Pendulum Effect
mSR and 120SR
A desirable trait of the Blade mSR and 120SR fixed pitch model helicopters which makes them easy for beginners to fly are their the ability to automatically self-stabilize and return to a hover "hands-off" when the control sticks are released. A related undesirable trait is the tendency for the model to rock back and forth like a the pendulum on a grand-father clock when the sticks are released. Below I will attempt to explain why this happens.

At hover the swash plate, flybar and rotor are all parallel to each other and the ground and the pitch on both sides of the rotor blade are equal. To initiate forward flight the pilot pushes the right "cyclic" stick forward which causes the "elevator" servo to move and tilt the swash plate.

The flybar is suspended between the swash plate and rotor with two sets of links - swash-to-flybar and fly-bar-to-rotor in what is known as a "Bell-Hiller" configuration. The flybar has heavy symmetrical airfoils on the end of its shaft which spins with the main shaft. It isn't connected to the shaft. A pair of guides for the links on shaft and rotor head turn the links and the links turn the flybar. When spinning at hover the flybar is level with the ground and the rotational forces and gravity work to try to keep it level to the ground at all times.

Due to the way the flybar floats suspended on the links and held level to the ground by the rotating mass on its ends, what happens when the elevator servo tilts the swash plate is that the swash links tilts the flybar and the second set of links on the flybar change the pitch of the main rotor blades.

The term "fixed pitch" means that there is no way to change the "collective" pitch angle of the rotor blades, but the blades pivot on a "feathering" shaft in the rotor head. At hover, when everything is parallel, the links keep the pitch and lift of both blades equal. But with the cyclic tilts the swash it tilts the flybar and the flybar link make the pitch of rotor blades unequal - temporarilly.

How all this happens is counter-intuitive due to something called "gyroscopic precession". Due to the spinning of the rotor and the earth's rotation and gravity when the rotor lift changes it does not immediately tilt the rotor. The change in direction of the rotor occurs 90 after the lift occurs. So for the rotor to tilt forward and pull the body of the helicopter forward the change in pitch must occur when the rotor is crosswise to the body of the helicopter not over the boom as you might assume.

If you take your mSR or 120SR and move the rotor so it is crosswise to the body you will notice that the flybar is 45 from the rotor and 90 from the elevator swash are. That is where the rotor is when it changes the pitch of the blades when forward stick movement tilts caused the elevator servo to tilt the swash.

From hover the initial stick forward input from hover changes the relative pitch of the blades and tips the rotor forward, but the flybar, due to the fact it is floating on the links and rotating tries to stay level to the ground. As the rotor tips the angle between the flybar rotor changes from parallel which causes the flybar-rotor links to rotate the rotor on the feathering shaft back to the point where the pitch of the blades is equal again, just as they were at hover.

Despite the fact the cyclic continues to be held forward because the flybar and swash tilts cancel each other out causing the pitch of the two rotors to revert back to more or less equal, give and take all the slop in the control linkages. Once tilted the pitch of the two blades being the same causes it to fly forward with the tilt remaining the same until the pilot release the stick.

What is actually happens in forward flight is that the initial forward stick movement changes the relative pitch of the blades, the difference in lift that causes tips the rotor, and the rotor tipping causes the flybar, which stays level, to pull the blades back to neutral as the helicopter flies forward with the rotor tilted at a constant forward-leaning angle. Continued forward pressure on the cyclic stick keeps the fly bar from automatically returning it back to the hover state.

When the cyclic is released the swash moves back to level, but as it does the equilibrium between the angled swash and level flybar causes the flybar to change the pitch of blades is it is released causing the rotor to return back to level. For a split second rotor, flybar and swash are all parallel and level with the ground again (hover trim) but the momentum keeps the rotor and tail tipping down. That in turn causes the flybar (which is level with the ground as always) to shift the pitch of the rotor the opposite way, causing it to go nose level then nose down again. The fly bar stays level with the ground (because its heavy and spinning) and the rotor teeter-totters above it and the body of the heli below it until it like a pendulum until comes to equilibrium again at hover.

Dealing with the Pendulum Effect

Two ways to deal with the pendulum effect are: 1) avoid it by not stopping or changing direction abruptly, 2) use it to change direction.

Due to the nature of the design the flybar will always cause an opposite reaction when movement is stopped suddenly in mid-air so a way to avoid that pendulum reaction is to plan the flight path to avoid such sudden stops.

The pendulum effect can be used to rapidly change direction 180. If after releasing the stick the tail is spun around 180 with the rudder as it is heads nose up for the first time. Spinning the tail 180 puts it nose down in the opposite direction, away from the crash it was heading for, rotor tilted for forward flight. Then push the stick forward again to maintain the forward rotor tilt add throttle, then and fly off fast in the opposite direction. The tail will flip around faster and then stop with better control CW (i.e. adding tail thust) than CCW (which slow/ stops the tail rotor). With practice you can stop it and flip it around in a split second. Its the most valuable crash avoidance maneuver you can learn.

RC Helicopters
Climbing Up The Learning Curve

This tutorial is copyrighted by © Charles E. Gardner.
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You can contact me at: Chuck Gardner

For other tutorials see the Tutorial Table of Contents