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Question about CG - 12/7/2001 7:25:03 AM   
Walt-RCU



Posts: 16
Joined: 12/1/2001
From: Coolidge, AZ, USA
Status: offline 		Consider a Delta wing or a Zagi type wing.
How is the Center of Gravity found?
Walt
       Post #: 1

Question about CG - 12/9/2001 1:26:57 AM   
gubbs3



Posts: 1403
Joined: 12/8/2001
From: Coon Rapids, MN, USA
Status: offline 		Try measuring from the leading edge closes to the fuse. If you have a Zagi, or foam plane you can glide test it before you fly to make sure its about right. If you have to use too much up elevator (elevon) the CG is too far forward.

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CG Location - 12/9/2001 7:36:27 PM   
Ollie


 

Posts: 958
Joined: 12/8/2001
From: Punta Gorda, FL
Status: offline 		The location of the CG relative to the neutral point(NP) determines the static stability. When the CG is at the NP the plane is neutrally stable and the plane goes where it is pointed without diverging or correcting itself. When the CG is aft of the NP the plane diverges and tucks under. When the CG is ahead of the NP, the plane returns to a trimmed flight speed and attitude after a disturbance. How strongly the plane returns to its trimmed flight condition depends on how far the CG is ahead of the NP. The desired stability and CG varies from pilot to pilot depending on reflexes and flying skill. It also varies with the type of flight characteristics the pilot expects.

The NP and CG can be calculated to arrive at a safe starting CG which should then be refined by adjustments between test flights.There is a small uncertainty in the aerodynamic center (AC) of the wing because it varies a little with airfoil. Some airfoils at low reynolds numbers have AC locations that shift a little with angle of attack. These variations and uncertanties make the final adjustments by flight testing necessary.

_____________________________

Regards, Ollie

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CG - 12/10/2001 11:28:00 PM   
transmission_dr


 

Posts: 83
Joined: 12/10/2001
From: Arlington, VA, USA
Status: offline 		Hi,

For a flying wing, the CG needs to about 10% of the mean average chord (MAC), for a conventional airplane the CG needs to be around 25% of the MAC. To calculate the MAC go here

http://www.palosrc.com/instructors/cg.htm

HTH

Jerry

_____________________________

Jerry Martire

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CG Location - 12/11/2001 5:13:16 AM   
Ollie


 

Posts: 958
Joined: 12/8/2001
From: Punta Gorda, FL
Status: offline 		A 10% of MAC CG will be nose heavy, have sluggish control response and require excessive reflex in the trailing edge to trim, in my opinion. I would favor a CG in the range of 17 to 20% of the MAC for a flying wing as a starting point for adjustment.

_____________________________

Regards, Ollie

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CG - 12/11/2001 6:47:33 PM   
transmission_dr


 

Posts: 83
Joined: 12/10/2001
From: Arlington, VA, USA
Status: offline 		Hi Ollie,

If you look at the recomended CG of planes like the Zagi, you will find that it is in the range of 10% of MAC. For the flying wings that I have built, 10% worked out OK. Much further back than that and they are virtually unflyable. My recommendation to the gentleman that asked the question, is to start at 10% or so and then move back if he does not like the response. It is easier to fly a nose heavy plane than one that is tail heavy.


My 2 cents

Jerry

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Jerry Martire

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CG Location - 12/11/2001 9:38:40 PM   
Ollie


 

Posts: 958
Joined: 12/8/2001
From: Punta Gorda, FL
Status: offline 		The Zagi would have a straight taper planform if the elevons extended all the way to the centerline, which they don’t. The missing length of elevons means that the Palos R/C website formulas don’t apply to the Zagi.

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Regards, Ollie

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Question about CG - 12/12/2001 6:24:16 PM   
transmission_dr


 

Posts: 83
Joined: 12/10/2001
From: Arlington, VA, USA
Status: offline 		Hi guys,

Ollie is correct in that the suggested CG for a flying wing should be in the 15% of MAC range. I ran the numbers on my Zagi and it comes out at 17% of MAC for the factory recomendation. I ran it on the Planos web site and on Design Calc with the same results. The missing section of aileron I do not think significantly affects the calculation. At any rate CG is a range of allowed values, not an absolute number.

Jerry

_____________________________

Jerry Martire

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Question about CG - 12/13/2001 4:02:47 AM   
Walt-RCU



Posts: 16
Joined: 12/1/2001
From: Coolidge, AZ, USA
Status: offline 		Ollie and Jerry
Been trying to post a picture
for a couple of days.
Finally It worked.

root chord (A) = 14"
tip chord (B) = 10"
sweep distence (S) = 18"
Half span (Y) = 28.5"
% MAC bal point = 12.1" ??????

sweep dist @ MAC (C) = 8.5"
MAC = 12.11"
MAC dist from root = 13.46"
bal point @ root chord = 9.97"

Attachments
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< Message edited by Walt-RCU -- Dec 12 2001 11:17PM >

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Question about CG - 12/13/2001 4:36:45 AM   
hattend



Posts: 2052
Joined: 11/29/2001
From: Redding, CA, USA
Status: offline 		Nice work, Walt!

Is that a slope/thermal ship or is it combat capable?

What do you call it?

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CG - 12/13/2001 5:01:29 AM   
Ollie


 

Posts: 958
Joined: 12/8/2001
From: Punta Gorda, FL
Status: offline 		There are two reasons for filling in the wing planform between the insides of the elevons. First, is that it will make your CG calculations valid. Second, It will improve performance appreciably by improving the profile drag of the wing center airfoil where it ends bluntly and it will also reduce induced drag because of a better lift distribution.

_____________________________

Regards, Ollie

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Question about CG - 12/13/2001 5:43:36 AM   
Walt-RCU



Posts: 16
Joined: 12/1/2001
From: Coolidge, AZ, USA
Status: offline 		Don
I've never played with wings before and basically it's strickly TLAR. Didn't have slope in mind, I'm to old to climb hills. As for combat, I think it would survive.
What I have in mind is a high start launch.

Ollie and Jerry are giving me an engineering course, and actually I understand most of what they're talking about. Just not sure how to apply it
Walt

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       Post #: 12

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