Airfoil Lift Coefficient
I have constructed an aerofoil and wind tunnel and researched the equation for lift for a physics class
experiment. I will be testing which angles of attack produce the most lift. However, I am unsure how to
actually calculate the lift coefficient. Can you help me?
- question from Matt Hodgkinson
I believe the answer to your query is pretty well explained in two previous questions on the
lift coefficient and
reference area. The basic
point of this latter discussion is that you measure the lift force in the wind tunnel using a force balance and
compute the lift coefficient from it by the following equation:
One difference that will be important to you is that the variables change slightly when considering an airfoil
as opposed to a wing or complete aircraft, for which the above equation is written. Instead of a reference area,
an airfoil has a reference length. This l _{ref} is the length of the airfoil from leading to
trailing edge, a quantity called the chord and represented by c. If using English units, this length is input in
feet, or in meters if using the Metric system. Also, we typically change the lift coefficient to lowercase letters
to indicate that we are calculating the lift on a two-dimensional airfoil section rather than a three-dimensional
wing. The above equation therefore becomes
You may notice that the units don't quite work out since we are replacing the area S_{ref} with the length
c. It is assumed that c is multiplied by a unit span dimension (1 foot or 1 meter) to make the units work out
properly. This is why the lift on an airfoil section is referred to as the lift per unit span.
- answer by Jeff Scott, 14 July 2002
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