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May 03, 2004, 12:12 PM
David Cairns
dcairns's Avatar

CG Calculators on a swept wing...


This might be good for a bit of debate Most CG calculator programs work off the root and tip chords as the amount of sweep. Do you think I should use the root chord that is buried in the fuselage or the root chord where it emerges from the fuselage?

On more normal planes with only slight sweeps and tapers this is not much of an issue, but my B-52 has a highly swept wing with good degree of taper, not to mention a rather fat fuselage for a sloper. Depending on what "root" chord I use, the calculated CG will vary quite a bit. My inclination is to disregard the "buried" part of the wing and use the dimensions of wing that is actually flying. On the other hand, the fuselage does provide some "flying surface"....
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May 03, 2004, 12:36 PM
Registered User
TFLG's Avatar
You DO factor in the wing that is inside the fuse.

TFLG
May 03, 2004, 03:16 PM
David Cairns
dcairns's Avatar
Ok, good answer. Care to explain why that is?
I am assuming these CG calculators are simply using a geometric solution. So if more of the wing is inside the fuselage, and not a "wing" there is more opportunity for error.
May 03, 2004, 03:17 PM
Blue skies forever!!
spitfirem9's Avatar
The CG is always factored at the root chord. Use TFLG's CG calculator on his site:

www.slopescale.com

I used it on my highly swept-wing P-51 ""Miss Ashely II" and the CG was at the exact location on it's maiden flight.

Jeff Vosburg
May 03, 2004, 03:52 PM
Registered User
Is that site up? Just tried it and got "coming soon" page.
May 03, 2004, 03:59 PM
David Cairns
dcairns's Avatar
I am willing to believe you guys, just being annoyingly curious .
I still don't see why a part of the wing that is not contributing to flying, should be counted in figuring the CG
Wish I had my wing dimensions handy so I could run the numbers both ways and see what the difference in CG comes out to be. Perhaps it is not significant.
May 03, 2004, 04:00 PM
Registered User
TFLG's Avatar
The address is wrong try this.

http://ourworld.compuserve.com/homepages/slope_scale/

or if you want to go right to the CG calculator

http://ourworld.compuserve.com/homep...le/page17.html

TFLG
May 03, 2004, 04:26 PM
Registered User
kingdavey's Avatar
Can that calculator also be used for non-standard planforms, like an elliptical wing?
What would you do different?
May 03, 2004, 04:40 PM
No fuse too fat
slopeiron's Avatar
Quote:
Originally Posted by dcairns
Perhaps it is not significant.
I think that's your answer. You may be right, but the difference is so small that it's not noticable.

Russ.
May 03, 2004, 04:49 PM
Registered User
TFLG's Avatar
NO IT IS NOT INSIGNIFICANT, especially on a swept wing! The difference on my Carvelle worked out to almost 5/8th of an inch if I figure the CG using all the wing versus just the panels outside the fuse.

I balanced it using the whole wing and it was spot on. Had I balance it 5/8" further back I'm quite sure the first flight would have been much less enjoyable.

TFLG
May 03, 2004, 04:55 PM
David Cairns
dcairns's Avatar
Found my drawing


so plugging these numbers gives me :
23.5 - 9.5 - 50.1 - 66.5 @ 30% = a CG of 26.75
Kind of funny, the CG is behind the trailing edge of the root due to the sweep

Doing some guesstimates for not including the fuselage area, I get:
22.5 - 9.5 - 45.0 - 61.0 @ 30% = a CG of 24.52

That works out to about a 2 1/4 inch difference. That is about 10% of the root chord, so it is significant. When I get home I will come up with some better numbers and see if the significance remains.
May 03, 2004, 09:08 PM
Registered User
What if I have a multi-panneled wing? So not just a single sweep but a double or tripple taper wing.

Any suggestions for a caculator for that?
May 04, 2004, 09:18 AM
David Cairns
dcairns's Avatar
If you did a CG calc on each panel and then averaged the CG points, weighting them based on the area of each panel, I would think that would work.
May 04, 2004, 10:14 AM
Registered User
TFLG's Avatar
Here's a spreadsheet of a double taper wing. I think I have a triple taper one too somewhere but I can't find it.

TFLG
May 04, 2004, 10:58 AM
Ascended Master
Sparky Paul's Avatar
Multi-paneled wings take a bit of mathematical finesse. Here's the best I've found.
http://perso.wanadoo.fr/scherrer/mat...glish/mce.html
.
For elliptical wings, use your best judgement and make up several panels in trapezoid format, eyeballing the overage and underage to be reasonable for each segment.