static thrust measuring

[g_kandylakis]

A friend of mine recently asked about it, so i thought I 'd post here as well, for others to see or to add their own ideas and suggestions...

So, of course the best way to judge a prop, rubber motor, electric motor etc. is in the air with a flying model, but I often found the need to do some home testing,.

To do that, a very basic thrust measuring device was built out of balsa, basically two levers at 90 degrees angle, with a pivot point. At equal lengths the one lever is attached to the model or test vehicle (later...), the other presses down a digital scale.

Being equal lengths, the indication on the scale is "the same" as the thrust produced by the prop at that time.

I cannot and will not claim originality of the idea, I am just showing what I am using, which itself is nothing spectacular anyway...

When I want to make a measurement I put a camera to film the indication on the scale and film the process. After that I view the film and write down the values at say 5 sec intervals, this way I can create a simple graph in Excel. Useful for visual comparisons.

Of course, for someone more clever and knowledgable, it would be a very simple thing to automate the data process, but I am happy enough with what I have.

The thrust limit is mainly the digital scale limit, although I imagine one could play with the levers as well if one wants to change the 1:1 ratio.

[flydean1]

I am interested in your camera setup to record the tests.  A picture would help.  Obviously, with a second camera.

[g_kandylakis]

no particular setup...

I just use the camera to film the digital scale indication. Then I play back the video...

The order is the following:

prepare the model (wind the rubber or connect the battery...)

turn the digital scale on, make sure the thread that is attached to the model is slack and that the scale is zeroed

turn the camera on to film the digital indication

release the propeller or start the motor... There is no set time for this, you 'll be able to define the starting time from the film later, when the thrust is the highest

continue filming until power runs down too low to make a difference...

I hope this makes it clearer...


The first graph shows two situations

different batteries used with a two-phase timer, hence the two discrete power phases (dH9 ff)

RC power (dH9 RC and Batboat RC)


different rubber thicknesses, weights and turns, with varying propellers...

George

[Slowmatch]

Hi George,

I tried this method but found that the friction in the pivot could make a difference to results, especially with low thrust and small motors.

Now I use a straight hanging method as shown in the picture below. It's simply a length of wood clamped to a shelf. The scale sits on this and a 'key shaped' strut dangles the motor (or whole model) This works very well for small motors but torque becomes an issue as the motor gets more powerful - say towards 100g of thrust and more. The advantage is that the motor can be suspended in clear air away from ground or recirculation effects. The idea was shown to me by a fellow PMFC member Rod Brigginshaw.


Jon

[g_kandylakis]

Hi Jon, I agree about the friction, although in my range of 10-50 grams I did not face this problem. Actually there is very little motion involved in the pivot.

Your solution certainly takes care of that completely. Good to keep in mind for the future...

I do think I would be scared hanging my model face down though 

George

[flydean1]

Do you mount the camera on a tripod or some other rigid mount?  What camera do you use?  Certainly not a phone.

[g_kandylakis]

yes, in order to keep hands free.

But indeed a phone. No need for a camera, you just video the digital display of the scale.

Using the sound helps you determine start end or whatever...

When you have the values you no longer need the video, so you erase it...

[lincoln]

When the model is slow and the pitch speed is high, static thrust is probably quite useful. For instance, on Yak52's model, which has small props and I assume, direct drive motors spinning at high rpm's. If you had a big, slow prop with low pitch on a faster model, airspeed becomes more critical to accurate, applicable test results. For instance, a model with a 12 ounce wing loading and a geared IPS-D, if that's even possible.

I'm guessing the "test vehicle" is intended to address that, and I look forward to the details. Is this test vehicle to be run indoors to avoid wind? Will the test rig be far enough away from the test vehicle that flow is not locally accelerated? (For instance, if the test prop is just above a car's windshield, airspeed will be much higher than if it was several feet over the hood, or Evan out in front.)

[g_kandylakis]

Oops, forgot about the test vehicle...

No, it is not a vehicle in itself. It does not move freely but is also fixed to the device with thread. It is more like an engine test bed on wheels, where you can install various power combinations quite before putting them on a model.

For rubber power that would mean a simple structure with a rear motor peg in variable positions and a front nosefor the nose bearing and propeller.

For electric, a simple base for the motor and a tray to put all the remaining electrnics on.

Mine had wheels, again only for minimizing friction... No photos possible, I do not have it with me, but imagine something very simple.

Taken into account that there are a lot of other factors that infuence the final result, my device is useful for some basic results, but pretty accurate based on my experience...

[flydean1]

I want to test props on my Cox Tee Dee .02 Pay Load model, which has yet to be completed.  I'm interested only in static thrust.  5 props, the standard Cox prop, the APC toothpick most use, a Graupner 5 X 2, a Grish Plasticote the same size, and a hand-carved Japanese prop.

I think a wheeled test "frame" with a fairly wide tread to deal with torque effects should do fine.

Thanks for posting an excellent solution.

[Heikki K]

I have been using the precision scales differently. I put weight on the scale (a bolt, battery etc) and attach a sewing thread to the weight. Upward tension in the thread pulls the weight up from the scale, reducing the reading. This makes the random measurements of small or big forces quite simple, just press "tare"-button and you get the readings directly without calculating. A pivot point for the thread is easier to arrange this way, just pivot the thread more or less friction free towards the plane. The plane or test carriage can be placed so, that it will not blow all the light stuff off the shelf. Placing and pivoting it to a safe distance from oil and dirt with this method, I believe a digital 2kg .. 5 lbs kitchen scale can be used even with glow planes, and returned back next to the recipe books with no fuzz at all.