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RC ROTOR HEADS AND BELL MIXINGTo understand what bell mixing is and how it works, you must first understand the differences between the two basic types of rotor heads that are used on most RC helicopters - The Hiller Rotor Head and the Bell Rotor Head. HILLER ROTOR HEADMost RC helicopters use Hiller Rotor Heads. These are very easy to identify because along with the main rotor blades, they also have a fly bar with small "paddles" at each end. This fly bar is orientated at 90 degrees to the main rotor blades. The fly bar is allowed to pivot up and down in the rotor head so it can stay in a level horizontal plane independent of what the main rotor and helicopter tilt angles are.
This picture shows a typical Hiller rotor head set up. Note the pushrods from the swashplate are attached to the flybar control arms - not the main rotor blade mounts. The main rotor blade mounts are only attached to the flybar control arms. This dampens the sensitivity and adds stability to the rotor.If a gust of wind hits the helicopter and tilts it sideways, the fly bar wants to stay in a horizontal plane. Since it is connected to the main rotor blade mounts, it automatically stabilizes the heli by changing the pitch angles of the main rotor blades to absorb the gust. This "self correction" also dampens the cyclic inputs so the helicopter is not as responsive or sensitive. The reason this happens is because the swash plate movements goto the fly bar and not directly to the main rotor blades. Another benefit is the cyclic servos don't have to work as hard to change the pitch angles of the main rotor blades, the fly bar does that. The sensitivity of the rotor head can be adjusted by moving weights on the fly bar further out towards the paddles. With more mass further out on the fly bar, the harder it will resist any tilting. When you are learning to fly, you will want the weights further out on the fly bars. Once you start performing simple aerobatics, you will want to move the weights closer in so the helicopter responds quicker with less dampening. BELL ROTOR HEADThe Bell rotor head has no fly bars, this is what is used on full size helicopters and in the RC heli world is often called a "flybarless" rotor head. The inputs from the swash pate go directly to the main rotor blades for fast and direct response.
There is no dampening and no self correction. This makes a Bell rotor head very sensitive - too sensitive in some cases for RC helicopters. There are also greater amounts of force on the cyclic servos because they have to be doing all the work in trying to change the pitch angles of the main rotor blades by themselves. Not really an issue with the digital, higher torque servos available today. Flybarless rotor heads are not used much on RC helicopters, but for people who fly larger scale RC helicopters they are desirable for a more scale like appearance since real helicopters don't use Hiller heads. Flybarless heads are the only option if you are using multi (more than two) bladed rotor heads for even more scale appearance. The sensitivity can be reduced by increasing the mass of the rotor blades, weight of the helicopter, and increasing the rotor blade dampening in the head (just like on a real helicopter). The latest way to deal with this extra sensitivity problem of using Bell rotor heads on RC helicopters is to use electronic stabilization. Basically a special two axis gyro that senses pitch and roll is used to stabilize the cyclic sensitivity, just like the tail rotor gyro. I have read great reviews of this new system and can't wait to try it out. It is not just for scale either - with no more fly bar, there is less overall drag and more power can go directly to the main rotors. This also means smaller sized helicopters can have Bell Rotor heads and loose the fly bar for both scale appearance or improved 3D performance. If this new gyro technology proves to be reliable, we may even see the end of the Hiller rotor head and Bell mixing all together. BELL MIXINGBell Mixing as the name suggests mixes both Hiller and Bell heads together. You get the self correction and dampening qualities of a Hiller head, and the direct response that is given by a Bell head. To sum up: The best of both worlds.
This picture shows a typical bell mixing set up. Note the pushrods from the swashplate goto one half of the "seesaw" bell mixer on the main blade mount. The other half of the bell mixer has a small pushrod that connects to the flybar. This way the input from the swashplate is dampened by the flybar, but also gives an immediate response to the main rotor blades.I must point out that the smaller the helicopter the more responsive and sensitive it is. So for small electrics and micros, Hiller heads with out mixing work just fine. The faster response of Bell mixed Hiller heads only comes into play with mid size and large RC helicopters. You don’t have to fully understand all the physics behind how this mixing works, but you should at least know what it does and realize that it is something you should have if you want quick responding cyclic controls. If you want a gentle flying larger helicopter or are only interested in scale type flying you will not need the quick and precise control provided by a bell mixed rotor head. If your goal is to move up to 3D aerobatic flying, then Bell Mixing becomes almost necessary, especially on helicopters with rotor diameters larger than 1000mm.
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