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I have been attempting to measure the static thrust of my EDF with a spring balance. It is part of a group university project. The EDF specs are as follows: 90mm freewing 9 blade 1900 Kv thats rated at 4000g (https://www.rc-castle.com/index.php?...roduct_id=8798). It is being powered by a 6s lipo battery. I made a wooden stand to screw the EDF on. The stand is then attached to the metal sliders. I attached a spring balance to the wooden stand. So when the EDF turns on, the stand slides foward. The spring balance is pulled and the value can be read off it. However, I am unsure if this method is viable as the spring balance is marked incrementally in Kg. I can see this being useful if I hung masses vertically off the spring balance and wanted to measure it, but in my case, I'm trying to measure the force ''horizontally'', if that makes sense.

In summary, I was not sure if I was using the spring balance correctly. Any advice is appreciated.

On the side here are some test results: The EDF at full throttle pulled the spring up to the 3.1kg mark. The lipo was fully charged. I have attached some pictures of it: enter image description here enter image description here . I assume the friction between the rails is responsible for the lower thrust value.

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    $\begingroup$ Please note that the 'kilogram-force' or kilopond is out of date. You should use newton... 1 kilopond = 9,8 newton $\endgroup$
    – xxavier
    Sep 3, 2021 at 18:59
  • $\begingroup$ @xxavier Thanks, will do so moving forward. Unfortunately I could only find these types of labelled spring balances $\endgroup$
    – Johan M
    Sep 3, 2021 at 19:05
  • $\begingroup$ The setup looks ok, you could check it using a weight on a string, attached somewhere on the thrust line of the fan and then run over a fixed pulley; whatever weight you need to produce an indication of 3.1kg on the scale is equal to the fan’s thrust. $\endgroup$
    – Frog
    Sep 4, 2021 at 0:41
  • $\begingroup$ In order to cancel friction effects, push the sliding stand forward as if adding thrust while the motor is running, then release and see to what value it moves back. The most credible thrust value is the average for measuring with and without pushing the stand. If your scale has been made for use on Earth (which I assume), the technique is well suited for thrust measurement once the Kilograms are converted into Newtons by multiplying with the Earth's gravitational acceleration. $\endgroup$ Sep 4, 2021 at 7:05
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    $\begingroup$ @Johan M, not quite, if the weight of a 3.1kg mass gives the same extension on the spring balance as the fan does then it’s fair to say that the fan has a thrust of 3.1kgf (30.4N). $\endgroup$
    – Frog
    Sep 4, 2021 at 10:44

2 Answers 2

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As pointed out by @xxavier, the conversion between the indicated kilograms and the desired newtons of force is 9.80; more exactly, the local acceleration of gravity, depending on your location and planet.

When used as designed, an object is hung on the hook below the scale. The force of gravity pulls on the object and stretches the spring, moving the pointer. The amount of motion is proportional to the force applied.

It is the manufacturer of the device who decides that, if a 1 kg mass stretches the spring to a certain point, then that point should be labelled with "1 kg".

There are many problems with such a system. It is only correct for one particular value of $g$. The spring extension may change with age, abusive use, or temperature, or because of external tampering. Such devices are often marked "Not legal for trade", and balance scales, that match an unknown mass against known masses are labelled "Honest weight; no springs".

There is one problem with the setup you are using. The spring scale is designed to indicate the mass hanging from the scale. The calibration takes into account the constant load of the moving scale and hook (basically the black stuff in the picture of the scale).

If the spring scale is used horizontally, it will read low, by an amount equal to the mass of the moving scale.

Another factor I would look at is any force acting on the sled from the prop wash of the EDF

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  • $\begingroup$ I did not even consider the spring scale mass itself, thanks! Would it be fair to measure the mass of the spring scale, and then add it to the measured thrust value itself to account for this? Lastly, my concern was that I'm unsure if the way the manufacturer created the kg labelling system is applicable for horizontal measurements. It would have been ideal if it was labelled in distance. I could work out the spring constant separately and then find the force with less hesitation $\endgroup$
    – Johan M
    Sep 4, 2021 at 10:28
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    $\begingroup$ Best way to find out is use a 5 kg test weight, mount the scale horizontally, and attach the rope over a wheel so that the weight hangs down. Use more test weights to calibrate the scale a bit. $\endgroup$
    – Koyovis
    Sep 4, 2021 at 14:44
  • $\begingroup$ @koyovis I initially vertically hung 900g masses on the spring balance hook and it did give a reasonably accurate reading btw $\endgroup$
    – Johan M
    Sep 4, 2021 at 19:59
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    $\begingroup$ 900g is not enough to calibrate the friction and inclination out of this elementary inexpensive spring device. $\endgroup$
    – Koyovis
    Sep 5, 2021 at 1:09
  • $\begingroup$ @Koyovis sorry, can you explain what calibrating for friction means? On a side note, do you think the manufacturer marked the balance in kg in such a manner: F=kx , mg=kx, then m=(kx)/g for various masses $\endgroup$
    – Johan M
    Sep 5, 2021 at 9:14
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Yes, basic principle of a spring is that it has a spring stiffness $C = \frac{F}{\Delta s}$. So you are measuring a force with a spring balance. The indication is in kg - but it is analogue, the reading error will be much larger than the variation in gravity along the earth's latitude.

The major issue to be verified is: how much internal friction is there in the spring balance, creating hysteresis in the measurement. When set up for gravity measurements, only the measurement in pull direction would reasonably account for internal friction. So only measuring in the pull direction, your indication would be OK-ish.

A more accurate set-up would be with a wheatstone bridge type force transducer. There are pretty inexpensive Arduino type circuits that can drive them nowadays.

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  • $\begingroup$ thanks for the reply. Yeah it is not the most accurate setup. We initially wanted to use a load cell, but we did not have any working ones at campus regrettably. $\endgroup$
    – Johan M
    Sep 4, 2021 at 10:22

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