[bongodriver]

Originally Posted by robtek

The fuel tank is right above the cog, so if it gets lighter the cog doesn't move at all, afaik.

CoG is not a stationary datum, the CoG is dynamic and will shift with change in weight, the datum you are talking about is the AC aerodynamic centre, the point at which all the forces act, if the CoG is behind the AC then the aircraft is unstable and vice versa, so....given the Mass of the engine sits ahead of the AC and the fuel is sligtly behind then any reduction in the combined weight will bring the CoG forward.

Sorry my mistake, AC is where lift acts and CoG where weight acts, but the point is the same.

I meant to add, the reason I believe this is the effect is because the only Weight and balance diagrams I have seen of the Spitfire place the fuel tank moment just behind the datum point, as fuel reduces then the moment weight is offset by the constant moment weight of the engine which I think we can all agree is ahead of the datum.