I cant think ofhow an aircraft can manage to fly upside down without crashing into the ground. I understand that the wings are designed to provide uplift when the plane is flying horizontally. However, when the plane flies on its back as some smaller jets often do, surely the uplift is working in reverse and forcing the plane back down towards the ground. Yet most types of small aircraft seem able to maintain the upside down position for long periods of flight. How do they do this?

It's all about the angle of attack and very little about the shap of the wings. Basically, planes don't fly with their wings parallel to the ground. They fly with their wings at a slight upward tilt. This means that air coming towards the wing will bounce off the bottom of the wing downwards and backwards. Jus like when you're sittig in a chair on wheels, throw a ball, and go sliding in the other direction, this propels the plane upwards and forwards. For more info, see this reply to another question.