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10-23-2008 | 06:29 AM

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da Rock

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From: Near Pfafftown NC

RE: help with C o G on .40 cap 232

Quote:

ORIGINAL: 4x4fab

Thanks for the help. I couldn't remember how to find the MAC so I looked it up. I'll post it here for anyone else's future reference.

Here is a link,
http://www.airfieldmodels.com/inform...amic_chord.htm

http://en.wikipedia.org/wiki/Mean_aerodynamic_chord

To Locate the Mean Aerodynamic Chord on a Tapered or Delta Wing
Measure the root and tip chord. Then draw the following lines on the plans:

At the root of the wing, draw a line parallel to the centerline of the fuselage extending forward from the leading edge and rearward from the trailing edge. Both lines should be the length of the tip chord.

Do the same thing at the tip but drawing the lines the length of the root chord.

Connect the ends of the lines so that they create an "X" over the wing panel. Where the two lines intersect is the spanwise location of the Mean Aerodynamic Chord.

If the plan indicates that the CG should be located at some percentage of the MAC, then measure the MAC and put the CG the given percentage back from the leading edge along the MAC. For example, if the MAC is 10" and the plan indicates the CG should be 25% back from the leading edge, then the CG is 2-1/2" back from the leading edge at the MAC.

This drawing should help you visualize what you need to do:

Note: The lines cross at the spanwise location of the MAC. It is not the fore/aft CG location (unless the CG happens to be located at 50% MAC).

The following formula will give the measurement (chord) of the MAC. It does not give the span wise location of the MAC.

rc = Root Chord
t = Taper Ratio = (Tip Chord ÷ Root Chord)

MAC = rc x 2/3 x (( 1 + t + t2 ) ÷ ( 1 + t ))

Using the drawing above, let's say the root chord is 11" and the tip chord is 6"

t = 6 ÷ 11 = .5455

Now plug t into the formula to find the MAC. Note that the wingspan and sweep do not matter. No matter what the span or how much the wing is swept, the MAC will always be the same length.

MAC = 11 x 2/3 x (( 1 + .5455 + .54552 ) ÷ ( 1 + .5455 ))

MAC = 22/3 x ( 1.8431 ÷ 1.5455 )

MAC = 7.3333 x ( 1.8431 ÷ 1.5455)

MAC = 7.3333 x 1.19254

MAC = 8.7453"

If you're going to go to that much work to find the MAC, then you're a believer in formulas.

Since the pitch control of a wing is a function of the tail, why not consider it too. And do almost no real work and let another guy do all the figuring. How do you do that?

Take 5 minutes and measure your model with a yardstick. Plug the measurements into http://www.geistware.com/rcmodeling/cg_super_calc.htm and click.

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