How Big is TOO Big

[Wingin' Wayne]

I read in a thread Dr. Drela's formula for calculating control surface area in relaytion to tail moment arm. I've seen planes with huge control surfacres. Not being able to scale pictures and calculate Ch and Cv, I was wondering how big can you go? I built my first scratch built pattern plane before seeing this formula. The Ch is .825 and the Cv is .073, big rudder. It flys well, I'd like to increase rudder area to improve knife edge. So, how much is too much? Thanks for your advice!

[Ollie]

For knife edge, don't use the formula in relation to wing lift function. The lift function in knife edge is by the vert. tail and the fuselage, not by the wing. The hor. tail at Ch is overkill. The Cv and Ch have to be enough for control function but when the Cv and Ch is too much for the drag question.

If the thrust is enough to overcome drag, the Cv and Ch can be large within the thrust over drag question.