Centre Of Gravity

    The CG is a 3 dimensional point, and this method finds a vertical line along which the CG of the aeroplane must lie, but it does not tell you how far up the line it is. Does this matter? Only if the model is tilted.

All we need to know is the fore and aft position of the CG, or the longitudinal balance point, and its height doesn’t really matter. As long as the model is held level, in the attitude in which it flies normally, you can extend the vertical line up to the wing and thus get an accurate position as % chord, which is what we get from instructions or calculate.

    So, support the Cub in a level flying attitude by placing some packing (a box or books etc) under the tailwheel. Now measure the wheelbase, from the mainwheel axle to the tailwheel axle (or tailskid point of contact). I called that distance d. Drop an accurate vertical line from the tailwheel, and for this I suggest a steel nut tied on a piece of thread as a plumb line.

Now you need to weigh the model (Figure 2). If the model is small enough I like to place a strip of wood across the middle of a kitchen scale (held with Blu-tac), then zero the scale and place both main wheels on the wood. My kitchen scales measure up to 5000 g in 5 g steps.

Then place another scale, a more accurate one if possible (mine measures up to 2000 g in 1 g increments) under the tailwheel, still maintaining the aeroplane in the same level attitude as shown. We measure W2, the weight on both mainwheels combined, and W1 the weight on the tailwheel. Then calculate distance X.

The formula I gave in the article was X = W1*d/(W1+W2)
Suppose the wheelbase (d) from mainwheel axle to tailwheel is 1050 mm.
Suppose the front balance gives W2 = 2500 grams and the rear balance gives W1=427 grams so the total weight of the model W1+W2 is 2500 + 427 = 2927 grams
Now we calculate X = 427 (tail weight) multiplied by 1050 (wheelbase) divided by 2927 (total weight) = 153 mm
So in the diagram above distance X turns out to be 153 mm. Use your plumb line again, still with the model in the same level attitude and stick the thread with Blu-tac onto the wing underside so that the line hangs exactly 153 mm aft of the mainwheel axle as shown below.

Mark the point where the thread meets the wing, and measure its distance behind the leading edge, as this is the CG reference usually given in instruction manuals or calculated using CG calculators.

Alasdair Sutherland