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HelicopterScale Model RC Helicopters › BIG scale electric power system questions
09-17-2017 01:01 AM  69 days agoPost 1
ssmith512

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Indianapolis, IN USA

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HI! Long time lurker, first time poster.

Question for you BIG electric guys (BIG is defined as a model spinning 2ea roughly 1000mm blades and/or weighing 40lbs plus.)

Can you guys give me some data on battery size (capacity) and flight times in your big birds? My electric experience is (was) limited to my Cobra - which was just a heavy"ish" 700 size bird. I ran 6000mAh packs (12S) in that bird and a full charge yielded me 2ea 6-1/2 minute flights.

I am contemplating building a big 1:6 scale electric, that will turn 1000(ish)mm blades and probably weigh over 40lbs and I need to learn more about power systems for said large heli.

In addition to battery info, would a "powerful" "standard" motor - say like a 700 size KDE motor or Kontronik Pyro 800 series motor running on 12S be a sufficient power power plant to handle the load of two big blades spinning around at 900 or so RPM?

Thanks in advance!!!

Steve

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09-17-2017 04:16 AM  69 days agoPost 2
Copter Doctor

rrProfessor

Enterprise/ft.rucker​,al- home of army​aviation

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um, Darrell? you out there?

drive a rotary, fly a rotorcraft

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09-17-2017 11:42 AM  68 days agoPost 3
TailKiller

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Panama City, FL

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Alright!! I'm tuned in to the "Bat" channel. I really would like to understand this myself.. Great question Steve.. Following.

,Dennis
http://pcrcheliclub.com/
700 Astar Super Scale T-600 Bell 206 Trex 600 Naza H DX8

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09-17-2017 02:11 PM  68 days agoPost 4
chopper37

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NJ and Long Island

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HI! Long time lurker, first time poster.
im in lol

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09-17-2017 02:54 PM  68 days agoPost 5
darrelluh1b

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Dalton Ga

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Steve
This is easy. The magic numbers are 50 watts per pound for a 2 blade rotor and 75 watts per pound for a 4 blade rotor system.
Gearing will make or break a set up the motor need to operate at 90-95% of total output.
Example my 1/4 jet ranger weight 43# flew with a castle 80HV pulling about 38 amps.
I have had good luck with the KDE line of motors.
I have started a new project it will have 6 1050mm blades and yes no tail rotor (tail rotors use 20-25 % and provide no lift)
Darrell

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09-17-2017 04:43 PM  68 days agoPost 6
ssmith512

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Indianapolis, IN USA

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Thanks Darrell.

So math check......

40lbs*50W/lb = 2000W

Assume loaded voltage of 3.7V per cell (3.7*12 = 44.4V pack voltage), then....

2000W/44.4V = 45A.

Using a 6000mAh pack, then

6Ah/45A = .13hours = 7.8minutes, but we only want to use 80% capacity so 7.8*.8 = expected run time of 6.24 minutes.

Is that right?

Steve

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09-17-2017 05:59 PM  68 days agoPost 7
darrelluh1b

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Dalton Ga

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You did good grass hopper. If you work on the system you can get lower watts per pound the jet ranger was around 38 watts per pound. The system will help with that that is why I like the castle stuff is because of the logging but some times the logging is off a little but it will help in the setup and tweaking. Did you try using a 4 blade setup? I would like to try maybe a 6 blade head and see what happens.
Darrell

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09-18-2017 03:15 AM  68 days agoPost 8
TailKiller

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Panama City, FL

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Hey Steve, Darrell... When Darrell says the motor at 90 to 95% output, Is that being determined by the % for that rpm in the castle esc or is it the actual logger graph Power Out % at the low voltage for 3.70 per cell..

90% / 95% at the end of flight or at the beginning of flight Hope I asked that right..

,Dennis
http://pcrcheliclub.com/
700 Astar Super Scale T-600 Bell 206 Trex 600 Naza H DX8

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09-18-2017 09:29 AM  67 days agoPost 9
Andy01

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Brisbane, Australia

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Steve

The most important criteria for the motor is the kV. Generally the power is the easy bit because nearly all larger 3D motors will have enough power, but many will be too fast.

IMO the formula I use for RPM works really well for calculating the governed speed, so;

RPM = kV x (# of cells x 3.7V) x 0.9 (efficiency) / gearing = headspeed at 100% throttle towards the end of the flight.

So, the 90-95% that Darrell is referring to would be the headspeed above x 0.9 or 0.95. So it is a speed related number, not related to the power out number in the logs.

I am a big fan of Xera / XNova motors - they don't have the raw power numbers of a KDE, but in my experience run smoother and cooler.

I think that if you take the 50W / lb (which tends to be a touch high for larger scale helis flown in a gentle scale fashion, so it is conservative) and multiply it out as you have done, it will give you a reasonable FF number. Hovering adds about 20%, and you would probably need double the original number to deal with Oh $hit moments, so any motor that could deliver around 4000W Nominal (the peak power is always a lot higher) would be fine.

I don't have a really big electric - my biggest is a 4x 820mm EC-145 (just over 20lbs), and it runs a 300kV XNova 4035 , and has PLENTY of power to spin the blades at 1050rpm. It does FF on about 950-1000W and hovers on 1200W. It uses about 3700mAh from a 12s lipo for a 8 minute flight.

Colin

Vario Long Ranger 700e
Seahawk 600
UH-1N 500
Baumann EC-145 800+ (coming soon )

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09-18-2017 01:54 PM  67 days agoPost 10
TailKiller

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Panama City, FL

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Steve.. I'm not trying to hijack here and thank you for asking the question..

Colin.. It's good to hear from you again. You had given me that equation in the past but I couldn't remember the sequence and couldn't find my note's from our talks. Could you please post an example? I'm not sure about the point where the gearing comes in such as pinion or main first / or *

Thanks

,Dennis
http://pcrcheliclub.com/
700 Astar Super Scale T-600 Bell 206 Trex 600 Naza H DX8

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09-18-2017 01:59 PM  67 days agoPost 11
darrelluh1b

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Dalton Ga

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Denis
There are two places you will see the # When you set up the ESC on the castle link you will see this in the rpm box this is a good guess for the esc. The data log will show the true % and yes as the voltage changes the % changes.
Darrell

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09-18-2017 11:36 PM  67 days agoPost 12
TailKiller

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Panama City, FL

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Thanks Darrell... Just wanted to verify.. I use the logger religiously. I've just always shot for around 85% in the CC set RPM but, will try the higher %. thank you bud..

,Dennis
http://pcrcheliclub.com/
700 Astar Super Scale T-600 Bell 206 Trex 600 Naza H DX8

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09-19-2017 09:17 AM  66 days agoPost 13
Andy01

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Brisbane, Australia

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Dennis

Example - a 470kV motor, on 12s, with a 13T pinion and 164T main gear

RPM = 470 x 44.4V x 0.9 x 13T / 164T = 1489rpm at 100% throttle towards the end of the flight.

The gearing is normally calculated as a ratio with the larger number first, so in this example it would 164T / 13T = 12.615 : 1, so in my original formula above you would have;

RPM = 470 x 44.4 x 0.9 / 12.615 = 1489rpm

If it is a two stage gearing like on many larger mechanics (including Goblin, then you calculate each pair of gears separately and multiply them together to get the final ratio.

So, running a CC ESC governor at a Set RPM of 1350rpm yields a governor % of 1350 / 1489 = 90.7%, which is a good setting for the ESC.

Of course, if you choose to run a active free-wheeling ESC like Kontroniks, then the motor kV doesn't matter nearly as much.

Colin

Vario Long Ranger 700e
Seahawk 600
UH-1N 500
Baumann EC-145 800+ (coming soon )

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09-19-2017 01:41 PM  66 days agoPost 14
ssmith512

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Indianapolis, IN USA

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Colin,

Thanks for the info. Very helpful!

Steve

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09-19-2017 04:03 PM  66 days agoPost 15
TailKiller

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Panama City, FL

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+1 thanks Colin. And Steve an Darrell... We now have all this great, needed info together in one short thread. Great time to print it an put on file for future.
Thanks again guy's.

,Dennis
http://pcrcheliclub.com/
700 Astar Super Scale T-600 Bell 206 Trex 600 Naza H DX8

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09-19-2017 09:11 PM  66 days agoPost 16
Mojave

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Palos Verdes, Ca.​USA

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I put a link to this topic in the scale tip of the week thread.
Barry

All helis and planes have an expiration date stamped on them...you only find it after you crash!!

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09-20-2017 06:03 PM  65 days agoPost 17
TailKiller

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Panama City, FL

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Colin.. Question.. Since you stated that the motor KV would not matter with a free wheeling ESC, Is there a difference in the equation for that type of ESC?

,Dennis
http://pcrcheliclub.com/
700 Astar Super Scale T-600 Bell 206 Trex 600 Naza H DX8

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09-20-2017 11:32 PM  65 days agoPost 18
Thumper217

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Slidell, LA - USA

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Steve, I hope you don't mind if I jump in here and ask a question.

What exactly is an active free wheeling ESC? I understand that it allows you more flexibility for the head speed range without the danger of over heating but how does it accomplish this and is that what makes Kontronik so darn expensive?

Andrew

Thumper

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09-20-2017 11:49 PM  65 days agoPost 19
ssmith512

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Indianapolis, IN USA

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Tailkiller - the equation does not change. All calculations would remain the same.

Andrew - the best worded explanation I know of....

https://rc.runryder.com/helicopter/...1/?p=6378697#RR

I can't comment on if the feature is what makes Kontronik so expensive relative to other brands. I imagine there are other factors that determine the cost of the Kontronik units.

Steve

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09-21-2017 12:21 AM  65 days agoPost 20
Thumper217

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Slidell, LA - USA

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Thanks Steve, interesting read and makes me want to try a Kontronik ESC but I just can't get over the price.

Thumper

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