Incidence vs. CofG is one of those neat sort of give and take relationships. They work together hand in hand to give you the flying characteristics you want. BMatthews has explained it pretty well, but he hasonly lightly touchedonone other very inportant factor that plays into the whole thing as well. Airspeed. He sort of alludes to it by mentioning to throw the glider hard, and when he talks about the trim speed. I am going to try and explain in practical terms how airspeed creates a neat balancing game between CG and angle on incidence.

Here is what happens, as we know, if you move the CG back, you have to decrease the incidence (or trim by adding down elevator trim). Now, when the aircraft is flying, the aft CG will try to "push the tail down", but the incidence (or down elevator trim) will hold it up, and away you go. But, here is where the airspeed comes in to play, you will have it trimmed to a certain speed as has already been explained, and you decide you want to slow down. The incidence or trim (aerodynamic forces) will be less effective due to less speed, but gravity of course staysthe same,so the aircraft will become unstable, wanting to nose up and stall. (less incidence or more down would help here) Then you get scared, push the nose down to pick up speed, and the aerodynamic forces become stronger again holding the nose down, no you find yourself in a dive gaining speed and you are thinking your elevator servo is jammed because it will not pull out. (Now your extra down trim or negative incidence is a major problem!!)

Lets look at a nose heavy airplane. You have it all trimmed out for a certain speed, and you want to slow down. Throttle comes back, airplane slows down so aerodynamics become less effective, and the nose starts to drop. A you slow down more you find yourself running out of up elevator to keep the nose up. (More incidence would help here) So, you giv'er full throttle, speed up so the aerodynamic forces can overcome gravity, but since you are in a dive you gain speed quickly, and next thing you know, the airplane is going straight up!! (Your extra up trim or positive incidence may even cause it to loop right over on its back). Of course it will only go straight up for so long before it stalls, and goes straight down, speeds up and starts over again setting up a wonderfull porpoise until....kerboom.

As you can see, it is all a big game of balancing. To keep basic trainers stable and easy to fly, designers typically go with the second scenario to provide a more stable aircraft. Once you are more proficient you will start to see these things all happen on your own. Once you have found a happy balance between incidence and CG that works at both ends of the speed envelope,then it all becomes a game of trimming for different speeds, depending on how you want to fly that day. Yes, elevator trim is constantly being adjusted.

With the neat little "aerodynamics lab" that BMatthews gave us, you can demonstrate all of these by setting up the gider to glide nicely at a certain speed with a certain CG, and the throwing it harder or softer. You will find that it will do what I mentioned above everytime, for the reasons that BMatthews has explained. Great fun. Another fun experiment is to build some good ole' rubber powered free flight planes and get good at trimming them. This is how I started out and I firmly believe that I am a better pilot because I learned all about trimming, CG, and lots of other things from them.

Hope this helps a bit to visualize a real life example of what is going on.

PS: I used bold type so that you can easily see where the aerodynamics change due to the airspeed.