cockpit could also be used for additional passengers on certain missions, Berger said.
Karem is not yet bending metal for a prototype and has not done wind-tunnel testing on a scale model, but has completed extensive digital modeling of its design, Berger said. It is buying up long-lead time materials and parts, “anticipating a favorable downselect for us, and we expect to meet the government/Army timeline for the program.”
Karem’s AR-40 is one of five designs vying for the FARA contract. The others are the Sikorsky Raider X based on its S-97 Raider coaxial-rotor prototype, the Bell Invictus tandem conventional helicopter, the AVX/L3 Compound Coaxial Helicopter (CCH) and a design developed by Boeing Phantom Works that the company refuses to reveal.
The Army plans to choose two teams by March 2020 that will then build competitive prototypes to participate in a government-sponsored fly-off in 2023.