Some Motor Test results.

[roycruse]

OK I just tested a couple of Thumper V2 2836-11 motors on 3S with 13X6.5 props with the following results.

Full throttle single motor 14.5 AMPs with almost exactly 1000g of thrust.

when i ran 2 motors bolted one above the other one with a pusher I still was getting 1900g of thrust at full throttle which surprised me.

what surprised me even more was that on measuring the current to each individual motor they are still drawing aprox 15 amps each.

1. I expected them to be sharing the workload of pushing the air and for it to not be anywhere near 2000g of thrust
2. I therefore expected the current of both motors to be less than when run individually
3. I expected the motor downstream of the airflow to be pulling considerably less amps than the other

any comments or discussion points welcome

how does this motor / prop setup sound for a Y6 copter ie 3 times this tested setup.

I did a plot of current vs thrust for a single motor - things was getting a bit hairy to do it with the 2 but i may try again tomorrow, if i can get my son to volunteer to help me, but this was the plot i got for the single motor - (does it seem an acceptable setup for multi rotor use)

View attachment 1307

 

[jes1111]

Do you have a link to data for that motor?

 

[roycruse]

the motor was bought from here http://www.overlander.co.uk/motors/brushless-motors/tornado-thumper-v2-5045-720kv-945.html

and there is some more info here http://www.overlander.co.uk/blog/?p=212

but I wouldnt go as far as calling the info a full data sheet.

One thing i noticed was that the stator was getting quite hot (no smell of melting enamel) but definately hotter than I had hoped for. this was especially surprising seing as i was only drawing 14/15 amps (150 watts) - this motor is supposed to be able to dish out up to 300W (but i gues only in a moving plane with fast airflow)

Ive decided to switch the plastic motor mounts i had planned to use on my multi rotor for some aluminium ones, that way the whole aluminium arm of my planned Y6 ship will become a very effective heatsink.

 

[roycruse]

looking at eCalc - the motor seems to have the same characteristics as the Turnigy SK2836-11 (750) so i presume its the same motor (or very very similar)

by my tests a Y6 should have a 100% full throttle thrust of 5700g and that exactly what eCalc says for that motor with 6 rotors and if i can keep the AUP below 4kg i should get 9 mins of flight time running with cheap as chips 4 x 3s 2100 packs

A question of the pro's

I am happy that from an electrical point of view running the same prop top and bottom of a coaxial setup is drawing almost identical amps from both motors (but this was without a flight controller just a normal receiver with the throttle channel mixed to the aux1 channel to give me two throttles) but what I dont know is if when the flight controller is in the picture trying to balance yaw - am I then going to see a difference in loading between the 2 motors???

The reason I ask is that many people talk of higher pitch or larger diameter props for the bottom motor, but with only my limited test run last night to go on - this seems to be unnecessary.

 

[jes1111]

ah! it was the "2836-11" in your original post that threw me

That looks like way too much prop for a 70g motor. eCalc list the max wattage at "100W for 20 seconds" for the RCTimer 2836-11. There's no mystery to this: the ability to dissipate heat is closely tied to the total mass of the motor, and 70g is a baby! Mounting it on an aluminium plate will help some, for sure (see the Droidworx mounts) - but you're still basically over-driving the motor. If Overlander told you it can handle 300W then they are being dishonest (or naive)! Remember that a brushless motor "sees" all the voltage and a high proportion of the maximum current even at partial throttle settings, so staying strictly within the (honest) ratings is essential for longevity.

 

[roycruse]

sorry about the typo

There are numerous motors on the eCalc database of size 28mm x 36mm that can handle over 150 watts so why not this one ??? Is pulling a full throttle 150 Watts really over loading it especially as it will practically never be at full throttle in normal use.

The most I can see being drawn through it continuously in normal use is 6 or 7 amps

are you suggesting running a shallower pitch or narrower diameter and requiring higher throttle to hover - or is it just unwise to expect to lift 4kg AUW with 6 of these.

I had considered trying some of the 3548/05 (900kv) motors but all up including beefier speed controllers and thicker wire etc its about 1000g heavier for a Y6 then theres probably an extra 400g of battery required to get my flight times back because of the extra weight. so im adding nearly a kilo and a half of extra weight and expense !!!!

 

[jes1111]

There are numerous motors on the eCalc database of size 28mm x 36mm that can handle over 150 watts so why not this one ???

Honesty of the ratings, higher mass (but same outer dimensions), design of the airflow through the motor, quality of the aluminium, quality of the copper, temperature tolerance of the magnets and their glue, etc. Lots of possible reasons. China does skew the price/quality equation somewhat, but nevertheless it remains basically true that you get what you pay for - so that is still a $10, 70g motor

Is pulling a full throttle 150 Watts really over loading it especially as it will practically never be at full throttle in normal use.

If it's getting hot (whilst sitting in its own airflow), then yes - it's overloaded Heat is consumed energy that is not producing torque - hence one of the most important figures that eCalc gives you is the efficiency (particularly at hover). Plus, like I said, the principle of operation of brushless motors is that the controller pulses the full voltage (and hence a high proportion of the current) in order to produce rotation. So, in loose mathematical/electrical terms, the current figure you're seeing is an "average".

are you suggesting running a shallower pitch or narrower diameter and requiring higher throttle to hover - or is it just unwise to expect to lift 4kg AUW with 6 of these.

Seeking that answer through eCalc will help you understand how these various factors inter-play. You're aiming for hover at 50% throttle, i.e. the total thrust available at full throttle should be double the AUW. In general you get valid answers more easily from eCalc if you set the weight in the top line to "without drive" (i.e. frame, gimbal, camera, landing gear etc.) and let it add the weight of motors, batteries and controllers.

I had considered trying some of the 3548/05 (900kv) motors but all up including beefier speed controllers and thicker wire etc its about 1000g heavier for a Y6 then theres probably an extra 400g of battery required to get my flight times back because of the extra weight. so im adding nearly a kilo and a half of extra weight and expense !!!!

Bigger motors will have more torque and can therefore spin bigger props. But higher Kv means more revs, so you become more limited on prop choices (especially with APC SlowFly, which have a very low maximum rpm rating). Those 13x6.5 props are at the edge of the envelope for multirotors: 50% pitch/diameter ratio means they'll pull lots of current for the amount of lift they produce. They've become popular only because they are available, not because they are ideal. They generally need a very beefy motor to work well with them.

 

[roycruse]

thanks for all your input.

ill do some thinking - I am on a tight budget - i need the cheapest way to lift 3 to 4kg AUP for around 10 minutes. i thought these motors would do it with the 13 x 6.5 at about 65% throttle and about 6 or 7 amps per motor - i already have loads of 2100 3s batteries which i use for all my rc stuff.

can anyone suggest a "safer bet" option for motors that will meet my needs for my Y6 setup - please dont suggest anything thats silly money - i just dont have it available to put into this project.

if i buy another 4 of these then its only going to cost me another 60 or so quid - but thats wasted if they start melting on me - so any suggestions gladly received.

 

[rtgaspar]

Let me suggest something that I think but did not tested yet.
If you put two motors one above the other then the lower will feel like its in high speed (it will have wind blowing in it), so to produce some thrust it must have a propeller with a higher pitch then the other so it can make some work in the air it takes.
When I have a chance I will try, some like a 9x5 above and 9x6 below or 9x4.7 with a 9x6.

 

[roycruse]

Rtgaspar..

I think that your theroy is true BUT... I also think that the effect the bottom motors has on the top one is ignored by most people.

It's easy for people to understand that air "blows" out the bottom end of the propellor as this is the "thrust" that we use. But this air had to come from some where.

Propellors don't manufacture air... They create a pressure difference above and below the blades which causes the higher pressure air above the propellor disc to move to fill the lower pressure void below the propellor disc.

Therefore in a coaxial setup the lower propellor is helping the top propellor make a larger presure diference just as much as the the higher propellor is assisting the lower one.

I believe both propellors need a slightly steeper pitch in a coaxial setup. Not just the lower one.

I need someone with a degree in fluid mechanics or something to confirm or deny my theory.

Sorry for any typos... I'm on my phone