Failsafe and Reverse Throttle

Not sure if I’ve found an issue. But here tis:

I’ve always used a throttle signal below a PPM value of 950 to trigger failsafe. This always worked well.

However, I’ve just added reverse throttle to a 2nd plane and noticed today that I wasn’t hearing Mission Planner nag me about FAILSAFE as I was testing. Sure enough, turning the radio off doesn’t activate failsafe because the throttle sits on 1500. And that’s the only value for an active throttle that doesn’t spin the propellers one way or another. Below 950 would be calling for full reverse thrust.

Presumably, Arduplane would notice the “below normal range” value for the throttle and go into failsafe and then control the throttle output accordingly, but it scares me to require that the software must react instantly to stop the motors from causing havoc. A sudden application of full reverse in flight would probably not guarantee a crash, but it would not be good. And a sudden reversal on the ground…even just for 50 millseconds might cause a nasty flesh wound.

With reverse thrust, one might as well use a PPM value above 2050 for failsafe on the throttle instead of using a threshold below 950. Both seem hazardous.

The use of the throttle channel for failsafe made perfect sense prior to supporting reverse thrust but I’m wondering what to do for failsafe detection now?

After sleeping on this and verifying identicale behavior (no failsafe) on my other reverse-capable plane, I realized that this should not be a problem if one is using a transmitter switch to control a channel with RCx_OPTION = 64 to command reverse. If the OPTION = 64 switching approach is used, the signal sent on the throttle channel from the Transmitter uses the full normal range (1000-2000 PPM) for controlling the throttle. So a “below range” TX signal should still be possible for flagging that a radio failsafe has occurred.

It’s odd that when I enabled reverse thrust on two planes, both of them lost their below-normal-range programming for the throttle channel. Before the change, both planes showed a failsafe throttle value in the low 900s when the radio was turned off. After the change, both had a failsafe throttle value of 1500? I will reprogram the proper failsafe for the R-XSR receiver today and will be hunting for the root cause as I do.

Two additional points

  1. The failsafe setting for the channel doing reversal shouldn’t go high (commanding reversal) when the radio signal is lost. Might not matter (since the FC shouldn’t care), but unwise to allow.

  2. The incompatibility between reverse thrust and failsafe still exists IF:

    • Pilot Control is enabled, and

    • RCx_OPTION 64 is not setup, and

    • Failsafe is signaled to the flight controller by a below-normal-range throttle signal (THR_FS_VALUE)

I will try to update the WIKI entry for Reverse Thrust so that this incompatibility is noted in the documentation.