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Electric Tail Rotor Drive


OBJECTIVE: Develop and demonstrate an electric tail rotor drive for a helicopter. DESCRIPTION: Recent years have seen significant advances in electric drive technology and increased interest in electrically-powered aircraft. It may now be possible to replace the mechanical tail rotor drivetrain in a helicopter with an all-electric system comprising a generator or generators driven by the main rotor gearbox, an electric motor at the tail rotor, and associated cabling and controllers. Potential advantages include reduced weight and maintenance, elimination of the intermediate gearbox (if any), simpler pylon folding, and electrodynamic braking to replace or augment mechanical rotor brakes. With the aircraft on the deck, an electrically driven tail rotor could be switched off entirely, providing an extra measure of safety to deck crew. Variable tail rotor speed could also be implemented with potential noise and performance improvements in forward flight. An electrically driven tail rotor could also be arranged to pivot in flight to provide forward thrust. Electrically driven tail rotors are common in small-scale Commercial Off-The-Shelf (COTS) Radio Controlled (R/C) helicopters, where adequate electrical power is already available from batteries sized to drive the main rotor. Technical challenges in implementing an electrical tail rotor drive system for full-scale helicopters include efficient generation of adequate electrical power and scaling the system to a relevant size while maintaining favorable weight relative to the mechanical system it replaces. The motor controls must have sufficient authority and bandwidth to prevent tail rotor RPM droop following large pedal inputs. This topic calls for the development and bench demonstration of an electric tail rotor drive system of size and operating parameters relevant to a naval helicopter selected by the proposer. The system should demonstrate technology for replacement of an existing mechanical drivetrain. Proposals should provide a credible review of relevant technologies and clearly outline sizing considerations underlying the selection of the subject helicopter. PHASE I: Define and develop a concept for an electric tail rotor drive system. The electric tail rotor drive need not be designed specifically for a military aircraft, but must be of a size relevant to an existing naval helicopter, and with similar operating parameters. Primary system attributes are weight, efficiency, and maintenance relative to the corresponding mechanical drivetrain. While the concept should assume a conventional tail rotor of appropriate size and operating speed, advantages deriving from a replacement tail rotor optimized in combination with the electrical drive system should be identified. Tail rotor speed transients due to yaw and pedal input should be similar to those seen in a mechanical drivetrain. The system should be powered by electrical devices interfacing with existing drivetrain components. A conventional tail rotor gearbox may be retained; if not, the system should include components necessary to react to tail rotor loads into the supporting structure. System impact on vehicle Center of Gravity (CG) should be investigated. The final concept should be evaluated to the greatest extent possible through high-fidelity modeling and simulation. PHASE II: Further develop and fabricate the electric tail rotor drive system defined in Phase I and demonstrate in a bench test. The demonstration should include the complete system and incorporate all drivetrain components between main rotor gearbox and tail rotor. In the bench test, gearbox-driven components may be driven by motors - an actual gearbox is not required. The tail rotor may be represented by an appropriate mechanical or aerodynamic load. No flight tests are contemplated in this Phase. PHASE III: Refine the concept to a full-scale flightworthy system design incorporating all details required for a successful retrofit to an existing naval helicopter. Fabricate and demonstrate the system in a ground test at an appropriate facility. PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: When operated in noise-sensitive urban environments, civilian rotorcraft may benefit significantly from noise reduction expected to accrue due to a variable speed tail rotor. Private-sector application may also include efficient distributed propulsion systems for large aircraft. The technology developed in this topic may help enable hybrid drive systems for aircraft and automobiles with improved efficiency. REFERENCES: 1. Jaenker, P., Hoffmann, F., Kloeppel, V., and Stuhlberger, J."Helicopter Hybridisation The Key for Drastic Reductions of Fuel Burn and Emissions", Proceedings of the American Helicopter Society 67th Annual Forum, Virginia Beach, VA, May 3-5, 2011. ISBN: 978-1-61782-881-2. 2. Prouty, Raymond W., Helicopter Performance, Stability, and Control, Krieger, Malabar, FL, 2002.
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