Yes, any aircraft be it of any sort will go unstable if the CG goes aft of the neutral point. But in many cases long before that happens you will find the stabilizer working with a positive lift coefficient and the aircraft will still be admirably stable. This is not an "us and them" situation between full sized and models. It just applies. And I'm also not talking about the aircraft with computer augemented stability either, just the regular stuff.

Without seeing a running of the numbers for the Cessna I'm not in a position to say yes or no to the question of if it has a positive lifting. And truthfully I'm not that comfy with some of the advanced math required to show what needs to be shown.

However I'm basing this on a few key bits of info.

The first is that a few aeronautical engineers hang around here and they have talked about this subject in the past. They often don't agree on much between themselves but they all agreed that the tail lift coefficient moves from negative with an extreme forward CG to neutal and into the positive values as the CG moves back towards a standard value for our models.

Mark Drela also had a lovely chart that showed the relationship of wing to tail area and the location of the nuetral point as the tail gets larger right from a very small stabl through to tandem wings and ending with a canard configuration. Oddly enough the same stability calculations can be used across the entire spectrum with the "front wing" of the canard being inputed as the "main wing" and the larger rear wing being inputed as the "stabilizer". The neutral point, and thus the related slightly forward CG position, comes out of the exercise just fine as it should. But the key point here was that the cross over of the tail lift went from negative to positive while the configuration was still very much like a normal aircraft should look.

The other kick in the pants was a version of the old David Fraser DOS program Sailplane that I have. In it the wing and tail lift coefficients are calculated by what I would assume is the standardly accepted formulas for such work. In designing my own sailplanes I was shocked to see that as I made the tail larger and pinched the CG back towards the 35 to 37% mark the tail was operating at a positive lift coefficient and that only when sped up to higher speeds did it shift to a negative value. I was so shocked that I inputed the values for my own and some buddies' sailplanes that were flying well and got the same sort of results.

So it is possible to operate at a postive lift coefficient and still have a stable flying platform. I see no reason to suspect that this would not apply to large models and up on to people carrying airplanes. But as I said before the earlier indications in previous discussions said that by agreement in the industry the designers out there choose to set the permissable CG range based on a number of CG related safety issues and the limits they set are very conservative. These conservative settings will USUALLY result in a negative lift coefficient at the tail of the majority of planes. However it does not specifically deny the idea that some of the aircraft out there may shift to a neutral or slightly positive lift at the stab if the CG is shifted to the rear of the permissable limits. As long as the aircraft still operates with the desired stability in pitch it isn't an issue.

I went through all this too and had to re-educate myself as all the books I ever read talked about the weight ahead of the wing being countered by the downwardl lift behind the wing. But in reality the answer is that "it depends". It depends on the tail size and tail length and the CG location. As I said it would not be possible to confirm or deny the Cessna example without running the numbers and doing so for the various speeds across the typical performance range.

I know I don't have any charts or other math formulas to show you but there's been a lot of discussion in the past about this and various proofs offered by the folks that have made the study of aerodynamics more than a hobby. This includes that old DOS program I have.