Flybars and paddles – what do they do?

Sponsored by Pete's Hobbies and Precision RC Products

Flybars, Paddles, Weights: Purpose and how they Function.

One of the main functions of the paddle-equipped flybar is to assist the servo in its function of changing blade pitch. The flybar, when spinning at speed (We will use the arbitrary figure of 1600 rpm for the purpose of this example) exhibits some of the characteristics of a gyro and as such responds to the basic laws of physics that apply to gyros.

 In the case of our models these gyroscopic effects are used to assist the servos in performance of their function. In effect, the flybar forces are applied at the same instance as the servo forces are applied to the main blade arm to change the pitch of the main blade and reduce the load on the servos and the mechanics.

To understand how this happens a few basics must be understood. One of the basics of a gyro is that it wants to stay in its rotational path. When force is applied to a mass spinning fast enough to act as a gyro, a large force is required to change its path of rotation quickly. When disturbed by applied forces, it will react to this disturbance but not in a manner you would expect. Imagine the flybar as a “disc” that is spinning flat and parallel with the path of the blades at zero pitch. The reaction of a spinning “gyro” from ANY input force takes place 90 degrees later, in the direction of rotation. This is called gyroscopic precession.

To stay oriented for this explanation, consider that the front of the heli is “North”; right is “R-East”; tail is “South”; left is “L-West”. To better visualize consider the flybar spinning at 1600 rpm as a disc. Visualize the main blades as a disc as well. Now let’s connect some linkage to this flybar/gyro disc. The location of the linkage attached to the disc and all of the components that rotate with the head is the “timing” of the flybar and head and is generally called swash-plate timing.  It is the timing of when the applied forces to the Flybar/gyro disc take effect on the main blade disc. We will not discuss timing here except as it applies to what the flybar influence does to the blades and servos as to pitch and loads etc.

It is the linkage that dictates two main conditions. One is direct mechanical swash-plate main blade pitch, the other is mechanical swash-plate flybar paddle pitch (also known as “angle of attack” for both). To see this mechanical pitch, place the main blades parallel to the boom at approximate zero pitch, one blade pointed “North”. Tilt the swash-plate simulating left cyclic input (left roll) while holding the fly bar in place at level. You will note the main blades pitch change, the "North" blade going positive pitch. Rotate the head and perform the same operation with a flybar paddle at “North” position, parallel to the boom. You will note the pitch of the paddles change as well, the "North" being opposite pitch as the "South". For the purpose of this discussion this "mechanical" pitch (also called direct swash-plate pitch/angle of attack) change is absolute.  

Start with the flybar in this position (paddle "North" and use a left roll swash-plate deflection. This requires, for this example, that the swash-plate be tilted up at “R-East" fully. Note that although the swash is tilted up at “R-East”, the paddle is at full “positive” or lifting pitch at “North” (as was  the blade). This is due to the timing of the linkage. Our heli’s gyroscopic and mechanical timing forces are set and controlled exactly as to when they are applied and when they take effect through the position of  the head in relation to the upper half of the swash-plate.  The "Washout" arms on the "Washout hub" and “Bell-Hiller” arms (sometimes called Bell Mixers or Hiller arms) at the top of the head, control ratios and not timing. The forces are applied by the servos through the tilt positioning of the swash-plate.

In this example, upward pressure is applied by these fully deflected paddles to the Flybar/gyro “disc” at point “North” Hold the flybar level at this point, as this is how the flybar actually is when rotating at  the example 1600 rpm. Even though the paddles have mechanically deflected to a steep angle of attack, our flybar has not moved out of its path due to gyroscopic effects described earlier. The effect of this force pressure exerted on the flybar disc comes into play  exactly 90 degrees later, in the direction of rotation, at “R-East” where the effect of the force  takes place. Now rotate the flybar clockwise. When the flybar paddle is rotated 90 degrees to a position exactly “R-East”, the force created at “North” is applied and the flybar disc will tilt upward. Note that the paddles are flat at zero pitch in the “R-East” position, yet this is where the flybar actually tilts..

Now we must look at the main blades to fully understand the flybars effect.

With the flybar paddle rotated to “R-East”, the Main blades are in the “North- South” position. Here is where the "swash plate direct force" input is applied to the main blades. To see this, hold the fly bar stationary and move the swash plate from tilted to level and back to tilt. You will see that if the flybar were locked in place, the blades would move in exact relation to the swash plate input. Note that the flybar paddles are now level. With the swash plate full tilted into a left cyclic input, tilt the flybar up on the “R-East” side. Remember that the input was at "North" and gyroscopic precession has made the disc actually tilt at “R-East”, where the flybar paddle is now. You will see that the input from this flybar "disc" tilt is added to the direct mechanical input of the swash plate to the main blades. This is the assist from the fly bar. The actual amount that the flybar tilts is far less than what is available. There is no possibility that paddles having been mechanically rotated to 30 or 40 degrees angle of attack at "North" will cause the flybar to tilt its full available swing at "R-East". The amount of movement out of its rotational path due to gyroscopic effect is a few degrees dependent on the amount of force applied at "North". The main fact is that the flybar is adding “force” to the force of the servo in order to change the pitch of the blades.

Keep in mind that a force is applied to the flybar/gyro on the opposite end of the flybar by the other paddle as well, creating the opposite (downward) force on the disc at the same time by the same 90 degree law and flybar deflection occurs downward on “L-West” in this example. The amount of force generated is a function of many factors.

Factors that increase this flybar generated force:

Faster rotation
Bar length, Longer = more leverage (until flex becomes a negating factor),
Lighter paddle = more force created by the paddle as it is not lost moving the
weight of a comparable heavier paddle
Airfoil shape = a good airfoil shape will contribute to less stall of the airfoil,
Aft the CG of the paddle = More "bite" 
Sharp leading edge= More “bite”
More square inches of paddle=more area=more force
Paddle designs that reduce drag
    Keep in mind that the paddle does not “fly up” at the time of rotation to angle of attack, it only applies force to our flybar/gyro at angles of attack. Bell Hiller ratios that create more LEVERAGE FORCE. (This can mean a flybar travels further with LESS blade input)

All of this equates to more PADDLE FORCE applied to the flybar-gyro at “North” and this equates to more FLYBAR~GYRO-FORCE applied to the blade pitch at “East” in conjunction with the direct servo/swash-plate input in our example)

Pete Schmidt
Pete’s Hobbies/Precision RC Products
AMA 6314

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