Kevin Horton
Well Known Member
I was playing around a bit with Xfoil, a 2-D airfoil analysis program, and wondered if it would give any sensible predictions of lift and drag vs flap angle. I selected a Reynolds number and Mach number appropriate to a stall at 1800 lb at sea level in a short wing RV. The predictions seem pretty much in line with what I would expect.
![](/community/proxy.php?image=http%3A%2F%2Fimg185.imageshack.us%2Fimg185%2F2527%2Fclmaxvsflapbs9.th.gif&hash=c68b6d20f8990ed605deb82769b84b52)
The first 10 degrees of flap gives a useful 13% increase in CLmax (note - only part of the wing has flaps, so the reduction in stall speed would be much less than this 13% increase in lift would suggest). The drag increase is quite small. But, once you get to 15 degrees of flap the drag has increased noticeably, and the drag increases quickly with flap angle. The amount of CL increase with additional flap angle gradually flattens out.
Notes: These CL max numbers are for a theoretical 2-D airfoil section - i.e. it extends to infinity in each direction. A real wing, with a finite span does not develop as much lift as these CL numbers would suggest, as there is a lot of potential lift lost due to the 3-D airflow effects.
![](/community/proxy.php?image=http%3A%2F%2Fimg185.imageshack.us%2Fimg185%2F2527%2Fclmaxvsflapbs9.th.gif&hash=c68b6d20f8990ed605deb82769b84b52)
The first 10 degrees of flap gives a useful 13% increase in CLmax (note - only part of the wing has flaps, so the reduction in stall speed would be much less than this 13% increase in lift would suggest). The drag increase is quite small. But, once you get to 15 degrees of flap the drag has increased noticeably, and the drag increases quickly with flap angle. The amount of CL increase with additional flap angle gradually flattens out.
Code:
flap Clmax AOA
angle Clmax
(deg) (deg)
0 1.718 17.8
5 1.837 16.8
10 1.936 15.8
15 2.017 15.0
20 2.085 14.0
25 2.138 12.9
30 2.179 12.0
35 2.206 11.3
40 2.225 10.7
Notes: These CL max numbers are for a theoretical 2-D airfoil section - i.e. it extends to infinity in each direction. A real wing, with a finite span does not develop as much lift as these CL numbers would suggest, as there is a lot of potential lift lost due to the 3-D airflow effects.