Why won't Mechjeb fly this plane?

[AeroGav]

 

Alright, I'm only a recent convert to the ways of  Mechjeb,  but was able to use it on my space plane like an autopilot, able to precisely command AoA , sideslip and roll angle in single degree increments.  You can see from this launch video how well it works.

On my newest design however, Mechjeb simply makes insane control inputs.   Sometimes it applies full aileron and rolls us over and over till we crash. In this example it puts in a bootfull of rudder and has us constantly sideslipping, but appears to command pitch very loosely.  Tried removing then reattaching the mechjeb module, orientating it differently.  What's going on?

 

[FleshJeb]

Your craft isn't very aerodynamically stable.

Personally, I use the MJs "absolute" surface controls (upper right SURF button). I find they result in more stable flight.

[AeroGav]

4 hours ago, FleshJeb said:

Your craft isn't very aerodynamically stable.

Personally, I use the MJs "absolute" surface controls (upper right SURF button). I find they result in more stable flight.

i can hand fly this airplane however.  Why is MJ applying full left rudder from the moment i enable it (before engine start) resulting in plane going off course and crabbing through air?  It is also making roll inputs I never asked for . As far as pitch is concerned, as soon as there is a discrepancy between requested pitch and AoA, in the first video MJ applies full inputs to achieve the desired angle.  In the second it more or less ignores me selecting +30 nose up in response to the terminal dive.

https://kerbalx.com/AeroGav/astrojet-sapphire

craft file of the airplane in the second video.

craft file of the airplane in the first -

https://kerbalx.com/AeroGav/Astrojet-Citation

moving the centre of lift further aft and adding a conventional control surface like the first airplane has does not help at all

[Pecan]

17 hours ago, AeroGav said:

...What's going on?...

I suspect MJ is seriously confused by you control-surface placement.  Poor software.

[AeroGav]

12 minutes ago, Pecan said:

I suspect MJ is seriously confused by you control-surface placement.  Poor software.

I thought this might be the case, so quickly put together a test vehicle with the same double delta wing and leading edge elevons with authority set to -100% (ie reversed) acting as elevators.    Mechjeb was able to fly this normally.

So, it's just a Kraken, nothing to see here, please move along.   The good news is that the design is viable.  The bad news is I have to build it over from scratch to not have whatever bug snuck into the craft file and mess with MJ.   The prospect does not exactly thrill me and i've been putting it off all day.

 

[Vanamonde]

Mod-use question moved to the mod-use subforum.