Helicopters

Traditional helicopters use three variables to control their flight: Collective, which is where the pitch of the main rotor's blades are all increased by a fixed amount to increase overall lift, cyclic, which is where the pitch of the main rotor's blades are temporarily increased every cycle during a certain part in their rotation to generate more lift in one direction and control roll & pitch, and the anti-torque pedals, which vary the pitch of the tail rotor blades to produce yaw.

There is a well-developed community of RC helicopter users. Control systems on helicopters are inherently suited to a single source of power, and this means that many of them use internal combustion engines rather than the batteries used in multirotors. The way that the miniaturization ended up being economical, hobbyist RC helicopters ended up being rather large, with dangerous blades. As such, they've been available to lift the relatively trivial weight of a camera for a long while now, and many commercial aerial photography professionals use them.

Anatomy

Flybar

• Bell-Hiller flybar explained - RCGroups

Rotor Pitch Concepts

Throttle

Throttle is not used for control in traditional helicopters like it is used in planes. The engine/transmission is optimized to work at one optimal main-rotor rotation rate. Throttle is adjusted manually or automatically to keep the main rotor spinning this fast.

Collective

The collective raises or lowers the main rotor's baseline pitch. This translates to a different climb or descent rate. It has some small induced change in yaw as well.

Cyclic

The thing that makes a helicopter a helicopter is the cyclic controls. Every blade of the main rotor is adjusted over a cycle every revolution of the blade. That cycle corresponds with a static "swash plate", whose angle of tilt and degree of tilt is changed by three servos. These servos are mixed electronically from two controls: pitch and roll. For example: If the pilot wants to roll left, the blades while they are on the right side are tilted to a more aggressive pitch, and while they are on the left side are tilted to a less aggressive pitch.

Yaw

Yaw is controlled by collectively varying the tail-rotor's pitch.

Types

Right now, the cost scaling of the mechanisms in a model helicopter appear to make the traditional type practical for anything large enough to carry a meaningful payload, while the coax and tandem designs are more common in small toy helicopters.

Traditional

One main rotor's yaw is adjusted by one tail rotor.

Fixed Pitch

Cyclic only, using two servos to control the swashplate, and throttle to control height.

Collective Pitch - Mechanical Mixing

Cyclic and collective are varied using complex mechanical means from three independent servos

Cyclic/Collective Pitch Mixing (CCPM)

Cyclic and collective are varied electronically, creating a swashplate orientation from three interdependent servos

Tailrotor

Throttle

These use an independent brushless motor to drive the tailrotor, and throttle to modify the thrust.

Collective

These use a driveshaft to connect to the main motor, and collective pitch to modify the thrust from the tailrotor, exactly like a full-sized heli.

Coaxial

Two main rotors placed on concentric shafts mostly cancel out each other's yaw, removing the need for a tail rotor. This does require some slightly complex differential collective calculations, since the bottom rotor is cutting through a shaft of already-moving air. It's also mechanically complex.

NOTAR

One main rotor's yaw is adjusted not by a rotor mounted on the tail, but by a ducted fan mounted inside the tubular tail, which pulls air through and out a hole in a pivoting drum at the end of the tail. May be more practical if that ducted fan is part of the exhaust of a turbine engine that's already being used for flight. Only seen on a very few models of RC helicopters.

Tandem

Two main rotors mounted parallel to each other use mechanically independent collective and cyclic pitch (6 DoF in total) for all control, from a complex mixing of 3 DoF pilot inputs.

Coanda Ducted-Fan

Very different from the other types of helicopters on this page. Coaxial, ducted main rotors are used which are *fixed pitch*, and control is through a combination of throttle and boundary-layer control flaps at the bottom of the duct.

Scale

Helicopters tend to be easier to fly and more laid back to control the larger they are. Adapting the design directly to RC scale has the problem that they tend to change characteristics faster than human reaction time is capable of adjusting for. Instead, a device rotating along with the rotor blade known as a 'flybar' is used to smooth out control inputs and provide some automatic feedback for changing wind conditions.

• Flybar - RC Heli Wiki

Flybarless designs that substitute an electronic stabilizer for this mechanical one also exist.

Links

• A mature wiki already exists which focuses primarily on electric helicopters at EFlightWiki