In reply to Trangia:
Yeah as far as we are all concerned it's simple. But the conservation of momentum changes things and it all gets wierd. Just like with a rocket that goes up because it is ejecting mass down, the same happens with a plane... the air is forced downwards after going over the wing.
Funnily enough on that note... if you have a rocket in space (ie. no gravity and no air resistance), the centre of mass of the rocket and all it's fuel always remains the same. Even if it flew to the edge of the solar system (in a straight line from earth ignoring all planetary and solar gravity), the centre of mass of the rocket and the ejected reaction products would still be where it started off from. I always found that pretty interesting.