||Description of the Freewing Tilt-Boom aircraft technology.
Here are the distinguishing features of this VTOL air vehicle:
- Dual pendulum control system. The vehicle operates as a thrust vectored pendulum in all flight modes. In vertical flight, this vehicle operates as a thrust vectored gravity stabilized pendulum (similar to helicopters). In horizontal flight, it operates as a thrust vectored drag stabilized pendulum. Any transitional modes are merely a blend of the two pendulums.
- The Tilt-Boom free-tilting rotor system. This is the common source of thrust, whether for vertical or horizontal flight. The rotor is free to tilt from vertical (plus or minus) to horizontal (plus or minus). Thus, the rotor is not motored up and down, as the rotors are with the V22 Osprey. Typical operation would be like this: (i) a collar holds the rotor system vertical when the system is under no thrust or low thrust conditions (exception to the free-to-tilt rule) but is released when the rotor system is powered up sufficiently to maintain position. The thrust is increased sufficient to achieve hover as the freewing weathervanes into the rotor downwash/relative wind; (ii) as the pilot vectors the thrust horizontally, the free wing simultaneously creates lift to counteract the gravity pendulum and creates drag to form the dynamic drag pendulum. The angle of the free-tilting rotor system at any given time is simply the angle which balances the thrust, lift and drag forces of the aircraft; (iii) this process is reversed to return to hover mode and land (re-engaging the rotor system collar before thrust shutdown).
- Method of thrust-vectoring. Several methods are possible, such as swash plate and cyclic or servo tabs. The method chosen for the Spirit series of Tilt-Boom UAVs is covered by a pending patent. A secondary freewing called a "Boomwing" is placed on the Tilt-Boom in the propwash (or rotor wash). The Boomwing replaces the mechanically complex prior methods, such as the swash plate and cyclic. Trailing-edge trim tabs, or elevons, are displaced to give the Boomwing either a positive angle of attack (boom trends up) or a negative angle of attack (boom trends down). The boom automatically assumes an angle which balances the thrust acting on one end of the boom with the lift and drag forces operating on the other.
Since the Tilt-Boom seeks an angle where it is dynamically balanced, only a small force is required to create a moment sufficient to displace a given balance and cause the boom to seek a new equilibrium angle. From a hover position, the thrust is vectored to cause forward flight. The forward flight generates lift and drag from the freewing, which, in turn causes a new, more forward, dynamic equilibrium angle for the Tilt-Boom. This more forward equilibrium angle, in turn, generates more forward thrust which creates more lift and drag from the freewing, which, in turn causes a new, more forward dynamic equilibrium angle for the Tilt-Boom. The process continues until the boom is horizontal, or back to vertical again. The rotor system remains free during cruise mode.
In any flight mode, the Boomwing can return to a neutral trim, with the result that the Tilt-Boom remains at whatever angle it currently has with respect to the fuselage.
- Freewing. The main wing of the vehicle is attached to the fuselage by bearings, and is free to float in pitch. The freewing sets its own angle of attack and holds it as a constant. In hover the freewing fairs itself into the propwash, providing no lift to the vehicle. As the Tilt-Boom is oriented from a vertical angle to a horizontal angle, the freewing automatically adjusts itself effectively “averaging” dynamic pressure effects from the flowfields of both the propwash and external relative wind. In hover, the rotors are carrying 100% of the weight of the vehicle. In cruise, the freewings are carrying 100% of the weight of the vehicle. During transition, the rotors and the main freewing automatically distribute the weight between them, generating the lift required for flight.
- Control during hover. Longitudinal movement is controlled by tilting the Tilt-Boom fore and aft. Roll control is by asymmetric deployment of “clamshell” control surfaces on the wingtips of the Boomwing. Yaw is by asymmetric displacement of the elevons on the trailing edge of the main freewing. Altitude is controlled by throttle.