Jan 4, 2002, 5:42 AM
Post #1 of 4
Dan/Quade ? Airfoil definition
Was reading some older posts, I quote....
<side note: <the spectre uses a stilletto airfoil, <the vx also is based on a stilletto airfoil...
<canopy designers in jest reffer to it as the "pd airfoil" as so <many "new" parachutes simply copy this airfoil.
What exactly is the definition of airfoil? the profile from topskin to bottom skin? The stilleto, spectre, FX and VX should all have different aspect ratios, but if that is not part of this definition what is? If you compare a stilleto 120 and an FX 120, will the FX be thinner fron topskin to bottom skin and can you say that it has a thinner airfoil, but it is the same airfoil?.
Is airfoil a mathematical relationship between the curve of the bottom skin (profile) and the topskin?
also what is a naca #. does this have anything to do with the definition of a naca duct (those air inlets comonly found on high perf cars or some aircraft).
Airfoil is a two dimensional measurement of the curvature of a rib. The definition of an airfoil includes the curvature of top and bottom surfaces, camber (curvature) and thickness. Airfoils are easy to copy, just flatten out a rib. A bewildering variety of airfoil sections are available for a bewildering variety of aircraft. For example, an airfoil section designed for a sailboat would be worse than useless on a supersonic fighter plane.
NACA is the United States government body that preceded NASA. Back in the 1930s, NACA tested hundreds of airfoil sections in two dimensional wind tunnels and published their findings. Since then dozens of new series of airfoils have been designed and some of them even made it into production. Newer low speed airfoils include; Lissaman, Wortmann, Riblett, Roncz, NLF and GAW.
Airfoil selection is just one small part of the entire design process. When you start considering aspect ratio, planform and line trim, you get into far more complex 3 dimensional aerodynamics.
quade (D 22635)
Jan 4, 2002, 2:45 PM
Post #3 of 4
When most people talk about an airfoil, they're talking about the cross-section shape of the wing that gets defined by the ribs a wing, but in fact, just about anything that interacts with the air stream can be considered an airfoil. So, on an airplane that might be the prop, wings, elevators and rudder -- or even the fuselage itself.
For a skydiver in free fall, that might be your arms and legs, or even your entire body during a tracking dive.
NACA (the forerunner of NASA) did some studies of wings back in the 1930s, and developed a "fairly" easy way to describe an airfoil. The original scheme defined an airfoil by it's chord, thickness, camber and camber placement. This is the "simple" 4 digit NACA number. A 2415 wing would have a 2% camber with the maximum camber placed 4/10ths back of the leading edge and a maximum thickness of 15 percent. From just the 4 digit number an engineer could, using a slide rule and some math, draw out exactly the wing and know it's performance.
Airplanes with wings of these "old school" wing designs are still in production -- the Cessna 172 for example.
NACA later developed more detailed descriptions of wings that included 5, 6, 7 and 8 digits . . . these are REALLY difficult to visualize and you need a computer to do all the proper math to draw them correctly.
It's really important to match the proper airfoil shape to the intended use. Some airfoils produce massive amounts of lift, but also create a lot of drag. That might be ok for a low speed heavy lifter, like the C-17, or maybe a bush plane, like an Aviat Husky, but completely inappropriate choice for a jet fighter, like an F-16.
Airfoils for ram air parachutes have the same considerations.
By the way, a perfectly flat wing is just as valid an airfoil as any other shape, but it might be inappropriate for it's drag and lifting capabilities.
P.S. Yes, the NACA group also developed the inlets you might be familiar with on race cars.