# Lift formula question

1. Dec 29, 2005

### itsjustme

About the lift formula i know its "F= Cl/2 P V^2 S" but my questions are what units do you use for each value (still SI?) and is there any formula for finding the caofiscient of lift, if not then how do you find it out?

2. Dec 29, 2005

### FredGarvin

Coefficient of lift is an experimentally derived value in most cases, i.e. wind tunnel tests. There are tabulated values in many text for basic shapes like spheres and flat plates.

As far as the units go, the standard units in SI are
$$\rho$$ is in $$\frac{kg}{m^3}$$

$$V$$ is in $$\frac{m}{s}$$

$$A$$ is in $$m^2$$

Of course, CL is a dimensionless number.

3. Jan 11, 2006

### jaap de vries

CL = L/(0.5*ro*S*V^2)

4. Jan 20, 2006

### itsjustme

thanks FredGarvin. you know jaap de vries i was actually thinking the same thing but i rearenged the equation and it read CL = 2L/(1*ro*S*V^2) anyway then i thought that if i measured the mass of the foil plus the straw (refer to diagram) and then timesed it by 9.8 i would get the ammoung of lift produced when the foil is neither climbing nor descending, after getting it in balance measure the airspeed at the foil.
Diagram (click on it to expand):

5. Jan 21, 2006

### jaap de vries

Itsjustme,

If this is the setup of your windtunnel it means that the airstream hits the airfoil with alot of swirl from the fan. That is because there are no flow straighteners. This can have significant effect on the stall onset point etc.
When you use SI units than everything becomes pretty straightforward.

Jaap

6. Jan 21, 2006

### itsjustme

Ok, if a fan wont work well then would an aircon or an air cooler?