ORIGINAL: Kevlar
How much weight is too much to add before it's start to comprise any possible flight characteristics?
Regardless of how much weight you need to add, you will never compromise flight characteristics by properly balancing your plane. A heavy, balanced plane will fly better than a light, poorly balanced plane.
ORIGINAL: theradioflyer
The more weight you add the worse your plane will fly. Lite planes fly the best and trim out better.
Jay
Sorry Jay, but this simply isn't true.
Light planes need less engine power to fly, and light planes 3-D better, but light planes do not "Fly" better, and they are also much more subject to windy days.
Your analogy of the penny is a bad one. A penny is not aerodynamic.
Wad up a piece of paper the size of a baseball and throw it. How far did it go?
Now throw a baseball.
A baseball will fly faster, smoother, and it will go farther than a paper ball because it is heavier.
So a lot depends on what type of plane you are flying. An RV-4 shouldn't fly like a Piper Cub - it shouldn't fly like a 747 either, but adding any required weight is not a no-no.
Case-in-point, Last year I needed to add 22oz of lead to the nose of a GP Seawind to get it to balance. That's almost a pound and a half to a plane with a 676sq in wing.... It flew great!