>> Why a Coaxial Drive Train?

Why the choice of a coaxial drive train?


The choice of a coaxial drive train for the Spirit was easy once the costs and benefits were considered.

  • Freewing Military UAV applicationsHigh Thrust to Weight Ratio: First, to take advantage of the vertical takeoff and landing capabilities of the Spirit, a slow turning proprotor was needed. This would require a gearbox reduction from the engine input. Likewise, a gearbox was also required to permit the free pivoting boom to translate from vertical to horizontal flight and vice versa. Since a gearbox would already be required, the only weight penalty which a coaxial system would incur would be the marginal weight of the second drive shaft and the second rotor. However, a coaxial system provides from 15% to 20% more thrust for the same horsepower. Therefore, if a given single rotor system were producing 1000 pounds of thrust, a coaxial system would deliver between 1150 pounds and 1200 pounds of thrust for the same horsepower. As long as the weight of the second rotor and shaft are less than the additional thrust produced, there is a net thrust benefit, in addition to the benefits listed below.
  • High Useful Load: The absence of a tail rotor from the Spirit permits all of the engine power to be used by the rotor system for lifting purposes. Even with a single propeller driven airplane, the airframe must be designed to address the torque of the engine through dynamic flight loads, which create drag and impair performance.
  • Absence of Torque Transfers: A coaxial system transfers all torque to the drive train and not the fuselage. There is no need to counter single rotor torque through the fuselage through a long boom and tail rotor, or a long fuselage with dynamic flight controls. The components of the coaxial control system are short thereby improving control responses and effectiveness. Accordingly, the wetted area of a coaxial system on the same payload basis is much smaller than either the tail rotor or tandem helicopter configuration.
  • Precession Loads Are Balanced: The free lever operational characteristics of the boom permits the inherently stable transition from vertical to horizontal flight. The coaxial rotor system balances the precession loads of each rotor during transition-- the loads of each rotor balanced against the other rotor. This minimizes bending moments imparted to the drivetrain.
  • Freedom From Propeller Size Limitations: The size of the propeller/rotor diameter in the Spirit is totally independent of the fuselage ground clearance, thereby freeing designers to increase propeller/rotor efficiency by utilizing larger diameter propellers and rotors. This feature, which disassociates the rotor diameter size from the fuselage ground clearance, becomes considerably important in increasing efficiency.
  • Complete Symmetry of the Rotor System: The symmetry of the coaxial rotor system permits the same aerodynamic efficiency and controllability for flight in any direction; a feature only found in the coaxial.
  • Simplified Rotor System: The Spirit utilizes a patent-pending control system which eliminates much of the mechanical complexity of the typical helicopter swashplate and cyclic system. The system is inherently no more complex than that of a fixed wing aircraft. The simplicity of design yields benefits such as lower structural weight, lower manufacturing, and maintenance costs.
  • Freedom from Control Cross-Coupling: Control cross-coupling exists in all other rotor configurations, causing control complexity, dangerous flight attitudes and vibration. In the Spirit, the loads imparted to the system by one rotor are countered by the loads imparted by the other counter rotating rotor.
  • Optimal Transmission Design: In a coaxial configuration, the inherent feature of splitting the power input into two paths results in a transmission design internally-balanced, compact and capable of handling greater horsepower per engine input than any other configuration. The design is naturally suited for multi-engine inputs without creating external torque reaction problems, and with only a small overall weight increase per added engine. This unique feature of the coaxial transmission achieves greater importance as the size of engines and number of engines increases as required to satisfy the need for larger aircraft.

    This unique feature also achieves greater importance in the event of an engine failure. With the coaxial system, there is no change in flying characteristics nor asymmetric thrust to deal with when an engine fails.
  • Modular Construction: The closed-loop torque-balanced coaxial rotor system eliminates torque reactions on the fuselage, thus making the fuselage a totally independent body from the rotor-transmission-engine combination. Therefore, adaptations of a given drive train to a new fuselage are significantly simpler.
  • Scale Potential: The unique features of a coaxial drive system permit scale increases as follows:

    As stated above, the fuselage design of the Spirit is purely functional and independent of the size of the lifting system (engine/drivetrain/rotors). Therefore, an increase in size of one of the major subassemblies does not affect the other.

    The coaxial transmission is naturally suited for multi-engine inputs without creating external torque reaction problems and with only a small weight increase in the transmission per additional engine. This feature permits a proportionately small change in the transmission for a substantial growth in engine input.

    The flying qualities of a coaxial system permit high disc loadings as a rotor system and low disc loadings as a propeller system without affecting controllability and fuselage design. This works better in a common propulsion system which functions both in vertical flight and in horizontal flight.

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Last Updated: Tuesday, 17 August, 2004 8:25 PM