Problem with 3D model of Wildcat

Hi,

I have a problem with aspect of a 3D model in game. It was good before I work on it. What I've done : I exported the 3D model of the Wildcat airplane as .obj (+ .mtl file). Then I opened it in wings 3D, I separated it and removed the pilot, then I reassembled it and exported as new .obj. But now, in game, I have no more a smooth surface as before, as you can see :





Why is there this problem ??? How can I solve it ?