Introduction: RC Aircraft Thrust Stand Version 2 - Hack of a Kitchen Scale for Only $10

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.

Comments

author
DM8 (author)2015-05-04

Bench tests complete. This measurement method proved to be as accurate as my other methods giving almost identical results within a couple counts difference here or there. This method also eliminates the need to subtract out the weight of components like is required with the equal arm balance method or other similar methods. This and it is really compact. Just be careful. In my comparison test I measured thrust of a Turnigy 2215J producing a maximum thrust of 705 grams on a 3S (I used a high current regulated supply to maintain voltage into the system throughout the test) using 147.5 watts and turning a 10x7 prop. This is a static thrust test so a slightly lower pitch prop may show better efficiency per gram of thrust (keep that in mind). Amazingly the power (W) to thrust (g) curve was basically linear; I expect the ends of the range to be rolled off. Print and enjoy:-)

author
DM8 (author)DM82015-06-24

Movie file attached. This is a video of an early test using a 3S battery and not the power supply discribed above.

MAH00291.m4v
author
sokamiwohali (author)2015-04-15

hahaha, it would be funny to see it n in use with an rc turbine engine!

author
DM8 (author)sokamiwohali2015-05-04

Actually, with a mod to hold the turbine (ducted fan; not fuel) it could be done. The thing to keep in mind about a ducted fan is that it has relatively low static thrust and deliveries it's power as airspeed pics up and airflow moves faster and faster through the turbine. It's like putting a high pitch prop on a plane for for high speed runs; it suffers from pure takeoff performance because the pitch is so high it is just slapping the air until the airspeed pics up.

author
Kiteman (author)2015-04-14

This is a really interesting project, but could you add some shots of it in use?

author
DM8 (author)Kiteman2015-04-14

Sure will. Will setup something later tonight and shoot some video.

author
Kiteman (author)DM82015-04-14

Cool.

author
DM8 (author)Kiteman2015-04-14

So instructables does not allow direct uploading of video images and I don't currently have a personal account setup on Vimeo or YouTube yet. I added a step 5 and took some images of the thrust stand in action. When I get a chance I will setup a video account and share that. Do you have any questions in particular about the setup?

author
Kiteman (author)DM82015-04-15

You can upload video files by pretending they are images, and then the site will play them for you.

One thing about the set-up; Without actually checking on the files (I don't have the software for that), it looks as though the motor is closer to the pivot than the load cells are to the pivot. If this is the case, your thrust readout will be lower than the actual value.

author
DM8 (author)Kiteman2015-04-15

With the L bracket being a hinge and a lever to press on the load cells you can change the mechanical math for the displayed value (ratio). The measured distance between the pivot points are exactly the same at 40mm so it should be a 1:1 relationship.

In using a meter like this, you can measure the amount of thrust produced, and then measure the weight of the model and get a relationship between the two. In RC, a 3D type aircraft in general wants to have a 2:1 thrust to weight ratio for good performance.

Uploading a video, what formats are accepted? I tried .MP4 but that seamed like a no go.

author
Kiteman (author)DM82015-04-15

MP4 should work.

If there are problems, drop an email to service@instructables.com, and they can help.

author
PhilippeG1 (author)2015-04-15

très belle créativité.

Pour vérifier l'exactitude de la mesure il faudrait tester un moteur à différents régimes sur un autre instrument de mesure, bref, étalonner l'appareil, sinon rien ne dit que votre mesure soit exacte.

Verser verify l'exactitude de la Mesure devrait Être tested moteur à des vitesses differentes sur Autre instrument non de Mesure, un tribunal, calibrer l'Appareil, sinon rien ne dit Que Votre Mesure is Précise

author
DM8 (author)PhilippeG12015-04-15

Agreed. I have a secondary device that I will be verifying all the measurements against. I am a day or two out from that test. Thank you for the comment.

author
EvolvedAwesome (author)2015-04-15

This is crazy hack, but its very cool and creative all the same.

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