Aerodynamically stable ship is not stable...?

[Brownhair2]

This is the plane in question:

Center of lift is behind center of mass, even with fuel drained. Tests near the KSC seem fine. But the INSTANT I start to slow down from hypersonic speed it just flips out. Literally. It flips and stuff.

[Signo]

How does the center of mass look if you empty the fuel tanks?  (and in my opinion the center of lift should be a little bit more toward the center of the plane - is it not nose heavy like that?)

Edited June 22, 2016 by Signo

[Brownhair2]

1 minute ago, Signo said:

How does the center of mass look if you empty the fuel tanks? 

Again, the center of mass is still in front of the center of lift. But here you go:

[Signo]

Give your canards a few AoA (5° or less) - it should be enough to move the CoL forward to the CoM (the CoL should however stay behind, just a little closer), the craft will be easier to pilot and more stable while you re enter.

I can not see any oxidizer onboard - nuke engine?

Edited June 22, 2016 by Signo

[Vaporized Steel]

The rule where COL is behind COM is a general one for most typical aircraft designs, in any way, COL should almost always be behind COM. This doesn't necessarily have to be true. If your engines are on the front the COM/COL relation flips around. A classical example of this are the WW1 planes, and many WW2 planes or many propeller planes in general.
If you have decent forward control surfaces like on your plane, you want the COL to be very close to the COM.

This is not the case on your aircraft or most KSP aircraft. What is a problem on your aircraft design is the position of your control surfaces in relation to the COM and COL.

First of all, your COL lift should be just behind the COM, but not anywhere close to well inside the COM bubble. Just on the edge of the COM. The COL on your plane is still far behind the COM. So position your wings to move them closer together.

I think your front control surfaces "Tail fin" is for pitch and the BIG-S tail fins are for rol? Am I right?

If the Tail fin is for pitch then in relation to the COM and COL it works as a enormous hinge once you put in pitch control input.
Theres a good chance that if you move the tail fins farther forward the COL moves forward far enough. Basically your pulling the ships mass to a point where it wants to flip backward (I assume the ship flips backward) thus pitches up and loses control.
So move either the tailfin close to the nose and if the COL is still not near the COM try to move the BIG-S tail fins also further to the front.

If your BIG-S tail fins are pitching and your tail fins are for roll you still have the exact same problem but then in reverse. That's because the BIG-S tial fins are very close to the center of lift, meaning that if they're assigned pitch control they cannot control the COL over the COM once your attitude changes to much. In more simple terms, it has a natural lift/mass relationship to pull up once you change attitude. Your pitch control surface whatever it is should be well away from the COM/COL relation as visible in the SPH or it lacks the control to control and hold that pitch.

It's likely to go haywire when you slow down because slowing down decreases lift and the natural lift of the aircraft will greater deduct the input from your control surfaces if the COM and COL are misaligned.

The relation of your COL and COM does work on many aircraft designs, but because your control surfaces are also misaligned to counteract the difference between your COM and COl it starts flipping.

No matter whether the BIG-S or the tail fins are for pitch or vice versa.
You can use the Move: Toolset in the SPH to change the position of both suraces within marginal corrections and test out what's most stable for you.

 

Edited June 22, 2016 by Vaporized Steel

[Snark]

There's a rampant myth in KSP airplane flying, the idea that "CoL behind CoM = stable."

No.  That's not what determines stability.

What's important is that your center of drag (not lift) is behind your CoM.

Look at your plane-- the CoM is way in the back.  It has all that bulky light draggy stuff sticking out in front.  You need to move your CoM way far forwards to make it stable.  (And then you'll need to move your CoL way far forwards, too, to stop it from wanting to nose-down all the time.)

 

[Boris-Barboris]

11 minutes ago, Snark said:

What's important is that your center of drag (not lift) is behind your CoM.

What is center of drag?

[Brownhair2]

29 minutes ago, Signo said:

I can not see any oxidizer onboard - nuke engine?

Yes

22 minutes ago, Snark said:

There's a rampant myth in KSP airplane flying, the idea that "CoL behind CoM = stable."

No.  That's not what determines stability.

What's important is that your center of drag (not lift) is behind your CoM.

Unfortunately you can't see the center of drag in KSP. Also I'm kinda limited in design since this thing is meant to go at about Mach 4.5 in the atmosphere.

EDIT: Might remove the Nerv since I don't need it right now

Edited June 22, 2016 by Brownhair2

[Signo]

I do not know if you already reached orbit with that, but I think you should discard a ramjet for a nuke (just my opinion, KSP is never "absolute").

 

 

 

Edited June 22, 2016 by Signo
"daypic"

[Snark]

19 minutes ago, Boris-Barboris said:

What is center of drag?

It's a conversational gambit to condense several pages' worth of detailed explanation and hard-to-grasp concepts into a short pithy phrase, the intent of which is to encapsulate the idea that "a CoM way in the back of the plane with a lot of lightweight draggy stuff in front of it is a bad idea." 

22 minutes ago, Brownhair2 said:

Unfortunately you can't see the center of drag in KSP.

That's right, for very complex reasons that are gone into in detail in the thread I link above, including some good explanations by Boris.

However, even if you can't see an actual marker, you can certainly eyeball it.  Just look at that plane:  the CoM is way in the back and there's a lot of big draggy stuff out in front of it.  It's a recipe for instability.

A few other things:

• Make sure that roll authority is turned off for your canards.  They're way out in front of your CoM and have the potential to make your flipping problem worse if they add drag up there.  You don't want them to do anything other than work to keep your pitch stable.
• Rotate your tail fins so they're perfectly vertical, and aren't doing anything other than yaw.  Especially make sure that they have roll authority turned off.  The fact that they're "unpaired" (i.e. have no counterpart on the underside) means that if they ever try to roll you, they're going to induce yaw as well (and pitch too, if you've got them slanted like that).

 

[Brownhair2]

Just now, Snark said:

• Make sure that roll authority is turned off for your canards.  They're way out in front of your CoM and have the potential to make your flipping problem worse if they add drag up there.  You don't want them to do anything other than work to keep your pitch

This is good to know

1 minute ago, Snark said:

• Rotate your tail fins so they're perfectly vertical, and aren't doing anything other than yaw.

They were originally, but in my desperate attempt to move the CoL back () I angled them

[Signo]

Snark, I am sorry to "steal" the thread, but why not adding ventral fins just like MiGs? I know that could be a pain in the "u know what" for landing, because we do not have any actuator that could fold'em, but it could be a possible solution to the roll/yaw issue too.

Edited June 22, 2016 by Signo
grammar

[Brownhair2]

Ok, it seems to be stable enough. On to the next problem: I can't land. It can't pitch up and it has a terrible glide speed or whatever the technical term is.

[Signo]

"Terrible" meaning too fast or too slow? Or what? (told you it was nose heavy  )

Edited June 22, 2016 by Signo

[Boris-Barboris]

36 minutes ago, Snark said:

"a CoM way in the back of the plane with a lot of lightweight draggy stuff in front of it is a bad idea."

But it's not.

And, pardon my agressiveness, but this subforum would perhaps be a better place, if you guys would stop throwing ideas on new people, based on concepts nobody of you can properly define, demonstrate or prove. Every time someone says CoD in gameplay questions & tutorials, boris gets triggered and starts quoting people, and he is yet to find a man to say anything substantial on the matter, besides those, who already know it's one of those "myths" you don't like.

rant out... perhaps we need to sticky some kind of big tutorial on this matter, i don't know. It's fun to rage while reading this, but I guess it's less fun for you guys to answer the same question every week

 

And for the OP:

1. Your semi-vertical fins need to be moved further back and turned vertical, just like Snark said.

2. There's old bug, that may inverse you rear control surfaces when your CoM moves a little back after your fuel burns. It will make your craft uncontrollable indefinetly of CoL\CoM.position etc. Empty, or halve maybe, fuel tanks in the editor, look at CoM\CoL, make adjustments, then launch and warm up your pitch and roll on the ground, look for incorrect behaviour on those Big-S'es. If they are indeed inversed, you'll have to redesign your craft heavily to move those Big-S'es away, further back from CoM. If it's not the case, only the video of the flip can help.

Edited June 22, 2016 by Boris-Barboris

[Brownhair2]

1 hour ago, Signo said:

I do not know if you already reached orbit with that, but I think you should discard a ramjet for a nuke (just my opinion, KSP is never "absolute").

Trying this. It definitely isn't reaching Mach 4, but it's still going fast. Once I get high enough the Nervs might help.

EDIT: "Oh my god IT'S ACTUALLY WOR--" *runs out of fuel*

Edited June 22, 2016 by Brownhair2

[Brownhair2]

26 minutes ago, Boris-Barboris said:

if you guys would stop throwing ideas on new people

I feel hurt

[Signo]

You can reach around 1200 m/s with a ramjet and a 25 to 30t craft at 20000m depending on the ascent profile. When reaching 20000 add the nukes, the ram will go out at around 24000.

Try with a lower ascent profile (around 5° pitch). Giving your intakes a -5° AoA will let you exploit a little bit the operative ceiling of the ramjet (500 to 1000m). Try to give +5° AoA to all of your wings, overall lift will be greatly improved. 1 lift per 3t is a good ratio on Kerbin.

Horizontal speed rules for planes, as soon as you are above 25/27km you might even go with a 0° pitch to reach orbit.

Try to minimize overall drag, anything that is not useful for ascent/descent should be inside a bay. (i.e. solars, RCS thrusters) 

Do not forget that nukes can not use fuel that is not directly connected to them, so you will have to manually move the fuel (fuel lines and struts add too much drag to be viable for a craft like that, less is more).

2400U of LF should do the trick, leaving you in orbit with around 2000 m/s d/v left (twr 0.70).

Your "prime" will be between 8000 and 15000m, try to maximize your speed inside this region. 

Do not be impatient, as long as you are gaining speed and height that's a good way to go, even if it looks really slow compared to a rocket or to a sportier plane.

Hope this can help - same as previously stated, I am pretty sure this could be done differently and maybe in a more efficient way. 

 

[Brownhair2]

52 minutes ago, Signo said:

"Terrible" meaning too fast or too slow? Or what? (told you it was nose heavy  )

Terrible meaning it falls out of the sky if it isn't going at 200 m/s

[cantab]

As to the original issue, try rotating the whole plane up a bit in the SPH and see if the CoL stays behind the CoM. If any of your wing pieces are angled it might not.

Secondly, as was mentioned, what really counts is the overall centre of pressure, and the CoL ball doesn't display that. Usually the wings are the main impact, but not always.

As for the landing issues, well "cannot pitch up" usually means you have the CoM too far forward compared to the CoL, combined with not enough pitch authority, which means you basically land dart into the ground.

[Boris-Barboris]

7 hours ago, Brownhair2 said:

I feel hurt

Don't be, we do this with anyone who comes here with this question, and quite often those are zero-postcounter guys with "why this rocket flips".