There's lots of things called "Nobler" out there -- better say what it is.

Probably start about 1/2" to 1" forward of the spar, then move to suit your tastes.

I scratch built a '52 Nobler and the plans did not show a C.G.  I calculated the MAC and set the C.G. at 20% of the MAC.  That measures out at 7 inches forward of the wing T.E.  It flies very well.

Paul

Has anyone used something like this to calculate C.G.?

http://www.scaleaero.com/CG_Calculator.htm

The online calculators are great. As long as we know the assumptions made and expect to make adjustments. They save time and are based on the best concept available. If you understand the underlying assumptions, you can adjust your outcome a bit ahead of time.

The graphical method is valid (based on the same mathematics), but the wing and auxilliary lines must be drawn VERY accurately. Otherwise the intersections that determine the MAC position will be inaccurate. The smaller the angle at which the lines intersect, the greater the error. I have found it difficult and painstaking to draw these as well as required. The on-line calculator takes the math on which the diagram is based and just computes the desired values directly from input of measurements already necessary to make the drawing anyway, no scaling being necessary. You can't go far wrong. and the answer come in seconds.

Incidentally, if you have elliptical wings, I have posted simple equations to get the same answers (some internet sites are wrong on these).

Edited to add that Paul's use of the full sized plans on a large enough table to extend the lines seems like a good idea and would result in a permanent plan record of the a.c. and c.g. positions. Accuracy at that scale would be much better than smaller scale drawings.

SK

Did you like the way it flew with the 35-S? If so, add the weight difference between the 35-S (light) and the .35 FP (heavy) to the tail and try it. Adjust as necessary.  May not be exact but it's a start.

Two comments:

1) CL planes require c.g.'s ahead of those used on R/C, FF, and full-sized aircraft.

2) If you want your c.g. to remain constant, you must know not only the change in engine weight, but the distance the engine c.g. is ahead of the c.g. of the entire aircraft and the tail moment length (c.g. to a.c. of horizontal tail). The tail weight added or subtracted is the engine weight change multiplied by the ratio of the engine moment to the tail moment arms. Ball park - add about a third of the engine weight change to the tail.

SK

 Define "problem". If you put your dry CG at 25% on a Nobler, you will likely have problems because the tail volume is insufficient to make that work very well. It needs to be more like 18-20% to start with, and very small adjustments from there. This is not new ground, people have been flying Noblers for 60 years or so, the necessary trim setup is not a mystery and doesn't really vary unless something is off somewhere.

   The Nobler (and many other classic airplanes) is *super-sensitive* to the correct CG position and the tolerance is relatively small for good performance. Too far forward and you won't be able to have satisfactory turns at the beginning of the flight, and too far aft, and it will go unstable towards the end of the flight.

In general, the bigger the tail, the bigger the useable CG range.  For transport airplanes, we want the tail to be as small as possible to keep drag down. We size the tail using what's called a V chart.  It's a plot with (as I recall) the CG position on the X axis and stabilizer size on the Y axis.  There's a big V on the chart.  The left side of the V is the forward-CG constraint: being able to rotate the airplane at low speed with flaps down.  The right side of the V is the aft-CG constraint, which used to be stability.  We make the tail just big enough to have enough distance between these lines that all the passengers have a place to sit.

I was told that one reason why planes with the engines in the back have fallen from favor is that the center of the passenger compartment is forward of the empty-airplane CG.  Hence there's a big CG difference between an empty airplane and a full airplane, so the tail has to be big. 

By "problem" I mean that as the CG approaches the CL, the aircraft becomes unstable, but also more maneuverable.
Thanks for your comments, gentlemen. Lots of knowledge and great information!