C.G. Where ?

[SJ]

Hwere should I hold the plane to get this CG ?

hold it under the wing at 75mm?

hold it over the wing at 75mm?

or, put my fingers aproximatly where the cg marks on the drawing are ?

There is a great difference when trying all three ways ?

[Sparky Paul]

Low wings are best balanced inverted. The fuselage mass then acts as a stabilizing penudulum instead of destabilizing.
.
Sparky Paul
http://www.angelfire.com/indie/aerostuff
PJB's Seriously Aeronautical Stuff
http://home.earthlink.net/~pjburke1/aindex.html

[MarkR]

Does it seem funny to anyone else when instructions direct you to "balance the model level with the nose slightly down"?
What the heck is that supposed to mean?
I've seen that several times.

Part of the problem is that balancing the plane on its wing doesn't really get you the CG. The real CG is the center of mass of the three dimensional object, and is at the crosses indicated in the plan.

Seems like on a high-wing plane, with the CG below the wing, the plane will always rotate until the 3D CG is directly below the balance point. So there are lots of fulcrum points on the wing where the plane will balance before it falls off your fingers.

As Sparky Paul said, with a low wing plane this is like an inverted pendulum, and maybe a little unstable to balance right side up.

On a high wing plane, if the plane is balanced on the wing so that the nose is slightly down, this means that the actual 3D CG is ahead of the balance point on the wing (the plane rotated down to bring the 3D CG from ahead of the fulcrum to directly below the fulcrum on the wing). The same thing on a low-wing plane upside down, but balancing a low-wing plane by the wing right-side-up and nose-down, means that the 3D CG is behind the fulcrum point on the wing! (i.e. the plane rotated nose down to rotate the 3D CG into place directly above the fulcrum point).

If the purpose of balancing is to put the CG at a certain percentage of the mean chord, then it seems like these changes could be relevant.

Mark

[SJ]

Ok thanks....

So Ill just hold it inverted and put my fingers on the cg spots on each side of the fuselage, like shown on the drawing....

SJ