RC Groups

RC Groups
    Multirotor Talk
        Discussion [calculation related] throttle vs current/power relation

#1 jy0933 Aug 15, 2012 01:42 AM

[calculation related] throttle vs current/power relation
 
after done a whole bunch of ecalc... i found something important.. but it is now shown in ecalc... so I wonder if anyone has the idea how to figure it out.

basically the question is. the charge for percentage of throttle VS total amp/power output

the relation does not appear to be linear.. how to I calculate it?


why it is important? >>> usually we dont go max throttle for any motor.. but ecalc kind of "limiting" it because the max throttle has reached/exceeded rated power limit.. in reality, since we do not go max throttle. then we might be able to use larger prop to reach rated limit at, for example, 40%. then use tx curve feature to prevent the output goes over 40% so that the motor stays in the rated range.. the benefit for this is that when you use larger prop, you have better hovering time. and as it is know, the longer hovering time you can extract from a given AUW& limited lipo mah the better
the KEY for this entire theory is to figure out at the given prop and motor, what % of throttle hits the rating limit. so we can use this to setup tx.

thanks

#2 diakatche Aug 15, 2012 04:31 AM

good observation but you should also take into account that the flight controller can command full throttle for stabilization effect. Normally, pilot commanded throttle cannot exceed 80% or so in order to leave a margin for the FC. This is particularly important in a failure mode where one (or 2 on an octo or higher) is inoperative for some reason.

#3 jy0933 Aug 15, 2012 06:39 AM

the maxthrottle is adjustable on FC... and pretty much mwc is only platform that i use.. so this is basically not the problem.. ( i can calibrate esc to 1095~1905 and set FC max throttle @ 1600. and then FC wont output anything more than 1600)

i'm looking for a way to find the relation based on calculation.. i know i can use a current sensor to check when it reaches the limit... but since i'll be trying a lot of props.. this is way too much work to do.. there must be some way to estimate it by calculation

#4 rampa202 Aug 15, 2012 08:42 AM

I just read in another thread that half throttle gives you 1/3 maximum thrust, so the throttle to thrust ratio can't be linear unless full throttle is only 66% of max thrust.

Regarding eCalc's "optimal efficiency." I just realized that it is a value that has nothing to do with your selected prop - it doesn't change when you change props. It seems like its a guide to selecting a prop to acheive optimal efficiency, which for me, is always not enough for adequate flight. Does this sound right?

Russ

#5 jy0933 Aug 15, 2012 08:50 AM

2 Attachment(s)
the optimal efficiency, afaik, is the max achievable efficiency at max throttle.. which is not useful at all.

this might sound BS to some people..
you might get 80% efficiency and 20 mins hovering time @ AABB prop
you can also get 75% efficiency and 25 mins of hovering @ CCDD prop..
what will you choose?

attached parameter might explain it clearer

(noted, max throttle might exceed motor limit.. but you can choose not to go that far to save motor)

#6 rimshotcopter Aug 15, 2012 11:13 AM

Interesting, reading the datasheet on many motors list a maximum efficiency zone however, I figured a multi-rotor would never reach this zone. Here is one for example that states 93%.

http://www.hobbypartz.com/96m601-big...20-1100kv.html

#7 jy0933 Aug 15, 2012 11:18 AM

i actually "reached" the theoretical zone on datasheet.. and the optimal efficiency on ecalc

BY REDUCING PITCH TO EXTREME LOW.. but that is just ridiculously low, say 0.6

for me, aside from efficient. if i can get more hovering time from it.. i say it is good better on performance side than efficiency side. cuz usually for ascending you just do about 5~10% more throttle than hovering.. which is not a big increase on power

#8 rampa202 Aug 15, 2012 05:05 PM

1 Attachment(s)
This is what I'm building now - the hover efficiency is very close to the optimal efficiency using the props that I actually have.

Russ

#9 jy0933 Aug 15, 2012 05:07 PM

Quote:

Originally Posted by rampa202 (Post 22456426)
This is what I'm building now - the hover efficiency is very close to the optimal efficiency using the props that I actually have.

Russ

i can get same hovering time with less than half battery capacity you have there

and if you increase the prop size by 2 inch. you might get less efficient.. but more hovering time and lower throttle at hovering

#10 rampa202 Aug 15, 2012 05:23 PM

Quote:

Originally Posted by jy0933 (Post 22456437)
i can get same hovering time with less than half battery capacity you have there

and if you increase the prop size by 2 inch. you might get less efficient.. but more hovering time and lower throttle at hovering

Its an 800mm hex, so 15 inches is maximum prop size - only about 0.75 inches between the prop tips.

Russ

#11 jy0933 Aug 15, 2012 05:31 PM

my point is...
1. good efficiency does not necessarily bring you good result(hovering time)
and seriously.. 6s 16000 for 20mins is just ridiculously low

2. that motor might be just an overkill

#12 rampa202 Aug 15, 2012 06:51 PM

Quote:

Originally Posted by jy0933 (Post 22456639)
my point is...
1. good efficiency does not necessarily bring you good result(hovering time)
and seriously.. 6s 16000 for 20mins is just ridiculously low

2. that motor might be just an overkill

Well, the DJI S800 gets 16 minutes on 10,000 mAh 6S with AUW 6kg. Mine (according to eCalc) with AUW of 6.2kg on 16,000 mAh 6S gets almost 22minutes - which does not sound rediculously low to me. I do agree that the motor might be overkill, but my payload capacity is probably more than twice that of the S800. Whether I would actually use that much is debatable.

Russ


All times are GMT -5. The time now is 07:16 AM.