Why does this space plan flip backwards after take off?

[THX1138]

Hi. I think this spaceplane used to work but now it flips over backwards shortly after take off. The centre of lift is behind the centre of mass. Any advice?

https://youtu.be/OaLAgJtXMMQ

 

[Gaarst]

Nevermind, got it messed up.

Edited January 27, 2016 by Gaarst

[NathanKell]

@Gaarst Nope, you've got it flipped. Think of a dart: fins go at the back, not the front.

 

@THX1138 Just because the CoL is behind the CoM at one angle of attack doesn't mean it always is. Try slowly rotating your plane in the SPH so it has more and more pitch (simulating AoA), and watch the CoL move. Remember that your Mk2 fuselage also provides list.

Finally, recall that what matters is the aerodynamic center (which, sadly, KSP won't show you)--i.e. the sum of all aerodynamic forces, not just lift. Just because the center of lift is behind the center of mass doesn't mean draggy bits forward of the center of mass aren't moving the aerodynamic center forward...

[MrChumley]

Your CoM can move forward or backwards depending on fuel levels, and fuel tank placement.  Also, intakes cause drag, too much drag on the front of the plane can cause it to flip.

[FullMetalMachinist]

49 minutes ago, NathanKell said:

Just because the center of lift is behind the center of mass doesn't mean draggy bits forward of the center of mass aren't moving the aerodynamic center forward...

Agreed.

You could probably get rid of most, if not all of those radial intakes. And if you can't ditch them all, at least move them back some. 

Another thing you could try is angling the wings up at the front (giving them some incidence). This will allow the wings to generate lift while the rest of the craft stays at zero angle of attack. This does two things. First it reduces drag (which if you want your plane to get to space, drag is the devil). Second, it prevents the body lift from the front of your craft from moving your aerodynamic center forward of your CoM, which as you discovered causes flipping. 

[Val]

@THX1138 You can get rid of all the slap-on intakes. A single Shock Cone Intake can comfortably feed 2-3 RAPIERs.

Edit: I'd also suggest moving the vertical tail fin further back. It's very close to CoM, so it'll have very little effect. Maybe double it? One attached to each RAPIER.

And I'd move the rear landing gear to the FL-T800 tanks and move it forward so the wheels touch the ground right behind CoM. That will make it much easier to lift the nose gently for take-off and will eliminate any sideways veering on the runway.

 

11 hours ago, FullMetalMachinist said:

Another thing you could try is angling the wings up at the front (giving them some incidence). This will allow the wings to generate lift while the rest of the craft stays at zero angle of attack. This does two things. First it reduces drag (which if you want your plane to get to space, drag is the devil).

This is correct and good advice.

 

11 hours ago, FullMetalMachinist said:

Second, it prevents the body lift from the front of your craft from moving your aerodynamic center forward of your CoM, which as you discovered causes flipping.

This is wrong. It does not help with with stabilizing the plane in general. It only moves the CoL back for a very small range of AoA. If you get outside that range it'll be just as unstable as before.

You'll see that if you pitch the plane up and down in the SPH with CoM and CoL showing as @NathanKell suggests. As soon as you pitch the craft either up or down CoL will move forward.

Edited January 27, 2016 by Val
Suggestions and typos

[Val]

7 hours ago, NathanKell said:

Finally, recall that what matters is the aerodynamic center (which, sadly, KSP won't show you)--i.e. the sum of all aerodynamic forces, not just lift. Just because the center of lift is behind the center of mass doesn't mean draggy bits forward of the center of mass aren't moving the aerodynamic center forward...

Any chance that this is on someone's TODO list? Please 

[FullMetalMachinist]

4 hours ago, Val said:

This is wrong. It does not help with with stabilizing the plane in general.

Perhaps I should have been more clear. My thinking was that at very small angle of attack the wings will generate lift while the fuselage parts will not. I should have noted that this is only true when that is the case. 

So yeah, should have been more clear, thanks for correcting me  

[AeroGav]

I am counting two long mk2 fuselage pieces and one half length fuselage piece ahead of CG, and only one half length mk2 fuselage piece behind.

These mk2 sections generate more lift than normal cylindrical fuselage, so much more so that the game gives them a lift rating, but i don't think the game is taking that into account.

Secondly, why do people insist on trying to get their Centre of lift so close to their centre of gravity?   Play it safe and let the centre of gravity be well forward.   No, it won't win any agility contests but it's so fustrating to spend 10 minutes flying a plane up to 25km then loose control and be reduced to subsonic and below 10km by the time you pull out.  Not enough fuel left to get to orbit , all you can do is fly back to KSC and start over.

The only drawback of having a forward CG is that the canards might have to work hard to keep the nose up.  Do you flight with f12 aero forces display on, if you see them making a lot of drag, try adding more canards with a lower max deflection angle in their properties.  Eg. advanced canard only deflects to 10 degrees,  standard canard goes to 30 and is very strong, but draggy.

Third, get TAC fuel balancer.  The rocket engines drain from the front tanks first, messing with your CG.    You can cover your craft in draggy fuel lines and mess about with locking tanks or just install this simple mod.

Fourth,  adding incidence to the wing can improve performance but will if anything make handling worse.    Your main wing will end up stalling before the canard, causing the nose to pitch UP at the stall.   I did use incidence for a bit but have now stopped for this reason.    I think it only benefits high wing loading designs.  My aircraft have a lot of wing area for their size and fly at low angles of attack,   at rather high altitude (thin air) for the speed they are doing (mach 1 only above 10km).   The combination of low AoA and thin air means fuselage drag is not a big deal for me.