Calculating Lift

[Ritchie]

Hi Guys,

I have data about the pressure around an airfoil under certian condition and i want to turn this information into the downforce that the wing is produceing.

Bassically i am using a CFD package called PHONEICS some of u CFD people my know of it. The only information after it has completed u can get is, the pressure and the velocity of the flow at certian points in the flow (probe value) so i have undergone the painstakeing process of taking the pressure at 100 points around differnt airfoils at differing angles of attack. Now i am confussed and stumped on how to turn these pressure values into lift figures, CL or actual physical lift data (lift would be negative as it is down force)

Any help greatly recived, startig to pull my hair out about it at the mo.

Ritchie

[Reca]

It’s the same work you have to do with experimental data, a discrete integral of the pressure around the airfoil. But... don’t you have a post processor to do it automatically ?

Anyway, some details for the calculation, the fastest way :
You should have the coordinates of each point where the code calculated the pressure value, starting from 0,0 calculate the length of the segment connecting 0,0 with the first point, then calculate the force (per unitary span) on that segment (pressure in the point x length of the segment). That force is perpendicular to the segment so you have to calculate the two components parallel and normal to the velocity, drag and lift, for that segment. Repeat it for all the segments. Once you have finished just sum the values of lift on each segment to obtain total lift and the values of drag on each segment to obtain drag. Only thing you have to be careful with is to use the right sign for the force on each segment, excluding that you shouldn’t have problems, it’s quite easy, typical work for Excel.
That’s the fastest way but you can do it in slightly different ways, for example assigning the pressure on each point as it was constant for half of the segment at the left and for half segment of the right. Variants follow the same basic idea, the only difference is how you decide to assign the pressure to the segments. Anyway if you have a big number of points (and 100 is big) and the points are correctly distributed (hope you did it with a bit of logic), the difference in the results with different methods is limited, stick with the faster way.