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John's R/C Flying Pages


Radios

Modern radio-control equipment is reliable and relatively inexpensive. When I first flew R/C models in the mid-1970's a good 6-channel radio would have cost around £180. Twenty-five years later the same is still true. Additionally, today's radios have features not even found on top of the range sets of the 70's.

Most radio gear is manufactured in the far east. In the UK at least, Futaba and JR dominate the scene. Where I fly most people use Futaba gear. It is probably wise to 'follow the herd' and use the make that is most popular where you intend to fly. Transmitters of the same make can usually be connected together for training. The student can then fly and the instructor can take over control at the flick of a switch. Much more reliable than hastily passing a single transmitter backwards and forwards! Furthermore there's plenty of expertise to hand on the flying field to help with any radio-related problems if you choose the most popular equipment.

A typical radio set consists of a transmitter, receiver, usually 4 servos, rechargeable batteries (with a charger) and possibly a few extra small accessories. Unless you're only going to fly simple gliders you'll want at least a 4-channel set. One channel is required for each 'control' on the plane. You may only need throttle, rudder and elevator on a trainer, but your next model will probably have ailerons as well. Later models may have flaps and retractable undercarriage - this would require a 6-channel radio.

The transmitter

A typical (though now slightly outdated) transmitter is shown here. Most transmitters are broadly similar in layout, whilst differing in details. The frequency that a transmitter operates on is controlled by a plug-in crystal.

In the UK 36 frequencies are available in the 35Mhz band between 34.950 and 35.300 Mhz. These frequencies are often referred to as channels and are numbered (bizzarely) from 55 to 90. They are legal only for aircraft. Boats and cars have a frequency allocation around 40Mhz. Another frequency band is available at 27Mhz, though it is shared by land and air models and is more liable to interference from 'other' sources, consequently it is not much used now. Almost totally unused is a further frequency band at 459Mhz.

Transmitters (and receivers) are sold for one 'band', ie. 35, 40 or 27 Mhz and the specific frequency is determined by the crystal that is used. This may be changed if required to operate on a different frequency within the same band. Note that imported American radios working on 72Mhz cannot be legally used in the UK without conversion. More expensive transmitters often have the components specific to the frequency band contained in a plug-in module which could be replaced to convert a 72 Mhz transmitter to 35Mhz operation.

The transmitter will have the 4 main functions controlled by two joysticks. There are many possible configurations but in practice only 2 are common. In Mode 1 the throttle and aileron controls are on the right-hand stick with rudder and elevator on the left. This mode is said to be popular in Europe. Nearly everybody I've ever flown with uses Mode 2. This has elevator and aileron on the right and throttle and rudder on the left. N.B. If a plane does not have ailerons then the rudder should be controlled by the aileron stick, the rudder stick will then have no function.

Whilst it is obvious to most people that pushing the rudder or aileron stick to the right should make the plane turn to the right, the orientation of throttle and elevator controls is not so obvious. In fact full throttle is got by pushing the throttle stick forward or up. Down elevator - to make the plane dive - is also got by pushing the elevator stick forward or up.

'Computer' radios now cost little more than a non-computer radio and are now worth considering right from the start. These permit functions that, whilst not essential, can make setting-up, trimming and flying a model much easier. Some of these functions (typically dual-rates and servo-reversing) can be found on non-computer transmitters as well.

Some Transmitter Features

Servo Reversing Reverses the direction a servo moves for a given movement of the stick. Usually available on all channels.
Dual Rates At the flick of a switch reduces the amount the servo moves in response to a given movement of the control stick. Often limited to ailerons and elevator only
ATV Sets the distance the servo moves in response to the control stick. Particularly useful for throttle control to get a reliable idle with the throttle stick fully back and full throttle with the stick fully forward
Exponential Softens the response when the control stick is near the centre.
Mixing Probably the 'key' feature of computer radios for many people. Mixing allows one function to influence another one. This is required for two reasons.

Firstly it may be essential due to unconventional control surfaces on the model. Eg. a V-tail model has neither elevators nor a rudder, the two tail surfaces each take part of the job of both. A V-tail mix mixes the rudder and elevator channels at the transmitter to produce the required combination of movements of the two tail surfaces on the model.

Secondly mixes can be set-up to make a plane easier to fly. Biplanes often need to be turned using both aileron and rudder. A mix can be set up to automatically apply some rudder when the ailerons are moved. Unwanted interactions between controls can also be reduced. Eg. On many planes application of full rudder, as well as yawing the plane can cause the nose to pitch down. This makes learning knife-edge flight (where the plane is rolled through 90 degrees and continues flying with the wings vertical and the rudder acting like the elevator) rather difficult. A little up elevator mixed-in with rudder can improve this.
Buddy Box Connecting two transmitters together for training purposes. One transmitter is the 'master' and can pass control to the 'slave', and rapidly take control back again!

Receivers

The receiver (as you'd imagine) receives the radio signal sent from the transmitter, decodes it and passes information to the servos to move the controls. All receivers in common use are 'superhets.' They work by mixing the incoming signal with an internally-generated radio signal. One of the products of this is a signal whose frequency is the difference of these two signals. This can easily be selected and processed. Ordinary domestic radios also work this way. This process allows the radio to receive a signal on, for instance, 35.010Mhz whilst ignoring one on 35.020Mhz.

The frequency that a receiver picks-up is controlled by a crystal, just as the transmitter is. However the crystal is used to generate the other signal that is mixed in the receiver and so is different to the transmitter crystal. In practice all crystals are marked with the channel number and an indication of whether it is for the transmitter or the receiver. Don't swap them over!

There is a further complication! Double-conversion receivers do the mixing bit twice. This gives even rejection of adjacent channels but requires a different crystal again. 'Ordinary' and double-conversion receiver crystals are not interchangeable with each other!

OK so far? Well there's more! Different receivers exist for PCM radios. What's that then? PCM is a newer and - in theory - better way of encoding the information (stick positions) passed from transmitter to receiver.This information is sent as a series of 'packets' of data. If a packet gets corrupted by radio interferance the receiver will sense this and ignore the package. If after a short time (say half a second) it hasn't received any 'good' data it will switch to 'fail-safe' mode. This will either mean holding the controls at the last known position or (in my opinion the preferred option) selecting low throttle and centralising the other controls. The 'ordinary' receiver (or PPM as it's usually known) will attempt to respond to any interferance it may pick-up, usually resulting in rapid, random movements of the controls until the interferance ceases.

Two final points on receivers. The receiver needs to be protected from vibration and shock, and must be wrapped in foam rubber. Secondly the aerial wire should be kept reasonably straight, kept away from other metal objects, must not be folded back on itself and must certainly never be cut shorter! Doing any of these can dramatically reduce the distance that the radio will work at.

Servos

The final link in the radio is the servo. The servos are plugged in to the receiver and convert the signal from the radio to a rotational movement which is used to move a control surface on the model.

A vast range are available for different uses. Most manufacturers produce a standard servo for general use at less than £10. For around three or four pounds more the same servo is usually available fitted with a small ballrace to reduce the wear on the output shaft which will cause unwanted movement of the servo arm. Miniature and micro servos are available for small (and even indoor) models. Larger, more powerful servos are also available for bigger models.