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Finding unknown CG

Old 02-28-2023, 01:26 PM
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Default Finding unknown CG

I have a scratch built giant scale Aero master around 65" span forty-two years old.
I'm in the process of resurrecting this bird.
The top wing is forward of the bottom.
can I just block the aircraft up at flight attitude
and put a mark on my workbench at the trailing edge of the straight bottom wing
and the lead edge of the top wing midway out from the center of half the wing
to accommodate for the sweep and put a mark on the bench at that distance and
balance at 30% back of the total distance from the lead edge mark?
I know you're going to tell me to find the MAC of the top wing but is that absolutely necessary.
At seventy-six I'm getting lazy; I'm looking for an easy way out!
Thanks, Bob
Old 03-08-2023, 07:46 PM
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Hi Bob,

Maybe this will help.

gpma0502-manual.pdf

At any rate, I'd expect the CG to be near the center of the top wing.

Cheers
Red
Old 03-20-2023, 02:15 PM
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Originally Posted by RapidRobert
I have a scratch built giant scale Aero master around 65" span forty-two years old.
I'm in the process of resurrecting this bird.
The top wing is forward of the bottom.
can I just block the aircraft up at flight attitude
and put a mark on my workbench at the trailing edge of the straight bottom wing
and the lead edge of the top wing midway out from the center of half the wing
to accommodate for the sweep and put a mark on the bench at that distance and
balance at 30% back of the total distance from the lead edge mark?
I know you're going to tell me to find the MAC of the top wing but is that absolutely necessary.
At seventy-six I'm getting lazy; I'm looking for an easy way out!
Thanks, Bob
Robert..it's been many years since I had to calculate the CG of a bipe.
Yes..working off your work bench you can use a carpenter's square to figure out what the combined outline [in the overhead view] is of both wings..then take those dimensions to an on line CG calculator.
Old 05-16-2023, 01:09 PM
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Default Unknown CG

Well folks, I did it the easy way! I just blocked the aircraft up at flight attitude and put a mark on my workbench at the trailing edge of the straight bottom wing. Then a line on my bench at the lead edge of the top wing midway out from where the wing starts to sweep to accommodate for the sweep and put a mark on the bench at that distance and balance at 30% back from the total distance from the lead edge mark and the trail edge.

It fly's and I brought it home with the same number of pieces I went with! It was a little scary,
I used the G-62 I took out of my 1/3 scale Laser 200 before installing a DA70 wasn't running as good as I had hoped,
I didn't make any tight turns for fear of it dropping out of the sky it wasn't running well.
I had a 20/8 Zinger up front, next time I'll lean it a little more and try a 20/12 Zinger and get a buddy to help me hold the nose up to make sure it's not too lean.
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Old 05-16-2023, 06:31 PM
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Now that you know it flies and the model is trimmed for straight and level hands-off flight, the final CG for acrobatic flying can be quickly determined as follows: Pull up into a 45 degree nose-up climb, then roll inverted and release the elevator stick. If the aircraft continues on a straight line, then the CG is spot on. If the model begins to descend like part of an inside loop, then the CG is too far forward. It it tries to push up into an outside loop, then the CG is too far aft.
Old 05-18-2023, 10:53 AM
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I remember that from a video on utube but I couldn't remember all of it Thanks, Bob

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