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Measuring CG on a symmetrical wing.

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Measuring CG on a symmetrical wing.

Old 08-20-2016, 11:39 AM
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carddfann
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Default Measuring CG on a symmetrical wing.

Hey there, got a basic question about locating CG. If you have a flat or nearly flat bottom wing, measuring CG from the leading edge is easy. If you have a symmetrical wing and say the plans call for a CG of 2" from the LE, is there a good way to measure and mark the wing? A person can try eyeballing the edge of a ruler to the LE but that doesn't seem to accurate depending on how good your eyes are. If you try a sewing (cloth) tape measure or piece of paper to follow the wing contour from the LE center, then you'll end up a little forward of the specified CG (because the CG is based along the chord). If you use a sewing tape to follow the contour, I guess you'll be a little nose heavy, which is safer than tail heavy. I tried marking my wing using a straight ruler and eyeballing it then did it with a sewing tape measure and the marks were over a 1/4" off.
Anyone here have a better way to mark off the CG on a symmetrical or non-flat wing?
Thanks,
Gary
Old 08-20-2016, 12:51 PM
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A. J. Clark
 
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Just put another straight edge at the front of the wing and measure from it. Your eye can make it perpendicular enough for this purpose.
Old 08-20-2016, 03:00 PM
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RBACONS
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I use a carpenter's square and set one edge one the bottom of the wing and butt the other up against the leading edge. Just need to keep the front edge relatively perpendicular to the leading edge.
Old 08-21-2016, 08:26 AM
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jester_s1
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Yep, a square makes it easy. But it's not a real crucial measurement. The real setting is found by flight testing, so if you are a percent or two off it won't really matter. Precise CG measurements are really only relevant when you are resetting a known good configuration after a recover or repairs.
Old 08-22-2016, 07:56 AM
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carddfann
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Thanks everyone for your suggestions. I went out and got a carpenters square. Still have to eyeball it a little to get the LE perpendicular to the square, but it works and gets me in the ball park.

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