Wobbly rear on twin-boom aircraft

[SuperTechmarine]

In my endeavor to produce a P-38/Sea Vixen hybrid, I ended up running into a slight problem.

The "spoiler wings" keep clipping into each other while the wings wobble up and down as well, destroying any semblance of stability.

The center of lift is a little above and behind the CoM.

Mods used: FAR, B9 Aerospace, KW Rocketry, Chatterer and BD Armory.

Edited November 30, 2014 by SuperTechmarine
answered

[Wanderfound]

What do you have your wing mass tweakables set to? And do you run into the same problem when you aren't flying that low, or if you back off the throttle until the high dynamic pressure warning goes away? 1,000km/h at 400m altitude is a fairly extreme situation.

[SuperTechmarine]

The wing mass is about 2.0. And it does that at low speeds yes.

[steve_v]

Just stating the obvious: I don't see any struts on that aircraft

I would at least strut the 2 tailplane sections together.

[Master Tao]

Short version:

The two parts of the horizontal stabilizer intersect, but do not connect or even collide with each other. Strut them together.

Long version:

KSP crafts have a tree structure starting from the first-placed part. Each part you place after that is attached to only one other part until you attach parts to it. Branches, like the split for your two wings can only be reconnected by the two special parts: struts and fuel lines.

[Hodo]

The simple fix for this is to add the B9 invisible struts to the parts to connect them and make sure they don't shift around on you.

[SuperTechmarine]

The struts fixed the tailplane. Now I just have to fix it stalling all the time I try to turn.

[SuperTechmarine]

The struts fixed it, thanks for the advice. Now I just have to fix it stalling every time I turn and the "nose-pitching up all the time" issue.

Welp.

Edited November 29, 2014 by SuperTechmarine

[Hodo]

The struts fixed it, thanks for the advice. Now I just have to fix it stalling every time I turn and the "nose-pitching up all the time" issue.

http://i.imgur.com/bdPMvxx.jpg

Welp.

The big question is what exactly does the FAR graphs and diagnostic charts say in the SPH? Check the AOA numbers the Mach numbers, and the diagnostic tab. Post pictures of all three of those and I am sure we can help you with that problem also.

Good luck and have fun.

[ratchet freak]

also check CoM and CoL indicators besides the FAR graphs

[SuperTechmarine]

I added a metric crapton of heavy struts (220.000 strength) to reinforce the fuselage, wings and booms and it cannot break. I did a 180 on elevators at Mach 1.5 and it did not disintegrate, I consider that a success. I will put up screenshots of the CoM and CoL as well as flight data soon.

Also, would adding Lerx (canards that control the airflow) work in FAR and reduce the stalling issues?

EDIT: Added canards beside the nose, rotated the rear stabilizers so it faced downwards at 45° and added wing-tips to the wings. Still stalls on elevator turns and pitches up if I turn off SAS.

Link to album of stalling screenshots and CoM, CoL and CoT.

Edited November 30, 2014 by SuperTechmarine

[Hodo]

I added a metric crapton of heavy struts (220.000 strength) to reinforce the fuselage, wings and booms and it cannot break. I did a 180 on elevators at Mach 1.5 and it did not disintegrate, I consider that a success. I will put up screenshots of the CoM and CoL as well as flight data soon.

Also, would adding Lerx (canards that control the airflow) work in FAR and reduce the stalling issues?

EDIT: Added canards beside the nose, rotated the rear stabilizers so it faced downwards at 45° and added wing-tips to the wings. Still stalls on elevator turns and pitches up if I turn off SAS.

Link to album of stalling screenshots and CoM, CoL and CoT.

Looks like the issue, is your CoL is a hair in front of your CoM, this is only made worse as the craft drains fuel. This will shift your CoM further back towards your tail making your plane very nose light and unstable. Ideally you want your CoL just behind your CoM.

While this is a bit further back than I would care for it is about where you would want it for a stable flight profile at subsonic speeds.

The CoL shifts back towards the tail at supersonic speeds so that is something to be aware of when designing a craft, this affect is often called "Mach Tuck". There is a very simple way of getting around this, design your craft with one half full or empty tank near the rear of your craft, so when you reach supersonic speeds you can shift your fuel load shifting it back to keep it stable at supersonic speeds.

To fix your craft would be pretty simple, lose the canards at the nose, they are just shifting your CoL forwards to much. If your CoL is to far to the back of the craft, then move your wings forward a bit, or widen the base of then a bit. You will also want to check your FAR information screen, all of the numbers in the second tab on the top should be in the green at .35 all the way up to mach 2.

If you have something that is just slightly in the red you will be ok, but if something is showing more than +2 then you have an issue and should go back to fine tuning the design. Ultimately for a highly nimble craft you want your numebers pretty close to zero but still in the negatives.

EDIT #2- One last thing, I would move the rear landing gear forward towards your CoM. The are the pivot point for when you are rotating your nose up to get off the ground. The further back they are the harder it is to lift the nose, because you will be pushing the gear into the ground. Most aircraft rotate around the CoL so if you place your gear almost directly under that you should be fine. But do it after you move your CoL to be just behind your CoM.

Edited November 30, 2014 by Hodo

[SuperTechmarine]

Will try that tomorrow. BTW, what mod has that F-22 style cockpit with the intakes? The first one from the left on the fourth row from the top, I mean. If it works I might replace my superonic inlets with that, saving me space and parts and RAM.

[Hodo]

Will try that tomorrow. BTW, what mod has that F-22 style cockpit with the intakes? The first one from the left on the fourth row from the top, I mean. If it works I might replace my superonic inlets with that, saving me space and parts and RAM.

Do you mean this one?

http://forum.kerbalspaceprogram.com/threads/96766

I like that cockpit but it is a bit of a pain because it is long. I rarely use it now. But the other mods are listed in my thread in my forum post.