Help With Reversing Flaps

[aminal]

Im having a problem with an aircraft that I built and it flies really well up until a certain point. The aircraft is pretty big, with multiple flaps. When they all work together, things are good, but a few minutes after taking off one or more of the flaps seems to reverse which immediately sends me into a forward spin which this plane can never recover from. I am not 100% sure what is causing but I think it could be fuel related. I have played around moving bits around but nothing seems to cure the problem. Which leads me on to a question which I hope makes sense as I am struggling to explain myself!

Does the CoG within a fuel tank move as fuel is drained?

If so, would a fuel tank that drains top to bottom drain the same way if it was on its side? eg - left to right as if it was ignoring any force of gravity. Or would this fuel tank on its side still drain top to bottom?

Any other advice would be greatly appreciated

[Neotician]

A fuel tanks own center of gravity doesn't change as fuel is drained, but it's total mass does, meaning the center of gravity of your plane shifts.

[spaceBrezel]

how high did you go?

maybe you need more air intakes.

at high height i try to compensate with rcs help. rudders get to soft

2 "place anywhere" rcs controls on nose up/down adds a lot of torque.

[aminal]

Thanks for the reply. This actually confuses me more because if it isnt the fuel balancing that is causing the flap(s) to reverse, then I am unsure what else could be causing it. When searching I could find very little information in the subject

Has anyone else at all suffered from reversing flaps?

[aminal]

Without a payload I can get to about 15km altitude before te engines start playing up, but with a 70T payload, 6-7000m is about all I can get. This is fine for me as it was originally built for a trip to the north pole. So I dont think the engines are causing the issue

The Rudders seem fine, its a pair of elevators that seem to reverse and tell me if im wrong but with 18 air intakes and 10 nacelles I would hope that to be more than sufficient

Edited June 11, 2013 by aminal

[Nurph]

There are no 'fuel physics' in KSP, it's just numbers. Resources will only change a part's mass, not where it's CoG is. I assume you know about the CoG, CoL, and CoT markers in the editor? As fuel drains the CoG of your plane (the whole plane, not individual parts) can shift around and cause instabilities, so you'll need to design around that or transfer fuel between tanks to keep balance.

[koshelenkovv]

Control surfaces behavior depends from their placement. Surfaces, placed if front of CoM will flap in reverse to those placed behind of CoM.

You can check this - take a cockpit, then add 5 fuel tanks in row to it.

Place some wings on firs and last tank, then surfaces on them.

Launch, then press WS, AD, QE and look.

Your plane's CoM shifted with fuel usage and some surfaces which was behind of CoM now is in front of it.

[aminal]

Well the fuel tanks are placed on the CoM so hopefully there should be minimal a weight shift when draining fuel. When i completely remove the fuel tanks the CoM barely moves. All 4 tanks are draining at the same rate.

With regard to the flaps, I think it is unlikely the the CoM would ever move enough for the flaps to be infront. Also the wings are 90degs opposed to the fuselage with a 10deg pitch so I am confused as to why it seems to be only one pair of flaps that invert. I would have though it would be all or nothing.

EDIT - Just went for another flight adding extra tanks infront of the CoM, and this time all aelerons decided to invert at the same time.

EDIT 2 - Moved the tanks forward to the point of making takeoff a problem. Same thing happened at roughly the same time/fuel level

Is it possible the craft file could be bugged causing this?

Edited June 12, 2013 by aminal