Originally Posted by Bozarth
Does this apply with a conventional design with positive stability?
The lift from a wing through the normal range of flight is focused on the 25% chord point. Then if there is some camber then you also see some pitching moment. But if we look at symmetrical airfoils for simplicity the lift occurs at the 25% point with no pitching moment.
So if the CG of the model is set to some point behind this 25% point as is very often the case then the horizontal tail will be lifting upward by some amount. It is purely related to the location of the CG with respect to the wing. The rest such as airfoils used on the stabilizer and angles between the wing and stabilizer only act to trim the model for a given stability margin and flight speed. This can alter slightly when we use camber in the wing. At some flight trims a CG at something like 30% with a strongly cambered airfoil might be lifting upwards. At some other speed it might be creating no lift and at yet a third speed might be lifting downwards. It depends on how strong the pitching moment is and the flight speed at any given time.
But even with a strong pitching moment as we enlarge the stabilizer or lengthen the tail moment and use that to allow us to shift the CG rearwards the stabilizer will shift to lifting upwards all the time and more and more strongly the more the CG shifts to the rear.
This is one reason why old time free flight models so often use lifting airfoils on the stabilizers. The use of an airfoil does not automatically make the tail a lifting tail. Instead it is related to the strongly rearward CG locations such models typically used. Anything from 35 to 60% for a location of the CG was not at all unusual depending on individual designs. One of my own models is an electric powered Henry Struck Record Hound. And at 50% CG it is actually still more nose heavy than is optimal for the glide. I'm waiting on a brushless motor to arrive and then I can install it along with the swap to light Lipo packs and finally get the CG back where it should be. I've only avoided ballasting it further back up to now because it was already quite heavy thanks to the old school geared brush motor it has lived with through the late 80's and all of the 90's.