The Art of the Helicopter John Watkinson - Karatunov.net

Fig. 9.7 The Bell-Boeing Osprey tilt rotor is a true side-by-side helicopter in the hover.
Other types of rotorcraft 357
either engine can drive both rotors. In the wing centre section the cross shaft drives
accessories.
In the hover, the pitch axis is controlled with fore-and-aft cyclic applied equally to
both rotors. Yaw is controlled by applying fore-and-aft cyclic in opposite directions. The
roll control requires differential collective pitch. In addition to the cyclic and collective
controls, the pilot has a control that can alter the angle of the two engine nacelles.
These are synchronized so they will always have the same angle. From the hover, thrust
is increased and the nacelles are brought down. Initially the download on the tail plane
will cause the nose to rise, but as speed builds up and the dynamic pressure on the
wing and tail increases, the machine becomes an aeroplane and responds to its control
surfaces. The cyclic control of the rotor pitch is unnecessary in forward flight.
The wing forms an obstruction in the hover and in the Osprey the leading and trailing
edges will both fold to present the least area to the downwash. As the angle of relative
airflow seen by the wing changes, first the leading edge and then the trailing edge will
fold back to the forward flight position. A disadvantage of the tilt rotor is that in the
hover the lift is applied at the wing tips and the wing is highly stressed.
One of the enabling technologies that turned a research tool into the Osprey is an
extremely complex electronic control system that recognizes the flight conditions and
automatically directs the pilot’s control movements to swashplates or control surfaces
or both as necessary. This could probably not have been implemented mechanically.
The extensive use of composites has led to an airframe light enough to allow a useful
payload.