Question

A new two-dimensional airfoil constant chord model is being tested in a wind tunnel. The model chord is 3ft. The atmosphere is standard. The velocity of the airflow is 250 ft/s. The forces are resolved as 150lb of perpendicular to the relative wind and 10lb parallel to the relative wind. Find the dynamic pressure and the section coefficient of lift (c_l) and the section coefficient of drag (c_d).

Answer 1

The fluid dynamic pressure

Static air pressure at standard atmosphere = 101.325 kPa

velocity 250 ft/s = 76.2 m/s

speed of sound = 340.3 m/s. thus Mach number

for air. Thus fluid dynamic pressure can be given by this formula

  

Now we find the section coefficient of lift

here 150 lb is converted to kg by multiplication with 0.4535 and 3ft is converted to m by multiplication with 0.3048 as I find it more convenient to use SI units. The final answer will be given in terms of ft and lb

And drag coefficient

Now in case we consider air to be non compressible, then

  

The difference in values with considering air flow to be compressible and incompressible is negligible