Not getting full power out of motors?


#1

My FC has a current sensor which I calibrated to within 10 mA, so I can get instantaneous amps as I fly.

At max throttle I get about 18A using Emax 1106 4500KV and GF3020 props and Tattu 3S 450 maH 75C lipos.

The specs here http://emax-usa.com/emax-rs1106-micro-brushless-motor-1-pcs/ say that I should get about 30A at max throttle. I also think there are some tests on YouTube.

Am I missing something?


#2

Test bench amp numbers are always higher, as the air is at rest with regards to the motor.

If you have a 10krpm motor with a 2.5" pitch that means it wants to travel 10,000 x 2.5" every minute. If it did it would be at zero load. In reality your motors are ~54k rpm at 12v.
It can’t of course, so it has a load.
The difference between it’s “ideal” air speed and it’s real air speed is a huge factor in its loading and therefore power draw.
In the bench is at zero air speed, so max load.
Flying, if you got full throttle you’ll be at 30-50mph in no time (1mph is about 1,056 inches per minute, 50mph would be about 50k inches per minute, that number should look familiar), so the load decreases dramatically almost instantly.

The above assumes perfectly efficient props as well as other things we very much do not have, but it’s the majority of why amp draw is so much higher on the bench. Battery voltage droop under the load is the other major contributor.


#3

So if I understand correctly, I’d have lower amps and lower thrust in the real case. However, the lower thrust would not follow the specs given for that ampage?


#4

Essentially, yeah. Nothing in the charts really matches in flight reality. Especially because the props behave differently when faced with moving air than non-moving air aerodynamically too.

Static tests will give you an idea of thrust level compared to other similar motors tested exactly the same way, and will give you an idea what props a given motor likes, and over a broad selection they’ll give you an idea of how a given prop behaves.

They are very much not real world though. It’s like saying “this motor makes 500hp and these tires are big fat and sticky! It’ll do X seconds in the quarter mile and Y mph top speed”, without taking anything but the motor and tires into account.
Not weight, not transmission, not aero, not whether it’s a dirt track, etc.

Basically, thrust tests only give you a very broad view of the motor/prop combo.

Which is all to say, once you’ve bought the thing, don’t use thrust tests for anything except choosing props to try, and some assume that because it thrust tests well it’ll fly well.


#5

@Bobnova Thanks!


#6

Now all that said, for picking motors thrust tests are extremely useful, they just have to be compared to each other, not to in flight video.


#7

I chuckle to myself when I see motor specs. Especially if they are putting different voltages up. If kv=rpm/v, @ 10k rpm and 3s, the kv is ~793. At the same rpm and 2s, the kv is ~1388. It’s a conspiracy…


#8

Having benched a shit ton, I agree, the numbers on most spec sheets are as meaningful as the horoscope in the sunday newspaper…Very few are repeatable even with the same hardware…I almost gave up myself lol.


#9

Ryan at mqtb does great work, but unless stuff has changed he doesn’t do micro’s.