It is also good practice to make use of the "Safety CHx" custom function to define a safety switch for the throttle
channel of electric models. Select your throttle lock switch, select the correct function for your throttle channel, set
the value to -100, then tick the box. While you should always set up your model without it being powered, or at
least without a prop mounted, the safety box is there to avoid forcing the channel to the default value of 0 (mid
throttle) while browsing the function list if your switch is active.
The "Instant Trim" custom function can be used if you expect your model could be badly out of trim on the first
flight, see the
Custom Functions
section for a full description.
Once the flight is over, the "Trims -> Offsets" function at the bottom of the SERVOS page can be used to transfer
the trim contents into the subtrim settings. Be aware that unless the servo mode is set to "=" an excessive subtrim
amount can lead to dissymmetric throws and influences settings like differential.
Advanced features
Flight modes
Flight modes in OpenTX are relatively simple compared to most radios. The settings are simple: A name (displayed
on the main views), a switch to activate them, a trim setting and 2 fade in/out settings. Yet they are very powerful,
because the main settings are actually just located somewhere else: in the D/Rs and mixers. Each of these has a
flight mode selection list, that will determine in which one(s) they are active. So (nearly) everything is done through
dedicated mixers. The mixers that are controlled by a flight mode will see their activation fade in/out according to
the mode's settings. As the trims can be made flight-mode specific (they are by default), using flight modes to
activate things like gear or flaps allows using the separate set of trims to counter the extra drag that often causes
an effect on pitch.
Telemetry values
The following will assume your radio is loaded with a microSD card and a valid voice pack, which should be the
case as it is supplied with the radio. Probably the most important telemetry value is RSSI, which is an indication of
how much signal the receiver in the model is seeing from the radio, and will warn you in advance if you are at risk
of losing control be it due to external interference, excessive distance, badly oriented or damaged antennas etc.
The
telemetry settings
page gives you 2 alarm levels you can set that will be announced in clear voice ("RF signal
low" and "RF signal critical"). They are set by default to levels that have been considered suitable and safe for
normal line of sight flight (45 and 42), but if you want to adjust them yourself the following explanations will be
useful:
RSSI on FrSky equipment is represented using a logarithmic scale (dB), not in %. This means that when RSSI is
high, a small difference in distance between the transmitter and receiver will lead to a big change of the RSSI
value. It is perfectly normal to see a value of about 100 when next to the model, and already down in the 70's by
the time you've walked to the other end of the field. When you have a reading of 50, it will however take a lot of
extra distance to reach the alarm level of 45. The basic approximation rule is that a doubling of distance between
pilot and model will result in a drop of 6dB of the RSSI value, which should make the previous explanation clear: If
you are 5m away from the model it only takes another 5m to reduce RSSI by 6dB, but if you are 600m away from
the model it will then take another 600m to reduce the value by the "same" 6dB.
Loss of control will happen when RSSI reaches a value of about 38, so setting an alarm at 10 is useless. From the
above explanation you can see that between the default critical alarm (42) and usual practical loss of control we
have a margin of about 4dB, or range factor of around 1.5. The alarms are thus rather conservative, and in normal
conditions even if you heard the critical alarm you would still be far from losing control (you should be at around
>1000m distance, with another 500m to spare). Intermittent loss due to local fades and antenna orientation
mismatches could however get more and more frequent.
As mentioned the default alarms are deemed safe for usual line of sight flight, but for example with FPV setups
especially when coupled with automatic return to home systems and properly configured failsafes the safety margin
can be reduced and you should be able to extract more or less double of the standard range out of the system. It is
up to you to (safely!) experiment as once the margin gets reduced influence of external interference sources will
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Summary of Contents for OpenTX 2.0+
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