A simple RTL controler for RC models

I think I’m going to have to pull finger and look at building a simple RTL controller for RC model planes. This post is a series of notes just to put it all in one place and provide some food for thought.

Comments or questions are welcome.

So what is a Return-To-Launch Controller (RTLC)?

It’s a small module or circuit board that reads the servo positions for aileron, elevator, rudder and throttle directly from the receiver. The other side of the RTLC connects to the servos. A 5th channel can be used to turn the RTLC on/off from your transmitter; highly recommended for setup/testing but not required for normal operation.


  1. As a simple safety device to prevent flyaways and disorientation crashes.
  2. As a stability controller for FPV etc.


What does it do?

Normally it does nothing (or almost nothing), but simply passes the servo signals from the RC receiver straight through to the servos.

When the RTLC takes over, it only monitors the RC receiver and produces its own servo signals to fly the plane. Using 3D gyros and accelerometers it knows what level flight is. Using GPS, it knows where it is and where it took off from.

During normal flying you don’t know it’s there. If your RC fails due to distance, interference or a simple failure, the RTLC knows because it’s monitoring the RC channels, and if switched on it takes over control of the plane. The RTLC resumes level flight heads for home and loiters at a safe altitude above the home location.

How to install it

Mount it using the vibration reducing material inside the plane. It must be isolated from excessive vibration for the gyros etc to work correctly. The GPS antenna must face up and be clear of major metal obstructions and radio transmitters. Keep it away from FPV and telemetry transmitters.

Connect the RC receiver and servos.

The 5th channel should be a 3-position switch on the transmitter:
1. off        RTLC is disabled and does nothing
2. help    RTLC helps with stability only
3. home    RTLC takes over and brings the plane home

A 2-position switch gives you 1 and 3.

How to operate it

The 5th channel can turn the RTLC on/off. When off, it will not attempt to save the plane. If the 5th channel is not connected, it is assumed to be on.

Before flying, power the plane and wait for the green LED. This indicates that the GPS has a fix and home position is recorded.

Fly normally. If you loose the plane for any reason simply turn off the transmitter and wait for the plane to return. As soon as it’s within control range turn the transmitter back on and take over.

If you do not take over, it will circle until the flight battery if flat or it runs out of fuel. If there is still power to the RTLC, it will circle down to the ground. Be sure to fly in a large clear area.


The RTLC needs to know your plane so that it can maintain level flight.

Mount the plane so that it is sitting level; wings level and the nose at about level flight attitude. Press the button until the green LED starts flashing and leave it alone for 1 minute or until the green LED stops flashing and stays on. Now launch the plane and fly it normally. The RTLC has recorded what it believes to be the plane sitting level and when first launched, which way is forward.


Note: at any time you can switch the 5th channel to “off” and you have full manual control with no interference from the RTLC.

Pre-flight check the plane before launch. Fly up to a safe altitude and switch the RTLC to “help”. You may notice a bit of pitching at first as the plane finds level flight. Now you can fly around normally but releasing the controls the plane should return to level flight. You will still have manual control but the controls will feel sluggish or damped and you won’t be able to loop or roll as it’s now attitude limited.

When this seems OK you can switch to “home”. The RTLC now takes over and will turn the plane back towards the home position, fly back and circle.

To take over at any time switch back to “off” or “help”.

This is why the 5th control channel is highly recommended. Without it, you have to trust that it will work. The main reason for failure would be excessive vibration getting to the gyros and accelerometers.

1. if the RTLC is turned powered-off before the first flight during calibration, no changes are recorded.
2. recorded calibration is saved even when power is off between flights.

What can go wrong?

Obviously, if you fly too low or behind things, you will likely crash.
The RTLC must be correctly mounted.
The GPS antenna and receiver must be working. No GPS = No RTL.
If GPS is lost during flight, the RTLC will loiter over the spot that RC was lost. This keeps it in the area and allows time to get closer or change transmitter orientation etc. to regain control. Wind will cause the loiter circle to drift.
If the plane is launched without a GPS fix it’s the same as if GPS is lost during flight.

Technical features

Just a few points and notes. Some of these are obvious requirements, some would just be nice if possible within size and cost limits.

  1. small size, light weight, low cost
  2. simple to use, no complex setup or calibration
  3. 3 axis gyro and accelerometers
  4. pressure sensor for altitude
  5. GPS receiver
  6. 4 RC channels pass through + 1 control channel in
  7. battery monitor input
  8. aux servo output
  9. data connection for fpv-osd
  10. sd card flight recorder
  11. 1 push button and 2 LEDs (red/green)

Possible Extra Features:

Battery voltage monitoring. I’m not sure of the benefit but it may be useful to modify operation based on battery status.

An extra power connector so that a backup battery could be plugged in. If the main RC power fails or with electric models you run from the flight battery and run out, the RTLC can still point the plane in the home direction and make some effort to reduce the distance. I’ll have to think about this.


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