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 Member articles
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A Word About Radio Performance
by Dallas Carter
Many have pondered the mysteries of how these amazing little boxes can
control our fantastic flying machines with such precision and reliability.
I am not here to unveil these mysteries, but to shed some light on our day
to day interaction with these little magic boxes. We all know that the
antenna must be fully extended and the batteries fully charged in order to
ensure reliable performance. What we don't all know is what is going on
when we are trying to get these little monsters..... erh... magic boxes
pre-flight checked. We see such things as control surfaces that flutter
erratically or that don't move at all. What's going on here?
What's a Channel?
Let's look at some of the basics of radio control without getting too
technical. That is to say that I am not going to try to explain the
technical nuances of PPM or PCM modulation techniques. Suffice it to say
that these radios are very efficient in interpreting our manual thumb
movements and applying them to functions in our models. First let me
discuss a term that gets transposed in some of our discussions. That term
is channel. The reason it is confusing is that there are different
applications of the term within our own sport. There are two types of
channels that we normally talk about. One is the
frequency channel,
or the numbered radio frequency on which the radio transmits it's signals.
The second is the servo
channel. This refers to the
assigned position of the control signals within the radio frequency
channel. Here I am talking about PPM radios, as PCM gets much more
complicated. The result is the same and, other than the inherent advantages
associated with PCM, is transparent to the user. When discussing servo
operation, I will be discussing the more common analog servos rather than
digital servos. The position within the radio channel, the servo channel,
is reserved for the operation of a specified servo. In other words, the
first servo channel (normally Aileron) only causes an action to be applied
to the servo that is connected to the receiver servo channel of the same
number. Applying control inputs to channel three therefore will not have
any affect on servos connected to servo channel four. The exception to all
of this is of course when servo channels are coupled or mixed in the
transmitter. Control signals generated on a frequency channel like 25 by a
properly aligned transmitter, cannot cause any servo channel response
or interference to a properly aligned receiver on channel 24, 26, or any
other channel.
Who's on First?
One of the standard practices that you may have heard of is to ensure that
you turn on your transmitter before turning on the receiver. When you have
completed your task the standard practice calls for the receiver to be
turned of first, and then the transmitter. The reasoning behind this
recommendation hinges on the premise that when the receiver is powered on
first and as the receiver stabilizes, random energy may be applied to
various servo channels. This may result in one or more servos moving to an
undefined position. As the transmitter is energized, with the controls
centered, the servos on the model will be immediately commanded to go to the
centered position. This sudden, possibly radical movement might cause
damage to the gearing within the servo. Turning the transmitter on first
generates a centered signal on each servo channel so that when the receiver
is turned on (with the control surfaces manually centered) little or no
change in position will occur. Turning the receiver off before the
transmitter is just a bit of insurance against any unanticipated control
input while the plane might be running.
Why am I Fluttering?
Have you ever noticed that when you are checking out your system and the
receiver is powered up with no transmitter on that the controls are stable,
as they are when the transmitter is on. As you proceed to perform your
range checks, certain control surfaces may begin to flutter. Why is it that
with a good signal, as with no signal, the controls surfaces are stable, but
with a marginal or weak signal the surfaces flutter? The reason is that in
order to cause a servo to change position, it requires a specific signal in
the receiver's servo channel. When there is no signal in the channel, the
servo position will remain constant. When the transmitter is on and
providing a strong signal, the servo channels will contain a control signal
that is relative to the position of the control stick on the transmitter,
which is normally centered or fully down in the case of throttle. As the
transmitted signal starts getting weak, it reaches a point at which the
servo channel control signals fade and the signal that is interpreted in the
receiver becomes unstable. As a result, the control surface will begin to
flutter. If the signal decreases further, a point will be reached when the
flutter stops. This occurs when the receiver now sees no control signal and
returns to the same stable condition as when the receiver was turned on
without a transmit signal present. It is only when the signal is marginal
that the flutter occurs, as the receiver cannot properly interpret the
desired servo control signal. Therefore, flutter in a control surface when
performing range checks indicates that the signal is not being received at
sufficient level to provide stable control.
Range Check Flutter
What does it mean if, as you are conducting your range checks, certain
control surfaces start to flutter within the prescribed 100 foot range?
There are actually a great number of possible causes. At the risk of
leaving some out, I will try to enumerate a few. The first step in resolving
the problem is to identify the source of the problem. This should be done
in a sequential process starting with the simplest to evaluate to the most
difficult. I have tried to list these in order of simplicity, although some
times, when you look at the list, the answer will just jump out at you.
1. Weak transmitter battery
2. Weak receiver battery
3. Receive antenna obstruction
4. Broken antenna wire inside transmitter
5. Broken receiver antenna wire inside or outside of model
6. Damaged transmitter
7. Damaged receiver
8. Transmitter out of alignment
9. Receiver out of alignment
Evaluate The Problem
Weak batteries are by far the easiest thing to check. Going down the list,
other causes may be more elusive. Receive antenna obstruction is by far the
most common cause. It is not always the easiest to evaluate and there are
many items that can cause this problem. If it is not obvious that this is
the problem, you might want to eliminate the other causes and then come back
to this one. Remember that your receive antenna should be routed as far as
possible from other metal conducting parts. These include steel control
push rods, servo and other electronic wiring. Consider that if your model
is covered with a simulated metal finish, such as chrome or aluminum
covering or metallic paints, these will form a shield around an antenna
routed within the model, blocking the signal. Antennas routed inside the
model are always suspect, but certainly some of these installations are
perfectly acceptably. An internally routed antenna on the bottom of the
fuselage may also place it so close to the ground, that while conducting
range testing, the ground absorbs enough signal to induce flutter. I am not
too concerned about this effect, as when the model is in the air, it's no
longer a problem. One test for this is to tilt the model up on it's nose
and see if the flutter goes away. Remember that no two installations are
alike. What works in one case might not in another.
Antennas
Broken antenna wires can only be found by physically inspecting them. It's
not a common problem in either transmitters or receivers, but is a
possibility. It's chance of occurring in a receiver increases as the
receiver is reused in model after model. In the transmitter, it might occur
if you are inside the unit replacing a battery or something like that. The
wire that I refer to is one that is soldered to the transmitter printed
circuit board and then is usually fastened to the antenna assembly with a
screw and lug. Damage and alignment issues are normally indicated by
reviewing the history of the unit. If you have crashed a model and are
performing the range test in the repaired model, (and it checked ok before),
it is likely that the receiver was damaged.
Crystals
If you have changed the frequency channel crystal in either the transmitter
or the receiver, that might be an indication that there is an alignment
problem. Most transmitters require that a qualified service facility align
the transmitter on a new frequency channel. As the transmitter frequency is
changed, the alignment of the power amplifier must be checked in order to
ensure that full power is coupled to the antenna assembly. This does not
apply to transmitters that have a "Frequency Module". These units have all
of the necessary alignment done within this module.
Frequency Excursions
Many modern receivers are broad enough to accept frequency channel changes
within certain ranges. The most common are referred to as "Hi" and "Low".
Receivers that have been adjusted for Hi range are aligned near the center
of the high number channels, around channel 55. Receivers that have been
adjusted for Low range are aligned near channel 25. If you have re-crystalled
a receiver to a channel outside the band to which it was aligned,
performance may have been degraded enough to cause the flutter.
Beware of That False Sense of Security
Note that whereas swapping receivers or transmitters may correct the
problem, it does not necessarily mean that the replaced unit caused the
original problem. Receiver sensitivity and transmitter power output may
vary enough to show an improvement in a specific case without addressing the
real problem. It's ok to fly in the new configuration, but don't assume
that the replaced unit is defective. It might work just fine in another
model. When in doubt, have the unit serviced.
What's All The Buzz About?
The next item is that annoying buzzing that you here from the control
surface servos when you have powered everything on. There are two primary
causes of this, and they may be related in a subtle way. The first, and
simplest to explain away is those cases in larger models where 6 volt
battery packs are employed to develop more servo power and speed. The
servos are designed to operate on 4.8 volts, and the control signals from
the receiver are a series of pulses. The width of these pulses determines
the precise position of the servo at any given point in time. When the
servos are significantly over powered, minute fluctuations in pulse width
are amplified to the point that slight movement occurs. As the fluctuation
varies to the other extreme, again these are minute, a slight movement
occurs in the opposite direction. As these fluctuations are very rapid, the
result is a buzz. This situation is normally self correcting. As the
battery voltage decreases slightly, the buzzing will stop. The other common
cause is with control surfaces that have tandem, ganged, push-push or
push-pull servo arrangements. If the servos are not very carefully centered
when coupled to the control surfaces, they will tend to fight against each
other. This can also occur when pressure is applied to a control surface in
one particular direction while at rest. This is most commonly associated
with a tail wheel that has some side pressure applied while on the ground.
The side pressure is transferred to the servo which tries to maintain a
centered position. As it applies a correcting mechanical action, it is
countered by an opposing electrical action. A rapid oscillation occurs and
a state of equilibrium cannot be reached. The condition is usually
corrected by tapping the rudder control in one direction or the other.
I hope that this information is found to be useful. I welcome your comments
and or corrections. If you have additional questions, I would be pleased to
try to shed more light on the subject. I can be contacted at <sacrc@sacrc.com>.
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