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Author Topic: How do I locate the proper C/G for a delta wing?  (Read 1992 times)
Jim Rhoades
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« on: July 24, 2010, 09:54:08 AM »

I plan to build a delta wing Vintage Speed model and the plan does not show the C/G location.  I'd like to have some info so I can try to be somewhere near the proper location.  The wing is of a double delta configuration similar to a Saab Viggen.  I suspect that the original model was probably fairly nose heavy which is not too bad on a speed ship as stability is what were looking for.

A good rule of thumb answer will probably suffice as I won't have a lot of leeway to change things.  Leadout position is of course going to be based on the actual C/G position.  If it is going to be nose heavy I'd like to know up front.

Jim Rhoades
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c.maikis
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« Reply #1 on: July 24, 2010, 03:53:58 PM »

Hello Jim,
nice to meet you here.
There's a formula ( graphic solution ) to find CG on wings with trapeze planforms. I think the same should work for delta configurations. After all a delta is a trapeze with tip chord very small or even zero. I have added a sketch; I hope it will get through!
If it doesn't I will send you a private mail.
Kind regards,  Claus

PS:  sorry: in the text on the drawing  it should read RC = root chord.



* delta CG 2.gif (49.52 KB, 551x794 - viewed 259 times.)
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don Burke
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« Reply #2 on: July 26, 2010, 09:57:08 AM »

I think it's possible to do a double delta by finding the MAC of each section seperately, then using the MACs from both, find an MAC for the combination by the same graphical method.  Just an intuitive guess, hope I'm right.
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don Burke AMA 843
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Jim Rhoades
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« Reply #3 on: July 26, 2010, 10:09:47 AM »

Thanks Don,

The shape is not too far from a true delta so I can probably just average things out to a straight delta.  I'll see how that cojmpares with using both shapes as you suggest.  It should be a fun little project.  At least it will be cute.

Jim Rhoades
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Serge_Krauss
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« Reply #4 on: August 07, 2010, 09:32:44 AM »

Jim-

The graphical method is valid, but can be quite inaccurate in application, since the line intersections can vary greatly due to slight inaccuracies in line placement or measurememnt. For a cropped delta, you are much better off to calculate it with accepted formulas (yes, a true delta is a trapezoidal wing with zero tip chord, as far as these formulas are concerned). This can easily be done for you by on-line calculators like the MAC/cg calculators at the Palos Verde R/C site:

http://www.palosrc.com/index.php?option=com_content&view=article&id=50:cg&catid=41:ic&Itemid=50

Find the 24%-MAC position, and place the c.g. ahead of it a reasonable distance. FWIW, the MAC of a true delta is exactly 1/3 of the half-span out from the root (meaning that the MAC is 2/3 of the root chord). CL planes seem to like static margins well in excess of R/C models, but with the delta you don't have flaps to complicate things; so perhapsstarting at 10%-15% of your delta's MAC would be more than safe.

(Edited for typos and an added comment)

SK

P.s. I'm back for only one afternoon to do the club newsletter; so I may not be able to respond to any comments until the end of the month.
« Last Edit: August 08, 2010, 02:42:56 PM by Serge_Krauss » Logged

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