Is there a way to turn off throttle slew limiting for takeoff?

Did my first catapult takeoff with my XUAV Clouds today. Went really well. But it’s a big and heavy airframe and my catapult can only accelerate it to “just about” stall speed. I set the takeoff throttle slew to 120, which should allow it to throttle up in just under a second. But I’d like to be able to have it throttle up more quickly. It’s a twin with counter-rotating props, so no concerns over torque roll effects.

There’s nothing in the parameter documentation that indicates any special values turn off the slew limiting. I wouldn’t want to do that for THR_SLEWRATE, but it would be nice if I could turn off limiting entirely for TKOFF_THR_SLEW. I’m hoping that zero () might be a special value for that feature…but there’s nothing written to say that.


A value of -1 will set it unlimited for takeoff.
I’ve added it to the param docs
Cheers, Tridge

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Thanks for the reply and the doc update!

I’ve set this value in the parms and will test on my next flight.

Mission planner notes and the valid range for the parameter value should also be updated. Should I cross post the request to the Mission Planner list?


It gets that limit from the ArduPilot code, which I have updated, so it will flow through to MP automatically on the next release.


Due to a few mistakes on my own part, I still haven’t done a launch with THKOFF_THR_SLEW set to -1 (for me, a value of 120 works fine with a strong head wind, although it fails in calm air).

I tested at home today to see the behavior of the “-1” setting (with props removed!). With “-1” as the parameter for TKOFF_THR_SLEW, it slews fairly quickly, but it does not advance from 1000 to 2000 all at once. The graph shows one of my throttle channel signals. Throttle appears to take about .6 of a second to slew from off to full on…gated by a pause at half throttle.

However, if one fully advances the throttle on the transmitter prior to firing the catapult, the slew is much faster. In this case, there are no intermediate values recorded between fully off (1000) and fully on (1980) in the log. It appears to take ~100 ms on the graph. But, I suspect that’s probably faster than can be accurately determined by logs. So, it’s probably instantaneous.

Is it a planned behavior that throttle “nudge” works during takeoff? Or that throttle nudge is needed to allow the throttle to slew at the full rate?