Hm, sorry, I see somehow part of my intended first post got lost...thought I already wrote what I tried...
As I'm doing this CSM, I started from scratch. I made a copy of the B17 called B17Robot. I eliminated all Wing- and Flaplifts (and now even removed them alltogether).
I remmed out all four active engines and put in a single dummy engine on 0/0/0 to make sure it's centered.
I tried adding all variations of floaters, no good, usually spread out along the X and Z axis, tried different distances too (doesn't do much good).
I now think Hullheight is the buffer zone, when the object gets below the floater y minus Hullheight, the FloaterLift activates and starts pushing the thing up (tested that by using radical Lift values. Setting the hullheight to 0 thus causes the object to be fixed, not the slightest movement there.
The problem is not that the B17 is making turns or something (how should it

), the problem is it's not flying level. It starts with small bounces (as its speed is still slow) and as speed increases, it describes huge parables and ultimately crashes into the ground. Low speed values somehow cause the plane to go tailheavy and make pretty weird slow-mo loops on the spot. Tail heaviness is not due to masses ( I think), but to the inital low speed. As the engine kicks in, it pulls forward, which, as the floater activates, becomes "up".
Tried playing around with inertiaModifier, gravitymodifier, setting mass to 0 etc. etc., no good.
Also tried tinkering a bit with the engine position, but somehow still to no avail

. I thought 0/0/0 described the absolute center of the object, deducted from the bbox? Or is 0/0/0 only the center of the first PCO geometry, not the whole PCO complex? That would explain a lot, then I'd just have to figure out the absolute center of the complex (should be a charm with 3dsmax).