Why does it fail?

[dino1984]

Why my super-duper design is failing?

Above 60 m/s it starts to wobble around and is completely out of control... Even if I manage to take it somehow airborne it is rotating and flipping out of control and in the end crashes. I've tried everything - bigger and samller wings, giving less power to engines, etc. every time the same.

I use realism overhaul and RSS and other mods.

 

 

mods that I use:

[Hupf]

I would guess that the foremost fuel tanks are depleted first, moving the CoM behind the CoL or the Aerodynamic Center at some time in flight.
 You may want to consult e.g.

 

To find out if this might be the issue, try locking the fuel flow on all but the rear fuel tanks. If the situation is better then, the proper fix would be moving the mass more to the front and the lifting surfaces more to the rear of the vessel.

[RizzoTheRat]

Air breathing engines will draw fuel from all tanks simultaneously, so the CoM shouldn't move too much. 

The CoM and CoL are right on top of each other though, getting the CoM a bit further forward than the CoL should improve things a bit, but short planes are generally less stable than long ones, a tailboom on the back of the central fuselage with the fin and tailplane on it might be worth a try.

With the swept wing those ailerons are a long way behind the CoM and CoL, so they'll give you quite a lot of pitch, but probably some yaw as well, so you might be getting some nasty interactions between control surfaces.  Try setting the rudders to yaw only, the ailerons to roll only, and the elevators to pitch only.  I'd also try moving the ailerons to the central wing section, you'll still get plenty of roll control and they'd be pretty much on the CoM/CoL so should cause very little pitch.

If you're losing control when still on the ground then check your landing gear is straight, if it's mounted on a sloped component it can cause problems

Edited March 11, 2016 by RizzoTheRat

[dino1984]

  On 3/11/2016 at 11:39 AM, RizzoTheRat said:

ry setting the rudders to yaw only, the ailerons to roll only, and the elevators to pitch only.

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I always do this

 

  On 3/11/2016 at 11:39 AM, RizzoTheRat said:

check your landing gear is straight

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checked, it is.

 

  On 3/11/2016 at 11:39 AM, RizzoTheRat said:

short planes are generally less stable than long ones, a tailboom on the back of the central fuselage with the fin and tailplane on it might be worth a try

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will try.

[Nich]

Too many joints between your ailerons will cause flapping.  You have 3 I normally only allow 1 but max is 2.

[dino1984]

It's not flapping. It's more like sliding, flipping and spinning out of control.

[eurybaric]

Just move those swept wings back until the CoL and CoM spheres aren't overlapping. Should do it.

Cheers.

[Rocket In My Pocket]

How heavy is this thing? Heftier planes need more time to turn, sounds like you're getting too far away from your prograde marker while turning maybe?

Hard to tell from your picture but looks sort of like the two rear landing gears are kicked out slightly? Make sure they are perfectly vertical. I'm not crazy about your wing setup either, looks wobbly, I hate to suggest struts but...yeah struts. Those swept wings are hurting more than they are helping I think, try a straight wing design it won't put your roll surfaces so far behind your COM. (They are practically in line with your pitch control atm.)

You use so many mods that it's honestly kind of hard to diagnose the problem, the COM/COL looks alright, maybe they could be moved forward a touch. Maybe someone out there has a similar mod setup and could shed some light on this, or try out your .craft file. Best of luck!

 

[Geschosskopf]

  On 3/11/2016 at 10:42 AM, dino1984 said:

Why my super-duper design is failing?

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Well, this is KSP, where failure isn't an option, it's standard equipment

But seriously....

  2 hours ago, dino1984 said:

Above 60 m/s it starts to wobble around and is completely out of control... Even if I manage to take it somehow airborne it is rotating and flipping out of control and in the end crashes. I've tried everything - bigger and samller wings, giving less power to engines, etc. every time the same.

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From this, I take it that the death wobble is happening on the runway?  That is usually caused by misaligned landing gear so be sure they're attached perfectly straight ahead and perfectly vertically.  Also make sure you didn't accidentally put 2 overlapping nosewheels on with symmetry.

Even if the wheels are on straight, however, if the structure of the plane is flexible enough, bends to the structure can misalign the wheels, also causing death wobbles.  So maybe add struts between the central fuselage and the tanks that the main gear are under.  Or use Kerbal Joint Reinforcement.

Another thing that can be causing problems is engine gimbals combined with SAS, because engine gimbal usually has way too much control authority for its own good.  For aircraft, engine gimbal is great for doing aerobatics but HORRIBLE for going in a straight line, such as when taking off or flying long distances.  So if you're taking off and have both SAS and engine gimbal enabled, that will often induce death wobbles, so you shouldn't use both at the same time.  I usually leave SAS on and disable gimbal.

Now, as to why it continues to flip out in the air, this is probably an aerodynamic balance problem.  From your screenshot, it appears that the CoL is actually slightly forward of the CoM.  The CoL actually needs to be a bit behind the CoM.  Their indicators can overlap somewhat but the CoL should be noticeably aft of the CoM.  Of course, the aforementioned issue with gimbals and SAS will continue once airborne so if the plane is already unstable due to the CoL too far forward, that will only make things worse.

[Nich]

Oh I see death wobble is on the runway yes it is your landing gear

[Plusck]

What happens if you rotate the whole plane (i.e root part) up 15° or so? By the look of it, body lift will push CoL even further fowards.

For runway stability, I'd move the rear landing gear forward a bit more - let aerodynamics rotate the aircraft cleanly up, rather than having to push down hard on the elevators. For stability once your AoA is positive, either add a small fuel tank to the nose or move all of the wing and control surfaces slightly back. In any event, I'd move the elevators back as far as possible.