Understanding RC helicopter controls

Learning the fundamentals of rc helicopter controls isn't as scary as it might seem, and understanding which function does what on a radio control helicopter goes a long way to making you a better rc heli pilot in the early days of your new hobby!

The primary method of making a real helicopter change direction while flying is through pitch control of the main rotor blades, either independently or collectively.
However, in the rc world in recent years there has been a huge influx of electric rc helicopters that do not have complete independent pitch control - these are known as fixed pitch (FP) rc helicopters and have proven to be very popular with beginners.
Emulating the real helicopters are the more complex collective pitch (CP) models which, although harder to learn on, are more agile and smoother to fly.

FP or CP, what's the difference?

To control an rc helicopter the pitch angle of the rotor blades must be changed in relation to the air flowing over them; this change varies the amount of lift generated by the blades (pitch angle is referred to as Angle of Attack when the blade is moving through the air).

But in truth, the terms 'FP' and 'CP' are a little misleading because both terms only refer to the collective (i.e. altitude) pitch control of the helicopter. The cyclic (directional) pitch control method is basically the same for FP and CP helicopters.

This cyclic control changes the pitch angle of the whole rotor disc, which is the imaginary circle in the air drawn by the tips of the spinning blades, and both FP and CP rc helicopters use a flybar* to influence the angle of the rotor disc.
The flybar is a short rod perpendicular to the main rotor blades. At each end of the flybar is an airfoil paddle and the flybar moves in direct response to the tilting of the swashplate, controlled by the servos - one for sideways movement and the other for fore/aft movement. A flybar also acts to stabilise the helicopter in flight.
*NB: While flybars have been almost universally used on rc helicopters for many years, more recently the development of flybarless systems has taken place, whereby electronic stabilisation units (gyros) are used to act on the rotor disc, and no flybar is used.

Above: the rotor head assembly of a fixed pitch rc helicopter

As the flybar rotates in response to the tilting of the swashplate, so the Angle of Attack of the paddles changes. This effects the amount of lift being generated by them and so they rise and fall accordingly. The paddles always work against each other i.e. if one rises then the other is forced downwards, and vice versa.
This rise and fall of the paddles in turn exaggerates the movement of the flybar assembly, and the end result is that the whole rotor disc tilts in response to the changes in lift being experienced at the paddles. The helicopter becomes 'unbalanced' and leans to the side that is experiencing lesser lift, thus changing sideways and/or fore-aft direction.

So the primary difference between FP and CP helicopters is in the collective pitch control, and this is influenced by the lift generated by the main blades acting together i.e. 'collectively'.

On a fixed pitch rc helicopter the main blades are fixed to the main rotor holder and cannot be pivoted about their longitudinal axis. Altitude has to be controlled by the speed of the blades (i.e. motor speed) - faster spinning blades generate more lift and vice versa.
But on a collective pitch rc helicopter the main blades can be pivoted about their longitudinal axis in relation to the main rotor head, thus changing the pitch angle of them and the associated amounts of lift. The motor can therefore be kept at a more constant speed, and altitude is controlled by changing the pitch angle of the blades.
This set-up gives much more agile performance and altitude control is so much easier and more precise, compared to an FP heli.

Above left, an FP rotor head compared to the more complex CP one, right

Collective pitch control is essential for any form of aerobatic flying, especially inverted flight where negative blade pitch is a necessity.

Radio Control helicopter channels

For an rc helicopter to have proper control there needs to be at least 4 channels - left/right cyclic, fore/aft cyclic, left/right yaw and collective pitch and/or throttle. If these sound confusing, compare them to the 4 primary airplane controls and you'll see the relationship:

Helicopter control	Airplane control	Action
left/right cyclic	left/right aileron	roll
fore/aft cyclic	elevator/thrust*	airspeed
left/right yaw	left/right rudder	yaw
collective pitch/throttle	elevator/thrust*	climb/dive

* Airplane elevator and thrust are shown together because
both influence airspeed and climb/descent.

Taking a basic 4 channel FP rc helicopter as an example, there will be 2 servos controlling the cyclic pitch - one for left/right and the other for fore/aft. The 3rd channel will be main motor speed control and the 4th channel will be tail rotor motor speed (left/right yaw).
This left/right yaw control is used in conjunction with, or against, the natural torque force that is generated by the spinning main rotors; as a natural reaction to the spinning blades, the fuselage of the helicopter will always want to spin rapidly in the opposite direction.

The tail rotor generates sideways thrust in the same way as a normal airplane propeller generates thrust. If the amount of thrust equals the level of torque then the helicopter fuselage won't spin round. If the thrust exceeds the torque, the fuselage will yaw one way and if the torque exceeds the thrust then the fuselage will naturally yaw the other way.

The Gyro

A radio control helicopter gyro The use of yaw control in rc helicopters is made easier by a gyro which is an electronic device that is connected between the receiver and the tail rotor control.
The gyro, technically also called an accelerometer, senses any rotational movement of the helicopter that isn't a result of a signal to the receiver, and it makes fine adjustments to the tail rotor speed or blade pitch to suit the torque force at that precise moment, hence dampening out any unwanted yaw. Gyros make these calculations and corrections at lightning speed, so much so that the pilot doesn't notice anything other than a stable helicopter!

The gyro sensitivity ('gain') can be adjusted by the pilot either directly on the gyro itself or remotely from the transmitter, and normal stick movements made at the transmitter send the receiver rudder signal through the gyro so that the helicopter can be turned onto the desired heading by the pilot.

Heading Hold Gyros go one step further than a standard gyro by performing more complex calculations to keep the helicopter pointing in the direction that the pilot intended. A more definite input is required from the pilot to overcome an HHG, and the new change will be memorized by the gyro which will maintain this new heading to a fairly accurate degree, until a further change is made by the pilot.

Heading Hold Gyros are an ideal solution to flying rc helicopters outdoors in any degree of crosswind. The gyro will prevent the helicopter from swinging naturally round in to the wind, in the same way as a weather vane does (the wind pushing on the tail rotor will force the helicopter round to point in to wind).
HHGs, once an expensive luxury, are now commonplace and almost all rc helicopter gyros have this feature.

RC helicopter control units

An rc helicopter RX/ESC/gyro control unit Most rc helicopters use micro-processor controlled printed circuit boards (PCBs) to save space and weight. The PCB based unit is a device that can potentially (depending on the helicopter type) combine the receiver, gyro and motor electronic speed control (ESC) on electric powered helicopters. A further function can be pitch mixing, again depending on the helicopter in question.

Separate components can of course be used, and often are, when the helicopter is big enough to carry the extra weight without problem, but on smaller RTF electric helis, particularly the cheaper fixed pitch models, a 3-in-1 or 4-in-1 control unit is almost always used to save weight and space.

CCPM

CCPM stands for Cyclic/Collective Pitch Mixing and is commonly found on electric powered CP helicopters. The swashplate has three servos controlling it (aileron, elevator and pitch), typically spaced at 120° to each other, and the servos all work together to control both cyclic and collective pitch as the inputs are made by the pilot.
The mixing is done at the transmitter and a computer radio is needed for this function to be available.

Coaxial RC helicopter controls

Coaxial rc helicopters have 2 sets of main rotors mounted one above the other, and no tail rotor.

Contra-rotating blades on a coaxial rc helicopter

The main rotors spin in opposite directions to each other, thus cancelling out each other's torque force. So, because there is no torque when both rotors are spinning at the same speed, the fuselage of the helicopter has no tendency to spin round in any particular direction.
Directional control is achieved by changing the speed of one of the rotors in relation to the other. In doing so, a small amount of torque is then generated and that force will naturally cause the helicopter to yaw one way or another, hence changing direction.

Coaxial rc helicopters are fixed pitch and so altitude is controlled by adjusting the motor speed and hence the main rotor RPM, thus changing the associated amounts of lift generated.

Less controllable RC helicopters

Less controllable 2 channel rc heli There are many 2 channel rc helicopters available these days but they aren't very controllable and you need to be aware of this when considering buying such a radio control helicopter.

The 2 channel helis have no pitch control whatsoever - only main and tail motor speed control, and sometimes this might only be a simple on/off function. Their simplicity means they are cheap to buy, which is great, and they are still good fun to fly, but they don't give you an accurate experience of flying a radio control helicopter.

The downside to such helicopters is that the torque force is only vaguely compensated, and as a result the helicopter will often spin round during flight, particularly when motor speeds are changed and at take-off.
With some degree of trimming and tweaking though, it is possible to get them flying in wide circles in a controlled way to produce some realistic looking flights, as my brother demonstrates in this video [opens Windows Media Player] with a Syma Dragonfly 2 channel helicopter.

With any 2 channel rc helicopter, stable, precise hovering is pretty much out of the question - but they're still good fun!

The important thing to remember when flying any kind of rc helicopter is safety. Even the soft plastic blades of a cheap 2 channel one can do a lot of damage. It goes without saying that understanding how the primary rc helicopter controls influence the helicopter itself is of paramount importance, if you're to learn to fly your heli safely, properly and with confidence.

RC heli glossary

Listed below are some of the more common terms that you'll need to know to get a better understanding of rc helicopter controls...

Angle of Attack - the angle of the rotating rotor blades in relation to the air flowing over it, as viewed from the end of the blade.
Coaxial - a type of helicopter that has two sets of main rotors, one mounted above the other, and no tail rotor.
Collective pitch control - all main rotor blades are moved to the same pitch angle at the same time, thus increasing or decreasing the amount of lift evenly throughout the rotor disc.
Cyclic pitch control - the angle of the rotor disc is changed to 'unbalance' the lift within the disc. As a result, the helicopter leans to the side that is experiencing less lift.
Downwash - air that is forced downwards by the spinning main rotor blades.
Flybar - short bar perpendicular to the main rotor blades, with small paddles at each end. Connected to the swash plate, the flybar tilt/twist influences the angle of attack of the rotor disc.
Flybarless - a new stabilisation technology that allows rc helicopter main rotors to not need a flybar.
Gyro - a small electronic microprocessor-controlled device that makes minute and rapid adjustments to the tail rotor speed/blade pitch, to dampen out any unwanted yaw.
Main rotor - the horizontally mounted blades above the fuselage.
Pitch angle - the angle of the rotor blades in relation to the horizontal, when viewed end-on.
Rotor disc - the imaginary horizontal circle above the helicopter created by the spinning rotor blades.
Rotor head - the central assembly from which the main blades extend.
RX/ESC/Mixer - an rc receiver-looking component that incorporates the Rx, gyro, electronic speed control and pitch mixing capabilities where applicable.
Swashplate - found at the base of the rotor head, it tilts in any direction to control the cyclic pitch of the rotor disc and moves up and down for collective control.
Tail rotor - the vertically mounted blade at the very rear of the helicopter.
Tail boom - the horizontal shaft that runs outwards from the main frame of the heli.
Torque - a natural reaction to any spinning object. Spinning main rotor blades create torque, which forces the helicopter fuselage to spin round in the opposite direction to the blades.
Yaw - the rotational movement of the helicopter about its vertical axis, controlled by the tail rotor.