What exactly is "unstable" about it? (Besides when toggling Auto Pilot)
For me, it flies better than the majority. There was a slight wobble during take-off, but after that only minimal automatic roll with a little bit of pitch-up while in flight. Both of which are lower than a lot of the planes I make, which I then have a lot of tweaking to do afterwards lol
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BTW out of curiosity, what was your intention with that Rotator in the back? It's only connected to the Fuselage around the engine, and wouldn't do anything from what I can tell... If you were trying to add Thrust Vectoring for the engine, I can try to add that for ya.
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EDIT: Aah ok, I think I see where you're coming from now... At insanely high speeds, under "constant-input-control" (aka, Mouse, or a touchscreen in your case), it gets a horrible case of the Speed-Wobbles. Particularly when in lower altitudes with denser atmosphere.
The real solution, there, is simple: Don't use such an absurd Throttle setting as "sum(Throttle*9+sum(0)" lol
I don't quite see the purpose of it anyhow, besides......... I assume..... giving infinite "Max" throttle, and thus unlimited speed. The downside I'm seeing, is when used on a jet-engine, it's not allowing you to throttle down, and leaves you stuck at max-throttle.
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I removed all the fuel (and Deadweight) from the body panels, restored their Scale to 1, and added a "Fuel Tank" block, then modified its capacity to 3x what you had, changed its Mass scale to be the same as what it had been... and now it can fly much much longer! :P (I also added rear landing gear, but that was mostly so I could take off trouble free.)