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Bench test Amp draw vs in-the-air comparison?

#1
I'm new to this, but got so far as matching up my motor and prop and doing a full throttle bench test to test the Amp draw. I got 15A at full throttle which is within the generally recommended ~80% max Amp rating, though only just (17A in this case).

I was wondering what the difference is between the bench test and actual flying measurement (I don't have any kind of telemetry) in terms of the Amp draw. Will the Amp measurement be lower because on a bench the prop is pushing stationary air vs at steady speed when flying there would be less stress on the motor? Or, due to the already moving air (relative to the plane), would the motor be able to spin faster and consequently pull more Amps?

Do I need to allow for the difference in calculating the draw when I set up the plane? I don't want to burn anything up.

It's times like these I wish I'd concentrated more in physics lessons.

Confused.
Tim.

Motor 2826-10 1400kv
8x3.8 prop
3C 2200 battery
30A ESC
Turnigy 9XR with OpenTX
 
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Foam Addict

Squirrel member
#2
The prop will "unload" slightly (10-25%) while in the air leading to lower current draw. You should have no problems with that setup since you have a big ESC.
 
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xuzme720

Dedicated foam bender
Mentor
#3
More load on the motor will increase the amp draw, so bench testing should give you the worst case scenario in terms of amp draw due to stationary positioning where in flight, the motor will "unload" to a degree due to moving airflow. In other words, bench will be a bit higher than flight draw.

Seems we posted at the same time, but at least you have an answer!
 
#4
Perfect, thanks guys. That's good to know. I've learnt so much, this is such a great community on FliteTest. :)

I've been taking photos all along the build and learning curve so will create a build post once I'm done. Maybe a 'lessons learned' too for other newcomers.

I hope to get it in the air this week sometime, though the weather in the UK has been terrible (wettest months on record) for several months so we'll see if the storms subside.
 

Jaxx

Posted a thousand or more times
#5
I use that same motor/ESC/battery combination, but since I'm at high altitude(lower air density) I have a little more flexibility with the prop size. What airframe are you building?
 

pgerts

Old age member
Mentor
#7
More load on the motor will increase the amp draw, so bench testing should give you the worst case ..
I am just curious at what air speed the air flow will be slower than the theoretical and cavitation occurs? If you put your hand and stop the air into a vacuum cleaner the motor will speed up due to the lack of air transported and the motor will draw less power. The same with a centrifugal pump. If you stop the water flow the pump motor will go down a lot in amps. There is the same problem getting a really fast pylon plane or hotliner to get the propeller to grip the air and start giving power.
 

xuzme720

Dedicated foam bender
Mentor
#8
I am just curious at what air speed the air flow will be slower than the theoretical and cavitation occurs? If you put your hand and stop the air into a vacuum cleaner the motor will speed up due to the lack of air transported and the motor will draw less power. The same with a centrifugal pump. If you stop the water flow the pump motor will go down a lot in amps. There is the same problem getting a really fast pylon plane or hotliner to get the propeller to grip the air and start giving power.
On vacuums and pumps, I thought blocking the inlet creates a partial vacuum, which would, in effect, lower the resistance on the impeller/rotor but isn't cavitation basically the blades stalling. I know you know, pgerts, but for those that don't, blade stall is created by pushing the prop above it's operating RPM range, like throwing a high pitch prop on a high KV motor. Something like an 8 pitch prop on a 4000kv motor powered by a 6 cell...definite recipe for blade stall (if the prop doesn't explode!) or maybe lowering the air pressure the blades see can cause it as well. On the bench, operating within the normal parameters and using the proper prop types and motor speeds, I wouldn't think you'd hit the cavitation point due to airflow always being available, even on a static bench rig, but as far as what speed; that, I have no idea. Might be a nice experiment for someone to give a try to make sure we aren't misinformed...
 
#9
I use that same motor/ESC/battery combination, but since I'm at high altitude(lower air density) I have a little more flexibility with the prop size. What airframe are you building?
Nothing too exciting (though it is for me being the first two I've ever built!). A trainer based on the Power Pod (plans on here if you search FT Trainer), and a Versa Wing. Both built and ready to be crashed when the weather turns a bit better; very windy in the UK.

I will definitely post pictures once I've collated them.
 

Jaxx

Posted a thousand or more times
#11
Nothing too exciting (though it is for me being the first two I've ever built!). A trainer based on the Power Pod (plans on here if you search FT Trainer), and a Versa Wing. Both built and ready to be crashed when the weather turns a bit better; very windy in the UK.

I will definitely post pictures once I've collated them.
Snuffy,

I started doing this one year ago this month, and every build is still exciting. Still learning and crashing too. Great job with that wing!