A look at primary 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 will make life a lot easier for you in your early days as a model heli pilot.
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.
Emulating the real helicopters are the Collective Pitch (CP) models which, although harder to learn on, are more agile and smoother to fly.
FP or CP, what's the real 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).
But in truth, the terms 'FP' and 'CP' are a little misleading because both terms only refer to the collective (ie altitude) control of the helicopter. The cyclic (directional) 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.
This 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 swash plate, controlled by the servos - one for sideways movement and the other for fore/aft movement.
Above, the rotor head assembly of a Fixed Pitch rc helicopter
As the flybar rotates in response to the tilting of the swash plate, 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 ie if one rises then the other falls, and vice versa.
This in turn exaggerates the movement of the flybar, 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 so leans to the side that is experiencing lesser lift, thus changing direction.
So, the primary difference between FP and CP helicopters is in the collective control, and this is influenced by the lift generated by the main blades acting together ie 'collectively'.
In an FP model the main blades are fixed to the main rotor head and cannot be pivoted about their longitudinal axis. Altitude has to be controlled by the speed of the blades (ie motor speed) - faster spinning blades generate more lift and vice versa.
But in a CP model 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.
Above left, an FP rotor head compared to the more complex CP one, right
Collective pitch is essential for any form of aerobatic flying, especially inverted flight where negative blade pitch is a necessity.
RC helicopter control 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 does. 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
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 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, and normal stick movements 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 heading to a fairly accurate degree, until a further change is made.
HHGs 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).
RC helicopter mixer boards/RXs
Most rc helicopters use micro-processor mixer boards to save space and weight. The electronic circuit board based unit is a device that typically combines the receiver, gyro and motor electronic speed control (ESC for electric helicopters). A further function can be CCPM, or Cyclic/Collective Pitch Mixing, but this is usually just found on the more advanced helis.
The mixer board unit is about the same size and appearance as a standard rc receiver and weighs considerably less than using separate devices.
Coaxial RC helicopter controls
Coaxial rc helicopters have 2 sets of main rotors mounted one above the other, and no tail rotor.
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, so altitude is controlled by adjusting the motor speed and hence the main rotor RPM and the associated amounts of lift generated.
Less controllable RC helicopters
There are many 2 channel rc helicopters available these days, but they aren't that 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 piloting 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.
Related pages
Electric rc helicopters - an overview of this very popular section of the hobby.
Coaxial rc helicopters - what they are and how they work.
How helicopters fly - a look at how real helis do what they do.
Transmitter modes for rc helicopters - the different set-ups in use.
RC airplane controls - learn about the primary plane controls.
