Fluid reaction surfaces (i.e. – impellers) – Articulated – resiliently mounted or self-shifting impeller... – Nonmetallic resilient mounting
Reexamination Certificate
2001-05-03
2002-08-27
Ryznic, John E. (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
Articulated, resiliently mounted or self-shifting impeller...
Nonmetallic resilient mounting
C416S19800R
Reexamination Certificate
active
06439849
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to the field of aircraft rotors, and in particular to a rotor design for use in a helicopter or similar aircraft.
BACKGROUND OF THE INVENTION
Helicopters generally incorporate at least two rotors into their design. The large rotor providing thrust in the vertical direction is known as the main rotor. In addition to this main rotor, the traditional helicopter design incorporates a tail rotor system to counteract the torque from the main rotor system. Although operable helicopter designs have been produced without the traditional tail rotor geometry, the vast majority of helicopters use this design. The number of blades in the tail rotor itself will depend on the requirements of a particular application.
A significant limitation inherent in the design of prior multi-bladed tail rotors is their inability to satisfactorily accommodate potentially powerful Coriolis torque. A Coriolis torque is generated in a helicopter rotor whenever the rotor plane is tilted relative to the shaft. Since the 1/rev Coriolis torque is proportional to the coning angle, it is usually negligible for most tail rotors. For a two-bladed tail rotor, the 2/rev Coriolis torque is also not a problem because both blades speed up and slow down at the same time, and the drive system is generally sufficiently flexible to provide the necessary torsional freedom. The 2/rev Coriolis torque does, however, become a problem with a multi-bladed rotor if insufficient lead-lag articulation is provided.
Existing multi-bladed tail rotors use a variety of methods to provide the necessary relief for 2/rev Coriolis torque. One design, developed by Sikorsky, uses a fully articulated rotor, complete with lead-lag hinges and dampers. Another design incorporates a flexible spindle at the blade root combined with restricted flapping motion to limit stresses due to Coriolis torque. One design, used by Kaman, allows a small amount of lead-lag motion by using a “rocking pin” arrangement in its flapping hinge. Yet another design, developed by Lockheed, uses a gimbaled tail rotor hub that relieves the 2/rev Coriolis torque in the same manner as a two-bladed teetering rotor.
All of these designs suffer from limitations. In general, each of the above solutions is heavy and complex. Each requires the use of heavily-loaded bearings oscillating at tail rotor frequency, resulting in designs requiring high levels of maintenance and excessive downtime.
Accordingly, there is a need in the art for a tail rotor assembly overcoming the above-described limitations of the prior art designs, including reduction of tail rotor weight and mechanical complexity, reduction or elimination of catastrophic failure modes, and increased service life of the tail rotor mechanisms.
SUMMARY OF THE INVENTION
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention, and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
The present invention relates to a dual-trunnion hub-to-mast assembly that provides improved damage tolerance with extended life expectancy and reduced maintenance burden due to the use of composite and elastomeric materials. In certain embodiments, the assembly is useful as part of a tail rotor assembly consisting of two stacked two-bladed teetering rotors, mounted on a single output shaft.
The present invention makes use of a variety of novel features to overcome the inherent limitations of the prior art. In certain embodiments, the present invention achieves increased-service life of the tail rotor mechanisms. In certain embodiments, the present invention achieves a reduction or elimination of catastrophic failure modes by the incorporation of redundant load paths within the rotor structure. In certain embodiments, the tail rotor of the present invention may be employed in a “pusher” implementation for improved aerodynamic performance by minimizing vertical fin blockage effects.
In addition to the above advantages, in certain embodiments the teachings of the present invention may provide improved aerodynamic efficiency, higher maneuvering capability, improved mechanical flaw tolerance design, and extended life expectancy. In certain embodiments, the present invention allows for reduced maintenance due to the use of composites and elastomerics. In one embodiment, a tail rotor constructed according to the present invention has been designed to achieve a minimum life of 10,000 hours of severe duty use in ground-air-ground maneuvers, air combat maneuvers, and high cycle vibratory loads, with little or no maintenance.
In certain embodiments, the present invention makes extensive use of multiple primary load paths in order to provide a fail-safe structure. In certain embodiments, the present invention provides redundant load paths for critical metal parts to minimize catastrophic failure modes. Certain embodiments eliminate the use of the bearings traditionally required to carry the full centrifugal force of the blade while oscillating at tail rotor one-per-revolution. This is done in order to further increase life expectancy, improve reliability, and minimize maintenance. In certain embodiments, the present invention minimizes control washout to the blades due to control system softness.
As described above, a significant limitation inherent in the design of prior multi-bladed tail rotors is their inability to satisfactorily accommodate potentially powerful Coriolis torque. A Coriolis torque is generated in a helicopter rotor whenever the rotor plane is tilted relative to the shaft. Since the 1/rev Coriolis torque is proportional to the coning angle, it is usually negligible for most tail rotors. For a two-bladed tail rotor, the 2/rev Coriolis torque is also not a problem because both blades speed up and slow down at the same time, and the drive system is generally sufficiently flexible to provide the necessary torsional freedom. The 2/rev Coriolis torque does, however, become a problem with a multi-bladed rotor if insufficient lead-lag articulation is provided.
Existing multi-bladed tail rotors use a variety of methods to provide the necessary relief for 2/rev Coriolis torque. One design, developed by Sikorsky, uses a fully articulated rotor, complete with lead-lag hinges and dampers. Another design incorporates a flexible spindle at the blade root combined with restricted flapping motion to limit stresses due to Coriolis torque. Another design, used by Kaman, allows a small amount of lead-lag motion by using a “rocking pin” arrangement in its flapping hinge. Yet another design, developed by Lockheed, uses a gimbaled tail rotor hub that relieves the 2/rev Coriolis torque in the same manner as a two-bladed teetering rotor.
All of these designs suffer from inherent limitations. In general, each of the above solutions is heavy and complex. Each requires the use of highly-loaded bearings oscillating at tail rotor frequency, resulting in designs requiring high levels of maintenance and excessive downtime.
One manner of addressing this problem is to mount a pair of two-bladed rotors on the same shaft. This arrangement provides a four-bladed tail rotor with the mechanical and structural simplicity of a two-bladed teetering rotor. By using this concept, no bearings are required to oscillate while carrying the full centrifugal force of the blade.
Although this solution partially addresses the above-described problems, it does not inherently provide relief for the 2/rev Coriolis torque. With this design, whenever the tail rotor experiences first harmonic flapping, one pair of blades will be attempting to accelerate at the same instant in time that the other pair of blades is attempting to decelerate. Thus, the two rotors will try to move in the same manner as a pair of scissors, placing considerable stress on the rotor hub components.
In spite of these limitat
Sehgal Ajay
Shimek Glenn
Bell Helicopter Textron Inc.
Emanuelson Kenneth T.
Gardere Wynne & Sewell LLP
Ryznic John E.
Warren, Jr. Sanford E.
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