Karem TR36XP DARPA VTOL X-Plane
The TR36 is named as such because it has two 36 foot diameter side-by-side rotors. Karem announced that its JMR offer would be a TR36TD design featuring OSTR and would provide a “leap ahead” in terms of vertical lift. It would be powered with twin 36-foot-diameter variable-speed rotors and existing turboshaft engines. The speed would be around 360 knots (414 mph) – fast compared to the 280 kts predicted by Bell for its V-280 and a leap ahead of the 230 kts predicted by AVX and Sikorsky/Boeing for their coaxial.
Despite decades of attempts, successful VTOL transport remains elusive due to challenging technical obstacles. Among other things, prior art VTOL is prohibitively expensive to operate, has low flight speeds, limited ranges, is relatively fuel inefficient, and has a relatively poor safety record.
The majority of prior art rotorcraft and prior art vertical takeoff aircraft are conventional helicopters. Conventional helicopters, such as the modern Sikorsky S-92, are severely limited in terms of cruise speed and efficiency. A conventional helicopter is lifted and propelled by the same predominantly horizontal rotor or rotor, one side of which advances into the oncoming flow, and one side of which retreats away from it. During cruise, the airspeed towards the tip the advancing rotor blade is much higher than that of the helicopter itself. It is possible for the flow near the tip blade to achieve or exceed the speed of sound, and thus produce vastly increased drag and vibration. This limits the forward speed of helicopters. Additionally, a rotor is an inefficient way to generate lift as compared to a wing, partially due to the dis-symmetry of lift between advancing and retreating sides of the rotor.
A major step forward in the prior art was the tiltrotor configuration including, for example, the Bell/Augusta BA609. Tilt-rotors represent a major step forward because they generate most or all of the lift necessary for cruise flight with a wing instead of rotors, which is considerably more efficient than rotor borne flight. Prior art tilt-rotors have had short, low-aspect ratio wings that were relatively thick because they had to support heavy rotor systems, which results in lower efficiency, L/D, as compared to fixed-wing aircraft.
Despite marginal increases in speed and forward flight efficiency of tilt-rotor aircraft relative to helicopters, the prior art tilt-rotors have failed to improve on the productivity (how fast one transports a payload) of conventional helicopters. This is because the ability of a modern tiltrotor to cruise up to 50% faster than a modern helicopter is almost entirely offset by its relatively higher empty weight fraction (aircraft empty weight divided by maximum hover takeoff weight, typically around 0.60-0.65 for prior art tilt-rotor) as compared to conventional helicopters.
The phenomena of "flutter" and of "whirl flutter" are characterized by limit cycle vibration or by diverging vibration that can lead to breaking mechanical parts or structural elements. It is therefore essential to take these phenomena into account in the design of an aircraft in order to ensure that the critical speeds (forward speed, speed of rotation of the rotor) lie outside the limits of the flight envelope. In particular, with whirl flutter, manufacturers ensure that rotor vibration modes do not couple with carrier structure vibration modes, thereby ensuring that those two assemblies are mutually compatible. In general, this can be done by appropriately placing the resonance frequencies and the respective dampers of the vibration modes of the various assemblies.
Prior tiltrotor transport designs featured a smaller diameter conventional rotor to minimize the rotor system weight. This would result in higher rotor disc loading in hover, and constrained the wing to be short and thick to prevent the whirl flutter aeroelastic instability resulting from heavy conventional rotors at higher speeds. Lightweight rigid rotors can delay the onset of whirl flutter to a higher flight speed than a conventional rotor system.
The TR36TD is a an Optimum Speed Tilt Rotor (OSTR) with a long-span, high aspect-ratio wing and small tail for low drag in aeroplane mode. The outer section of the wing is attached to and tilts with the nacelle to reduce downwash from the large wing in hover. In-flight rotor speed ratios between the minimum rotational speed and maximum rotational speed of at most 80%, 60%, or even 40% are contemplated.
Prior art tiltrotor wing designs, the Bell V-22 and Bell BA609, were thick (23% thickness ratio) and of low aspect ratio, 5.5, to maintain a lower weight while remaining aero-structurally stable to Mach 0.45. As a result, these prior art designs were unable to achieve high cruise efficiency. By contrast, highly efficient transonic wings for a transport aircraft have high aspect ratio and low thickness ratio, with an appropriate degree of wing sweep. The combination of very low RPM in airplane mode and the low blade mass virtually eliminates whirl mode flutter and therefore allows for higher efficiency wings (larger span and narrower chord), thereby further increasing endurance, range, altitude and reducing fuel consumption and noise levels.
For hingeless or bearingless rotor types there are no flap or lag hinges, and a rotor can transmit some bending moments to the rotor mast, which supports the hub and rotor. In a hingeless rotor hub system, a rotor blade is connected to a pitch bearing and housing that attaches to a hub rotating mast. Helicopters usually operate at a rotor RPM much lower than that of a propeller; in either case the product of diameter and RPM is usually such that the tip approaches sonic speeds at some flight conditions. The ability to transfer bending moments necessitates a stiffer rotor to mast connection, which must be accommodated by the rotor hub. Most often, the rotor hub transfers the axial and bending loads down a small diameter mast structure into an internal frame structure or gearbox, and that structure is in turn mounted to the airframe.
Most hingeless rotor hubs are used in helicopter rotor applications where the total angular travel of blade feather is less than 40 degrees total. In this manner, many of these applications are able to use flexible elements for the feathering joint. Currently the rotorcraft industry trends toward hingeless rotor designs with lower total part count and fewer moving parts. Elastomeric bearing elements and flexible beam elements are common.
The rotors can operate in cruise flight at a rotational speed of at least 25% or even 40% slower than that used for hover flight, and the aircraft has a high aspect ratio wing, preferably between 10 and 22. In still more preferred embodiments the fuselage is sized and dimensioned to carry at least 20,000 pounds of payload, and the wing is sized and dimensioned to have a maximum wing loading of between 60 and 140 pounds per square foot. Especially preferred aircraft can cruise at speeds greater than Mach 0.5.
While all suitable wings are contemplated, preferred wings are relatively thin and unswept. For example, suitable wings could have a wing airfoil at mid span with a thickness ratio of between 19% and 22%, and could have a leading edge sweep angle of less than 15 degrees. It is contemplated that a single wing could carry first and second tilting rotors.
Rotors have blades that are preferably long and stiff, with a quarter-span thickness ratio of less than 22%, and having size and dimensions to hover the aircraft with a rotor disc loading of at most 60 pounds per square foot at a maximum hover weight. Rotors can advantageously be equipped with a spinner having a shaped region to slow local airflow in the vicinity of the blades. A preferred nacelle could have a spinner with a concave region with a diameter that is reduced by 3%, 5%, 10% or even more from a maximum spinner diameter. The aircraft's fuselage is relatively small in comparison with the wing, for example having a frontal area that is between 10 and 16 percent of the planform area of the wing. Viewed from another aspect, the fuselage is sufficiently small that the aircraft has an empty weight that is at most 60% of the aircraft maximum hover weight. Preferred aircraft also have a relatively small empennage, where the wetted area of the empennage is between 14 and 40 percent of the wetted area of the wing.
All suitable power plants, gearboxes, and drive trains are contemplated. Especially preferred aircraft have a turbine engine sized and dimensioned to provide sufficient power to hover the aircraft, and also overcome an aerodynamic drag of the aircraft even in high-speed cruise flight mode. Preferred aircraft also have a fuel capacity sufficient to provide a maximum range of at least 1,000 miles while carrying its maximum payload.
Abe Karem-owned Frontier Aircraft was purchased by Boeing in May 2004. Frontier and its founder, Abe Karem, were known in the UAV field for innovation, along with rapid design and prototyping of aircraft. Boeing continued a contract that Frontier had from the Defense Advanced Research Projects Agency to develop the A160 Hummingbird. Among Karem's achievements ware designing the Predator unmanned aerial vehicle. He also served as an officer in the Israeli Air Force. At Israel Aircraft Industries he oversaw projects such as upgrades to the Super Mystere fighter before immigrating to the United States in 1977.
On February 29th, 2008 Lockheed Martin and Karem Aircraft Incorporated signed a teaming agreement to jointly develop Karem Aircraft’s Optimum Speed Tilt-Rotor (OSTR) design. The OSTR concept is one of three approaches selected by the Department of Defense (DoD)'s Joint Heavy Lift program office to receive a Concept Design and Analysis extension contract. Abe Karem, president of Karem Aircraft, said, “Teaming allows us to provide our DoD customer with a highly innovative Joint Heavy Lift solution that leverages Karem’s innovative OSTR aircraft design and Lockheed Martin’s expertise in system integration, advanced composite design and tactical transport aircraft.” Lockheed Martin’s effort will be led by Lockheed Martin Aeronautics’ Advanced Development Programs organization (The Skunk Works®). With offices in Lake Forest, Calif., and Fort Worth, Tex., Karem Aircraft is a developer of advanced tilt-rotor transport aircraft. Led by Abe Karem, this aircraft rapid development team continues the tradition of innovation of his previous teams, which developed the Amber and Gnat 750 (of which the General Atomics Predator is a derivative), unmanned aerial vehicle (UAV) systems, as well as the Hummingbird A160 Optimum-Speed Rotor UAV, now a Boeing product.
With offices in Lake Forest, California, Karem Aircraft is led by renowned aircraft designer, Abe Karem, widely recognized for his innovations in manned tactical fighter jets, aircraft retrofits and mission-conversions, and fixed- and rotary-wing unmanned aerial systems (UASs). Abe’s previous companies developed the Amber, the Gnat 750 (of which the General Atomics Predator-A is a derivative), and the A160 Hummingbird Optimum Speed Rotor (OSR) helicopter (now a Boeing product).
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