Karem Aircraft develops and manufactures advanced fixed-wing and rotary-wing aircraft, including manned and unmanned high-efficiency tiltrotors. Karem’s patented Optimum Speed Tiltrotor (OSTR) technology combines the fast, inexpensive, safe operation of efficient fixed-wing airplanes with the robust hover capability of helicopters. The famous “Predator” military UAV used in various US missions is a project by Abraham Karem, a Jew born in Baghdad, Iraq, and now 82 years old.
It was initially assumed the company would pitch a tiltrotor to the Future Attack Reconnaissance Aircraft (FARA) program, but the Army’s requirement that the aircraft measure no more than 40 feet by 40 feet made that configuration untenable. Karem Aircraft unveiled its vision for the U.S. Army’s Future Attack Reconnaissance Aircraft (FARA) program: a rigid main rotor helicopter with a rotating wing and a rotating tail rotor, named AR40. The AR40 aircraft has a 12.2m (40ft) wingspan – wider than the helicopter’s 11m main rotor diameter. Powered by a GE Aviation T901 engine, the aircraft has two side-by-side seats plus a four-occupant rear cabin. The AR40 has tilting wings with 12.2 meter wingspan. The wing can provide the majority of the aircraft’s lift and tilts upwards during the helicopter’s descent or ascent in order to make its vertical flight more aerodynamic.
The AR40 also has a swiveling tail rotor, which in forward flight is angled backwards to be used as a pusher propeller. The company says in forward flight the aircraft’s vertical stabiliser compensates for torque from the main rotor blades. The swiveling tail rotor should allow for the aircraft “to manoeuvre aggressively at low speeds,” says Karem.
The design also has a three-blade main rotor that uses Karem’s Optimum Speed Rotor technology, which was initially developed using US Army research funds to create optimal efficiency for tiltrotors in vertical or horizontal flight. On the AR40 helicopter, the system would be used to control each individual blade as it rotates, instead of forcing the blades to move in unison as is the case with a traditional swashplate system.
Northrop Grumman and Raytheon partnered with Karem Aircraft on the AR40. Karem is contributing its rotor and drive technologies, and is leading the design and prototyping process. Northrop is providing production and product support, as well as avionics expertise. Raytheon is the mission systems integrator and modular open systems architect.
The Optimum speed rotor [US Patent 6,007,298 of December 28, 1999] provides a variable speed rotor and a method for using the same for improving helicopter performance and efficiency while reducing fuel consumption. The efficiency of an aircraft, whether fixed wing or rotorcraft, as expressed by the fuel consumption required to achieve a specific performance as for example, cruise, climb, or maximum speed, is directly proportional to the power required to achieve such performance. The power required is inversely proportional to the ratio of the aircraft lift to the drag (L/D). In order to increase an aircraft efficiency designers strive to increase the lift to drag ratio by minimizing the aircraft drag at lift levels required to counter the aircraft weight and to allow for aircraft maneuvering.
A helicopter in a substantial forward speed (e.g., 100-200 mph) experiences problems of control, vibration and limitations in performance resulting from the asymmetry in the speeds of the advancing and retreating blades. When traveling in a forward direction, the advancing blade has a speed equal to the rotational speed of the blade plus the forward speed of the helicopter, whereas the retreating blade has a speed equal to the rotational speed of the blade minus the forward speed of the helicopter. As a result, the advancing blade has more lift than the retreating blade. To avoid helicopter roll over due the airspeed asymmetry, the lift on the retreating blade has to be increased while the speed on the advancing blade has to be decreased. Because, lift is inversely proportional to the velocity (i.e., speed) of the blade squared (V.sup.2) a substantial increase in the coefficient of lift (C.sub.L) of the retreating blade is required. Consequently, the asymmetry in speeds between the advancing and retreating blades has to be limited thereby limiting the forward speed of the helicopter.
Increasing the RPM of the rotor reduces the relative asymmetry of the airspeed distribution, thus reducing the effects of forward speed on roll control limits. But such RPM increase is constrained by the maximum allowable rotor tip speed. The maximum allowable tip speed is typically lower than the speed of sound (i.e., Mach 1) so as to avoid the substantial increases in drag, vibration and noise encountered when the tip speed approaches Mach 1.
Current helicopter rotors turn at a constant RPM throughout the flight because of the complex and severe rotor dynamics problems. Generally, helicopter designers are content if they succeed in the development of a single speed rotor, which can go from zero to design RPM when not loaded on the ground during start and stop without encountering vibration loads which overstress the helicopter and rotor structure. When the blades of a conventional rotor are producing lift, a significant change of the rotor blade RPM from the design RPM may yield catastrophic results.
Some research helicopters such as the Lockheed XH-51A compound helicopter have experimented with rotor RPM reduction at certain flight conditions by incorporating a wing for producing most of the required lift and a jet or a propeller driving engine for producing the required forward thrust. The use of the wings and engine relieve the rotor of its duty to produce lift and thrust, thus allowing the unloaded rotor to operate at reduced RPM. In this regard, a helicopter can fly at higher speeds before the tip of the advancing blade approaches the speed of sound and encounters the increased levels of vibration and noise as well as drag.
The advantage of OSR is dramatic at the lower speeds and light weight range (loiter at the end of fuel and with light payload). The reduction of 60%-70% in power required at 1400 lbs. at 40-80 knots provide an equal impact on fuel consumption. The reduction in tip Mach number (about 40%-50%) of the advancing blade may provide 10-15 dBA reduction rotor noise levels. The 15 knot increases in speed at constant power of 270 HP and the 50 knot increase at a constant power of 120 HP are dramatic and indicative of the level of inefficiency of conventional constant RPM rotors especially for a low weight helicopter loitering at low speed.
A similar power required analysis conducted for hover Out of Ground Effect (OGE) indicated that OSR offers 23% increase in take-off weight with constant engine power (may provide double the payload weight in most helicopters) and 30% reduction in tip speed (may reduce noise level 8 dBA). The reduction in power required offers 7,000 feet increase in hover OGE ceiling out of ground effect with the same engine. The exemplary OSR can reduce its angular velocity to as low as 150 RPM (tip Mach number of 0.25) or at any other interim RPM to optimize lift/drag ratio, reduce power and achieve longer endurance and range or achieve higher altitude and forward speed for the same power level.
A Optimum speed rotor system allow the helicopter rotor to be operated at an optimal angular velocity in revolutions per minute (RPM) minimizing the power required to turn the rotor and thereby resulting in helicopter performance efficiency improvements, reduction in noise, and improvements in rotor, helicopter transmission and engine life. The system and method provide for an increase in helicopter endurance and. The system and method also provide a substantial improvement in helicopter performance during take-off, hover and maneuver.
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