What is an airfoil?
An airfoil is a two-dimensional tear-drop shape that, when extruded into the third dimension, will generate forces when moved through air. As any streamlined shape travels through the air, it might generate a retarding force (DRAG), a lifting force (LIFT), and/or a pitching torque (MOMENT). Those forces are a function of the cross-sectional shape. For example, if you take the cross-sectional shape of a generic wing, it would have an elongated teardrop shape. This is called an airfoil. In Europe the spelling is sometimes aerofoil and sometimes it's even called a profile. With proper selection, you can choose airfoils to either create lift or minimize the drag (i.e. fairings around landing gear, wires, etc...). If you are short on time, you may find the new Airfoil Primer very helpful.
Does the airfoil I choose really matter?
For the most part, yes. Whether you are designing airplane wings or race-car wings, the airfoil matters. In those situations you want an airfoil that will generate just the right amount of lift while minimizing the parasitic drag forces. From 1915 until 1959, the N.A.C.A organization used wind tunnels to develop several family sets of airfoils, many of which are still in use today. It is very likely that the one you need has already been created; DesignFOIL includes all of the popular NACA airfoil generators.
There is an anecdote floating around about an old graybeard engineer running a wind tunnel test whom had to postpone things because a new engineer tasked with designing the wing became obsessed with finding the perfect airfoil. Supposedly, he had the young man cut out the wing from plywood and attached that to the fuselage in the wind tunnel; his goal was to show the young engineer that the airfoil cross-section didn't matter that much. While I do love a good anecdotal story, this one most likely never happened. How do I know? Two-reasons: First, no wind tunnel test conductor would allow such flimsily dangerous attachments on his test stand (airloads could rip a plywood wing to pieces and suck it downwind from the test section). Lastly, wind tunnel testing is EXPENSIVE and no company rep would allow so much time to be completely wasted just for a "lesson" to a young engineer. Neat story though.
Does DesignFOIL work for all airfoils?
No. DesignFOIL only works for subsonic airfoils. This means they should be round on the nose and sharp at the trailing edge. Sharp-nosed airfoils are for supersonic flight and the DesignFOIL analysis routines are invalid above Mach 0.7.
Does DesignFOIL predict maximum lift coefficients?
Yes. It uses a novel approach to predict maximum lift coefficients for Reynolds Numbers between 90K and 9million. However, it does not predict max lift coefficients for flapped airfoils.
I want a wing that flies around at maximum lift coefficient all the time. Is that a problem?
I get this question a lot. In theory, it is not a problem. But driving a Porsche around at redline RPM all the time shouldn't be a problem either; however, it's not recommended. Instead of operating a wing continuously at the very hairy edge of it's performance, you may want to either enlarge your wing area or choose a better airfoil that allows higher -operational- lift coefficients (Selig, Liebeck, etc...). You'll pay the price in drag and limited performance envelope, but at least you'll have some safety margin either way to account for gusts and other surprises.
Is there a real wind tunnel to test my airfoils and wings?
Yes, there are many wind tunnels that offer testing rental services to designers like yourself. Costs vary depending on what speeds you are looking for and whether or not you provide the model. I recommend the wind tunnel laboratory at The Ohio State University. For daily and weekly rates, contact them here:
The Ohio State University
Aero/Astro Research Laboratory
Attn: Dr. Janiszewska
2300 West Case Road
Columbus, OH 43235
Email: JANISZEWSKA.1 (AT) OSU.EDU