RC Brushless Motor Thrust Meter

Overview

It was always a hit and trial while selecting a brushless motor - propeller combination for my RC models till I built this Thrust Meter. Now I can try out different propellers to find the optimum pair for my model.

This is what I built

The thrust reading is read off from the kitchen scale, the current consumption is read off from the power meter, the rpm from the Digital Tachometer and the throttle setting can be continuously varied using the servo tester.

Principle

Electric RC models are built either with the propeller facing forward (Tractor mode) or facing backwards (Pusher mode). In either case the motor rotation and propeller orientation is adjusted so that the thrust is always in the forward direction.

For measuring the thrust there is a need to change the direction of the thrust. This is achieved by using a L shaped lever. The distance from the pivot (fulcrum) of the thrust line is exactly equal to the distance of the downward force measuring point from the pivot. Using the Law of Moments, if the two distance are equal, the downward force will be equal to the thrust provided the friction at the pivot is zero.

Build

I started by making the L shaped lever. Initially I had made it using an aluminum L shaped shelf bracket, but the vibrations were unacceptable. So rebuilt it by using wood strips.

I have used a 1 inch by 1 1/2 inch wide teak wood strip I had lying im scrap box. The base arm is 11 inches long while the vertical arm is 10 inches long. The L shape is achieved by joining the two pieces. I have used pocket hole/screws for this. Any other joint style can be adopted.

The pivot is made using a 2 inch wide door hinge. Since the wood strip is only one inch wide, I extended the width by attaching two pieces of aluminum angle. Wood pieces could also have been used.

The rocker arm is attached to the Thrust Meter base while using it and can be removed for ease in storage. I have drilled two holes in the vertical arm and the Thrust Meter base, where I attach the Rocker arm for storage as shown below

For the base I took a 20 inch by 8 inch piece of 3/4 inch ply and added the side support for attaching the Rocker arm.

(The above photo is my prototype while developing the design)

The Rocker arm is attached to the vertical piece using two bolts and wing nuts

Next attach a screw to the bottom of the Rocker arm at exactly 10 inches from the pivot (the pivot of the hinge). Cut of the head and slide a piece of silicon rubber (fuel tube) over the screw. Trim the length of the silicon tube that when the Rocker arm rests on the kitchen scale, the Rocker arm is horizontal. The silicon tube is required to absorb any vibration from the motor passing on to the scale.

(The above photo is my prototype while developing the design)

Next step was to build the Power meter support and battery box.

After leaving space for the kitchen scale, a similar box was made for supporting the Servo tester (used as speed control for the motor). The empty box can be used for storing the Engine mounts.

Next step was to make the motor mounts. I have made a few of these as the distance between the mounting holes vary with different motors. The motor mount has been designed so that the same mount can be turned around for either the tractor configuration or the pusher configuration.

The engine mounts were made by gluing three pieces of 1/4 inch ply. Before doing so 1/8 inch T nuts were fixed to the front plate at distance dictated by the motor mounting holes. 

It has to be ensured that the shaft passes through the centre of this plate so that the thrust line is at exactly 10 inches from the pivot point. I used 3/8 inch balsa triangle strip to reinforce the glue joints.

Last step was to make a mount for the tachometer. I made the mount by bending a 1 inch wide strip of 1/8 inch thick aluminum strip. As this support is in front of the propeller, I wanted this mount to disturb the airflow to the minimum. I mounted it on the Rocker arm, though a better place could be the by mounting it to the front box.

The tachometer is held in place with rubber bands.

The electrical connections are standard. The battery is connected to the IN port of the power meter. The OUT port of the power meter goes to the ESC. The three OUT wires of the ESC are connected to the motor. The three pin throttle connector  of the ESC is connected to the Servo Tester. Since the servo tester is spaced apart from the ESC, I have used an extension wire for the connection.

The tachometer has its own battery and the sensor being optical, no wiring is required.

How to Measure Thrust

(video coming shortly)

Sketchup Model

(Model can be downloaded by clicking on the 3D Warehouse icon above)