i'm working on MQ-1 predator i try to make one solid body from the tail and the body by using surface knit command in solidworks............it gives me a solid surface not solid body
There is probably a gap somewhere in the knit. The surfaces must fully enclose a volume before surface knit can generate a solid.
By default SolidWorks will color an open edge as blue. It is very hard to see, so set the background/scene to be black. You'll usually see the defect right away.