DIY Motor Dynamometer

[Inq]

I'd like to look into making a Dynamometer to test motors, ESC and propellers. Something like:

It gave 305g thrust with an efficiency of 4.13 g/W.

I have a handle on making the mechanical part of the rig.
I can come up with the required electronics to get the RPM's of the propellers and the current draw, but I'm wondering if there are any products you all use that can give me current draw and/or RPM.

Thanks.

 

[Mr NCT]

I use this for power & current draw:
https://www.amazon.com/gp/product/B06XPVKY13/?tag=lstir-20

 

[Inq]

I use this for power & current draw:
https://www.amazon.com/gp/product/B06XPVKY13/?tag=lstir-20

Perfect... Just ordered!

I wish it had the connectors on it, but I guess there's too many different versions. I have trouble soldering such heavy wiring to these little red T-plugs.

Thanks

 

[Merv]

... any products you all use that can give me current draw and/or RPM...

Here is a tachometer
https://www.ebay.com/itm/303949187290

 

[Inq]

Here is a tachometer
https://www.ebay.com/itm/303949187290

At some point (when I can fly with a better than average chance of landing in one piece) I want to bring my electronics hobby into the mix. I've read I can integrate a microprocessor into the plane's receiver and send back data to the transmitter and display it on the little screen. I'm thinking one of my first projects would be to get main battery voltage, current draw, propeller speed... maybe altitude, GPS or air speed. But for now... I think as cheap as your device is, for ground use... I'll go with it!

Thanks.

 

[Merv]

...I can integrate a microprocessor into the plane's receiver and send back data to the transmitter and display it on the little screen...

Why reinvent the wheel? Other brands of receivers also have telemetry and sensors.
https://www.amazon.com/dp/B01FJX3DR4/?tag=lstir-20

 

[Inq]

Why reinvent the wheel? Other brands of receivers also have telemetry and sensors.
https://www.amazon.com/dp/B01FJX3DR4/?tag=lstir-20

It's called a hobby. I don't buy any planes that are premade even though my flight instructor thinks I'm crazy to make my own airplanes even if they only cost a few bucks. He keeps nagging me to buy a $400 Carbon Cub. I don't always go buy electronics that are premade. I do it because I like messing with stuff. Occasionally, I actually make something better or cheaper. The sensors and MPU to do altitude, airspeed, RPM, and battery voltage will cost... oh... about $3 a little soldering and poof... telemetry. It also leads to other sensors, maybe active control, maybe waypoint autopilot... who knows, maybe auto landing. Yes, I know all that stuff is readily available, but, where's the fun in that?

I do realize, I have a different concept of fun. My instructor can't seem to understand that I like building, designing, tinkering with the airplanes more than flying them. He thinks people are ONLY in this sport to fly planes. The flying is simply to prove my projects. I have no interest in flying 5" above the ground while upside down and doing barrel rolls. I just want to be a good enough test pilot to save a sub-standard design from destroying the electrical equipment inside.

 

[danskis]

Besides that just look at his avatar - I like suffering. (me too sometimes)

 

[Inq]

Besides that just look at his avatar - I like suffering. (me too sometimes)

Avatar - "Odysseus and the Sirens"

Now... I know you all understand this. You don't build these foam and paper airplanes because they're easier than what comes in a box... you do it because you're into self abuse also.

 

[Piotrsko]

Ummm, close. I do it because there's a thing in my head that is annoying and won't shut-up until it gets what it wants, but is shiney object controlled. The self abuse is just an added bonus.. and DTFB is a finance officer (wife) and medicare doctor approved expenditure.

This does explain my making of beer, roasting coffee, homebuilding, gourmet cooking, whatever.

 

[Bricks]

Way beyond me but probably right up your alley


https://www.rcgroups.com/forums/sho...dio-RC-FPV-system-(v-6-7-adaptive-video-link)

 

[Inq]

I was looking at several of the electronic, hand-held tachometers. Some of them come with reflective tape. Is that required to put on the propeller? Will it still work without it?

 

[WillL84]

I was looking at several of the electronic, hand-held tachometers. Some of them come with reflective tape. Is that required to put on the propeller? Will it still work without it?

You'll need the tape as the tach uses a laser that the tape reflects back to the tach. Just s small square close to the hub will be fine though.

 

[Inq]

Wanted to conclude with the setup. It confirmed an issue that might have lead to my poor showing and eventual crash. Here is the motor tester getting Thrust, RPM and Volts and Current. And here are some results of my first testing with it: https://forum.flitetest.com/index.php?threads/what-did-you-crash-today.23426/page-83#post-735948

 

[Inq]

Now, I'd like to share what I'm learning and if anyone has any input, please jump in. I may be way off base in my assumptions or interpretations. I doing this so I can better guestimate what motor I need in my planes and how they'll fly.

First, off, I've re-oriented the load cell so that I don't have to hang out over the spinning prop to see what the thrust load is on the scale. I run the propeller as a pusher. Now I can read the thrust, voltage, watts and rpm all from the same side.

 

[Inq]

My first series of tests involve wanting to learn about KV and propeller sizing. I'm using the budget sets found on Amazon for around $20 that include a 2212 Motor, 30A ESC and an appropriate propeller. Since these are the only motors I have, I'll go with what I have...

Here are the three sets I have. All run on 3S batteries. Edit: Tests are performed at 2200 feet elevation.

Motor	KV	Prop
A2212/13T	1000​	10 x 4.5​
A2212/10T	1400​	8 x 6​
A2212/6T	2200​	6 x 3.5​

The first lesson (learned from this forum) was learning that the ESC can be calibrated.

1. Turn transmitter on.
2. Advance to full throttle
3. Turn on the receiver
4. Let any beeping finish
5. Retard the throttle back to zero
6. Let any beeping finish
7. Good to go!

My FT Storch was way underpowered and lead to a crash (yeah! let's go with that and not my lack of skill). I still had the motor in the un-calibrated state from the crash day. Both test runs are using the A2212/13T with 1000 KV and the standard, black 10 x 4.5 propeller that came with the set and using a fresh 2200 mAhr battery showing 12.5V

Calibration	RPM	Thrust	Watts
Pre calibration	6620​	584​	105​
Post calibration	7370​	857​	160​

Yeah baby! A whopping 47% increase in thrust. I'll give the Storch another chance to plow the field.

The second test was just to get a baseline for what these three motor sets can do with their included propellers. All are calibrated.

Motor Set	RPM	Thrust	Watts	Eff (g/W)	Calculated KV
A2212/13T - 1000 KV	7370​	857​	160​	5.35​	590 (59%)​
A2212/10T - 1400 KV	10190​	729​	197​	3.70​	840 (60%)​
A2212/6T - 2200 KV	20660​	634​	162​	3.92​	1674 (76%)​

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.

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?

 

[danskis]

Thanks for doing the math!!! This is good info.

It would be nice to know what props you're using.

Also - just because the props came with the motors doesn't mean they are optimized for those motors.

And I gotta ask - are you doing this at sea level?

Why use a higher KV motor? You might want to use a smaller prop on a flying wing pusher because a longer prop won't fit between the wings.

BTW - the ESC is what you're calibrating to the transmitter.

 

[danskis]

This is interesting

Leopard Motor Data Chart - Altitude Hobbies (shopify.com)

 

[Piotrsko]

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?

 

[Inq]

It would be nice to know what props you're using.
Also - just because the props came with the motors doesn't mean they are optimized for those motors.

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.

And I gotta ask - are you doing this at sea level?

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.

BTW - the ESC is what you're calibrating to the transmitter.

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