It’s not the first time I heard someone had a quadcopter, lost radio signal, and it just flew away and never be found. This is why setting up failsafe properly is important when there is problem with your radio connection. There is also safety concern regarding what failsafe behaviour should be which we will discuss later.
What triggers Failsafe?
There are many scenarios where failsafe can happen on a multirotor (if failsafe is supported and setup properly).
- Radio is out of range.
- Receiver gets disconnected from flight controller.
- Receiver loses power or malfunctions.
- Pilot accidentally switch off transmitter, or transmitter lost power.
When failsafe is triggered the following could happen, which are user determined.
- All motors just stop spinning, and the vehicle drops out of sky like a stone.
- Small amount of throttle (20%-30%), motors spinning slowly and ideally the vehicle descends smoothly to land.
- Some RX/FC allows certain flight modes to kick in when failsafe happens, such as auto levelling, loiter or even return to home.
- However, nothing will happen if quadcopter is already disarmed.
Setting up Failsafe and Testing
Between all the ideas of different failsafe behaviours, I prefer “shutting down all motors”. A quadcopter with uncontrolled spinning motors are extremely dangerous, it could end up going anywhere. it’s better to crash your quad and repair it later, then hurting someone on the ground.
Stopping all motors is a safer way to handle situation like radio signal lost, you have a lower chance of hitting someone as it should just crash vertically into the ground. Also in case of lost video signal, you can just switch off transmitter to emergency land your multirotor, preventing it from wondering around / ascending unexpectedly.
Find out how failsafe is setup on your radio receiver. Many receivers failsafe is setup when binding happens, some has a dedicated failsafe button. They remember transmitter throttle position when they are bind together, or when the failsafe button is pressed. That means when failsafe happens it defaults to output thatt throttle setting to your flight controller. Rebinding should reset the failsafe function.
Many Flight controllers also supports failsafe. It’s a good idea to setup failsafe on both FC and RX to have an extra layer of safety. In the next part I will describe how I setup failsafe on my gear, Naze32 and D4R-II.
Don’t rely solely on any guide/article on the internet, including this one. It’s IMPORTANAT that you always test it and make sure it works on your multirotor!
To test failsafe, REMOVE all props and simulate all possible radio problematic situations. Arm it, throttle up to 50%, then turning your radio off and seeing what happens. You might also want to remove the servo lead between FC and RX for another test.
Failsafe on Frsky D4R-II Receiver
I was told the Frsky D4R-II has two failsafe options.
- User defined throttle on lost signal (however this will not be detected by FC, and will not trigger Failsafe on FC)
- No pulses on lost signal (this is the same as unplugging the servo lead between RX and FC, this will trigger failsafe on FC)
If failsafe is not set/enabled, the default behaviour is to hold last position before signal was lost (Avoid! Dangerous!)
For options 1, It’s explained in the RX manual:
- Bind the receiver first and turn on both the transmitter and receiver;
- Move the controls to the desired failsafe position for all channels;
- Held down the F/S button of the receiver (for a second). The GREEN LED of the receiver will
flash twice, indicating the failsafe position has been set in the receiver.
- To disable/reset the failsafe function, re-bind the receiver.
For reference, here is how option 2 is done. If you want to send no pulses, you can bind transmitter and receiver, turn off the transmitter, and power cycle the RX (switch off and on again), and finally press the F/S button. This will trigger failsafe on Baseflight/Cleanflight (if failsafe feature is enabled).
Setup Failsafe on Naze32
The Naze32 has a built-in failsafe that can be set via the Command Line Interface (CLI) in the Chrome configurator App. With FS enabled in Flight controller, it ensures the aircraft behaves exactly what we want in case of RX failsafe fails, such as connection cable unplugged or RX powered down.
First enable Failsafe feature:
These are some basic example commands:
set failsafe_delay = 10 set failsafe_throttle = 1000
“Failsafe_delay” means when failsafe is detected, it will kick in after a delay of 1 second (10 x 0.1 second), with throttle value of 1000. You should change “failsafe_throttle” to a value lower than your min throttle setting and the motors will stop in a failsafe situation. So if your “min_throttle= 1150” then “set failsafe_throttle= 1100” or below will ensure they stop.
Note that after FC failsafe, you won’t be able to arm the board, unless you power cycle the FC. However if it was just a RX failsafe you can (if FC didn’t detect it).