Ben:

Nice diagram. Glad you did it. Less work for me.

Ben has basically nailed the deal here, and I'll use his diagram to illustrate what I was talking about. If you balance the CG slightly behind the CP of the wing, the tail will produce an up force (lifting tail) in level flight with no elevator deflection, and zero incidence angles. Of course, the angle of attack in the diagram is exagerated like everything else. Now roll the plane over and if it was truly balanced for zero elevator deflection and zero incidence, the plane will fly inverted at the same level speed with no elevator deflection. This is the only CG configuration (CG slightly behind wing CP) for which this will work. If the CG is forward in a more conventional location where stab incidence or (up) elevator trim is required to balance the forward weight - the plane will pull towards the ground when flipped over. If the CG is any further back, negative incidence or (down) elevator would be needed for level upright flight. When this plane is flipped over, it would tend to pitch nose-up.

The distance between wing CP and aircraft CG for this condition is determined by the size of the stab. This is a condition often sought after by aerobatic flyers for obvious reasons. This condition does not automatically mean that the airplane will be overly responsive or touchy on the controls. It all depends on the size of the stab. Airplanes with a too-small stab may not be able to fly like this at all, however aiplanes with a larger stab will be able to.

All this so far to explain why it is quite common for planes to fly with "lifting" tails, as anyone flying with a "pitch-neutral" plane like this is flyiing with a lifting tail. It is common to balance for this for aerobatics (pattern, IMAC, etc. )


The larger the stab, the farther rearword the CG can move. Stab size and moment arm are the variables many overlook when using "rule-of-thumb" CG location. The larger either of these two are, the more you can move the CG back and still have a flyable airplane. Imagine that the stab keeps growing until it is almost as big as the main wing - well now you have a tandem wing and the CG will be loacted somewhere around halfway between the AC's of the two. Keep stretching the "stab" and it becomes bigger than the main wing and now you have a canard. With this stretch you may move the CG further back still. SO, you see - the larger the stab/rear wing the further behind the main/front wing the CG can go and still have stable flight.

Now, to tie this into the question of what to do with a CAP kit.[>:] I have had the 80" Cap from Lanier and it could not be balanced so that it would fly hands-off inverted. Not that and still be able to land under control. I also had flown an 80" Stinger which could be balanced for zero trim upright and inverted. The difference was that the CAP had relatively much less stab area than the Stinger. SO, if you are looking for a mod to do to make the CAP a better flying plane, enlarge the stab!