Sd7037

[er1cw]

Hi,

Sorry for the noob question, I have a SD7037 foil with 4m span, 0.4m chord. May i know how much CL it has ?

[Ollie]

The airfoil (2D) coefficient of lift depends on angle of attack, chord and airspeed.

See:
http://www.nasg.com/afdb/show-airfoil-e.phtml?id=1078

The wing's (3D) spanwise distribution of coefficient of lift depends on the airfoil (including angle of attack and airspeed) and planform (including aspect ratio, sweep back, taper ratio and twist).

See:
http://www.geocities.com/jebbushell/COOKBOOK.htm

[BMatthews]

For level flight it'll lift with enough Cl to equal the weight of the model.

Go to www.grc.nasa.gov/WWW/K-12/airplane/foil2.html and let it load up Foilsim2. Learn to use it and then input the size of your wing into the program. You can't specify the actual airfoil but you can modify the one that is there to the same thickness and camber. Then play with airspeed and angle of attack to reach the weight of your expected model as seen in the lift value in lbs. Alter the readout to Cl and there you have it.

As suggested by Ollie as you alter airspeed you then need to similary change the angle of attack to come back to the lift value that matches the weight. As the speed changes the Cl changes since at higher speeds the airfoil does not need to work as hard. The Cl value is actually a measure of how hard the airfoil is working and not a single number.

Foilsim is a fantastic way to study how all these various parameters fit together and to find out what Cl's our models are really operating at.

[Sail 'n Soar]

Question was, "What CL?" I assume er1cw is referring to a reasonable operating upper limit, and not some wing average CL at stall or some other value dependent on the model trim angle.

From Ollie's first URL reference, a reasonable opereating useful CL max is ~.9, although the nasg data shows that the airfoil will generate CL a little greater than 1.0, but at a high CD price. From your description, the wing sounds like a constant cord, AR=10 wing. Given that, CL average/CL max at root ~ .9, or wing CL average = ~.9*CL max ~ .9^2, or ~.8.

Plan on .8 as a reasonable max CL for the wing.

[er1cw]

Yeah.. thanks for the input... my initial calculation was .9 CL however my parter started to doubt the foil ability to generate such a high Cl. ( he was thinking of something like .3 ). Anyway.. doubts cleared... thanks mate !