Setting the neutral point of the ailerons 'up' will compound the effect by pushing down on the trailing edge of the wing thus forcing the tail down even more! If you go this route, lower them if anything.

Jazzy,

I don't believe you thought about the statement above or you don't know how a wing works. Following your reasoning above lowering flaps would cause the tail to pitch up and the airplane would dive, or the wing half with the aileron deflected down would pitch down and cause the plane to roll in the direction of the down going aileron and we all know that doesn't happen.

An aileron or flap going up or down changes the camber and effective angle of attack (AOA) of the wing and thus changes the lift being generated by that wing. An aileron deflected downward increases the camber on that side of the wing increasing the effective AOA and creating more lift on that wing half which results in that wing half rolling upward.

Getting back to the original question about needing more down elevator after repairing the airplane. The first part of my suggestion was to raise the trailing edge by 1/8 inch or so. On a flat bottom airfoil I designed for a modified Great Planes Trainer 60 a number of years ago I measured the change from a 1/8 inch shim and the incidence decreased by less than 2 degrees. This is for a chord of about 12 inches so for a trainer with a 10 inch chord the change would still only be about 3 degrees. This is not enough to create any drastic unexpected changes in flight characteristics, but rather a noticeable change in required elevator deflection to know if the change is moving things in the desired direction.

The second half of my suggestion was to move the ailerons up from their geometric neutral position by 1/8 inch. This would have the effect of reducing the wing camber and affective angle of attack. Again, in the case of Trainer 60 wing, the effect is about a 1 degree reduction in the effective AOA, enough to be noticeable but without unpleasant surprises.

This technique is called reflexing the trailing edge and is widely used by sailplane pilots to increase the penetration of their airplane when moving out to seek another thermal for one example. Just because this is something common to sailplanes doesn't mean it can't be useful to prop turners. In fact Dow Lowe has in numerous articles on trimming pattern planes discussed this very same technique. More on this topic can also be found in Martin Simons' book MODEL AIRCRAFT AERODYNAMICS.

Another respondent wrote about setting the bottom of a flat bottom airfoil parallel to the horizontal stab and using 3-5 degrees of engine down thrust to keep from having to re-trim the elevator for each throttle/speed change. Finding the correct amount of down thrust can be a pain. I would suggest increasing the angle of the wing bottom relative to the horizontal stab and positive from leading edge to trailing edge. Do this by either raising the trailing edge or lowering the leading edge and set the mean chord line parallel to the horizontal stab or even something close to the zero lift AOA. This will eliminate the need for down thrust and the wing will act more like a symmetric airfoil over changing speeds while the plane retains the gentle stall and flying characteristics of the flat bottom wing.

Steve Woodruff