That is mostly correct. There is also a visual effect that occurs when you render a 3D scene with the camera tracking an object. The point you are tracking always appears stationary. Examples of this in FlightGear are the "helicopter view" and the "tower view". If the origin is the nose of the aircraft then the camera moves up and down with the nose. In the air or from a distance this makes it look like the airplane is wagging like a dog's tail from the nose, when it really is not.
Take a look at the p51d as an example of an aircraft with 0,0,0 at the nose. In the file p51d-yasim-set.xml there are several "target offset" settings (one
for each view) that represent the distance from the nose to a very approximate
center of gravity. If you want to see the effect, then take those target
offsets out.
As you will see, this is a design weakness (my fault). The model configuration will always need to be updated whenever more views are configured. Note that this is a problem with the camera (the viewer code), and not the model. It only needs to be defined per model because of the different sizes aircraft come in.
This does seem to have been an odd choice in how things are arranged.
Take a basketball for a simple example. CG in the middle. Now hang it by a point on the sphere. Hang point is above, CG has a lever arm against it and makes it hang down. Now spin the basketball and balance it on a finger. The point of suspension is then below the CG, and gyroscopic effects keep things stable with the CG above the POS. Note that CG is not the reference point for what is holding the ball up.
A plane 'hangs' on the wing. CG and control moment arms and everything else work around the point that it's hanging from in the air. CG is usually slightly forward, and a small down force on the tail. This makes a line from the CG to the tail through the POS that helps keep the plane more stablilized on the POS, like the fulcrum of a seesaw with some weight on each side. POS is the point all the calculations work around. It's also the point that the whole plane moves around when viewing.
The 'nose' is a bad choice for either the viewing center or the FDM center. Everything works around the POS. The nose is just an arbitrary point how ever many feet ahead. That means there will be unneeded translations in all calculations since it's using a point that is not the actual 'center'. A point 27 miles to the right and 33 miles ahead of the POS could also be used, and the FDM calculate from that and the viewing model also translate it's view from that. The computer can do the calculations, it's just a pretty useless thing to do when both models actually center around the POS. Who cares about the point 5 or 10 feet ahead at the nose, any more than the point that's 27 miles right and 33 miles ahead? Until this thread it never would have occurred to me that another point was chosen as the basis for calculations or viewing. It can be done, but it adds extra calculations.
POS end up being COL for a plane. It suspends itself by the aerodynamic forces it generates. But POS is a better general term for working out how things hang, it works on anything. Take a person. Your POS is through your hips. And your CG is roughly in your waist. We're slightly unbalanced, that's why we have to have feet and active correction to stand up.
A planes nose pitches all over the place when flying level at different angles of attack. The forward vector of a plane in straight and level flight extends out from the POS regardless of angle of attack. While the POS of the plane changes slightly as the CG and and aerodynamic forces shift with attitude changes, it's mainly near COL of the wing. The nose and engine just pull the wing forward. The tail just gives leveraged control. The wing does almost all of the flying by itself in most planes, it takes some consideration and work to get any other part of the plane to help contribute to the flying of the plane and thus shift the POS a bit.
I can sort of see the choice if this started from RC, since some people fly by the nose. But the best place of reference to fly a plane in 3D from outside is really the center of the wing, just where you'd expect from all the forces actually working around that point.
Alan
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