Using a standard kitchen scale purchased from amazon (Item # B00MN0NI90 or search for "DecoBros Digital Food Scale") for $10 and some 3D printed parts, you can make an RC Aircraft Thrust Stand to measure thrust produced across the operating range of an RC motor and prop combination. Using a standard RC watt meter you can now also measure efficiency and better match props, batteries, and motors. Building a custom Quad, a 3D aircraft, or gentle flyer that you want ultimate flight time on, this is a must have.
Step 1: Pop the back off the Scale
A couple screws and some tabs and you have gained entry into the scale. The arrows show the scales feet attached to the sensor plates (metal plates with strain gages). Each one of these provides input in to the central processor that sum the strain (bending of the metal plates) and displays that force as pounds or grams. Bend back the tabs and remove the sensors from the scales frame. Careful of the wires and do not bend the metal plates.
Step 2: Cut the plastic frame of the scale
Leaving the electronics in tack, cut the plastic frame of the scale as shown. I used a cut off wheel on a dremel type tool. Clean up the edge to the webbing frame as shown. Using a pair of flush cut pliers, (not the dremel), nip the plastic nibs that hold the plastic feet on the sensor plates. You do not want to damage or bend the sensor plates, these are calibrate to measure force and any change to the way they bend will cause false readings.
Side note: I clip off the coin cell holder and used a 2 AAA battery hold from RS in it's place. (Don't solder until later, if you decide to do this; but you don't have to.)
You're scale is ready to be reused as a thrust measuring device.
Step 3: 3D Printed Parts
The STL files are attached. Print or have someone print the attached 3D files for you. The base is made of 3 pieces to make them more printable, glue them together when you are done. The scale fits under the cover and locks into the rear plate. The force measuring plates fit in pockets in the center section. Make sure the adhesive covering the sensor is face down in the pocket. There is a place for the 2 AAA battery holder and wire to pass through. I purchased 12GA aluminum wire from a local craft store as the hinge pin material (shown in the picture and slid in place without the hinges). It's soft and easy to straighten. When done, make sure the hinges move smoothly. If they don't you can work them back and forth a bit and the aluminum will work in straight. The motor mount, shown printed in blue, mounts to the front of the table with screws and nuts.
As the motor and prop combination flies forward, it pulls the table forward, rotating the rear hinges (L shape) into the force measuring plates. (MAKE SURE YOU CLAMP THE THRUST STAND TO A SURFACE!) The scale displays the force in grams or lbs. Mix in a watt meter, RPM gauge, and go to town measuring the effects of motor, battery, and prop combinations. If your a school, mix in a vacuum chamber and simulate altitude.
Step 4: STL Files
Step 5: The thrust stand in action
Here is a view of the setup running. The camera used a flash so the prop is frozen in time and shows no blur, but it was running. You can see that we are measuring 212grams of thrust for the servo input provided. Please note that I used tie warps in this setup to hold the motor, but I would not suggest this as a good practice. I only did this to take a few working pictures to show; use real nuts and bolts, be responsible.