DIY Motor Dynamometer

[Piotrsko]

Altitude/pressure corrected to standard day sea level at some temprature so your current alt doesnt matter.

Yes, once the esc is calibrated, it doesn't matter what motor. However it should be recalibrated every turn on.

 

[Inq]

Thanks also, are you going far enough to build a general table, or are you using just what you have?

I think the KV rating is theoretical, and does not account for various heat losses IN system... Freezing a motor prior to test might quantify the loss or actually getting data on how KV is calculated by the motor maker.

I also guess you are using battery packs instead of a large current precision regulated power supply and very high current ESC. Calibrated measuring sensors?

Table - I'll only do the ones that I can on this load-cell. I do not think it would make sense for me to incorporate other people's results that I have no idea how well they would correlate to my stuff. I do plan on getting other motors and if someone recommends a motor I need for a particular plane, I'd probably get it and test it too. But... even though I've asked on two different planes, I've got no advice... even when it was on a common plane that I know you all have "souped up" - Josh even comments that you can put a lot of power in a Storch and make it quite acrobatic... but no advice on motors for that.

Equipment - All the equipment is shown in the pictures above. I think doing a power supply would fudge the data... I want what my planes will be experiencing, not some lab result and certainly not freezing the motors ahead of time. The scale is a jeweler's accurate to 0.1 grams, the ratio of the lever arm is adjustable to improve accuracy or to divide force so it doesn't exceed the scales 1KG limit. Even so, the variability due to vibration and estimating must be at least +/- 10 grams error. The tachometer, is as good as I'm willing to purchase. Same thing for the V/A/W meter.

I have two of the 1000KV and two of the 2200KV and will see what kind of manufacturing variability I get between the two. I dare say, that variability will be larger than my "crude" equipment. I also have multiple props, and will test those as well.

Stay tuned!

 

[Piotrsko]

The power supply was for repeatability. Your results will be only accurate for your specific battery pack on that specific run, and you find the data drift is all over the place. Battery packs are sensitive, finicky subjects.

do chive on

 

[danskis]

Piotr ski is probably right in that you should recalibrate frequently. The only problem with that is that you might go into the motor programing schedule and really screw up something.

 

[Bricks]

Sizes - The prop sizes are what's given in the first table. They have no labels except prop size and I assume are cheap Chinese clones. I'll have more on some surprises with these in my next report of tests that I'm doing now. If you look on Amazon and search for "brushless ESC propeller" you'd get these class of motor kits. I've looked at a bunch of these listings and the sizes vs KV values seems to be consistent. The 1000KV / 10x4.5 is always black plastic and the others are the bright orange. That's about as good info as I can provide. I'm guessing they are all not nylon as they break easily with nose overs onto grass if running.... Ask me how I know?

Optimized - Maybe not but I would think they would have vested interest in not putting "too much" prop on a motor, burning it out and get bad reviews AND likewise, they wouldn't put too small as they would underperform. It might have been trial and error, but I'd say it Chinaoptomized.



Geeze! I know better than that. I should have included that. No - I'm at 2200 feet. How much does this make a difference? Nitro, gas -YES, but electric? The prop meets with less resistance so it speeds up till it gets the resistance... making the same thrust??? I know technically from a theoretical standpoint it should be slightly off, but in a practical standpoint, is it really that much measurably different? I'll edit the post above, Thanks.



Does this mean I do it once and it doesn't matter which motor is on the ESC???

Correct... the ESC is calibrated to the transmitter unless you get a lot of drift in the gimbals on your radio it should not be needed more then once.

And I think you mentioned as to RPM is static as the plane gains speed the prop will unload and RPM will climb.

High KV small props for speed a low KV motor swinging a larger prop will not get the plane to the same speeds....Plane Dependent of course.

 

[Tench745]

What I'm Deducing

1. The same size "can" doesn't produce the same thrust.
2. Bigger propeller, slower speed produces more thrust and is more efficient than smaller propeller higher speed.
3. Drag of the propeller brings down the real KV value.

Prop diameter is the primary variable of the thrust equation. For a given motor and battery combination, larger diameter props will give more thrust than smaller diameter props, assuming the motor can tolerate the amp draw. (With some outliers and exceptions, of course.)

A point of note about KV. It is a measure of the RPM of the motor per volt applied when the motor is under no load ie. no prop installed. As soon as you add a load, you are not measuring KV.

Questions - Can anyone help explain?

1. Why would one use the higher KV motor when it clearly creates less thrust (26%)?
2. Someone here told me that the motor will just suck more current, but will "pretty much" be running at its rated KV. I did not find that to be true of these "kits"... by a LOT! Only reaching 59% of the rated KV on the 1000KV motor. Does that mean these propellers are too large for these motors or is there something else I should be reading into this?

To quote from an article at RotorDronePro.com
"A low Kv motor has more winds of thinner wire—it will carry more volts at fewer amps, produce higher torque, and swing a bigger prop. A high Kv motor has fewer winds of thicker wire that carry more amps at fewer volts and spin a smaller prop at high revolutions."

If you want to spin a prop fast, you want a high KV motor. Racing airplanes, for example, are aerodynamically clean and don't need a lot of thrust to fly but they want to fly fast. So they take the efficiency tradeoff of less thrust for a higher RPM and thus a higher theoretical maximum (propeller pitch x RPM).
If you have a draggy airplane, or one that needs to accelerate and decelerate quickly, you want thrust more than speed so you need bigger props. Big props require a lot of torque and a low KV motor produces more torque.
To muddy the waters further, you can have two motors of the same KV but different sizes. A larger can/stator size will be able to create more torque and spin a larger prop.

 

[Inq]

I don't imagine many will want to use something like this except for those wanting an indoor airplane (and weak one at that). This is a geared motor that seems to come in three motor sizes... 716, 720 and 8520. This test is for the 716. This may help someone designing a plane. Let me know if you do.

I've included the motor/props above in this table, so you (and I) don't have to go digging through the whole thread.

 

[Inq]

70mm EDF
...to add to the data overload.

Specs

• 70 mm, 6 bladed EDF
• Motor QF2822D / 3000KV
• 80 Amp ESC
• LiPo 4S, 1500 mAh

I do not have watts used or RPM. It uses different connectors than is on my watt meter.

Thrust: 1159 grams

 

[Inq]

FlashHobby BE1806, 1400KV

Here is the official page of specifications - https://www.flashhobby.com/be1806-rc-brushless-motor.html

Motor: BE1806, 1400KV
Prop: Uxcell 6x4 Nylon
LiPo: 3S, 12.4V, 2200 mAh

Results
Thrust: 317 grams
Speed: 13,200 rpm
Power: 60.1W
Efficiency: 5.28 g/W

 

[Piotrsko]

Nice rig. You need a servo tester, less hardware to adjust.

 

[Piotrsko]

New question: does reversing the prop to a tractor and reversing the motor make a thrust difference?

 

[Inq]

New question: does reversing the prop to a tractor and reversing the motor make a thrust difference?

I guess I could test that. Obviously, the rig as it's setup as shown blows as a pusher configuration so that it presses down on the jeweler's scale. I can turn the arm around so the prop is hanging over wood platform, turn the prop around and reverse the motor direction for tractor configuration and still be able get compression on the scale.

Yeah, I'll give it a try.

More to come...

 

[Piotrsko]

Didn't want you to go through all that effort, just swapping the prop and changing the motor rotation with 2 of the three motor wires. Should give the same effect, right?

 

[Inq]

Didn't want you to go through all that effort, just swapping the prop and changing the motor rotation with 2 of the three motor wires. Should give the same effect, right?

It's no trouble. I'm curious also. I asked the question on this forum about motor direction and was told that for these cheap motors direction shouldn't matter. That more expensive motors actually have "advance" on them. In my mind, I interpreted that as like I used a timing light to set the advance on my old cars. I also assumed you might be hinting at the pylon itself would screw up the flow going into the pusher prop and reduce thrust.

But no, I can't simply do as you suggest. If you look at the configuration... it relies on the horizontal beam pressing down on the scale. If I simply swap the prop and wires to reverse the flow, the motor/prop will fly forward and crash into the floor. ... which I've done several times because of a wrong guess of the wires. Actually now, I have a stop build into it so at least no crashing of props occur. Looking at the EDF picture above, it might be a little clearer. As you can see, I can simply remove the bolt at the pivot, slide the vertical arm off the horizontal arm and reverse the vertical arm, then I can do the things you said and all will be copasetic!

Apologies for not getting to it sooner, but I'm using it on the other thread https://forum.flitetest.com/index.php?threads/designing-propellers.73649/ and if I go changing it around and the results are different, I can't compare them to numbers I already have.

 

[Piotrsko]

Not to worry. People say I sometimes talk/ write oddly, been like this all my life. Stuff that looks obviously easy to me looks seriously wrong to others.