Does COL shift when one engine type is out of fuel? i.e. Rocket engines vs wings

[JoeSchmuckatelli]

I built a spaceplane in the SPH that looked like it should fly.  Put 'Thud' type engines on it so I could maneuver in space.  With all fuel removed from the rocket engines, the COM and COL looked good for atmospheric flight.  Plan was to use rocket boosters to get to space, the thuds to help line me up on my trajectory, and then a long glide path back down to my destination.

 

However, once in the VAB, my COL shifted to a point near the Thud rockets - well behind and above my COL (were I in level flight).  You can imagine my flight char... okay, there were no flight characteristics - the plane didn't fly.  I got it into space, and only killed a few fish when I crashed into the ocean.

 

Is there a rational reason behind this - or is this a quirk of the game?  I figured that once the Thuds were out of fuel, and the plane back in the atmosphere - I figured the COL would shift to where it showed me it was in the SPH with all fuel empty from the rockets...

[Sean Mirrsen]

The COL should not "shift" unless you have a modded variable-geometry wing set up or some very creative use of staging. COM shifts as you run out of fuel, and COT shifts as you engage and disable various engines, but COL is entirely dependent on your craft geometry.

A screenshot of how the COM, COL, and COT look in the SPH and the VAB would be helpful here.

[Venusgate]

CoL also shifts in the editor when you rotate the craft. Usually, i'll try to build a spaceplane horizontally to get everything balanced, and then take the whole mess and tilt it back 5 degrees at a time toward verticle to make sure my Col:CoM don't go nuts as my AoA rises.

[pandaman]

1 hour ago, Venusgate said:

CoL also shifts in the editor when you rotate the craft. Usually, i'll try to build a spaceplane horizontally to get everything balanced, and then take the whole mess and tilt it back 5 degrees at a time toward verticle to make sure my Col:CoM don't go nuts as my AoA rises.

Didn't know this,  thats handy info. thanks.   I'll try to remember that trick. 

[JoeSchmuckatelli]

3 hours ago, Venusgate said:

CoL also shifts in the editor when you rotate the craft. Usually, i'll try to build a spaceplane horizontally to get everything balanced, and then take the whole mess and tilt it back 5 degrees at a time toward verticle to make sure my Col:CoM don't go nuts as my AoA rises.

I think this is what I think happened.  The COL, which had been close to and behind the unfueled COM shifted in the VAB after I rotated it.  The automatic nature of it made me suspect the VAB was assigning the lift to the thud rocket, but it sounds like it'sa  90 degree snap to verticali n the VAB thing... I just clicked the plane and it went from horizontal to vertical in an instant.

[martinborgen]

I think its a bug or strange effect. I find that when flying, its the SPH col that matters. 

[JoeSchmuckatelli]

1 hour ago, martinborgen said:

I think its a bug or strange effect. I find that when flying, its the SPH col that matters. 

I think it's the last editor applied COL.  Haven't been able to test fromS PH, but I have seen the same thingh appen to a different plaane.  

 

Stupid phone induced text errors.

[Claw]

If you've manually rotated any of the wings or control surfaces, you can run into this sort of strange behavior. It can also happen if you connect wings/control surfaces to a rounded part, because the angle snap sometimes attaches a little tilted, due to the model.

So then what basically happens is you reach a point where one of the lift surfaces could be at zero AoA, and generates zero lift. But the rotated lift surface still has some AoA, so it's lift might become a significant factor and cause instability.

As mentioned above, if I'm rotating lifting surfaces on the ship, I will select the root part and rotate the ship through a range of AoAs. You can watch how the CoL shifts. If there's a dramatic jump, you just need to make sure it's at an AoA that you either aren't going to be near, or that the resultant lift is small and won't cause an out of control situation.

You can rotate the ship by either SHIFT+CLICK and SHIFT+WASD the whole thing, or you can select the rotation gizmo and then the root part (I think you can also SHIFT+CLICK when using the rotate gizmo). I tend to use the rotate gizmo because it gives finer adjustment.

[JoeSchmuckatelli]

I have confirmed it to be an editor problem - i.e. the VAB applied a totally different COL to the plane than it had in the SPH, probably during the auto-rotation after loading the SPH file.

 

I took the plane out of the SPH, managed to get it up in the air and flight tested it - it flew about like I expected.  However - when launched from the VAB... flies like a rock with a tail.