The flybar has a very important job for our model helis. A term commonly used that is related to the flybar is its "mixing ratio," or "flybar ratio." Have you ever looked at a heli manual and noticed that there are installment location options for flybar mixing arms? Have you ever wondered what that adjustment is really doing? The flybar ratio is one of those adjustable parameters that can dramatically change the way a heli moves through the air. A better understanding of this topic will allow you to make such adjustments with confidence.
Background:
Most RC helicopters in use today have a flybar. The flybar provides stability without the expense of maneuverability to our helis. In order to better understand why flybar ratio adjustments do what they do, we need to understand how they work through the history of its design.
Bell and Hiller
There are two systems that are combined and mixed together to control our model heli rotor heads: the Bell and Hiller systems. Both the Bell and the Hiller systems get their names and designs from full-size helis. Let's explain them both briefly:
The Bell control system is based on the Bell Stabilizer bar. The Bell stabilizer bar was used in many early Bell helicopters and is most commonly seen on the Bell UH-1 Huey. The stabilizer bar is basically a weighted flybar without paddles that spins with the rotor head providing gyroscopic stability. A mixing arm on the stabilizer bar takes inputs directly from the swashplate and mixes them with the gyroscopically stabilized bar to the blade grips. Flight control systems using a stabilizer bar benefit from direct control input, but offer limited maneuverability due to the overriding stability of the system.
• On a pure Hiller setup, the cyclic input from the swashplate is sent to the flybar; the flybar is then flown to the desired disk angle. The main rotor grips are attached to the flybar, and the tilting flybar will move the main grips to follow the same circular path of the flybar. The Hiller flybar is gyroscopically stabilized as the Bell Stabilizer Bar but now offers increased maneuverability as it can be flown to a new position. The downside to a pure Hiller setup is the lag in controls. The flybar needs to move itself into its new position before the main blades do, as the flybar in a Hiller system pulls the main disk behind it. This results in a small delay before the effect is realized at the main rotor disk.
Bell-Hiller System: The best of both worlds
How do you enjoy the pros of each system while diminishing the cons? Simple. Just combine them! Most RC helis use a combination of the above systems called a Bell-Hiller setup. By offering a direct input from the swash to the main grips via a mixing arm (Bell input) and controlling a maneuverable flybar (Hiller input) you get stability and maneuverability without any delay in control input.
The Bell-Hiller mixer arm is where all the magic happens. It takes the combined input from the swash and the flybar and transmits the resulting output to the blade grips. The use of a mixer allows the blade grips to get the desired instant input from the swashplate while also getting the needed control and stabilization input from the flybar. When the flybar tilts, it raises or lowers the mix arm, and will thus raise or lower the pitch on the main blades. By making adjustments to the mixing arm you can fine-tune how much influence you want the flybar to have over the main blades.
Bell input Adjustment:
The Bell component is usually set at factory to what the manufacturer has determined to be the best for the given heli. There are very few model helis on the market with adjustable Bell input; most let you adjust only the flybar (Hiller) side of the mixer arm. If you have a machine that allows Bell input changes, and you decide to experiment with these settings, be sure to check the total collective pitch range as you do so. Changes in the Bell input affect total pitch range much more than Hiller changes. This is because the washout removes all collective pitch influence from the flybar (Hiller side), leaving the Bell input to transmit all collective pitch changes to the grips. Making changes to the Bell mix is generally done by more experienced pilots looking for the perfect mix for their flying style. Making changes to the Bell side of the input is done using the same steps as making changes to the Hiller input, so the procedure below is more or less the same.
It can seem confusing at first, but if you read the manual and try the different settings out on the bench, it will make more sense. Try tilting the flybar with the heli on the bench while watching how much the blade grips rotate on the spindle. The amount of movement generated at the grips by the flybar input increases as the flybar ratio is changed to a higher setting—just as lowering the flybar ratio will reduce the amount of movement you see with this exercise.
How to Measure Flybar Ratio:
With all the prerequisite info behind us, we can move on to working with the flybar ratio. First, let's define what exactly we are measuring, here, so it's not confused with the Bell and Hiller ratios. The flybar ratio is simply the correlation between the flybar tilt and main blade pitch, measured in degrees. The standard form is to measure how much main blade pitch is given per each degree of flybar tilt. For example, a Raptor 50 Titan is equipped stock with a 1:1 flybar ratio. What this means is that for each degree of flybar tilt, the main blades actuate one degree as well. It's a 1 to 1 relationship in this case. 1:1 is considered a fairly high flybar ratio, while .5:1 would be a low flybar ratio, as the main blades would only move .5 of a degree for each degree of flybar input.
An interesting sidenote on the Raptor is that due to its 1:1 ratio, the flybar does not need to be level when setting the pitch up on this heli if you use the flybar as a guide to set pitch to. Any heli with a flybar ratio other than 1:1 needs to have the flybar leveled while setting pitch.
Calculating the flybar ratio on your heli is quite simple as long as you have a pitch gauge with a bubble level on it. If your pitch gauge does not have a level, you can simply attach a small one to the top or bottom rail. The other tool you'll need is a protractor, or anything else that measures angles.
1. The first thing you do is tilt the flybar in one direction (it doesn't matter how much tilt you put in). Measure the angle that the flybar is tilting by using your protractor (make sure that the protractor is level). Keep the flybar locked at that position and use your pitch gauge to find out what pitch the main blades are at, and again check for level. Write both of these numbers down.
2. Tilt the flybar in the other direction and re-measure in its new location. Also check the main blade pitch at this new position. Write these numbers down as well.
3. Use the two main blade pitch numbers to figure out the total travel between the two. For example, if one end was -3 and the other end was +4, you would have 7 degrees of travel between these two points. Now do the same with both flybar angle readings. Let's say, for our example, that we had 9 degrees total range on the flybar tilt.
4. Simply divide the main blade travel by the flybar tilt travel we just found to get the ratio. In this exercise it would be 7/9 or .78, meaning we have a .78:1 flybar ratio in this exercise. You want the main blade travel in the numerator because you are trying to get a ratio of how much main blade pitch each degree of flybar tilt creates. In our example, you get .78 degrees of pitch change for each degree of flybar tilt.
It's wise to run this calculation a couple of times using different flybar positions, in order to verify your results. While you do not need to know the flybar ratio of your machine in order to fly, it's interesting data to have and can be useful for fine-tuning a machine.
The flybar ratio is only one part of the equation regarding head adjustments. The length of the flybar and the dampers used will affect how a heli flies, as does the size, weight, and shape of the paddles. A flybar paddle that has a large surface area placed farther out from the main shaft (longer flybar) will have more influence than a small paddle placed closer in. For example, on a machine with a high flybar ratio you can get more powerful cyclic by using light paddles that have a large surface area, or use a longer flybar. To experiment, start with the stock heli setup and change one item at a time to get a feeling for how each option changes the heli's flight style.
Conclusion
The flybar is one of those seemingly simple, yet important parts to a model helicopter. Knowing what your flybar ratio is and what options you have on your heli makes you a more knowledgeable pilot in the long run. Next time you have a hankering to tinker, try adjusting the flybar a bit. The results achieved by small changes are amazing. In fact, a simple change on the flybar can make a heli feel like a completely new machine in flight. Whatever you do, enjoy!
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