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
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
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
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
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.
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
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 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
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 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
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
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.