Regarding decalage and stability..

There is a mix up between cause and effect and between stability and trimgoing on here. The cause of stability is having the CG located ahead of the plane's neutral point, simple as that. The effect of having the CG ahead of the neutral point other than producing stability is that youneed toadd some decalage to achieve balanced (a.k.a. 'timmed') straight and level flight (i.e. to trim the plane). Decalage creates a nose up pitch that balances the nose down pitch caused by the forward CG.

Decalage is not the cause of the stability, it's an effect of stability and isrequired if the plane is to be trimmed for level flight.

I'll give you a couple of examples that hopefully illustrate this point.
<ul>[*]Take a plane and add weight to the nose to make a forward 'stable' CG. Now set the decalage to zero. Drop the plane from a great height.. It wont fly as such, it will plummet vertically down, but it IS stable. i.e. whatever way you dropped it, and however it's disturbed on it's way down, it will always return to a vertical dive. A vertical dive with zero AoA is it's trimmed state and it's stability ensures that it always returns to it's trimmed state. This plane is stable in the same way a bomb dropped from a B-17 is 'stable'.[*]Now take another identical plane (the first having been destroyed when it dived vertically into the ground) and add some decalage. Now the plane is trimmed to fly at some positive AoA. It's stability (due to forward CG)ensures that whatever way it's disturbed it will return to it's trimmed state. By adding decalage we did not change the stability, we changed the trim of the plane.[/list]The point here is <u>stability is the tendency for a plane to return to it's trimmed state after being disturbed</u>. Decalage sets what that trimmed state will be. Decalage does not make the plane stable, it's CG location that does that.

A third example;<ul>[*]Take the same plane and move the CG way back. You can set the decalage to whatever you like. If you are very careful is should be possible to come up with a balanced (trimmed) condition even with the CG way back,howeverany disturbance will see the plane deviate away from this balanced state. This plane will be incapable of maintaing steady state flight. This plane is unstable and nothing you can do with decalage will make it stable, only CG location can do that.[/list]<u>Anyone who doubts any of this I'll repeat my challenge</u>: If you truly believe that decalage makes a plane stable -Take a plane that you know flies well. Add a lump of weight in the tail so that the CG moves maybe half the wing chord back from it's normal position. Do whatever you like with the decalage to makethe plane stable. Now go fly it and tell me it's reallystable

Steve

PS.. i accept the point made by rmh earlier that these days it's possible to add 'artificial stability' with gyros and the like but that's another topic, lets assume we are talking conventional aerodynamic stability here...