John Charlton, via Facebook asked:
What keeps jets in the air? Pressure differentials, force from the air, or a combination?
Dave - I guess he means jets in a general aeroplane?
Essentially, when an aeroplane moves through the air, if you have ever tried to walk from the back of the plane to the front of the plane, you'll have noticed it is always uphill. Itís an awful lot more uphill than you're expecting. Thatís because the nose of the plane is pointed upwards, so the wings are pointing slightly downwards. This means that when air hits them underneath it bounces off and gets deflected downwards a bit, and also, due to an effect called the Coanda effect, the air on the top of the wing tends to stick to it and it also gets deflected downwards. So, the wing is pushing air downwards; Newtonís laws mean that every action has an equal and opposite reaction, so the air is pushing the wing upwards and the plane stays up.