I agree also on the Gull winged Stuka Models...and would agree..they would be unstable.
The Real Stukas of the Luftwaffe were built with alot more balance..and of course the materials used to keep them in flight. It boils down to weight displacement and Balance.
These RC Planes you have built are awesome. Your craft ability surely overcomes a disheartening crash and your will to rebuild. Great Job . Regards Larry