I mentioned the way I balance my big models in another thread and some guys asked some questions, so I'll summarize here:

Any plane with landing gear can be balanced this way. Modern airliners have on-board systems that do it automatically.

It's very accurate and takes the trauma off the wing under-skins when balancing a large model. Double-check it with a mechanical balance, if you like, and if done correctly will come out within 1/16" of an inch the same.

1) First, you need three good scales. Mine are electronic and measure in grams or ounces. I use grams because it makes me feel more precise, but it doesn't matter.

2) Second, a nice flat floor which meets a perpendicular wall or other vertical suface.

3) Place the plane with one LG on each scale, putting it in flying attitude, zero'ing cardboard boxes or whatever on the scales to achieve this.

4) Push the nose or the spinner of the plane firmly onto the vertical wall: this is your "zero reference point" Both main LG axles should be aprox. equidistant from the wall.

5) Now obtain the three weights and the three lengths necessary to calculate the CG: As long as you don't change them in mid-calculation, it doesn't matter what units you use. I just happen to use grams and inches.

a) left main gear weight...(we'll call that WL) b) right main gear weight (WR) c) tail or nose gear weight (W3)


d) distance wall to left main axle (DL) e) distance wall to Rt main (DR) and f) distance wall to nose or tail axle (D3)

6) Now to get the critical "total lever product," add (WL) X (DL) + (WR) X (DR) + (W3) X (D3) = Lever Product in (grams) (inches), for example

7) Add total weight: (WL) + (WR) + (W3) = Total Weight...in (grams), for example

8) Divide Lever Product by Total Weight to get CG...in (inches), for example. This number is where your CG is, measured back from the "Zero Reference Point," of course, not the leading edge

It seems a little convoluted when first tried, but it's really only 6 basic measurements and some simple math. It is very handy when you are playing, for example, with the placement of a battery pack or some other heavy, moveable component, you can create a short "constant" for a particular plane, thus abbreviating the necessary work for each different CG computation to just 3 different weights. No hanging and re-hanging the plane.

Of course I always measure and calculate twice before quitting to make sure I'm not having a daffy moment the first time.


mt

interesting

kram-RCU, Thank you. This is very useful and should be posted in the Tips and Techniques forum as a sticky.

Bruce

What capacity scales do you find to be most useful?

Standard weight and balance calculation, I've done the same thing myself on some big models, I used a scale designed to measure Freon weight for charging AC systems, good to 1/4 ounce. Be aware that there is a certain amount of error involved with measurement taking and therefore one should be careful not to assume the results are spot on.

My scales are 7-10 years old, so I'll have to go out to the shop to see the exact brand/model.....Tanika? They measure up to 15 kg each, supposedly accurate to +/- 5 gm.

I feel I should mention again that it's very important to position the plane on the scales in flying attitude....as close as possible to a horizontal thrust line. Because CG changes very little with up or down attitude those last few degrees +/- horizontal, eye-balling it is usually good enough.

Thanks,


mt

You would want to make absolutely sure the plane was level on the datum line specified by the designer, otherwise the relationship of various components changes adding to the overall error which even at 1% could be close to + or - .75 inch on a large planes CG.

I use one scale with two blocks of wood the same thickness as the scale.

Joe

So, just to clear the air of all confusing details, the equation is:


(WL)(DL) + (WR)(DR) + (W3)(D3)= CG
(WL) + (WR) + (W3)

if (WL)= left main gear weight,
(WR)=right main gear weight
(W3)=nose or tail wheel weight
(DL)=reference point to left main distance
(DR)=reference point to right main distance
(D3)=reference point to nose or tail distance


Equation in Action. Garage door isn't the best vertical reference point in the world, but the best I had at the time.

Note the weight-zeroed blocks on the scales to get the thrust line level.

I wish I had that idea about using one scale and two blocks! Would saved me some money to buy balsa and glue. Gotta make sure the plane doesn't move while you're switching things around, though.



mt