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Optimum Speed Tilt Rotor OSTR

On 19 September 2005 the US Army, in cooperation with its Joint Service and NASA partners, announced the award of five agreements/contracts for the Concept Design and Analysis (CDA) of a Vertical Takeoff and Landing (VTOL) Joint Heavy Lift (JHL) rotorcraft.

Abe Karem-owned Frontier Aircraft was awarded a contract for the Optimum Speed Tilt Rotor [OSTR], with a maximum speed of 310 knots. Frontier Aircraft was formed after Karem sold Frontier Systems to Boeing. Boeing acquired the rights to Karem's optimum-speed rotor technology used for unmanned aircraft, such as the A-160 Hummingbird. But Karem is free to apply the concept to manned vehicles.

Frontier 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.

The baseline design specification was to maneuver an FCS/Stryker/LAV Vehicle over a 250 nautical mile (nm) radius, under 4000 foot density altitude and 950 Fahrenheit (4k95) conditions, from/to land or sea bases and operating areas. Eight specific excursions to these conditions will also be investigated that include lighter and heavier cargo (16 - 26 tons), shorter and longer mission radii (210 - 500 nm), more extreme environmental conditions (6k95), and full compatibility with a future ship. These design variations populate the desired trade space in the joint requirements process.

Karem Optimum Speed Tilt Rotor Technology OSTR Performance

  • Optimum Speed technology for enhanced performance
  • More cruise efficient than existing cargo aircraft
  • Affordable to develop, acquire, and operate
  • Capable of carrying M2 Bradley [25 tons] and Stryker [19 tons] payloads
  • High survivability for future conflicts
  • High-speed cruise capability - Mach 0.65+ / 330+ knots
  • 45,000 foot cruise ceiling, pressurized cabin
  • Flexible high capacity aerial tanker - land or sea based refueling of helicopters, bombers, and loaded fighters
  • Hover out of ground effect in high/hot landing zones - at mission weights with one engine inoperative
  • 36 ton payload capacity
  • Deploy with troops over global distances in 24 hours
  • Transport armor over large radii in high/hot conditions

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 8, the advancing blade has a speed equal the rotational speed of the blade plus the forward speed of the helicopter, whereas the retreating blade has a speed equal 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 (V2) a substantial increase in the coefficient of lift (CL) of the retreating blade is required. The available lift coefficient for a given blade is limited. 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.

A variable speed helicopter tilt rotor system and method for operating such a system would 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 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 range. The system and method also provide a substantial improvement in helicopter performance during take-off, hover and maneuver. All such improvements are valid in helicopter mode when the helicopter is supported by rotor vertical lift, in airplane mode when the helicopter is supported by wing lift and the rotor is tilted forward to provide propulsive thrust and in conversion from helicopter mode to airplane mode.

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.

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.

Other attempts have been made in improving helicopter maximum forward speeds and/or reducing noise at maximum speed by using 2-speed gearboxes. These gearboxes allow the rotor to rotate at two RPM values while maintaining a constant engine RPM. The rotor is set to rotate at a lower RPM when at high forward speed so as to reduce the rotor tip speed. In all other conditions, the rotor is set to rotate at the higher RPM. However, these attempts do not substantially improve the efficiency of the helicopter by reducing fuel consumption.

Tilt rotor type rotorcraft incorporate wings which produce lift in forward flight, and, at forward speeds which is adequate to support the weight of the rotorcraft. The rotors (usually 2 or 4) are "tilted" from a first position where their axis of rotation is vertical and where the rotors act as a regular helicopter rotors to a second position where their axis of the rotation is relatively horizontal and the rotors act as propellers producing forward thrust. A tilt rotor type rotorcraft converts from helicopter mode to airplane mode (wing borne with propellers) after vertical take-off and converts back to helicopter mode for hover or vertical landing.

The best-known tilt rotor rotorcraft is the V-22 Osprey. The V-22 Osprey uses 2-speed rotors, a 412 RPM for helicopter mode and for conversion to airplane mode and 333 RPM when the rotors are locked in propeller mode for forward flight.

The experimental predecessor to the V-22, the XV-15, attempted the same type of 2-speed rotor but was not successful in achieving such 2-speed because of rotor dynamics, which caused high vibration and loads at RPM other than 100%.

The constant RPM or the close-ratio (100% and 81%) RPMs of the current tilt rotors result in excessive RPM in forward flight and excessive blade "twist" (variance of blade angle at the tip of the blade vs. the angle at the root of the blade) for hover flight. Both of these limit the performance and efficiency of the current tilt rotors. As such, there is a need for a tilt rotor system which will improve the tilt rotor type rotorcraft maximum hover weight, cruise range, altitude and speed performance while reducing fuel consumption and noise levels. The optimum speed tilt rotor system, when incorporated on tilt rotor rotorcraft, allows for a substantial improvement in range, altitude and airspeed with less fuel consumption and noise levels.



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