The owner of Karem Aircraft, which was passed over by the US Army for a spot in the joint multi-role technology demonstration (JMR-TD) programme, says the company could still contribute to the future of US military rotorcraft.
“When things are not in the public domain, I don’t go first. I let the government go first,” Abe Karem tells Flightglobal about future US government business opportunities beyond JMR. “I’m not confirming, denying or saying anything. I put $25 million after tax – that’s $50 billion in Sikorsky speak. So yes, we are fully committed to that. This is not something that will be brushed aside. I can say that for me. I am not saying anything for the government.”
Karem and another small competitor, AVX Aircraft, were beat out of JMR contracts by Bell Helicopter and a team of industry giants Boeing and Sikorsky. Karem and Bell each submitted designs for tiltrotor aircraft for the demonstration, which is seeking a vertical-list platform that can cruise at 230kt (496km/h) and hover at 6,000ft above sea level on a 35˚C (95˚F). AVX and the Boeing-Sikorsky team went with coaxial-rotor designs.
JMR is expected to feed into the future vertical lift (FVL) programme that could replace all of the army’s helicopters beginning around 2030. While Karem has not built a full-size manned aircraft, his claim to fame – beyond developing the iconic MQ-1 Predator, which he sold to General Atomics Aeronautical Systems – is inventing an optimal-speed tiltrotor with a clutch that halves blade rotation speed to improve efficiency in forward flight.
“When you are talking about a tactical aircraft flying at high density, if you don’t change the RPM, you are living with an ineffective glide ratio like the helicopters,” Karem says.
A glide ratio, also called lift-to-drag, of about 4 or 5 is about the best aerodynamic efficiency rate current helicopters can achieve. Karem’s Hummingbird UAS, developed in concert with Boeing, achieved a glide ratio of 7, he says.
“The non-tilting [optimum-speed project, meaning the A160] went through a substantial programme – the US government spent some $30 million and another $150 million between them and Boeing and we know it works,” Karem says.
In the compound configuration – which uses a rotor to produce vertical lift for take-off and hover, then off-loads some lift to the wing during forward flight – the Hummingbird would have reached a glide ratio of 11, Karem says. With a tiltrotor design, Karem claims to have achieved an efficiency rate of 18 in ground tests.
“Here you are with the lift you need for helicopter mode up to 2G manoeuvre and you need 1/18th of the lift to cruise,” he says. “Basically whatever we analyse happening on the ground happens in the air exactly.”
Achieving that performance would meet the requirements for the Defense Advanced Research Projects Agency’s (DARPA) vertical takeoff and landing (VTOL) X-plane. DARPA is seeking a 4.4t-5.5t (10,000-12,000lb) gross weight aircraft that can cruise above 300-400kt with a lift-to-drag ratio of at least 10. But Karem also was cut from that competition earlier this year when DARPA chose Aurora Flight Sciences and Boeing to progress to Phase 1B.
Helicopters are relatively inefficient in horizontal flight because the massive amount of thrust needed to takeoff vertically an hover is wasted. The relatively massive rotors needed for vertical lift create drag in forward flight and the power required to generate lift is not needed to keep the aircraft flying horizontally.
The lift-to-drag equation can be solved either by reducing the engine power in horizontal flight or somehow varying the surface area of the rotors, Karem says. But telescoping rotor blades present nightmarish engineering and safety challenges, he adds. He cited 1993 patent by Sikorsky for a tiltrotor aircraft driven by blades that extend and retract on command.
“Sikorsky has a patent on variable diameter, which they don’t want to touch, and I don’t want to touch,” Karem says. “But, unless you shrink the diameter, you must shrink the RPM.”
Source: FlightGlobal.com
