||The Freewing Tilt-Boom concept
The Tilt-Boom Technology is a breakthrough in how to achieve flight. It marries the time-tested concept of the freewing a wing that floats freely in pitch, like a weathervane turned horizontally with a “free lever” - a lever that floats freely in pitch, (angle of orientation), like a pendulum, to produce a truly novel method of controlling an aircraft and transitioning from vertical to horizontal flight.
A freewing freely adjusts to the direction of the relative wind, inherently balancing the various forces operating on its surfaces to maintain a constant angle of attack.
Similarly, a “free lever” freely adjusts its angular orientation to balance the forces operating on each end of the lever. In the Spirit UAV, the thrust acts on one end of the lever and the lift and drag forces of the aircraft operate on the other end. For a more in-depth discussion, please see Briefing Slides.
The result is an aircraft that can hover for takeoff and landing, and also transition back and forth to high-speed cruise flight.
But the real breakthrough is that it does all this while
- remaining completely stable throughout every point in the flight envelope including all through the transition it can even stop at any point in the transition and be in a stable flight mode
- staying mechanically simple nearly as simple as a comparable fixed wing vehicle
- neutralizing turbulence from takeoff to landing to provide the best platform for cameras and other mission instruments
How Does It Work?
The Spirit operates as a thrust vectored pendulum in all flight modes. Though the principle is simplicity itself, the technology produces an aircraft design vastly different in function from traditional fixed wing or rotary wing vehicles.
In a traditional fixed wing aircraft, the aircraft must have a forward center of gravity and operate with dynamic flight controls. In a traditional helicopter, the helicopter must have a pendulum center of gravity and operate with vectored thrust flight controls.
These fundamental differences between these two aircraft types have doomed hybrid vehicles to either be operational failures or have overwhelming mechanical complexity. Hybrid aircraft encounter a period of fundamental instability when transitioning from operating with a pendulum center of gravity to a forward center of gravity.
However, the Spirit operates as a thrust-vectored pendulum in all flight modes, so there is no period of instability. The Spirit operates as a thrust vectored gravity stabilized pendulum in vertical flight and as a thrust vectored drag stabilized pendulum in horizontal flight. Any transitional flight is merely the force vector resolution of the two pendulums. For more information please see Briefing Slides.
The key to this inherent stability is the “free lever” operational characteristics of the rotor shaft. Since the rotor shaft is totally free to pivot about its mounting axis, it is free to assume that angle which balances the lift and drag forces operating on the aircraft. Also, since the rotor shaft is free to balance the forces acting upon it, the shaft undergoes torque and tensile forces, but minimal bending moments.