# Aerospace Question on Bernoulli's law

1. Jul 24, 2008

### 50936

Hi everyone,

I read on a website that the Bernoulli's expression can be applied to airplane wings by using the equation:

Difference in pressure above and below wing
= 1/2 x air density x [(airflow velocity below wing)^2 - (airflow velocity above wing)^2]

I tried using it but the units don't seem to add up correctly when I use the metric system (kg/m^3 for density, m/s for velocity).
The website uses (slugs/ft^3) for density, and (ft/s) for velocity.

Can anybody tell me if this equation is correct?

2. Jul 24, 2008

### FredGarvin

What they express is simply the difference in dynamic pressure. Dynamic pressure is the first term in the Bernoulli equation. This makes the assumptions that there is no friction, no potential energy change and no compressibility effects.

Since it is dynamic pressure, the units do work out.

$$\frac{1}{2}\rho V^2$$

$$\frac{kg}{m^3}\frac{m^2}{s^2}$$

$$\frac{kg*m^2}{m^3*s^2}$$

$$\frac{kg*m}{s^2}\frac{m}{m^3}$$

$$\frac{N}{m^2}$$