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Old 06-02-2014, 07:40 PM   #1
mclarkson
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Default Building a test stand to measure thrust

I'm starting a new build and have two potential motors available, but am not sure which one to use.

I'm using this as an excuse to build a test stand to measure their respective thrust with various props. This was inspired by a Flite Test video I remembered watching some time ago.

It's form is an Isosceles triangle. The idea is that thrust is applied by the motor at the top left corner (B), and is transferred to a digital scale which sits under the bottom right corner (C). The frame is hinged at the remaining corner (A), directly beneath the motor, allowing the frame to rotate and apply pressure to the scale. The horizontal arm (AC) and the vertical arm (AB) are the exact same length.


Here’s the finished frame, made from basswood, balsa, a Popsicle stick and some bamboo skewers. The long arms are 18" each; I cut a single piece of 1/2"x1/2"x36" balsa in half at a 45 degree angle and glued it back together to form the right angle of the frame.


The thrust is directed onto the scale via this short length of bamboo skewer.


The third corner is attached to the frame’s “foot” via a small hinge. The 'foot' plus hinge have approximately the same height as the digital scale.



The motor(s) mount to a stick mount at the top corner of the frame. The ESC and other components are held in place with zip ties.


Here’s the full setup. 1-Motor; 2-Peg for motor mount; 3-ESC; 4-LiPo; 5-Watt meter; 6-Digital Scale; 7-Rx.

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Old 06-02-2014, 07:48 PM   #2
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I just made my first measurement and it seems to work great!

AX-2308N-1100 with an EP-9050 prop on an 850mAh 3-cell: 20+ ounces at 9.7 Amps.

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Old 06-03-2014, 12:25 AM   #3
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Originally Posted by mclarkson View Post
I just made my first measurement and it seems to work great!

AX-2308N-1100 with an EP-9050 prop on an 850mAh 3-cell: 20+ ounces at 9.7 Amps.

Don't know if APC has an equivalent prop size or not, but APC has a lot of data on their various props. That includes torque, power input, and thrust.

Take a look:
http://www.apcprop.com/v/downloads/P...B/datalist.asp

What RPM did your above setup turn the propeller?

Running the numbers, your motor is putting out about 100 Watts at 9.7 Amps. Divide that by 746 to get horsepower, that is 0.136 HP. Looking at APC's charts, turning a 9X5 prop at 8000 RPM takes about 0.14 HP. Per APC' charts, that is 1.35 pounds or 21 ounces of thrust. That would be at zero forward speed, as measures by your setup.

If you got slightly over 20 ounces of thrust, that is pretty close!
http://www.apcprop.com/v/downloads/P...B/PER3_9x5.dat

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Old 06-03-2014, 01:37 AM   #4
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Sounds like it's working, then. I've seen a lot of charts and tables for various setups, but I wanted to be able to actually measure what I'm getting on my end with my equipment. And the total expenditure was <$10, so it was worth a try anyways.

I figured I can at least compare various props, motors, etc., to each other.

I don't have a tach, so I can't say for sure what speed I was turning the prop. It's an 1100kV motor, so it's theoretical RPM would be 8000+ but, realistically, it was probably closer to 6500RPM or thereabouts.

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Old 06-03-2014, 02:02 AM   #5
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Mark,

I recently had to test a twin setup and did a similar thrust tool. Made two of them out of 1/2" MDF. I only have one digital scale so had to test them one at a time for thrust while both were powered but it worked great and I got good numbers from the setup. Although it's pretty simple I do have the plans if anyone is interested.

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Old 06-03-2014, 02:56 AM   #6
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Ooooh - push pin. Great idea!

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Old 06-03-2014, 05:53 AM   #7
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I use the hanging fish scale - basically I have an old house with exposed beams so hanging a model is no big deal.

It is useful to test a motor combo before installation though which this bench system does.

My only concern is 'beam moments' ..... and pivots. If the beams are not identical lengths from motor centre to pivot / scale pressure pin to pivot - the results will be affected.

I also note that the set-up is based on Pusher ..

I think I would be inclined to create an inverted T .. where the horizontal sits on scale .. at one end. The other end has a pin that is set at same height as scale height. Fitting a motor will upset the vertical CoG of the stand .. so i would have a way of adding small weights to opposite side vertical to motor to bring scale to zero. Pressing Zero on the scale is not correct as that only adds in the inbalance error. This way tractor and pusher can be tested.

OK - so maybe it's taking it to extreme limits ...

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Old 06-03-2014, 06:34 AM   #8
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Originally Posted by solentlife View Post
I use the hanging fish scale

Nigel
The hanging fish scale would also show the net propeller thrust, minus what ever drag is present from the propeller prop blast past the wing and fuselage. My guess is that drag is a substantial percentage of the total prop thrust on the motor shaft. But, when getting to the larger models, like my 3000 watt Hackers, that would be one way to dust off the ceiling joists in my workshop!

I've taken those giant scale models outside on their wheels, and measured prop thrust with a fish scale. The numbers came out to about 80-85% or so of the calculated values.

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Old 06-03-2014, 07:31 AM   #9
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I know lots of people who say they do horizontal tests with Fish Scales ... my thought is that some friction / losses may occur that way.

For really big stuff - I have a 300sq.m barn with beams up high that you can hang your Chevy of ! In fact I have a tackle on one beam that I lift my garden tractor with.

The only trouble with my vertical system - is the model wants to torque spin, so I have to stop that ...

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Old 06-03-2014, 08:00 AM   #10
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Nigel,

Moments were taken into account in my design as shown. Any imbalance due to the weight of the motor, ESC, wiring, etc. is taken care of with the tare function on my HF digital scale, at least close enough for comparison figures. It's not a lab instrument but it seems to work pretty nicely for some pieces of junk wood I had laying around.....

This is the system I used to test and trouble shoot the variable thrust setup I've mentioned on other threads.

I believe that you may be correct on the "pusher" configuration of Marks but mine is for tractor installations.

Mark,

Just a note of caution....the knob on the bottom of the arm I show bears on the base and keeps the system from backfiring on you if you happen to have the motor reversed on initial setup. I do have some experience with that....lol Not sure if yours can flip backwards or if you've already taken that into consideration.


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Old 06-03-2014, 08:28 AM   #11
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Hi Deg ... I noted in photo that things were reasonably similar in lengths.

My post was really to comment so that any others who consider doing similar are aware that different lengths will affect the results.

To those who are still wondering what we are on about .......... take a garden Seasaw ... put a person on one end and similar weight person on other end at same distance it will balance. Move one of those inward towards pivot and other end immediately falls and you are out of balance. Just because the thrust frame is not straight - does not mean same does not hold true.
On the seasaw - half distance means 2x weight to balance.

Deg - using tare function is fine as we are not as you say in to Lab accuracy. If we were - we would be looking to get arm truly vertical and also balanced with scale showing zero without tare function used.

......... while you're around - can you have a look at my Megawatt idea in Scratchbuild ? http://www.wattflyer.com/forums/showthread.php?t=73830 ..... Maybe you have some ideas / sketch's that can help on this. But I really do need to stay with 4mm ply and Depron.

Cheers
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Old 06-03-2014, 08:57 AM   #12
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I was meticulous (for me at least) in getting the arms exactly the same length. The post which sits on the scale is in the exact same (relative) location as the motor shaft, etc.

Since the amount of actual movement is nill, even at the extremes, I'm not too worried about shifts in CG, etc.

As to pusher/tractor, isn't that just a case of prop orientation and motor rotation?

I've thought about the 'hang them from a beam' approach but, frankly, it scares me to death.

Deg, I'll give some thought to adding a safety to prevent it flopping over the wrong way.

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Old 06-03-2014, 09:12 AM   #13
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Originally Posted by mclarkson View Post
I was meticulous (for me at least) in getting the arms exactly the same length. The post which sits on the scale is in the exact same (relative) location as the motor shaft, etc.

As to pusher/tractor, isn't that just a case of prop orientation and motor rotation?

I've thought about the 'hang them from a beam' approach but, frankly, it scares me to death.

Deg, I'll give some thought to adding a safety to prevent it flopping over the wrong way.
Please don't think I was getting at you ... far from it ! I was posting so others can appreciate the idea and explain a few points that need to be taken care of.

I think the tractor / pusher side of it may need some thought .. I agree that its a matter of rotation of motor ... not really sure if all motors are equal both ways .......... I know that you cannot reverse the Turnigy EDF motors as the shafts are not locked ... reverse it and the motor comes apart ...... ask me how I know !! You'd have to put a screw collet on the shaft to stop it pulling through.

Hanging from the beam ceiling ? I agree that its frightening for larger models ... and that's when the horizontal pull method I feel is called for.

The beauty of OP beam method is the trial and solution to which motor is best for a model, before committing to installation. I for one have installed motors and then had to change due to insufficient thrust.

Now here's an interesting idea : How about extending the idea to include a full model ?

Using beams or T / L frame - a 'dolly' holding a full model could be used - the results are displayed the same by beam pressing on scale ... just because the model weighs something - that's taken care of by the base.

A removable bolt on model holder exchanges with motor mount holder ?

It could even be set-up to sort CoG ....

WOW my heads spinning with ideas flowing out for this gear !!

Nigel

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Old 06-03-2014, 10:10 AM   #14
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No worries.

Here is my first set of data from an Emax CF2822 1300kV outrunner. Each thrust measurement is actually the average of four separate thrust measurements. Amps is the average of two separate peak amps measurements. I've defined 'efficiency' as thrust/amps.

//edit: this was with a 2-cell, 850mAh LiPo.

Code:
		thrust	amps	efficiency
APC 1138 SF	17.60	10.60	1.66
EP 1047 SF	16.90	9.75	1.73
EP 1060		15.18	8.45	1.80
EP 1080 SF	15.23	11.50	1.32
EP 1180 SF	16.43	12.65	1.30


Based on this data, I'm liking the 1047 SlowFly.

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Old 06-03-2014, 11:25 AM   #15
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All you need now is a tachometer to register RPM and you're laughing ! I bought an LED Digital of ebay for peanuts ...



http://www.ebay.co.uk/itm/Digital-LE...item1c0fd8152d

Uses a small piece of reflective tape stuck on one blade ... shine the LED light on the spinning blade ... bingo - up comes rpm.

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Old 06-03-2014, 04:51 PM   #16
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Now that tach is what I need to complete my setup....looking on Ebay now....
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Old 06-03-2014, 05:16 PM   #17
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Originally Posted by DEG View Post
Now that tach is what I need to complete my setup....looking on Ebay now....
What I like about - you can stand BEHIND the model and use it ... instead of my old one that was pressed onto the spinner or prop nut.

I've also checked mine against known rpm gear and it's been remarkably accurate through it's range. Good enough for us anyway !

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Old 06-04-2014, 02:05 AM   #18
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Here's my first real set of data, comparing the two possible motor choices for my next build along with all the appropriate 10" - 11" propellers I have on hand.


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Old 06-04-2014, 03:33 AM   #19
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Just for fun, try reversing the prop.

slow stock prop reversal. it flies! easily! 543 watt dual motor bipe slow stick. push-me-pull-you. 242 watt 3 channel slow stick. 365 watt mini ultra stick. 415 watt mini contender. 810 watt ultra stick .25e. 220 watt alpha 450 sport (retired).
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Old 06-04-2014, 04:21 AM   #20
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Oh, you!

Alright. I just ran a pair of tests with a GWS EP 1047 slowfly prop on the EMax CF2822 1300kV outrunner powered by an 850mAh 2-cell LiPo.

With the prop on the right way, I got 18 ounces of thrust at 10.3 amps.
With the prop on the wrong way round, I got 6.9 ounces of thrust at 10.3 amps.

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Old 06-04-2014, 07:40 AM   #21
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Now this is something I'm intrigued by ...

Just for fun ...

Can you run motor other rotation and swap prop round so its correct ... see if motor does have preference rotation ?

I believe that some motors do.

Nigel

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Old 06-04-2014, 05:54 PM   #22
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Originally Posted by solentlife View Post
Now this is something I'm intrigued by ...

Just for fun ...

Can you run motor other rotation and swap prop round so its correct ... see if motor does have preference rotation ?

I believe that some motors do.

Nigel
Those brush type motors definitely can have rotation preference. As an example, my Astroflight brush motors absolutely must be re-timed before mounting a gear box. Not doing so can fry the motor. Other fractional horsepower brush type motors that I have can run either way with no problems. These use permanent magnets for the field.

Long ago, I made a DC motor out of an old automotive generator for use in a band saw. The RPM was varied by varying both the Armature voltage and field winding voltage. It worked very well, but that thing would only turn in one direction. Reversing polarity to the brushes (and not reversing the field windings) resulted in the motor pulling a lot of amps with no load. It also caused a lot of arcing on the brushes.

As for brushless motors, those I've run across can turn either way without notable differences in power output, idle current draw and so on.

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Old 06-04-2014, 06:12 PM   #23
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Brushed - many of those I used over years of racing cars etc. had an arrow on for designed rotation.

I have heard that some brushless are better one way than another ... hard to understand why ... but ??

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Old 06-04-2014, 08:18 PM   #24
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Originally Posted by solentlife View Post
Brushed - many of those I used over years of racing cars etc. had an arrow on for designed rotation.

I have heard that some brushless are better one way than another ... hard to understand why ... but ??

Nigel
Some brushless motors prefer one rotation direction? Having wound and rewound a number of brushless motors some years ago, I don't see where the motor can tell which direction it is turning. That would be entirely under the control of the ESC. The no load current pulled in either direction was identical.

In fact, in the late 1990's I rewound a common automotive alternator to put out 60 Volts DC at 5 Amps for a 38 cell Nicad charger. Again, that could be turned in either direction with no difference. Just to prove a point, I took that thing to work, and energized it with a low voltage three phase AC power source, just to show that it would run like a motor. It did.

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Old 06-04-2014, 08:56 PM   #25
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Originally Posted by solentlife View Post
Can you run motor other rotation and swap prop round so its correct ... see if motor does have preference rotation ?
Seems to me if I reverse the motor's rotation, the thrust will be in the wrong direction.

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