Originally Posted by
momo95
OK ... so essentially the ideal situation is to have 0 trim, but this is rarely possible due numerous other factors such as pax seating, cargo in the hold etc?
That offers me a different perspective, because the impression of gotten so far in my studying of stability is that aircraft are intended to have a forward CoG in flight, to give positive stability in case of an upgust etc and that the horizontal stabilizer is designed to counteract this by producing a downforce. I was wondering what happened in the rare instance that it wasn't possible to have a fwd CoG and so my initial question arose ...
There are a several airplane components in the airstream: The wing, the tail, the fuselage, nacelles, etc. Each has its own contribution to stability.
The issue of CG vs. center of lift, is only the wing's contribution. Since it's the total that matters, and the tail is always a huge huge contributor of positive stability, the wing can be slightly unstable and the total still be stable.
How much of each? Well, good thing the engineers figured all that out and gave us, as an end result, the envelope that we have to keep the CG in. Doesn't matter to us why the limits are they are, we just have to stay within them.
As an aside, you'll see that float planes that are converted from land planes, (instead of designed as float planes from a clean sheet) almost always have extra tail surfaces added. Because the majority of the float is ahead of the CG and therefore destabilizing. So we need more stabilizing surfaces to compensate.