RC airplane CoG scale

Introduction

I was looking for a design of a center of gravity scale for small RC gliders without the need of much additional hardware and couldn't find any existing models that fit my criteria. So, out came the version you see here. It's for small RC airplanes. I didn't test it with models heavier than 300g but my guess is that it will work well for anything up to 500g, possibly 750g and a wind span of up to 1.5m (no guarantee ;)). Check out 3D-Muc's design for something more sturdy. I also didn't want to go a route that requires electronics and/or actual weight scales. Check out freedom2000's designs if you want to go full nerd on that :)

My version is rather small scale. It has a range from 40mm to a little bit less than 140mm. The scales have markings in 2mm steps which should be accurate enough to get to 1mm precision. In case of doubt, check with a ruler. The balance has endstops at +/- 10 degrees. The distance between the left and right arm can be set from 34mm to 98mm.

The large range for the CoG comes in handy for small delta wing gliders.

The design is fully available on Onshape.

Print Settings

I printed all parts in PLA with a layer height of 0.2mm and a 0.4mm nozzle. I use OrcaSlicer and exported a generic 3mf file with all parts distributed over three build plates.

Required hardware

• 6 x M4x25mm bolts (longer threads work too, I used 30mm)

• 4 x M3x10mm bolts (threads shouldn't be longer than 12mm)

• 4 x M3 and 6 x M4 threaded inserts

• 2 x 608 bearings (22mm OD, 8mm ID, 7mm depth)

• 4 x rubber bumpers and some felt pads

Assembly

The M4 threaded inserts

Insert four M4 inserts into the two main stands. Use a soldering iron with a specialized tip or any other way that suits you. The holes for the inserts go through the parts, so you can insert the threads on the opposite side. It also means that the bolts on the main stands can go fully through and the actual length doesn't matter that much.

For the bases, this is different. We don't want bolts to stick out the bottom side that will rest on a table. That's why the inserts are not inserted into the base part but into the M4 fly nut. The bolts are glued in from the bottom side. I used super glue for that.

Insert the M4 inserts into the two knobs that have the holes with the larger diameter. Threads are inserted from the top, like shown on the picture.

The gluing part

Glue the main stands to their bases. Take care that you glue the right parts together, there's a left and right handed version of both. Check the picture for the right orientation, it shows the right version. Also make sure that the angle is 90 degrees. The recessed part in the bases should make it easy to get the angle right by eye but better check with a square and clamp everything into position while letting the glue dry.

Finally, glue the M4 bolts into the knobs with the smaller holes. Be careful not to get any glue on the threads.

I used UHU hart. But 5-minute expoxy should be a good option, and probably better for heavier models. Gel super glue might work, too. Also, it's a good idea to roughen up the parts on the gluing surfaces with some coarse sand paper.

The balances

Insert two M3 threaded inserts into each of the two balances like on the photo. Be careful not to melt the plastic on the sides like I did. Although a little bit won't do any harm.

Then insert the bearings and tighten the M3 bolts until you feel some resistance. There's not much room for overdoing it but it's not necessary to go crazy tight because the main forces on the part will be vertical. The bearing just needs a bit of friction to stay in place.

The final assembly

Start with the two balances and insert the M4 bolt, washer and bushing in the order shown on the picture.

then screw them into the main stands. Assemble the remaining parts like shown on the picture below. Use some rubber bumpers on the balances and some felt pads on the bottom side of the bases and the main stands.