RE: Telemaster 40 Mod
This is going to be a fun one!
First of all, the Telemasters tails DO NOT LIFT in the same direction as the wings. If this were true, the airplane would have an extremely aft c of g, and would fly very funny. Having had the 40 size, I know what I'm talking about. Let me give you some theory, before I actually answer your question.
It all has to do with the center of lift with relation to the center of gravity. Center of gravity is where the model actually balances. The center of lift is the point on the wing where the lifting force is averaged. By this, I mean that if you think of lift as the opposite of gravity, this is the point where the lift is balanced on the wing. Just replace gravity with lift, and balance the model again. Of course we can't actually do this, because there is no lift with no airflow. The other reason we can't do this is because the center of lift MOVES with the speed of the air, the angle of attack, and the position of the controls on the wing.
Now, think about landing a sport model or a trainer. The slower you fly, the more up elevator you need to hold the nose up. You are actually pulling the tail down, but the effect is the same. The reason for this is that the center of lift is aft of the center of gravity. Meaning that the model is wanting to drop the nose because the balance point is forward. The center of lift moves aft as the model slows and the angle of attack increases. Conversely, the center of lift moves forward as the model speeds up and the angle of attack is reduced. This arrangement is what gives the plane stability.
If you're familiar with 3D models, those guys move the cg aft as far as they can stand it and still be able to control the plane. In some cases, they move it aft of the center of lift and the plane becomes unstable. These planes are hard to land because the nose wants to rise as the speed bleeds off. This is because the balancing force of the tail actually has to hold the tail up, instead of holding the nose up.
Having said all that, we both know that the Telemasters are stable fliers with no bad tendencies, especially on landings. That's because it is balanced as the first paragraph describes. That being the case, the tail is actually installed upside down from the way it should be to provide the lift in the right direction. I know this, because the Telemaster I had would rotate much faster in the down stick direction than in the up stick direction. Why? Because the tail is more efficient in lifting that direction. The wing is flat bottomed, so it should, you would think, pitch up easier than down, but it doesn't.
Now that I have given you a crash course in aerodynamics, I would say that to make the Telemaster a better flying plane, use a slab type or symmetrical airfoil on the horizontal tail. Because of the type of airplane, and the speed envelope it flies in, there is no advantage to a symmetrical tail other than looks. Flat sided tails work just fine at these low reynolds numbers, and they react symmetrically to control inputs. To answer you question directly, the center of gravity should be at the same point no matter which type of airfoil you choose. The rule of thumb on a plane like this is around 30% chord of the wing. You can start there, and fine tune it to your liking.