Expert: Paul Soderman - 7/4/2009

Question
Hello Paul.
I am making a twin fuselage remote control model. The wingspan will be 57 inches and the cord is 10 3/16 inches. The cord of the tail will be 45/16 inches but that can be made larger or smaller if need be. What I don't know is the formula needed to make the span of the tail or the ratio of horizontal tail to elevator, but have been working on 25% on that one. I would like the model to be fully aerobatic and be able to do knife edges. And to stretch my luck nice and easy to land as well. Appreciate your help
Glenn

Answer
Glenn
You are asking me about stability and control.  In most aircraft, the C.G. (center of gravity) is behind the wing so the wing lift times distance to the C.G. gives a nose up pitching moment.  The distance is measured from the wing quarter chord point.  In addition, the wing camber causes a nose-up pitching moment about the quarter chord.  These moments must be balanced by the tail lift times its distance from the C.G.  That determines the tail size and angle of attack.  The elevator must be big enough to trim the moments as well as change aircraft angle of attack for maneuvering.  The tail also introduces a small pitching moment about its quarter chord that needs to be considered.

A basic aerodynamics book will explain this better. To make the calculations you need the wing and tail pitching moment characteristics, C.G. location, moment arms, wing lift, and tail lift.  Wing plus tail lift equals weight in level flight.  Lift can be calculated from the same book.  Pitching moments for your airfoil can be found in Abbott and von Doenhoff:  Theory of Wing Sections.

Good luck.
Paul