23.3. From the upper swashplate to the rotor head

There are three systems which are widely used to control the rotor head from the upper swashplate:

23.3.1. The Bell control system

For a Bell control system, the upper swashplate is directly connected to the main blade grips. Usually there are two main rotor blades and these are directly connected to two control balls on the upper swashplate. This is sometimes called a "flybarless" control system.

One advantage of the Bell control system is very quick cyclic response. The control system directly controls the main blade pitch, so the system is very sensitive to swashplate changes.

One disadvantage of the Bell control system is the lack of stability. The system is very sensitive to minor gusts. It also stresses the control linkages because they control the heavy rotor blades directly and therefore strong servos must be used.

The early helicopters using Bell control systems had a "stabilizer bar" which was perpendicular to the main rotor blades, which had heavy weights attached to the ends (similar to a flybar but with weights instead of paddles and no tilting). This added stability to the system.

There are no popular helicopters which implement a pure Bell control system.

23.3.2. The Hiller control system

The Hiller control system was invented by Stanley Hiller in the 1940s. This was originally called the "Rotormatic" control system, and was so stable the first prototypes could be hovered hands-off for minutes at a time.

For a Hiller control system, the upper swashplate does not directly control the main blade pitch. Instead, it indirectly controls the main blade pitch by changing the pitch of the flybar paddles. As the flybar seesaws up and down, it changes the main blade pitch.

Two advantages of the Hiller system are:

  • It places less stress on the swashplate servos because they only control the pitch of the flybar paddles.
  • The flybar paddles dampen pitch and roll which improves stability.

One disadvantage of the Hiller system is the lag in control response. The flybar paddles must change their plane of rotation in order to change the main blade pitch.

Two popular helis which implement the Hiller control system are the Piccolo ECO/Fun and the GWS Dragonfly.

23.3.3. The Bell-Hiller control system

The Bell-Hiller control system is a hybrid of both the Bell and Hiller control systems. The key component of this system is the Bell-Hiller mixer which mechanically mixes both the flybar tilt and the swashplate tilt.

There are two basic types of Bell-Hiller control systems:

  • Nonmovable flybar systems
  • Movable Flybar systems

In a Bell-Hiller system with a nonmoving flybar, there are usually four control balls on the upper swashplate.

Two of the control balls on the upper swashplate are connected to the washout unit which is a mechanical isolation device which allows the flybar to tilt with the swashplate, but not move up and down.

The other two control balls are connected directly to the Bell-Hiller mixer. The tilt of the flybar also affects the Bell-Hiller mixer. The output of the Bell Hiller mixer controls the main blade pitch.

So, in a Bell-Hiller control system, the swashplate controls the flybar pitch and the flybar tilt affects the main blade pitch. This is the Hiller component of the Bell-Hiller control system.

The swashplate tilt also affects the main blade pitch through the Bell-Hiller mixer. This is the Bell component of the Bell-Hiller control system.

Two popular helis which use a Bell-Hiller system with a nonmoving flybar are the ECO 8 and Logo 10.

The Bell-Hiller system with a moving flybar works exactly the same as the nonmoving flybar system except the washout unit is eliminated and the flybar is allowed to move up and down with the swashplate.

Advantages of the Bell-Hiller system include:

  • Less control force required (as in the Hiller system)
  • More stable than a Bell system (as in the Hiller system)
  • Faster cyclic than the Hiller system due to some direct swashplate input

Disadvantage of the Bell-Hiller system include:

  • Slightly slower cyclic response than a pure Bell system
  • More complexity than either a Bell or Hiller system

The advantage of the moving flybar system is it has fewer parts (no washout unit) therefore has less slop in the control system.

Three popular helis which use a Bell-Hiller system with a moving flybar are the Hensleit 3DNT, Hensleit 3DMP and the Century Hummingbird Elite CP.