Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

Question about CG

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Old 12-07-2001, 02:25 AM
  #1  
Walt-RCU
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Default Question about CG

Consider a Delta wing or a Zagi type wing.
How is the Center of Gravity found?
Walt
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Old 12-08-2001, 08:26 PM
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Default Question about CG

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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Old 12-09-2001, 02:36 PM
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Default CG Location

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.
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Old 12-10-2001, 06:28 PM
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Default CG

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
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Old 12-11-2001, 12:13 AM
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Default CG Location

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.
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Old 12-11-2001, 01:47 PM
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Default CG

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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Old 12-11-2001, 04:38 PM
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Ollie
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Default CG Location

The Zagi would have a straight taper planform if the elevons extended all the way to the centerline, which they donít. The missing length of elevons means that the Palos R/C website formulas donít apply to the Zagi.
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Old 12-12-2001, 01:24 PM
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Default Question about CG

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.

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Old 12-12-2001, 11:02 PM
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Default Question about CG

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"
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Old 12-12-2001, 11:36 PM
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Default Question about CG

Nice work, Walt!

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

What do you call it?
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Old 12-13-2001, 12:01 AM
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Default CG

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.
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Old 12-13-2001, 12:43 AM
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Default Question about CG

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
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