How do I compute the proper angle of incidence to maximize cruise efficiency?

[MinMN]

I am designing an electric model and would like to align the angle of incidence of the wing so that the fuselage is most aerodynamic at cruise. This requires me to compute the most efficient angle of attack for my airfoil. Since I have yet to settle on a wing design, I would like to know how to ascertain this angle for any airfoil. Does someone know how to do this?

[nmasters]

Google for “profili”. The free download version doesn't let you do much but registration is a real bargain at about $15 and you can do it with paypal so it's instant although oversees mail is surprisingly fast too. Then plot the polar of the airfoil you want to use. If you aren't good at reading graphs you might be able to find a copy of a book titled “Understanding Polars Without Math”by Bill & Bunny Kuhlman

--Norm

[edit]Oh yeah! :p If you are pretty technically inclined you can download a somewhat less user friendly but much more capable program called “Xfoil”. But be warned the Linux version has a better interface and more IO than the MSWindows version. [edit]

[BMatthews]

Google for foilsim. LEarn to use it and set up the one airfoil it offers with the same thickness and camber value as the airfoil you want to use. Then input your planned wing size and speed. It's only a rectangular wing so shoot for close to the aspect ratio and area Then alter the angle of attack until the lift matches the anticipated weight of the model. That's the angle of attack your model will fly at at that speed and weight. Then switch to the Cl readout and take note of that. Because it's a generic airfoil the actual angle to get that particular lift coeffcient may not be the same with the airfoil you want. But the Cl will be the same. So take the Cl from Foilsim and go to the lift to angle of attack charts from something like Profili and see what angle of attack generates this Cl. This is the angle of incidence you want to use if you're looking for the ultimate efficiency.

Now all this is fine if this is for a UAV that will spend most of its time flying level at that speed over a long distance. But if this is for a sport RC model that'll just be used in the typical manner to buzz around the flying field then it's a waste of time since the model will easily spend less than 1/4 of it's time flying in a way and speed that'll gain you anything.

Even for a glider where stuff like this counts if you're using camber altering flaps and ailerons you need to consider which speed range you want to maximise the performance for. Because once you alter your flap and aileron angles the incidence of the wing alters along with it.