Can one fly up side down while a glass of water keeps full due to g-forces? If one makes a looping for example with a plane and the g-forces that the planes makes will keep the water in the glass, but if one is at the top of the loop is there still enough g-force to keep water in the glass?
Yes, as long as you continue the loop, and don't fly the plane in the upside-down position. 

See, for example: Barrel roll with a glass of water on a glider