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General control line discussion => Open Forum => Topic started by: Motorman on June 02, 2014, 06:03:01 PM

Title: Comparing CG Points
Post by: Motorman on June 02, 2014, 06:03:01 PM

I have a straight wing plane and my buddy has a tapered wing plane. How can I compare CG points? Can I just use percentage of root chord?

Thanks,
MM

Title: Re: Comparing CG Points
Post by: Tim Wescott on June 02, 2014, 07:45:59 PM

Nope.

If the taper is straight then use the chord at half span.  Do include the flaps on both planes, if they're there.

Title: Re: Comparing CG Points
Post by: Curare on June 02, 2014, 09:06:38 PM

I would have thought this would be a bit of an odd excercise to do, they're not the same plane are they?

What are you hoping to determine?

Title: Re: Comparing CG Points
Post by: Larry Fernandez on June 02, 2014, 10:59:38 PM

Quote from: Motorman on June 02, 2014, 06:03:01 PM

I have a straight wing plane and my buddy has a tapered wing plane. How can I compare CG points? Can I just use percentage of root chord?

Thanks,
MM

You must also consider the area of the stab/elevator and the length of the aft moment.
If you look at the classic ships with the smaller tails, the CG would be around 20%

Todays hi zoot stunters have much larger tails and are happier with the CG around 25%. The larger tail ships will also have a larger CG envelope.

Larry, Buttafucco Stunt Team

Title: Re: Comparing CG Points
Post by: Trostle on June 02, 2014, 11:24:13 PM

Quote from: Tim Wescott on June 02, 2014, 07:45:59 PM

Nope.

If the taper is straight then use the chord at half span.  Do include the flaps on both planes, if they're there.

Well, really, to compare the CG points, technically, one should determine the CG relative to the mean aerodynamic chord (MAC) of each aircraft.  Of course, the MAC of a straight non-tapered wing is its chord throughout the span.  Also, for the relatively small taper ratio's usually found on our CLPA models, the MAC is not much different than the average chord.  The higher the taper ratio, the greater the difference between the average chord and the MAC.  If one is really wants to compare the CG of one model to another, then to do it correctly, it should be done by comparing the CG's relative to the respective MAC's.

Keith