A problem while shifting CG

[JMW]

Thanks Tblaxland.
OK. So if I am moving the CoG, my box should move along the fuselage of the plane about 90 metres, is that right? Because it doesn't move an inch!
It looks like it's moving, but what is actually happening is that the craft moves slowly across the screen from right to left. If I measure the position of Cog(Box) it doesn't actually move along the plane. It's like something is compensating and moving it back to its original position with each step.

If I use ShiftCg it messes up my internal view camera and/or mesh in that, if I start in VC view and then change to generic cockpit view, then back to VC view, the VC doesn't reappear. If I don't start in VC view, it never appears whatever I do.

I'm using FuelMFD to transfer the fuel from one tank to another. One being monitored by "fuel_change".

JMW

 

[tblaxland]

Thanks Tblaxland.
OK. So if I am moving the CoG, my box should move along the fuselage of the plane about 90 metres, is that right? Because it doesn't move an inch!
It looks like it's moving, but what is actually happening is that the craft moves slowly across the screen from right to left. If I measure the position of Cog(Box) it doesn't actually move along the plane. It's like something is compensating and moving it back to its original position with each step.

As stated previously, the external camera will always* be centred on your vessel's CoG. If the vessel is moving from right to left on the screen, that implies that the CoG is moving from left to right. It is important that you understand that, in Orbiter, the CoG is always at the vessel's coordinate system origin. When you call ShiftCG, you are not modifying a vessel parameter that tracks the CoG, you are shifting all your meshes, thruster locations, etc, etc, in the opposite direction.

* Also as stated previously, there is a smoothing function on the camera motion so, if the CoG movement is fast and far enough, you may notice the CoG moving relative to the camera which manifests itself as the little yellow box moving its position off of centre screen and then moving back to the the centre after you have finished moving the CoG.

If I use ShiftCg it messes up my internal view camera and/or mesh in that, if I start in VC view and then change to generic cockpit view, then back to VC view, the VC doesn't reappear. If I don't start in VC view, it never appears whatever I do.

I expect that is because in your callback function for the VC you set the camera position, right? Well, because of the shifted CoG (or more accurately, the shifted meshes) your VC mesh is no longer in the same position relative to the vessel coordinate system origin. So, you need to track, in a vessel member variable, the total CoG shift from the initial origin point and use that to modify the VC camera position.

 

[JMW]

Thanks.
So are we saying that basically my whole concept of altering cog to change craft trim is flawed?
Or is it that I need to alter the vessel's coordinate system origin as well, ('cause that's what I thought Shift CG was doing) and that that will then alter my trim?

I get the reason for the VC problem.
JMW

 

[tblaxland]

Thanks.
So are we saying that basically my whole concept of altering cog to change craft trim is flawed?

No, it is not flawed.

Or is it that I need to alter the vessel's coordinate system origin as well, ('cause that's what I thought Shift CG was doing) and that that will then alter my trim?

ShiftCG does move the coordinate origin, relative to the global coordinate system. It also moves the meshes etc in the opposite direction, relative to the local vessel coordinates, that is why you see the vessel moving relative to the camera. It all sounds like it is working OK to me.

BTW, 90m is a huge offset in CoG. I assume this is one monster vessel we are talking about.

 

[JMW]

Yes, that's my problem - even with a massive change of cog, the flight trim is not affected. That is, the aircraft doesn't incline down or up.
The craft is NOT massive either, it's Concorde ! (size = 30).
That's why I'm not achieving my object, and I don't think that cog can be effectively changing.

 

[tblaxland]

Yes, that's my problem - even with a massive change of cog, the flight trim is not affected. That is, the aircraft doesn't incline down or up.
The craft is NOT massive either, it's Concorde ! (size = 30).
That's why I'm not achieving my object, and I don't think that cog can be effectively changing.

If you see the mesh moving relative to the camera (as you describe), your CoG is shifting.

We are talking atmospheric flight here? Put your vessel in space, how does the CoG shift affect rotational RCS thruster performance? As would be expected, ie, the pitch thrusters are no longer as effective? I suspect that Orbiter might not be automatically shifting the attack point of your aerodynamic surfaces so you may have to adjust those in the same manner as required for the VC camera position (the documentation only talks about "thruster positions, docking ports, attachment points and to the cockpit camera position" being automatically shifted).

BTW, shifting the CoG 90m for a 30m vessel is obviously unrealistic, 15m is the max that could be physically achieved, and not even that in practice...