Expert: Mark Janus - 8/7/2008

Question
Hi,
Every time I watch some program about aerodynamics, when it comes to the question of how the airplanes fly, the answer is always: because the air under the wing passes slower than the air above it, and as a result of that, the pressure under the wing is higher, causing the upward force. But my question is this: Why does the pressure decreases with the speed of the air flow in the first place? Is there any kind of theory or equation that states this, or is it just an empirical knowledge?
I would really like to know the answer to this one, no matter how complicated it might be.

With regards,
Samir.

Answer
Hi Samir,

Yes there are equations that tell us the relation between pressure and velocity...  the equation is called Bernoulli's equation.... it comes from another equation called Euler's equation which is the equation that really shows us the qualitative relationship between pressure and velocity...that is, as the velocity goes up the pressure goes down and vice versa...  so that is why we know the pressure above and below a wing are lower and higher resulting in the lift

Thanks for the question,
mj