Lift and Drag vs Flap Deflection

[Kevin Horton]

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.


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.

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.

[hevansrv7a]

So Kevin, and all others with experience, using this data and practical experience:

What is the best technique for grass fields, especially rough ones (I fly an A model). Is 10 degrees of flap best or would more CL be better, even with the added drag, given that there's plenty of power? What flap setting will get me off the ground best while allowing me to keep the nosewheel off the ground? Isn't there a pitching moment from the flaps?

I have noticed that flap settings don't affect stall AOA much, but they somehow do seem to affect takeoff behavior. I once accidentally took off with full flaps from a paved runway and it seemed to go up more vertically, but would not build speed.

Good advice would be gratefully received. Thanks.

[Mel]

My recommendation for short field take off is 15 degrees flap on the short wing RVs. This is the same as the "down" aileron. My testing has shown this to be pretty optimum for short field TO. I use 13.3 degrees on my -6 because I have manual flaps and don't have a 15 degree notch.

[fodrv7]

Kevin,
I have heard of the mention technique of setting take-off flap to max aileron throw for Short-fields.

So far I have not found that I need any extra take-off performance..

Similarly, on down wind the problem is more to slow the aircraft down to the flap limit and so when I reach it I take full flap to help keep the speed down as I descend on base.

So to sum it up, I use the flaps only for drag and so use full flap or none.

But, your data is always interesting and welcome.
Pete.