RC ROTOR HEADS AND BELL MIXINGTo understand what Bell mixing is and how it works, we 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 flybar with small "paddles" at each end. This flybar is usually orientated at a 90 degree offset angle to the main rotor blades although smaller, easy to fly, micro RC helicopters will often orientate the flybar at approximately a 45 degree offset for stability reasons. The flybar is allowed to pivot up and down in the Hiller rotor head so it can stay in a level horizontal plane independent of what the main rotor and helicopter tilt angles are.
If a gust of wind hits the helicopter and tilts it sideways for example, the flybar wants to stay in a horizontal plane. Since the flybar stabilization movement is transmitted to the main rotor blade mounts, it automatically help stabilizes the heli by changing the pitch angles of the main rotor blades to absorb the gust. This doesn't mean the heli will hover "hands off" when a gust of wind hits it, but it does absorb the gust quick enough that a human's reaction time is now able to correct for it. This "absorption" also dampens the cyclic inputs so the helicopter is not as responsive or sensitive. The reason this happens is because the swashplate movements goto the flybar and not directly to the main rotor blades. Another benefit is the cyclic servos don't have to do all the work to change the cyclic pitch angles of the main rotor blades, the flybar also helps pitch the rotor blades and takes much of the load off the servos. The sensitivity (responsiveness) of the Hiller rotor head can be adjusted by moving weights on the flybar further out toward the paddles or in closer toward the head. With more mass further out on the flybar, the harder it will resist any tilting and the bird will be more docile and absorb more movement. This is why most beginners will want to move flybar weights further out. Once you start performing simple aerobatics, you will want to move the weights closer in toward the head so the cyclic control responds quicker with less flybar dampening. The size of the flybar paddles and how far out they're placed from the rotor head also has an effect on the cyclic response rate and the amount of overall stabilization/absorption, but for most RC helicopters and folks who are just starting out, those two variables are non adjustable so I'm not going to get into that. BELL ROTOR HEADThe Bell rotor head has no flybar, this is what is used on most full size helicopters and in the RC heli world is often called a "flybarless" rotor head. The inputs from the swashpate 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 most 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 haven't been used much on RC helicopters, but for people who fly larger scale RC helicopters they are desirable for a more scale like appearance since most full size helicopters don't use Hiller heads. Bell heads are also the only option if you are using multi (more than two) bladed rotor heads for even more scale appearance. Times are changing however, and with the improvements in electronic flybarless systems (also known as virtual flybars), more and more people are flying these days with true Bell type rotor heads. I to have converted over to flybarless on a couple birds and absolutely love it. If you want to learn more about electronic flybarless stabilization systems, that link above takes you to my write-up on them. So What Is Bell Mixing Then?Bell Mixing as the name suggests mixes both Hiller and Bell heads together. You get the stabilization and dampening qualities of a Hiller head, and the direct control response that is given by a Bell head. In other words - 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" 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. These mixing seesaws can also be mounted on the Flybar control frame as is common on all Align RC helicopters using what is called the SF mixing arms. Most higher end RC helicopters will also have the ability to adjust the amount of Bell Mixing using different spacing/pushrod geometry on the seesaw mixers or SF mixing arms to allow more or less flybar dampening. This can make the heli seem more responsive as if a true Bell type head was used or more docile as if a non Bell mixed Hiller head was used. It's pretty hard these days to find any Hiller type RC helicopter rotor head that doesn't employ Bell Mixing since Bell mixing works so well, but you may run across a non Bell mixed head on some of the smaller fixed pitch micro sized helis.
|
RC Heli Help E-Books
Beginners Guide To RC Helicopters
Getting The Most Out Of Your mSR/X & 120SR
Setup & Tips For RC Helis
Swash Setup & Levelling
How To Build Training Gear
