@ bzhyoyo:
here's an example image for a specular map I made for a C47 (very roughly, since the current TW engine doesn't support specular maps)
like you see I took out the specularity from the wing roots and the connection to the engine by darken this areas. (this is what I called corners

)
I set a higher specularity on the panel lines outer edges, the rivets and the aileron edge.
For the overall surface I added some overlays which are different from the diffuse, this way I'll have some nice effects when I'm rolling with the ac

.
The values on the specular map itself can be quite different, depending on the game engine. So you can take the image as an example for seeing the different parts in it, for brightness/contrast or even colors you may need a completely different appereance for your work
@ Blowhard: I'm not sure if I'd add reflection maps on a fabric surface. If you would like to do you have to use them really settle.