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grs-pms(at)comcast.net
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 Posted: Fri Nov 14, 2008 11:53 am Post subject: Mean Aerodynamic Chord |
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John Read, and others:
"Mean Aerodynamic Chord" is a length, rather than a point. For an untapered wing, the MAC is the length of the chord measured anywhere along the span. For a tapered planform the MAC is found at a spanwise location where the chord is sort of an average of all the chord lengths. On a wing with simple taper this is easily determined. For a planform with sweep and multiple tapers, typical of modern airliners, it becomes a bit more complicated.
I think the point to which you refer is the so-called "aerodynamic center", which is found on the MAC at a distance equaling approximately 25 percent of the MAC aft of the leading edge. The acceptable range of center of gravity locations is usually expressed in percent of MAC; for a particular airplane it might be from 10 percent to 30 percent. The acceptable CG range depends on a number of design features, one of the most powerful being the tendency of the wing to pitch or rotate about its spanwise axis. This tendency must be counteracted by other forces on the airplane, largely by a down force generated by the horizontal tail.
The popular old NACA 23000 series airfoils (23012, 23015 etc) were designed to have a low pitching moment. Symmetrical airfoil sections (for example 23000) have zero pitching moment and have often been used on helicopter blades and aerobatic airplanes for that reason. Incidentally, "symmetrical" means identical, top and bottom. Some refer to an airfoil as symmetrical just because it has some curvature on the lower surface, but this is incorrect.
You are correct in noting that flying wings typically use an airfoil section which has a reflexed trailing edge. This reduces the pitching tendency to near zero and deflection of the trailing edge surfaces does the rest. You are also correct in noting that changing the incidence of the stabilizer is not the way to correct a static balance situation, although it will have an effect on the overall static stability.
I hope this helps generate light, rather than heat.
George
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notsew_evets(at)frontiern
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| Posted: Fri Nov 14, 2008 12:23 pm Post subject: Mean Aerodynamic Chord |
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Right On !
[quote] ---
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