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A SPRING SCALE FOR WEIGHING BALSA (and OTHER LIGHT STUFF)
Roy Bourke
MAAC 204L
Previously, I published an article on building lighter aircraft by using balsa density as a criterion for selecting the wood that goes into the aircraft. That article also included a Balsa Density Chart giving the weights of standard balsa sheet stock for various densities of wood.
When buying balsa in a hobby store, there is no way of knowing the density of the balsa you are buying unless you have access to a scale to weigh the sheets as you select them. Since most hobby stores do not supply such scales you must take your own with you, a real inconvenience if you have a bulky or delicate scale which isn't very portable. Here is a plan for a simple, easy-to-build spring scale, which is quite rugged and portable, and accurate enough for selecting balsa of approximate required density (in conjunction with a balsa density table). The scale is an adaptation of a design that appeared in Ron Williams' excellent book "Building and Flying Indoor Model Airplanes".
First, let's review the formula for calculating density. Balsa density is usually expressed in lb/cu.ft. and the units I find most convenient for measuring the wood are grams (for weight) and inches and decimals (for dimensions). For strips, sheets and rectangular blocks, the formula becomes:
Density = Weight (grams) x 3.8096
Length x Width x Thickness
To accurately determine density, the purists will use a very accurate scale, measure the balsa with micrometers or calipers in several places, and calculate the density. When I buy balsa I usually estimate the various densities I am looking for (depending on structural function in the aircraft), then take only the portable spring balance and a density table to the hobby shop and sort through the balsa looking for a target weight according to the table. When I get the balsa home, I weigh and measure it more accurately using digital calipers and a digital scale, calculate the density, and mark the density and thickness on the end of the each piece.
The attached sketch shows the basic design of the spring scale and its carrying case. I made mine from 1/8" aircraft plywood edged with spruce, and the slipcase from "Corroplast", (sort of corrugated cardboard made of plastic, available in art stores). Note that the case is made with two spaces for the hook on the end of the music-wire spring, so that the scale can be slipped into the case either way. A hook is not suitable for holding a sheet of balsa, so I tie a loop of heavy thread to an alligator clip, which can grasp the balsa and facilitate hanging it from the weighing hook. A bit of shrinkable tubing on each jaw of the clip protects the balsa from the teeth on the jaws.
The gauge of the music wire determines the range of the scale. Using a 6" length of music wire I find that:
.015" wire will produce a scale with a range of about 2 grams
.025" wire " " " " " " " " " 15 grams
.032" wire " " " " " " " " " 35 grams
I find the 15 and 35-gram ranges the handiest, so I use both sides of the backboard with a different size spring and scale on each side. For sizes of balsa used in larger or heavier aircraft than I build, it may be necessary to experiment and make a spring of heavier music wire to produce a scale with a larger range.
The actual scale is a piece of bond paper glued onto the backboard. To calibrate the device, mount a blank paper scale temporarily in the proper position, hang the clip on the hook, and mark the zero point of the spring on the paper. With the clip still on the hook, hang a few known weights and mark each corresponding position of the spring. Then the paper can be removed, the scale completed by interpolating the points between the calibration marks, and the paper re-positioned and attached permanently. Obviously the more known weights you use in the calibration process, the more accurate the scale, but I find a scale can be interpolated fairly accurately with only three or four good calibration points.
If you have trouble finding some known weights with which to calibrate the scale, you could make use of the fact that 1 cc of water weighs 1 gram. Hang a small container on the hook or clip, and add known quantities of water to the container. (You need to know the weight of the container.)
All this sounds like a lot of work, but in reality the scale can be built in a few hours, and will last a long time. Mine has held its calibration for several years now in spite of some fairly rough usage and a few falls to the floor. And I certainly find the device much handier to pop into my pocket when I head for the hobby shop than carrying my bulky electronic scale or delicate beam balances as I used to do!
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