Di seguito le parole di Colin Mill sull'argomento principale di questo post, rigorosamente in inglese, ma sicuramente attendibili
evidenziato in rosso un pò quello che avevo detto io
:
The Flybar & Control Systems - Information Provide By Colin Mill (CSM)
Cyclic response of a model helicopter. We saw that the natural tendency is for the main blades to respond too quickly to cyclic commands. This happens because the aerodynamic forces acting on the blades are large compared to the weight of the blades. We can't do much about this because the lift on the blades has to be big enough to support the weight of the helicopter and so we can reduce the forces on the blades only at the expense of not having the helicopter fly at all!
The control systems employed on model helicopters almost without exception employ a flybar to overcome these difficulties.
The flybar as illustrated here consists of a rod carrying small aerofoils (paddles) and is pivoted so that it may rock. The angle of attack of the paddles is set by the cyclic control and they respond in much the way outlined for the main blades last time. Again, to roll the flybar to the right the angle of attack of the paddles is increased on going round the rear half of the rotor and reduced on going round the front half of the rotor. This is simply done by rotating the whole bar around its axis. Because the flybar is not responsible for lifting the helicopter the aerodynamic forces acting on the paddles can be tailored to give the required speed of response. It is best to think of the flybar as a gyroscope that can be steered by the cyclic controls but when not being steered tends to maintain its axis of rotation relative to the ground rather than the helicopter body or the air. The speed of response of the flybar to commands can be adjusted as follows:
Increasing the weight of the paddles slows it down.
Increasing the area of the paddles speeds it up
Increasing the rotor RPM speeds it up
Increasing the aspect ratio (span/chord) of the paddles speeds it up.
Increasing the length of the flybar speeds it up.
However its not obvious why this should be the case so let me just give my reasoning for it. If we take one size and weight of paddles and fit them to a flybar that has been lengthened by say 10%. We :-
1) increase the moment of inertia (the flywheel effect) of the flybar. This means that the flybar will need a greater torque to impose a given rolling or pitching rate on it.
2) However, in putting the paddles further out we have increased the leverage that they have so that, for a given aerodynamic force on the paddles, we have a bigger torque.
3) In addition, by putting the paddles further out we have, for a given head speed, increased the airspeed of the paddles and thus increase the aerodynamic forces they produce.
Now effect 1) acts to slow down the response of the flybar, and it involves a square law so a 10% increase in flybar length increases the torque needed for a given roll rate by about 20%. However, effect 3) also involves a square law so, with the paddles 10% further out they produce 20% more force for a given cyclic pitch. So effects 1) and 3) cancel one another out. This leaves effect 2) which is linear and so a 10% increase in flybar length speeds the flybar up by 10%.