The Bell Rotor Head, and the Hiller/Bell Mixed Rotor Head.
Bell Rotor Head:
The 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.
In the above photo, notice the pushrods go directly from the swashplate up to the main rotor blade mounts just like on most full size helicopters.
There is no dampening and no self correction offered from a flybar. This makes a Bell rotor head very sensitive – too sensitive in most cases for smaller size RC helicopters unless a flybarless stabilization system is used. 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. On a flybared helicopter, the flybar helps move the main rotor blades as well as the servos. Not really an issue with the digital, higher torque servos available today, but it’s worthwhile noting in case you were upgrading to a Bell head, but didn’t have very strong servos on your helicopter.
Bell heads on RC helicopters can also come in a few different flavors such as DFC or Driverless for example.
Having more blade mass with a multi-bladed head will dampen cyclic response a fair amount, and it actually is very possible to control a multi-bladed Bell rotor head without any electronic stabilization help. That said, it still poses certain challenges such as ballooning and lack of overall stability. I’ve flown a couple multi-bladed Bell heads without a flybarless system and although it’s very doable, it’s far from enjoyable. Without question, the majority of scale pilots today flying muilti-bladed heads will use electronic stabilization.
The photo above shows a typical Hiller/Bell Mixed rotor head. Note the pushrods from the swashplate are attached to the flybar Bell mixers, not directly to the main rotor blade mounts as they are on Bell heads. This dampens or absorbs the sensitivity of the rotor disc to a controllable level mechanically without the need for electronic stabilization. As the hobby progressed, Hiller/Bell mixing was introduced to combine the stability of a Hiller Head, with the response of a Bell head. This type of rotor head remained the popular bullet proof norm for well over 20 years until electronic flybarless stabilization systems started being introduced and went more or less main stream around 2010-2011.
Most higher end flybared collective pitch RC helicopters with Bell/Hiller mixing also give you the ability to adjust the amount of mixing using different spacing/pushrod geometry on the mixing arms to allow for more or less flybar dampening. This way you can shift the cyclic response toward the aggressive side of a pure Bell head, or slow it down towards the pure Hiller head type of response.