# What really causes a low-pressure gradient over the top of an aerofoil? [duplicate]

As air flows over the top of an aerofoil the pressure drops. Compared to the high / normal pressures on the underside, a pressure gradient is formed with higher pressure further above the wing. You get:

less pressure = less particles = less density

This sketch summarizes what I know so far:

My questions are:

• Does lower air density above the wing mean the air weighs less compared to the weight of the air underneath the wing? If the difference is negligible then it's OK, we can forget it.

• What forces act on the air particles causing them to speed up?

• Its the shape on top of the wing. Lets refer to newtons law the second that states that if you have a straight line airspeed will not change, but on a jet or airliners wing it is slanted downwards which increases the airflow speed and results Iin a high pressure under the wing and a low pressure above the wing. Think of yyourself going down hill on your bike and you dont have to pedal to get down the hill, it has the same effect as going down a hill on your bike, gravity pushes down on the airflow making it go faster on top and slower on the bottom. – Ethan Aug 31 '15 at 18:57
• is this right ??so the air particles speed up (due to gravitys downward pull) as they move arcoss towards the back of the wing due to the downwards slope of the rear of the wing as they speed up they are elastically pulled further apart this then cause less particles per unit vol in a given space and there for less localised pressure – 123 Aug 31 '15 at 19:08
• thanks Ethan I think its starting to make more sense now !! – 123 Aug 31 '15 at 19:28
• if your question is really of this kind I feel it should be better asked on physics.SE – Federico Aug 31 '15 at 19:37
• to me this is more and more about physics (and shouting won't achieve anything). – Federico Sep 1 '15 at 18:06