To scale a drawing (any size) to make a similar drawing to the size of your choice. In this case - an aircraft. Say you found a small three view drawing of an aircraft you would like to model and you decide so that it should have a span of 96 inches. The wing span of the small drawing measures 8 inches. Divide the 8 in. by 96 in. which gives you .0833333+ so use .084. Using a scale that is in thousands of an inch mark off the .084 dim. 96 times along the edge of a strip of light cardboard or heavy paper. If you have done this accurately your scale will be .064 long!!!! (96 x .084 = 8.064) and .064 is about 3/4 of .084 so your plane will have about 3/4 in. less span and all other dimension will also be proportionately less.
If you can't live with it add the 3/4 in. to the span or use the .0833333+ and make a new scale. In this event you will find that you will need a stronger magnifying glass, sharper pencils, more light and patience or forget that whole thing and go get a hamburger!
If, however the scale is acceptable, coat the marked edge with Hot-Stuf, varnish, dope or whatever to make it durable as it will get a lot of use. Hot-Stuf gets you going faster.
Use this scale to measure the small drawing and make your drawing using the full scale inch. But be careful - Many drawings have portions of them that are of a different scale. If that is the case you will have to make a scale to fit that particular portion.
A hint - Add the dimensions before you start to make your scale:
(.084+.084 =.168+.084 =.252+.084 =.336+.84 =.42) etc., 1 2 3 4 5etc., etc. Mark the first dozen or so then maybe the 20 in. (1.68), 30 in. (2.52), etc., etc., then if you feel up to it fill in between. If you have a calculator they add up easier.
I used this method for W. Rinkers Stolp Starlet and
Super-marine, my JRF-2 Goose, J2F-5 Duck, Douglas World
Cruiser and several others.