Talking about trainers. What is the proper way to bevel the ailerons? Should it be angled on either the top or bottom, or both, or neither.

You talking about the hinged side of the ailerons? If so,you'll have to bevel both sides. I generally just use a sanding block and pretty much do it slowly alittle on both sides equally until they come together in the middle. Just make sure you get a consistant,proper angle on each that will be enough that when the ailerons is moved up and down it doesnt touch and keep it from moving throughout its full range of motion.

Cut the hinge slots BEFORE doing any beveling. Cutting them after beveling is not only more difficult to do, but harder to have the slots on the centerline.

Ahhh!!! I was going to ask that question. Ya beat me to it. Thanks

Mark lines on them first . Lay a pin flat on the table. One line on top and bottom. Two lines for the thinckness part. This should give you a
general idea where the center is at.
use a long sanding block. Go side to side or horizontal, this will give you a more even bevel.

I use a centerline pin tool I got from tower run down the length of the aileron and trailing edge of the wing to mark the center, then according to the kit instructions I mark the pencil lines top and bottom of the ailerons and trailing edge, in most cases it is appx,1/8 of an inch back, and like Campy said cut your hinge slots first. then all you have to do is use the pencil mark you made and the scribe mark from the tool as a sanding guide. they turn out perfect every time when I do it this way.

A razor plane works really well to do this, the bevels turn out sharper with less dust and effort than sanding.

Some of us can accomplish the bevels with precision

The guide marks certainly help

Some of us just cannot do it but sloppy.

Each person is different