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Old Jul 16, 2012, 07:02 PM
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Quote:
Originally Posted by SkylineFlyer View Post
Those were my numbers in the airframe compared to your numbers on the test stand. Why lose a lb in an airframe?
Actually I don't think you "lose" a pound of thrust it's diverted (for lack of better description) in this case to move air faster out of the thrust tube.

If you have a plane that weighs 7lbs and you have 6lbs of thrust with faster airflow then you have some pretty good Oh Crap power (power to get out of bolter quickly) then when it's in the air it moves fast...

I think some of simple explanation of eflux is good here...I honestly think in some cases we might be slowing air down and losing total CFM vs speeding it up and gaining some performance.

I THINK I've done this on the StarMax F14...I had to use 3700 watts to power the twin 70mm plane with the the CS 10 fans...I was at the field and said screw it and took the little thrust tubes off with a hobby knife..I swear the plane took off in 10 feet and vertical'd until I felt uncomfortable...then floated down for a nose heavy landing....there was that much thrust.

With some of the lighter loaded fans at the same static thrust and watts the thrust tubes were a non issue, matter of fact I used to power the thing off of a 2200mah 4s cause it only took 60 amps on 4s to get it flying with lighter loaded rotors and the thrust tubes (the ones built in) on.

I would love to know why the lighter loaded rotors did better than the heavier loaded CS10 rotors and if the same could be happening in other planes with heavier loaded rotors....

TIA for any input

Regards
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Old Jul 16, 2012, 07:27 PM
Extreme CNC Alloy EDF
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More likely lost in the airframe, intake restrictions stopping the fan from breathing to the same level it was on the stand means less out the back.
I have tested the same fans both with and without thrust tubes on the bench simply to see the differences in figures, in all cases the RPM drops and the efflux goes up.

The thrust changes very little, dropping slightly if at all.

Not in all cases does the amp draw change by much, on lower powered setups it might change only 1-2 amps, on high power setups where the fan is really hammering such as my 64mm Alloy Wemo, it can rise by 7-9A.

Testing the 64mm 4s 2848 version open and with a 50mm thrust tube saw an amp increase of 7, an rpm drop of 1500, and thrust change of 1.4kg vs 1.38kg.
Testing the 64mm 6s 2858 version open and with a 50mm thrust tube saw an amp increase of 8-9, an rpm drop of only 320 (much bigger motor for a 64mm fan), and thrust was 1.95 vs 1.94kg.
This is serious power from a 64mm fan so the amp increase is logical.

In all tests I froze the meter data at the same voltage or as close to each other as possible, within 0.15 of a volt on the meter max difference. Tests were done to simply compare the same fan/motor/esc/batt at the same time with and without a thrust tube fitted.
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Old Jul 16, 2012, 07:32 PM
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This may be a little off topic, but I'd appreciate if somebody can help. I have this fan and ARC 36-75-1 with HobbyWing 150A OPTO 6S esc. My question is on the ESC setting, should I run it on 8hz or 12hz? 3.5, 7.5 or 15 degree?
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Old Jul 16, 2012, 08:00 PM
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low timing and 8khz
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Old Jul 16, 2012, 08:01 PM
Extreme CNC Alloy EDF
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I would run it on 8khz and 7.5 timing. You only want to increase the PWM if the motor is slotless, or ironless, 95% of the motors we use are not.
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Old Jul 16, 2012, 08:12 PM
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Timely discussion about timing (and pwm)!
Many times the ESC settings will affect fan performance measurably. Switching motors (innrunner vs. outrunner and different pole count)) will require changes made to get best efficiency and power.
We often focus on kv, C ratings and battery size but ESC settings are possibly even more important if the ESC was manually set at any time (not auto settings).
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Old Jul 16, 2012, 08:29 PM
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I run my arc motor on low timing and 8khz as specified by the mnfg. I tried all diff timings and pwm rates and found zero reasons to run it higher than the mnfg specified settings. these are just my findings others may vary
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Old Jul 16, 2012, 11:18 PM
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Quote:
Originally Posted by SkylineFlyer View Post
Those were my numbers in the airframe compared to your numbers on the test stand. Why lose a lb in an airframe?
I think it has be covered by the others.
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Old Jul 17, 2012, 12:08 AM
Lee Liddle
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For the RPM that this fan runs at, with an 8 pole outrunner 16hkz is needed for the motor to run smoothly. For inrunners with 2 or 4 poles 8hkz is plenty and there is no advantage to going higher with pwm.

You will see some increase in power with higher timming on inrunners, but it is often not worth the increased amp draw.

On outrunners, you need to run 15 to 30 degrees timming for them to work well at these rpm levels.
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Old Jul 17, 2012, 12:24 AM
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That sounds what I would expect Knife.
But I have some cases of testing where timing made no difference at all right across all values.
Technically it should matter, and make a difference in either, or both, power and current, you would expect, but there must be some reason it doesn't sometimes. (at least so low it is neglible).

1deg to 30deg sounds like a LOT to me! So you would think it should show up with reasonable note. Even to the point of too far advanced causing notable 'stuttering', but I rarely see that occur too.
I suspect that "1deg", "30deg" is a number that supposedly means an advance on some base value they already have anyway. Because pretty well every motor case will need some advance anyway, and then they give you that programmable setting to add some amount more.
Motors running 10k RPM (at least) by the time they reach any power level of their use, so I would guesstimate (very rough of course) they would need at least 10Deg sort of area anyway.
1 deg against a base value is minor... but 30deg is going to be a 'big' amount no matter what.
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Old Jul 17, 2012, 12:34 AM
Lee Liddle
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Yeah,, I know that they type od wind makes a diff. and that more poles need higher timming even at lower rpms so that`s part of it. Since I deal mostly with outrunners I`m pretty much stuck with high PWM and high timming on most motors.

On outrunners I start high and then work down. I use the lowest settings that still allow the motor to run smoothly. On inrunners I start low and work up a little on the timming to see if there is any benifit, but stay with 8khz pwm.
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Old Jul 17, 2012, 12:43 AM
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Well there is "what I do" and there are the actual electrical parameters, often its best to understand the electronics first before guesstimating things.

There is no simple formula in setting PWM, one thing to note is Kv and PWM dont really relate. For best motor efficiency, a high PWM is warranted, for best ESC efficiency you want to run a low PWM.

Hi inductance slotted stator motors (like outrunners) will not show very much efficiency difference above 8-12khz as the high inductance limits the peak currents during PWM.

Low inductance slotless motors will show a big jump in efficiency when the PWM frequency is raised as the PWM currents will get very high during 'on' times due to there being little inductance to limit the current rate of change. Because losses are current-squared, the integral of the current-squared (loss) waveform changes significantly with high PWM rates.

So to make it easy, slotted stator motors = motors with cogging effect when turned by hand, IE iron cored stator whether inrunner or outrunner.
Slotless = no cogging, IE perfectly smooth rotation of the shaft. Only found in some inrunners, some of the expensive EU brands were slotless, and some Asian made inrunners are too.

So with all the motors we use, basically its best to stick to the lowest PWM, it protects the ESC, and going above 12Khz will make no difference in your efficiency, only in the heat your ESC generates.
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Old Jul 17, 2012, 12:54 AM
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But all too often the "technically it should do this", or "technically this is what is best", does not even hold true! Which is more because other unknown parameters/reasons come into play to 'ruin' the main technical thought.
So TRYING... trial and error... for every given system, seems to work best. More so when options are limited enough - like ESC's and PWM/Timing total permutations. (but painful to reprogram!!)
Then there is the "I did this and it worked"... but was never truly measured/tested the right way to truly know 'it worked' properly, safely, or efficiently etc.
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Old Jul 17, 2012, 08:27 AM
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Reading some of this discussion with interest.

If a particular setup produces 1.0kg actual measured static thrust (AMST), for example, and the same basic setup produces 1.2kg AMST for the same amps through some form of tweaking, is not the second setup the better choice of the two?

Forget thrust-tube/no thrust tube, efflux blah blah blah - thrust is thrust.

The assumption here is that we're talking WOT thrust - no 1/4 throttle, 1/2 throttle or anything else. Pure WOT thrust.

How can something that produces 1.0kg AMST be better than something that produces 1.2kg AMST for the same amp draw on the ESC (all other things being equal) ??

BJ
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Old Jul 17, 2012, 07:33 PM
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Would be wonderful to be able to "tweak" a couple of things and have our fan/motor combo suddenly produce 20% more thrust with no change in materials or power consumption, I would certainly love to see that...

Big problem with that theory is that in order to get 20% more thrust you are going to need more RPM, and more RPM costs more in the form of power, your motor is not going to spin faster and push against a higher load without more power. Amps are not going to show you power consumption, you have to monitor the wattage consumption.

Playing around with ESC settings is only of use if you have enough equipment to be able to monitor the changes, most importantly RPM and ESC temp, seeing a rise in watts without knowing your RPM is pointless. And pushing the PWM of an ESC can be dangerous too, lots of guys run cheap ESC's near their limits, a rise in PWM makes the ESC work a LOT harder.
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