Hi ppl here I have a problem with my mesh 
When I upload my mesh to Unity, there is a normals problem, the red lines show it, and the blue ones when I flip normals
Here is the mesh
It seems I must make 2 faces to fix that, but there is a method to make with just 1 face polis?
You might be able to make Unity work with just 1 face in those locations. It all depends on if Unity will draw double sided polygons or not. You might look for features named “fill back face” or “double sided polygon”. The problem with turning on a flag like that is that it is likely that all faces in your model will be drawn double sided. This could effectively double the number of faces that Unity has to process each render pass.
Since you only have a few faces that seem to be messed up, it seems like you should just be able to clean those up. Either add faces behind the current ones or just flip the “red” faces. If you go into edit mode and select just those faces then the “flip normal” should only flip them and leave the blue ones alone.