I just completed a sportscale model of Spaceshipone and was wondering if anyone has any reccomendations as to where the CG should be? I was told that one way to do this is to draw a straight line from the leading edge where it meets the fuse to the outboard trailing edge. Then draw a second straight line from the trailing edge where it meets the fuse to the leading edge at the wing tip. Where these two lines meet is where you would establish a chord liine on the wing. From that line you would set the CG at say 25% back from the leading edge at that point. In the first picture the back edge of the stick on the wing is at 25% by the method I just described.

To me this CG position seems pretty far forward. In the second picture I have the plane balanced as it naturally came out. Here the CG is about .5" behind the 25% of the wing chord.

Since this aircraft is designed to be a glider and certainly not aerobatic a slightly nose heavy balance probably will not hurt anything.

Any ideas on if I am on the right track or if there is a better way to figure this would be appreciated.

Regards,

John B.

I believe the method suggested to you is in error and will result in a CG being too far BACK. The CG you show with the stick is not far enough forward. I know there are posts on this news group which will show the correct method so I suggest doing a search for "determine cg" or "find MAC" (Mean aerodynamic chord).
In the meantime, I suspect a more informed ng member than I will have the link for you before the day is over.
The model looks good, by the way.
Allan

http://www.rcuniverse.com/forum/m_19...tm.htm#1983706

Here's one such link > (Post#5).
Usually the wing is drawn as though it extends to the centerline, ignoring the fuselage shape.
Allan

I don't have anything to add to determining your CG, I just wanted to say nice job. Any plans to build the White Knight?

I would suggest making a little all sheet scale model and throwing it around until you get the CG right and then try that on your full scale model.

There were plans for a White Knight RC model in Flying Models magazine. Plans available from Carstens, I imagine.

Jim

John using the method in post #5 as suggested, the c.g. you have indicated is a tad forward of 25%.. should be OK for the first test tosses.

Here's an easy to use online calculator for MAC and CG. (Handy when you're doing swept wings, especially)
http://www.palosrc.com/instructors/mac.htm

Cool model. Keep us posted when you fly it.