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Old 03-02-2007, 09:29 AM   #11
Ed Smith

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Somebody will have to explain how this works. In the diagrams I have seen the airplane is suspended in a sling from a point above it. The sling is attached to the airplane either around the wing or at the extreme nose and tail.

How is the attitude of the airplane determined? If the attitude is changed then the plumb bob will point to different positions. Will it not?

The CG of the model is a function of the mass/weight of the model. The flying balance point is an aerodynamic function. The two are not necessarily related. Or are they?

Ed S
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Old 03-02-2007, 10:45 AM   #12
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I am not sure why but I also suspend my larger models on a pully. Especially in the 30% size. If the wings are removable I make a 1/8"ply template with the c of g holes just above the wing surface. I then suspend it from the ceiling with a single nylon cord to a piece of alum 1x8" with piano wire clips to the wing mounts. I make several holes to move the cg back or forward after a test flight. For my forty size or lower I use my digital's for rough then go to the makeshift pencil stand. And yes I still have my digital's.
This method seems to work for me.
John Davidson
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Old 03-02-2007, 11:16 AM   #13
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Originally Posted by Ed Smith
The CG of the model is a function of the mass/weight of the model. The flying balance point is an aerodynamic function. The two are not necessarily related. Or are they?
You want to start with your specific plane's CG at the design recommended CG. (i.e. someone flew the design sometime with this setting)

The sling and plumb bob are hanging from a common point, the CG of the plane will be under this point.

Simplest rule of thumb: 1/4 to 1/3 of the span back from the leading edge of the wing. More difficult with a swept or tapered wing.
desiderantes meliorem patriam
To err is human, to moo is bovine
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