Using UV Mapping -> Weird different texture mapping on similarly sized faces


I am trying to texture my Heinkel 111 Bomber I modeled. To make the plane’s metal skin structure visible, I am creating faces corresponding to the metal plates on the fuselage. I also created a texture tile (1024x512) which should apply to many faces of the model ( 1 Tile on 1 face) to fake the rivets. Before that i tried to view-projection map a picture with the metal skin structures on it onto the model (from top and from side view) but the lines were to thick and to blurry to apply a bump map on them. Ok, now my problem: Similarly sized faces have significantly different distorted textures on them. I appended a screenshot of the blender viewport and a test render here to demonstrate the effect.