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Old Apr 03, 2007, 10:23 PM
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Technoman's Avatar
Oregon
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Help!
Delta Wing Center of Gravity?

How does one calculate the center of lift and or gravity for a flying wing (Delta)? I have a buddy who has a wing cutter and he gave me several wings to play with. My first plane flew great with little effort. I test flew my latest creation and it seems to be very tail heavy. As a result my several flights were very short.

Thanks for any help,

Technoman
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Old Apr 04, 2007, 12:52 AM
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nmasters's Avatar
Colorado
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I haven't verified that this graphical method works for deltas yet but it works for high aspect ratio FWs and it's in close agreement with what I've been told about the MAC on deltas.

--
Norm
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Last edited by nmasters; Apr 04, 2007 at 11:29 PM. Reason: updated drawing
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Old Apr 04, 2007, 01:20 AM
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http://www.palosrc.com/instructors/cg.htm

or

http://adamone.rchomepage.com/cg_calc.htm

or for canards

http://adamone.rchomepage.com/cg_canard.htm

Good pages with lots of other data.

Larry
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Old Apr 04, 2007, 07:39 AM
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United States, NJ, Frenchtown
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And then there is the. Toss it in tall grass until the plane glides as slow as possible. Move the battery around with all control at neutral. It is easy to get hung up on a slower and slower glide. You want a fast , slightly angled down glide.

If you get that angled glide but you feel it is still to fast. Put a shim of cereal box under the front edge and do the battery shuffle again. You can add shims till you feel a maxed out setup.--------If the plane likes to always bank to the same side. put a piece of shim under that side of the wing to take the banking out.

Do this as many times as you want. If you are under 12 oz. per sq. ft. of wing loading, you can almost run along side the plane

ALL tests are done on a DEAD calm day, ONLY.
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Old Apr 04, 2007, 08:32 AM
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Thank you all for the great info... One of he problems I ran into was that I felt that I was tail heave and the battery was as far forward in the canopy as I could get it. I may need to move the whole canopy. Knowing where the center of gravity is supposed to be is going to be the key!

I will get to work on this is report back as soon as I see some results.

Technoman
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Old Apr 04, 2007, 11:12 AM
Texas Buzzard
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McAllen,Texas
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Deltas & CG

I dont have any fancy equations. I do have somethiing that works. I have built three different sizes of this Delta and the C of G is between 41% and 44% on all of them. Thats %age back from the nose, not the firwall. It's measured from the imaginary point up fron where the triangle would coincide.

My Delta's specs: Area = 600 sq. in.; W.S.= 42" ; length = 30, tip width = 6".
( The actual length is 28".....the 30" is before cutting away for firewall).

The C of G is 12.5" back from the nose. I would test fly at 12". It works
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Old Apr 04, 2007, 11:32 PM
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Quote:
Originally Posted by Texas Buzzard
I have built three different sizes of this Delta and the C of G is between 41% and 44% on all of them. Thats %age back from the nose, not the firwall. It's measured from the imaginary point up fron where the triangle would coincide.
Thanks. I was afraid that the graphical method I've been using might not work with high taper and low aspect ratio wings but hadn't checked it out yet. I could have done it yesterday but my research assistant (11 year old nephew) won't be here until friday afternoon. I added your CG range to the drawing in post #2 to show how far off I was.

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Norm
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Old Apr 05, 2007, 12:00 AM
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The few deltas I have done had a pretty wide flyable CG range.

41-44% back from the point is only about 15% MAC. That would be a very stable CG.

I've flown most of mine back at 50% (and even more in some cases) from the nose which is about 25% MAC.

Larry
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Old Apr 05, 2007, 09:50 PM
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I have been doing the math and I can see the error. My current CG on a wing with a Root Chord of 11" is right about 8.5" This is very bad. When I run this win g through the calculator using a 15% MAC I get CG of 5.5" from the tip.

I believe the problem is that the design that my buddy came up with uses foam that needs to be covered with packing tape to make it strong. My first wing flew great without me ever finding the CG. (Check out my 1 Blog entry) One of my problems is that I got a little carried away and coated my wing with a type of transparent duct tape. I was not as heavy as regular great duct tape but it was heavier than I realized. By using it for everything, (including the vertical stabilizers which are well behind the CG), I threw the weight too far to the back.

I have some 2 mm Corplast on order that I am going to use for the new stabilizers and I am going to remove the attached elavons and cut some from the trailing edge. That should help.

I want to thank you all for making my first post a positive experience.

Cheers!

Technoman
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