The completed cheap and cheerful CG scale doing its thing.

CG Scale on the Cheap

Looking to restore a little balance to your life?

Many here will no doubt be familiar with the nifty electronic center-of-gravity (CG) scales that have come onto the market in recent years. I first saw one in a build video by Paul Naton and marveled at the ingenuity and accuracy of the device. Quite reasonably priced at about $200 bucks, many builders and pilots will find the commercial units well worth the investment — they appear to be very well designed and constructed and very accurate.

That said, a CG scale is more of a convenience than a necessity and with a bit of trial and error folks can and always have been able to finely adjust CG manually, myself included. Still, the ability to easily determine and record CG in an accurate, repeatable way and to test model CG weight shifts quickly on the bench really is pretty nice. With that in mind, I wanted to try designing an inexpensive DIY scale made from common, easily accessible materials and parts. Several folks on RCGroups have worked out designs in wood or for 3D printing, generally emulating the designs of the commercial units. These are a good option too, but my goal was to avoid machining entirely and instead build something from that humblest of all RC hobby materials — foamboard.

Design

The idea here is to build the scale’s frame from foamboard to sit on a pair of inexpensive, widely available electronic gram scales (see *NOTE below for single scale use) which run from $10 to $20 bucks depending on the rated maximum weight and/or desired accuracy. $10 will get you a scale that can handle up to 5kg at a resolution of 1g, while $15-20 buys a scale that can handle up to 500g or 1kg at resolutions of 0.01g (see Resources below).

Going through several prototypes I began like others trying to emulate the commercial designs, but the results were unacceptably flimsy or inaccurate or both. Stepping back to consider the most fundamental design requirements, I found there are really just two elements of the scale where construction must be done with real precision, shown in the diagram below: 1) the distance ℓ between points a and b where the scale frame contacts the electronic scales — i.e. the ‘contact points’ — and 2) the 90° vertical alignment of the wing leading edge point c and contact point a on the front scale. (fig. 1)

With elements a, b, c and ℓ measured and positioned accurately, the CG distance from the wing leading edge is found with the following simple formula (see Resources below for calc spreadsheet) where w¹ and w² are the weight readings of the front and rear scales respectively:

fig. 1 Diagram of build elements requiring precision.

With this understanding I eventually came up with a simpler overall approach where foamboard’s inherent lack of rigidity and strength could be mitigated and the desired precision of 1) and 2) described above could be achieved easily. After a bit more prototyping I’m happy with the result detailed here and hope anyone with the desire can build this scale with not much more than foamboard and the most basic model building supplies.

*NOTE — the frame as designed/built for two scales can be used with a single scale, but there would be two measurement steps because you must first measure the total weight of the model — i.e. the simplified formula is (w² * ℓ) ÷ (total weight) so you would replace the front scale with a riser to match the height of the rear scale to keep the frame level and then use as described to record the rear scale w² value. To measure the total weight, simply use the frame placed fully on the scale — i.e. centered on the scale with neither contact point touching your workbench.

Supplies

• 1 sheet foam board
• 1/8" dowel (wood, carbon rod, bamboo skewers or similar)
• Hot glue gun
• Sharp razor knife or X-Acto + extra blades
• T-square or other right-angle gauge
• Ruler with mm spacing
• Straight edge for cutting foamboard
• Medium grit sandpaper (e.g. 120)

Measuring and Cutting

All parts are made of foamboard (I use Adams Readi-Board available at Dollar Tree stores in the US but any similar foamboard can be used) and a single 36" 1/8" dowel cut to the dimensions below. Cutting foamboard is best done with very sharp blades so plan to replace the blade a couple of times in cutting the parts list below.

Foamboard Parts (fig. 2)
It may be clear how to layout the foamboard cuts from fig. 2, but this is how I did it:

• First square up the bottom and right edges of the board using the t-square and a straight edge to cut the new edges.
• Use the t-square to lay out vertical lines at 10cm, 10cm, 12cm and 3 cm. and a horizontal line at 28cm.
• Cut along the four vertical lines to get four blanks.
• Lay out six horizontal lines at 1.5cm increments at the top of the first 10cm wide board for the four contact point platform pieces and the two leading edge stops.
• Lay out two horizontal lines at 5cm increments and 1 vertical line at 5cm at the top of the second 10cm wide board to get four 5cm x 5cm blanks for the eight right triangle reinforcements.
• Lay out 1 line at 12cm on the 12cm wide board for the 12cm x 12cm right angle tool.
• Draw 45° lines on the 5 right angle blanks in the previous laid out in the two steps.

Use straight edge to cut along layout lines to give you the following parts:

• (2) 10cm x 28cm
• (1) 12cm x 28cm
• (1) 3cm x 20cm
• (6) 1.5cm x 10cm
• (8) 5cm x 5cm (right triangles)
• (1) 12cm x 12cm (right triangle)

Dowel Parts (fig. 3)

• (2) 24cm x 1/8"
• (4) 10cm x 1/8"

fig. 2 Foamboard parts | fig. 3 Dowel parts

Construction

First up is preparing the pieces making up the base of the frame:

• Using a t-square or similar draw two parallel lines 240mm apart on the bottom of the 12cm x 28cm piece. (fig. 4)
• Strip the foamboard paper from one side of four of the 1.5cm x 10cm pieces. (fig. 4)
• Using sandpaper attached to some kind of flat block (I’m using a paint stick here), sand a small 45° chamfer on one bare foamboard edge of each of the four small pieces. (fig. 5)
• Two of each chamfered pieces will be used to create platforms for the dowel contact points with a small gully where the dowel will sit accurately centered over the two lines drawn on the base. (fig. 6, carbon rod shown to illustrate the gully and alignment more clearly)

fig. 4 Scale bottom parts | fig. 5 Chamfering contact point platform parts | fig. 6 Illustration of rod in chamfered gully

• Next, prepare the two 10cm x 28cm sides of the frame and the two 1.5cm x 10cm wing leading edge stops by hot gluing the 24cm and 10cm dowels to the foamboard pieces as shown at left. (fig. 7 and 8)
• Hot glue one side piece to the bottom piece at a 90° right angle. (fig. 9)
• Hot glue 4 of the small 5cm x 5cm right angle braces to reinforce and align the joint to 90°. (fig. 10)
• Repeat the previous two steps to glue the other side of the frame, resulting in the finished part shown at right. (fig. 10)

fig. 7 Hot glued dowel | fig. 8 Scale sides and leading edge stops | fig. 9 Frame side in place | fig. 10 Completed sides with triangle reinforcements

The following two image sequences detail the technique used to allow the wing leading edge stop point c to align precisely over the front scale’s contact point a. Please be sure your work table is level for this phase of the construction.

• Hot glue one chamfered piece with the chamfer facing up and it’s edge right up to the parallel line at the front of the frame. (fig. 11 & 12)
• (not shown) Hot glue two of the chamfered pieces to form the contact point b gulley at the 2nd parallel line at the rear of the frame.
• Temporarily affix the 3cm x 20cm piece with it’s edge contacting the front edge of the glued chamfered piece. The extension past the sides of the frame will then allows us to use a right angle to precisely align the leading edge stops’ points c with contact point a. (fig. 13)

fig. 11 Chamfered piece in position | fig. 12 Hot glued in place | fig. 13 Temporary extension piece taped in place

• Tape down the frame and the extension to the table and the large right angle piece to the frame side with its forward edge against the extension. (fig. 14)
• The forward edge of the triangle is then used to position and glue the leading edge stop to the side of the frame, aligning it accurately with the center of the platform gulley where the 10cm rod will become contact point a. (fig. 15 and 16)
• Repeat the same for the other side of the frame.

fig. 14 Preparing jig for wing stop | fig. 15 Stop in position | fig. 16 Stop hot glued in place

• Remove the temporary point a extension and hot glue the remaining chamfered piece to form the contact point a gulley. (fig. 17)
• Finally, carefully position and hot glue the 2 remaining 10cm rods into the gully’s of the two platforms, completing scale contact points a and b. (fig. 18 and 19)

fig. 17 Dowel platform gully | fig. 18 Dowel hot glued into gully | fig. 19 Both dowels in place

fig. 20 The completed CG frame + scales

Measuring CG With Your New Scale

fig. 21 1.5m Yellow Jacket F3RES on the scale.

Using the scale is quick and easy to do:

• Place the empty frame onto the scales and use the tare (T) function to zero them out.
• Place the model onto the frame with the leading edge forward and touching the stops.
• Wait for the scales to settle, then take the w¹ and w² readings.

With the known distance between the scale contact points a and b of 240mm, plug the values into the simple formula (w² * ℓ) ÷ (w¹ + w²) to get the CG. In the example, this F3RES model has a CG of 57.2mm from the wing leading edge (see Resources below for calc spreadsheet).

(54.3g * 240mm) ÷ (173.5g + 54.3g) = 57.2mm

I’ve tested this design with a range of models including a Dream-Flight Alula, 1.5m Yellow Jacket F3RES, 2m Radian and 3.5m F5J Supra and the results have been exceedingly accurate — on the order of +/- 0.5mm as checked against a commercial unit and manual balance points on the bench. Larger F5J models might benefit by simply scaling up the plans somewhat and perhaps doubling the frame walls to handle the added weight.

©2022 Chip Kaye

Resources

• CG Calculation Spreadsheets — In both Microsoft Excel and MacOS Numbers formats. Note that with most browsers the download will start automatically and will put the file in your Downloads folder.
• Trimming Model Aircraft by Peter Scott for the May, 2022 issue of the New RC Soaring Digest. — “We wouldn’t drive a car that drifted across the centre of the road. We wouldn’t ride a bike at speed that had a warped wheel. We shouldn’t fly our models that are not trimmed correctly…”
• F3X CG Scale by Olav Kallhovd on GitHub. — “Arduino based Open Source CG scale for F3X gliders (and other model airplanes) The scale can be used for most modern F3F/F3B gliders with slim fuselages and will calculate the CG and weight…”
• Foamboard from R.L. Adams Plastics and branded as Readi-Board. However, your local stationery store will have something equivalent even if you can’t find the Adams’ product.
• Scale with 500g max, 0.01g resolution — $16.99 on Amazon. Note that these are just examples which exemplify the specs required. Of course, you may find equivalents closer to home and/or at a better price.
• Scale with 1kg max weight, 0.01g resolution — $20.99 on Amazon.
• Scale with 5kg max, 1g resolution — $9.99 on Amazon.

Also by the Author

• J’Adore PicaSim! from the July, 2021 issue of the New RC Soaring Digest. — “Flight simulators are, of course, no substitute for the natural-world pleasures of RC soaring. But they can be loads of fun to fly and can serve as an excellent, risk-free training tool…”

All images by the author. Read the next article in this issue, return to the previous article in this issue or go to the table of contents. A PDF version of this article, or the entire issue, is available upon request.