Airfield Models - Radio Control Systems

Transmitters for Radio Control Model Airplanes

May 05, 2015

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Back to Model Airplane Radio Control Systems


Airfield Models ( for Radio Control Model Aircraft

Trying to make sense of the feature list of various radios is enough to bewilder anybody.  Making recommendations is also difficult.

Some people advocate purchasing an entry-level, 4-channel radio so that if a beginner chooses not to stay in the hobby, he will not be out as much money.

Others recommend a low-end 6-channel, computer radio because anyone who sticks with the hobby will soon grow into the additional features.

Some people say that even a 6-channel radio will be out-grown and a new person should buy as much radio as they can afford.

My answer to this is to be honest with yourself.  If you've taken on interests in the past and quickly put them aside for something new, then go with a radio on the lower end.

However, if you know this is the hobby for you and you've historically remained involved in your interests for long periods, then go with a better radio.

All of us can use more than one radio eventually, so you really can not go wrong with an entry level unit unless it would be more of a financial burden to buy a second radio in a year or two.


Common Transmitter Features

The most basic 4-channel radio has 2 sticks and an analog trim for each channel.  I would not recommend a radio this basic to anyone.

As radios go up in price, more features are added:


Trims are used to make the aircraft fly in a prescribed manner with the sticks at neutral.  Normally that means straight and level.  For example, if the plane wants to dive then you would have to hold the stick back for the entire flight to keep the plane level if you didn't have trims.

Setting the trim for the elevator channel will tell the servo to move the elevator in whatever direction you specify (up or down) and hold it there.  The stick will still move the elevator up and down, but when you release the stick, the elevator will move back to the position that was set by the trim.

Trims do not have to be used to make an airplane fly straight and level, however.  For example, pilots who fly sailplanes often put in a little trim so that the plane will make large circles without using the sticks to steer it.

Other than radio control toys, I've never seen a set of R/C equipment that did not have trim levers on the transmitter.  Don't even consider purchasing equipment that doesn't have trim levers on all 4 primary controls.

Servo Reversing

Often a servo moves in the opposite direction from what is needed.  For example, moving the rudder stick to the right should make the rudder move to the right.  If the rudder moves to the left then the control is backwards.  Sometimes the problem can be fixed by moving the pushrod to the opposite side of the servo.

Unfortunately that is often not possible.  Servo reversing is the most basic of features added to radio systems and probably the best.  A simple flip of a switch changes the servo direction.  What that means is that we do not have to open servos and switch wires around like we used to.

I would not buy a radio that did not have servo reversing on all of the primary channels.

Dual Rates

Before rates came along we had to make compromises in setting up control surface throws.  If we wanted a plane to be wildly aerobatic then we needed a lot of throw.  Unfortunately, this also made the plane more difficult to control in normal flight because the controls are too sensitive.  The use of exponential (described farther down the page) can help in this regard if the radio doesn't have dual rates.

Dual rates allow us to have two control surface throw settings.  For normal flight we might want the elevator to move 1/2" both up and down.  But for aerobatics we may want to double that.  Flipping the rate switch changes between rates.  Dual rates can be on any or all channels depending on what the manufacturer chose to include.

I would not buy a radio that did not have dual rates on the aileron, elevator and rudder channels.

Triple Rates

The same as dual rates but has three rates.  I doubt I'll ever be a good enough pilot to use triple rates but you may be.  This is not a feature you'll need for your first radio.

End Point Adjustments (aka Adjustable Travel Volume)

The absolute best way to set control surface throws is mechanically.  Any time we use an electronic system to achieve the same goal we are not giving a servo the best mechanical advantage.  However, it is sometimes not possible to get the exact amount of throw we want.

End point adjustments allow us to set how far a servo can move in either direction.  Some systems have a setting for the full servo range.  In other words, if the end point is set to 90% of full throw, it is 90% in each direction (or possibly 95% of the total throw).

Most systems allow the setting of each direction of travel independently.  That means we can set one direction at 90% and the other at 65%.


When exponential and end point adjustments first became available the choice was one or the other.  Now we can have both.

Exponential allows us to change the proportion in which the servo moves with the stick.  For example, if we move the stick halfway in one direction and do not have exponential then the servo moves halfway through its full travel.

With exponential we can set the servo to move less around neutral and more at the extremes or vice-versa.  For example, we can set it so that 1/2 stick movement translates into 1/4 servo movement.  The servo catches up at the end.

Digital Trims

Digital trims use a memory in the transmitter to remember trim settings.  Digital trims are basically an off/on switch.  The trim does not actually move from center.

Pushing it in one direction or the other makes the transmitter trim adjustment electronically rather than mechanically.

Digital trims can be adjusted only when the transmitter is on.  If you bump the trims when the transmitter is off nothing happens.

Analog trims are mechanical.  They are moved from center to trim the plane.  If you bump the trims whether or not the transmitter is off then the trim settings will be wrong.

This is why it's a very good idea to make adjustments to the pushrods until the trims are back at center.  Then you don't need to think about it.  If you bump the trims just center them again and they'll be very close to being right.

Memory for multiple models

Every airplane you will ever fly will have unique settings.  It used to be that when we used a single transmitter for more than one plane we had to reset all the trims each time.

With model memories we can fly several planes from the same transmitter by simply telling the transmitter which plane we are going to fly.  The settings for that plane are saved in the memory so that no adjustments need to be made after they are initially set.

If you make trim adjustments during the flight they get saved in that model memory but do not affect the other models.  You can switch to fly a different plane and then come back to the other and the settings will be just as you left them.

Channel Mixing

Channel mixing is pretty much what defines a "computer" radio.  Mixing instructs one channel to command another.  For example, the rudder channel may be mixed to the aileron channel so that when the ailerons are moved the rudder moves too.

Another common use of channel mixing is flaperons.

Trainer switch

Beginners do not need a trainer switch on their radio.  The trainer switch is on the instructor's radio.  By hooking up the instructor's radio to the student's radio using a buddy cord the instructor can hold the switch to give control the student.  The instructor simply releases the switch to regain control.

Before buddy boxes training was performed by handing the transmitter back and forth.

Buddy box compatibility

A radio that is buddy box compatible has a receptacle to receive a trainer cord.  I would not buy a radio that is not buddy box compatible.

Spread spectrum technology which has been in use for many years for military radios is now available to us.  I've never used it but I sort of understand how it works.  Instead of having a dedicated frequency (channel) for your radio you don't use any single frequency at all.  Instead the system constantly changes frequency many time per second.  The receiver is "bound" to the transmitter so it knows what signal to accept and what to reject.  In theory it's nearly impossible for your receiver to respond to the wrong transmitter.  The odds of it happening are astronomically low.

Spread spectrum has been in mainstream R/C use for only a few years and has been very successful.  The people I know who use it have nothing but good things to say about it.  No glitches, no shoot-downs, no problems.



Overview of Radio Control System Components
Servos for Radio Control Model Airplanes

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Copyright 2004 Paul K. Johnson