My 3d printable Bell Cobra helicopter which you can use for 3d printing,but also as hard surafce model for your 3d project.It was created with Moi3d CAD software 3dm is their extension.It is like liteweight version of Rhinoceros 3d.

It is simplified version of real Cobra helicopter.Rotors made to spin I left some gaps betwwen objects.

In my software it showed me that all objects are solid watertight objects

OBJ file useful only as NGONS,that is how I imported to blender3d and created my renderings