I have a hexacopter, When I set the Wpnav_speed to 1000, the maximum horizontal speed in this mode (Auto) goes up to 1.7 meters per second. But if I set this value to 1020, The speed reaches 10 m/s correctly.
I have had the same issue with acceleration before.
This is a long time ignored bug, i reported it dozens of times already but completely ignored. There is something wrong on navigation speed, sometimes setting not round numbers works, sometimes doesn’t.
I have had that problem since 2 years ago and still having it on latest release. Setting navigation speed is a gamble each time.
Navigation speeds, accelerations and limits have greatly been improved in ArduCopter 4.2.1
But please do use the workaround described by @Corrado_Steri after updating.
Thanks for your replies, I will try the new version.