On a control line plane why does the airfoil have to be fully symettrical to work? Many R/C planes and real ones don't have this kind of airfoil and do unbelievable tricks and stunts. They enen fly inverted realy well. I just don't understand. Thanks Rod

Control line planes don't have to use symmetrical airfoils, but decent aerobatics are tough with flat bottom or semi-symmetrical airfoils. If you look at a plane flying inverted with a flat bottom airfoil you will see that the nose is pitched quite high to mainain level flight. This doesn't leave much room for error. In control line flight, if you have a flat bottom or semi-symmetrical airfoil and you get close to the ground, you need all the down elevator you can get to recover and get back up and over the top of the outside loop so you can return to normal upright flight. If you're already using all of the "down" elevator travel to stay level, you are not going to have a chance at recovery unless you use "UP" elevator and pull out above the ground. The plane and the ground typically will not cooperate in such cases.

I believe Al Rabe used an airfoil with about 2% camber.  The theory was that the fourth corner of the hourglass really needed some extra lift, since the plane often approaches this corner at high speed due to the long inverted, downwind dive.  It's the only seriously assymetrical point in the pattern.

I wrote more about asymmetric airfols in my two part autobiography published in Control Line World October 2007 and January 2008 issues.  I said:

"Asymmetric airfoils

While testing airfoils, it became apparent that thicker airfoils were providing more lift.  It also raised the question of whether we need as much lift for inverted flight and outside maneuvers.   There was no clear reason why asymmetric airfoils wouldn’t work well for stunt.  My thoughts were that the two most difficult corners in the pattern, demanding of lift were the lower right triangle and hourglass corners.  These corners are all inside corners.  We need more lift for these inside corners than we do for the less demanding inside and outside corners in the rest of the pattern.  Through airfoil testing, I selected two airfoils having the same general curvature but differing slightly in maximum thickness for the Sea Fury wings.  I used the thicker one for the top of the rib and the thinner one for the bottom.  The Sea Fury used this asymmetric airfoil from the wing root, out to the dihedral breaks, then smoothly transitions to fully symmetrical at the tips.  In flight, there is no detectable difference in the handling characteristics of the asymmetric airfoil in inside and outside maneuvers.

It raises the question of what would motivate me to use such a radical airfoil on a competition airplane.  Remember, at the time the use of a 25% section was unheard of.  I was frankly afraid that an airfoil this fat would result in an ugly airplane. Thinning the airfoil by reducing the camber on the bottom half of the airfoil would give a more pleasing appearance to the wing in flight while preserving the highly cambered top for necessary lift in those demanding triangle corners.  The wing design met my goal of creating a winning airplane, whatever the effort required."

Al

THANKS!!! You have revived my interest, I had given up hope on this, now I think I will! What the heck, the worst I can do is crash. Best I can do is make a wonderfull bird! I will settle for somewhere in the middle.