Thrust Stands for Micro Motors - RC Groups
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Aug 11, 2005, 12:37 AM
Realtired User
mharms's Avatar
Started work on cutting down the GWS 3x2 prop to 50mm. I drew up 8 configurations to try and went over to Aeromicro (I'm lucky to live nearby) and bought 10 props. Perry did look at me a little funny. So far I've got an initial version of Nick's cut-down prop done, but it probably needs to be thinned more. I've named this particular prop version "prop A". I took some preliminary static thrust measurements (all DD) and it looks like this:

SS Red/Prop A:
Vm = 3.5V, Im = 0.745A, Thrust = 9.6g, RPM = 14580

SS Orange/Prop A:
Vm = 3.5V, Im = 0.485A, Thrust = 7.8g, RPM = 13260

For comparison, here's my results for some stock prop combinations:

SS Red/GWS 2510:
Vm = 3.5V, Im = 0.695A, Thrust = 7.6g, RPM = 15700

SS Orange/GWS 2510:
Vm = 3.5V, Im = 0.457A, Thrust = 6.0g, RPM = 13870

SS Red/Balsa Products Yellow Prop:
Vm = 3.5V, Im = 0.756A, Thrust = 6.8g, RPM = 15200

All these cases are over-propping the motors (RPM is below the Max Power RPM per Gordon Johnson's numbers). Prop A does give significantly more thrust than the GWS 2510 (with a little more current draw). With more thinning, I think I can get the RPMs up and get even better results. Nick -- it looks pretty good.

I took measurements 15 seconds after applying power and took the average of at least 3 separate measurements for the numbers, above. I verified the motor voltage, Vm, directly at the motor pads. Beers were limited to 1 while measurements were taken.

My thrust stand is the "L" style, with ball bearings at the pivot point and equal arm lengths (from pivot to thrust line and pivot to scale contact point). I checked my thrust stand using known weights, tied with a thread to the motor mount, with the thread pulled out along the thrust line and going across a very low friction pulley so the calibration weight hangs down below the pulley. This applies a known force to the motor mount which transfers to the scale just like thrust. I don't see much of an error at all (maybe 0.1g or 0.2g at the lowest forces -- mainly repeatability gets a bit worse below 6g or so). I can't explain why my thrust numbers are quite a bit lower than Gordon's values (he reports 10g thrust for the orange SS/GWS2510, and 12.9g for the red SS/GWS2510). I wonder... if Nick's prop was measured on Gordon's stand... would it read nearly 15g thrust with the red SS???!! Wow, that would be really good thrust!

I am going to further thin my prop A. Today I got a clay-molding tool with a small concave wooden surface that may help me some with the sanding -- I plan to glue a small piece of sandpaper to the inside curve of the tool. Take a look at the picture to see what I'm talking about. The tool was only 59 cents at the LHS. Over the next few days I will cut and test my other 7 prop configurations and then post the final results. If anything interesting pops up along the way I'll post it.

BTW: How do people balance such small props? I tried placing the prop on a slightly undersized piece of music wire (which allows prop to swing freely) but, even when I add a tiny bit of imbalance (piece of tape) to one side of the prop, I can't detect it. One thing that showed promise: I tried vibrating the music wire by placing one end on an aquarium air pump and the low amplitude vibrations did seem to cause the heavy blade to swing down. Any other methods out there?

Gordon -- Yes, I'm getting to like those props. They are so thin -- I can do a lot of the cutting with a good pair of scissors! They sand well too. BTW: Do you have any ideas why my thrust results are lower than yours? Do you have a really streamlined motor mounting method? I think anything really close to the prop can have a significant effect on readings.

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Aug 11, 2005, 12:52 AM
Realtired User
mharms's Avatar
Forgot to post a pic of my "Prop A" (my version of Nick's cut down GWS 3020). Here it is.

Aug 11, 2005, 01:33 AM
No more nicotine for me!
micro_builder's Avatar

the thrust numbers look good, glad to hear that. i've been using the 3x2 props for quite a while now and have never done any tests on it to determine how well they're actually performing, thanks for taking the time to check it all out. i'm sure there is a "sweet spot" with these props, and i think removing some of the excess weight from them is part of the key. the blades are just so thick near the hubs, i'd think removing some of the mass would help get those RPMs up where they should be.

if the RPMs didnt drop, i wonder if using an M20HV would be a better match since its max power and efficiency is in between 11,400 and 18,900 rpms which seems to be close to the red SSs RPM with the A prop.

i've had a hard time balancing the props in the past also. i had tried music wire as you did but it didnt seem to do anything noticable. i tried a thin peice of CF and that seemed to work a little better, but its still hard to tell how ballanced it really is. i guess one way to figure it out is to stick it on the plane and see if there's any excess vibration. maybe with props this small getting a near perfect balance isnt as nescessary as with a bigger prop.

i hope further testing proves worthwhile, thanks again for putting in the effort

Aug 11, 2005, 07:15 AM
in persuit of low wing loading
Gordon Johnson's Avatar
I don't know why your numbers and mine are different. I am also measuring volts right at the motor terminals. My test stand is L shaped like yours. My mount for the motors is a bit smaller. It's possible on small DD props that might be some affect, but I kind of doubt it. From comparing my tests with other people's tests my results are sometimes spot on with what they get, and other times my results are a bit different. For example, when testing Uttam's Mighty Midget BL motors my results agreed virtually exactly with his. At about the same time I was getting lower thrust for the Gary Jones BL motor than Gary got. I think (from memory) that my results for the 4.5 ohm pager were very very close to what Epilot and a couple of other people measured, this was when we first started using that motor. I think I wrote in an article somewhere that my results may differ slightly from what someone else gets, but that at least my results are comparable to my other results since nothing changes on my setup.

It's hard to say what causes the differences. I use a $20 radio shack volt meter to measure volts at the terminals, and a much more expensive radio shack meter to measure amps. It's possible I'm giving the motor more volts than I think I am. But, the volts on my volt meter are not much higher than what my power supply shows. My stand pivots on 2mm id ball bearings, so I doubt that is it. And, the arms on my stand are equal in length. If your top arm was slightly shorter that would give you less thrust, but they look equal in your pictures.

Its hard to say why any two test stands may give the same results or may not. And, I think for micro motors it's harder to get things right than it is when testing larger park flyer sized motors.

Bruce Abbott has a nice thrust calculator. With GWS props you can put in the rpm and it will tell you the thrust. Even though our prop rpm tends to bounce around about +/- 150 rpm or so with our cheap meters, that doesn't make much difference. I haven't checked it in a while, but for the small GWS props like 5x3's etc the static thrust I get at a given rpm is very close to what his calculator predicts.

As an aside, Astro Bob of Astro Flight was doing static testing of his motors with various props and posting them in the Power Systems forum. Then it turned out there was some error creeping in and I think he acknowledged that. For his Firefly motor and gearbox (this is all from memory) he was getting more thrust than I did, and than what Bruce's calculator predicted. So, I think there can be differences and we may not always be able to figure out where it comes from.

Nice work, BTW, and nice stand.

Aug 11, 2005, 07:53 PM
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mharms's Avatar
Thanks Gordon. I'll search for Bruce Abbott's calculator.

The reason I wonder about the effects of near-in obstacles to airflow is that if I bring the head of a pencil an inch behind the prop (steady hands!), I see a few tenths of a gram reduction in the scale reading. The vertical arm also has a big flat surface in the lower part of the prop wash -- maybe I can make it into a more streamlined shape.

My wiring is also a concern. Not the gauge of the wire, but the fact that it is rather stiff. Where the wires leave the arm and go over to my little junction box -- maybe I effectively have a spring that pulls against any small movement away from the tared position. I don't know how much the scale platform really moves when these types of forces are applied to it. Maybe the movement is so small that this "wire-spring error" is negligible. Also, I should have the wires leaving the arm at the pivot axis, rather than a few inches up the arm, so the moment arm is less. More flexible wire might also be good (the silicone insulation type).

I am using some rather large sealed bearings for my pivot. I found them surplus (plus the aluminum block they are mounted in) and they were the most free-moving I could find. I use a 3/8 inch shaft through the bearings and the arm assembly is U-bolted to the shaft. I am not too "bearing-savvy", but it seems that the force needed to initially get the bearings moving (to overcome "static friction", grease viscosity, etc.) would be important in thrust stands like these. A knife-edge pivot might be ideal. Still, when I "calibrate" my stand with known forces along the thrust line, it seems I don't have much of an issue. I've attached a diagram of my cal method and the results I got using it. Does this look like a valid method?

Many little things to try. For now, I can at least make relative comparisons of prop static thrust.

Aug 11, 2005, 08:05 PM
My member is not
Zlatko's Avatar

Lots of nice work there.

Just a quick observation on your thrust stand.

Can you move the motor to the left so that the vertical of the prop goes vertically to the pivot point?

Your calibration is fine but the prop is forward of the right angle pivot line and so the thrust from the prop is not at the maximum / optimum.

Aug 11, 2005, 09:16 PM
in persuit of low wing loading
Gordon Johnson's Avatar
On the wires, yes you don't want them pulling on the arm. Mine are the flexible silicone type. They go to the portion where my pivot is, and are attached there, then have a slack portion of wire, and then are attached to the vertical arm. I've attempted to make sure they don't pull on the arm in either direction. My vertical arm is 1/4 inch brass tubing, so is pretty thin. I mount the motors to a small block of wood which is rounded and streamlined and barely wider than the brass tubing, and is drilled to press fit over the top of the vertical arm. I took my tach apart, and removed the sensor part, extended the wires and made a streamlined probe that I embeded it in, and then that is mounted on a brass tube that positions it behind the prop about 3 inches and run the wires down the tube and onto the bench where the tach readout now is. I was attempting to keep the tach from disrupting the airflow. And, I also needed a way to read the tach when the test stand is inserted up in the bottom of my wind tunnel. This way the LCD and main part of the tach are outside the wind tunnel, but the sensor is inside. I have a 12V halogen car headlight bulb mounted on a (of course) brass tube about 12 inches ahead of the test motor and off to one side. This is what I shine through the prop blades towards the tach pickup. I turn off the lights in my workshop and turn on the 12V bulb when doing tests so the tach isn't influenced by my florescent lights. My ball bearings are 2mm id bearings from Dave Lewis, but are not the sealed variety. So, I don't think I'm getting any binding there. I suspect that with any of our test stands any of these things would not matter at all if we were testing speed 400 motors. But, for small pager motors we can get errors creeping in from these things. For example, I think Zlatko and I get different results. I think Bruce's web site is listed in the personal web sites thread listed in the Micro Threads Index. Keep it up. It's nice to see others doing this. I should probably recalibrate my test stand. Maybe something has changed.

Aug 12, 2005, 01:15 AM
Realtired User
mharms's Avatar
Thanks for the comments and info. This is kinda fun.

I was thinking it would not matter if the motor was offset from the pivot point axis and that what mattered was the distance from the pivot up to the thrustline. I'll have to sit back and think about this. Most of the thrust stand designs I have seen have at least some offset like I have. Now I am wondering if the plane of the scale should also be in line with the pivot point (horizontally). Hmmmm.... clink, clank, clunk (sound of thinking).

I am sensing a revision 2 of my thrust stand coming up, if for no other reason than to reduce the cross-section of the vertcal arm near the motor. I can easily offset the upper portion of the vertical arm (which will become some kind of small diameter tubing) to the left and align the prop with the pivot point.

I think I will re-do the wiring like you suggest. I did search for silicone wire when I was building the stand, but could not find any. However, I did see some the other day, so I may go back and pick some up tomorrow. I've seen some "streamlined" aluminum tubing at the LHS. I might change the upper portion of vertcal arm to use this -- if its strong enough. Brass tubing would be stronger. I hope my bearings are not the problem -- they are so nice (but maybe not right for this application). Maybe all these little things add up and, like you say, matter for these little motor measurements.

Seems to me that if my calibration shows my stand will accurately measure known forces applied along the thrust line at the prop hub, then the bearings and geometry (even the wiring effects) are probably not issues. I'm really suspecting the airflow around the prop and motor mount are major culprits.

I was inspired to take the photocell out of my tach from your design. I put the sensor in a pen tube that is magnetically mounted on the side of my desklamp that sits behind the stand (back about 1 foot). I don't turn on the desklamp, I just use it to position the tach sensor. I measured thrust with/without the desklamp behind the prop and I saw no difference, but I may rework this to eliminate possible effects. I use a cheap LED flashlight for the tach light source. This flashlight uses 3 cells and sends real DC to the LED (not AC like some of the voltage boosted LED flashlights). The flashlight is about 2 feet ahead of the prop.

Looks like I may do some thrust stand mods tomorrow.

Aug 12, 2005, 01:58 AM
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mharms's Avatar

Regarding the calculator on Bruce Abbott's website: I checked Bruce's site, but I didn't see a calculator there. I did see his prop constants and a link to the Motocalc website. Was there once another calculator on his site, or were you refering to the use of his prop constants with Motocalc?

Aug 12, 2005, 07:02 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Sorry, I forgot the thrust calculator is not on Bruce's web site. It can be downloaded from this post in the Power Systems forum. I've added the link to his calculator spreadsheet to his blurb in the Personal Websites thread so we can find it easily next time. BTW, it does not include the tiny DD props we are talking about here. But, it does have props like the GWS 5x3.

I'm going to split of the portion of this thread dealing with thrust stands into a new thread on Thrust Stands for Micro Motors. It's probably time for such a thread. When I get around to it I'll post a picture of my test setup. I've added this thread to the micro threads index.

Unfortunately your thrust numbers are in your first post about your test stand. You'll need to repost those numbers in the cut down prop thread.

Last edited by Gordon Johnson; Aug 12, 2005 at 07:12 AM.
Aug 12, 2005, 07:50 AM
Thanks for the Fish!
aerogel's Avatar
Good idea gordon there are way to many Thrust Stand's out there in the thread's Im going to build a new one this weekend and Im looking for the best design.
Aug 12, 2005, 12:28 PM
Only nerd in the village
I use a simple see-saw. The motor end is held over the edge of the table so the readings don't get distorted by backwash from the table top.

Aug 12, 2005, 01:08 PM
Registered User
stormbird's Avatar

My thrust rig is also a right angle but has a piano wire brass tube bearing and my wires are also stiffer than I would like ! small world aint it !

I have noticed when measuring thrust that the motor runs faster [ sound increases in pitch [ no tacho ]] when my amp meter is removed from the circuit.So I measure thrust with the amp meter out of circuit before I put it into the circuit to measure the current current.

regards paul
Last edited by stormbird; Aug 12, 2005 at 01:09 PM. Reason: dam the faulty left shift key 1 <grin>
Aug 12, 2005, 02:07 PM
Realtired User
mharms's Avatar
Thanks for splitting this off as a separate thread. I was thinking to ask you to do so. I'll repost the thrust numbers in the prop cutting thread.

Glad to hear from a fellow builder! These stands seem so simple at first... (haha).
By design, the amp meter has a low value resistance inside it. This resistance is in series with your motor. When the motor draws current, there will be a small voltage drop (proportional to how much current) across the amp meter. So your motor gets a little less voltage when the amp meter is in the circuit. If you have another meter to measure voltage, you can connect it directly to the motor (rather than to the power supply) and read the true voltage the motor is getting. Then you would just tweak the power supply up a bit until you got the desired voltage at the motor terminals. If you don't have a 2nd meter, then what you are doing is fine.

Aug 12, 2005, 02:18 PM
Realtired User
mharms's Avatar

Your design sounds like it has advantages: no propwash effects and a pivot rather than a bearing. Do you have to worry about props coming off? I've had a couple fly across the room, but, I always sit behind the prop (unless its a pusher). Got any photos of your see-saw (especially the pivot area)?


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