Originally Posted by Dave Jackson's ref
... hovering, the longitudinal oscillations .... values of 12 seconds were ascertained. With this time of oscillation of a dynamically unstable character, the helicopter could be flown without difficulty.
The Fa223 is statically and dynamically completely stable around all axes other than the longitudinal one. At traveling speed of 140 to 150 km/h all controls can be released, because longitudinal instability disappears at about 120 km/h. Then this aircraft behaves just like a normal airplane and is automatically stable.
OK, point accepted for lateral stability. Longitudinal stability was only accomplished above 120 kph by use of a tail plane though, which would not allow hands off in hover - a 12 second cycle would be reduced for a smaller machine. I suspect that the simple mechanics of how the pilot holds the stick makes it easier to correct for longitudinal instability.
A gyro would allow stability in all conditions. Interestingly longitudinal stability in a fixed wing suffers at high altitude, since the higher speed for same dynamic pressure results in lower elevator AOA change with pitch. Many swept wing jet-liners have put up with gyro yaw damping for a while. All modern FBW jetliners have laser gyro stabilisation on all axes - primarily to reduce the otherwise necessary aerodynamic compromises.
It has also to be said that the Fa223 has a very wide seperation between the rotors. This would adversely affect the utility function of a helicopter, since you don't want to constrain landing sites. A smaller seperation would begin to diminish lateral stability. Not sure you want the complexity of the George de Bothezat machine...
Mart