[BUG] CoG calculations in SPH (and possibly VAB) dead wrong!

[DocMoriarty]

Windows 7, KSP 1.0.4

Was just working on a new VTOL (after a longer break from KSP since 1.0.2 due to burn out and bug ridden KSP) just to notice that CoG calculations (and CoT) are obviously totally bugged. Not sure this is also the case in the game when flying.

To reproduce start building a VTOL:

1. Start new plane in SPH.

2. Use Mk2 Liquid Fuel Fuselage as root part.

3. Add BZ-52 Radial Attachment Point in the center below the tank so that a VTOL engine can be attached to it.

4. Switch on CoG display, CoG is still in center of tank.

5. Attach Basic Jet Engine to BZ-52 Radial Attachment Point below tank -> CoG moves UPWARD although heavy weight is added BELOW the tank. CoG should move DOWN!

The sad part is, that the same calculation seems to be used when flying the craft.

Guys get your very very basic physics straight! I consider this a game breaking bug because it makes serious space plane construction practically impossible at least for VTOLs or planes that have significant weight above or below the center of the plane.

Edit: It seems that CoG is always moved in the wrong direction when weight is added above or below the center. Removing fuel from tanks even moves the CoG further up.

Edited June 26, 2015 by DocMoriarty

[Rhomphaia]

Not a bug, the Mass of the jet engines has been intentionally offset.

[clivman]

It is to represent the weight of the other parts of the engine, the unrealistic engines we have in game were causing COG issues which made it hard to build normal (non-canard) aircraft.

[Claw]

Yes, VTOL jets have taken a bit of a beating in the last couple updates. The CoM for jet engines is indeed intentionally offset for the reasons stated above.

Cheers,

~Claw

[DocMoriarty]

I never had an issue building non-canard planes. Total crap.

[Claw]

Well, the reasons are more than just canard aircraft. But primarily it was a way to move the CoG forward on standard aircraft configurations. Several changes were made to make CoM management easier (such as air breathing engines sucking fuel from every single tank). These changes make CoM management a little easier for basic planes, but make it much more difficult for anything even slightly complex.

Personally I'm not on board with all of the changes. The CoM for the jet engines needed to be moved forward some, but it's way too far now. Also, I'm not a fan of jets sucking fuel from every tank.

Cheers,

~Claw