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Old Jul 27, 2009, 09:33 PM
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Morgan City/Patterson, Louisiana, United States
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Why aren't RPMs and prop size ever listed in electric motor specs??

I'm thinking about converting a Nitro Planes Falcon 120 pusher jet to electric but I'm having a hard time finding information that makes sense to me on which motor and battery set up will work for my application. I'm new to electrics, but have been in R/C for about 17 years so I know prop size and RPMs, not watts. I just read a review where the author tells what prop, and what battery produced so many watts, but that tells me nothing about its performance relative to a glow motor. I asked about this conversion in the glow conversion section and got asnwers like "you'll need at least 2,000 watts" but no specifics on how to get there. This plane is going to be about speed so I need to spin a 10" pitch prop at about 13,200 or 12" pitch prop at 11,000 rpms. About the only place I can find that gives me any prop/RPM/battery info is the Electrifly Rimfire site. Can some one give me some guidance on where to look to find some relavant information or recommend specific combinations? The specs for the plane are below.

Wing Span : 63 in / 1600mm
Wing Area : 1007sq in / 65sq dm
Flying Weight : 12.3 lb / 5600 g
Fuselage Length: 76.8in / 1950mm
Engine Required : 1.20 2c or 1.60 4c
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Old Jul 28, 2009, 12:58 AM
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Xpress..'s Avatar
United States, CA, Alpine
Joined Oct 2007
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Regarding that plane, you're probably going to need to run a 14" prop or so, say a 14x8 prop.

Regarding watts, firstly, you select your prop range you can use, then we can help you using the watts per pound rule. For pushers, 150 watts per pound is plenty for sporting around, and good strong climbs. 200 watts per pound wih the proper prop will send the plane vertically.
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Old Jul 28, 2009, 07:31 AM
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Ken Myers's Avatar
Commerce Township, MI
Joined Aug 2001
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Why aren't RPMs and prop size ever listed in electric motor specs??

They are if you look at Scorpion motors at http://www.innov8tivedesigns.com, because Lucien Miller tests each motor that he sells and posts the results. He is, honestly, the only motor provider that I know of, that gives a whoop about us modelers. The other folks would be just as happy selling us toasters or TVs.

This is an example, but NOT for your big pusher, just an example.

The specs are here:
http://www.innov8tivedesigns.com/pro...roducts_id=520

The tests are here:
http://innov8tivedesigns.com/Scorpio...20%20Specs.htm

The drawings are here:
http://innov8tivedesigns.com/images/SII-3014%20ASM.pdf

No one else, that I know of, puts so much effort and care into presenting their motor information to us.

This is why so many folks are using Scorpion motors nowadays.

Pop in on the Scorpion Motors thread in the Power Systems section and ask Lucien what would be an appropriate motor for your plane. He doesn't have all the "big" motors tested yet, but he'll make a great and useful recommendation.
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Old Jul 28, 2009, 10:20 PM
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Morgan City/Patterson, Louisiana, United States
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Quote:
Originally Posted by Xpress..
Regarding that plane, you're probably going to need to run a 14" prop or so, say a 14x8 prop.

Regarding watts, firstly, you select your prop range you can use, then we can help you using the watts per pound rule. For pushers, 150 watts per pound is plenty for sporting around, and good strong climbs. 200 watts per pound wih the proper prop will send the plane vertically.

A 14" prop will be the largest that I can run due to ground clearence. Based on my experinece with gas turbine powered models I'm shooting for a pitch speed of 125 mph. A 14 X 8 needs 16,500, 14 X 10 needs 13,200 rpms and 11,000 for a 14 X 12. Sounds like 2,000 to 2,400 watts will be needed as the plane will be 10-12 pounds based on what others have ended up with. Motor and battery selection should be based on these numbers.
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Old Jul 30, 2009, 07:52 AM
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RPM and prop size are not listed commonly because they totally depend on what battery you use. However, Scorpion, Hyperion and a few others publish test result tables for different voltage/prop combinations.

The better way is to use Web-o-calc, Drive Calculator or Mototcalc and do the calculation based on the motor and battery parameters.

In this particular case, I'm figuring a Hyperion ZS4025 on 6s or 7s, and the problem boils down to picking a motor model from that series and a matching prop. Browsing Hyperion's data, it looks like a ZS4025-10 and a 13x10e should be within limits on 7s and turn somewhere in the region of 12000 RPM at 105A, 2500W... that's a back-of-the-envelope extrapolation, though.
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Old Jul 30, 2009, 11:29 AM
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First off, unless you measure the rpm of your setup, even with IC, you really don't know what you have. And a static measure of rpm/power is not exactly what you get in the air since static tests are not the same as dynamic.

What you have done over the years is determine what setups fly well and then relate that to mfg specs.

With electric you will do the same kind of thing, but, there are many more variables to consider. Basic motor specs show an rpm/v and max current. Some will show a max voltage as well. But, like different mixes of glow fuel, different batteries will give different performance with the same motor. Add in the different props and you can see there is a lot more to experiment with.

So, we pick something in the ball park and then fine tune it. To do the initial selection you can just assume some losses, like 80% for the prop and 80 or 90% for the motor. So, for 150mph pitch speed, you will 'need' 16,500 rpm from the motor (80% for the prop eff), then 19krpm for the motor (85% eff). Using a 4s LiPo that means a working voltage of around 14.8V gives a required rpm/v of about 1300.

The power required would be a second calculation to determine the current you would need. You can use your IC experience, ie, what power did you get? Hp converts to kW. Say 1hp. Thats 735W. At 14.8V that's 50Amps.

So, you need a motor that is higher than 1300kV and is capable of 50A current on 15V or more. Add a bit more just for 'fudge' factors.

The motor calc programs will try to predict your aircraft drag to determine the power requirement for your app. They are also a good place to start.

charlie

PS If you shop for the cheap motors, expect lower performance, mainly due to poorer quality bearings. Same for cheaper batteries.
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Old Jul 30, 2009, 05:43 PM
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Morgan City/Patterson, Louisiana, United States
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Quote:
Originally Posted by portablevcb
First off, unless you measure the rpm of your setup, even with IC, you really don't know what you have. And a static measure of rpm/power is not exactly what you get in the air since static tests are not the same as dynamic.

What you have done over the years is determine what setups fly well and then relate that to mfg specs.

With electric you will do the same kind of thing, but, there are many more variables to consider. Basic motor specs show an rpm/v and max current. Some will show a max voltage as well. But, like different mixes of glow fuel, different batteries will give different performance with the same motor. Add in the different props and you can see there is a lot more to experiment with.

So, we pick something in the ball park and then fine tune it. To do the initial selection you can just assume some losses, like 80% for the prop and 80 or 90% for the motor. So, for 150mph pitch speed, you will 'need' 16,500 rpm from the motor (80% for the prop eff), then 19krpm for the motor (85% eff). Using a 4s LiPo that means a working voltage of around 14.8V gives a required rpm/v of about 1300.

The power required would be a second calculation to determine the current you would need. You can use your IC experience, ie, what power did you get? Hp converts to kW. Say 1hp. Thats 735W. At 14.8V that's 50Amps.

So, you need a motor that is higher than 1300kV and is capable of 50A current on 15V or more. Add a bit more just for 'fudge' factors.

The motor calc programs will try to predict your aircraft drag to determine the power requirement for your app. They are also a good place to start.

charlie

PS If you shop for the cheap motors, expect lower performance, mainly due to poorer quality bearings. Same for cheaper batteries.


All you've wrtitten makes sense. The amount of variables in electric setups is unbeleaveable. Problem is I know very little about electrics and I don't have time, energy or funds to do unlimited testing of set ups. I'm needing recommendations on specific motor/battery combinations.
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Old Jul 30, 2009, 07:03 PM
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Joined Dec 2005
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Have to agree with Ken!!
Lucien and his staff, are really super. I called asking questions. The fellow spent 10 minutes talking to me. I asked the dumbest questions, I got straight answers.

I wish all manufactures or distributors would do what Lucien does.

I am pretty bad with math and for me to do math problems, well let us say that me and math, we don't get along. Being dyslixic and spelling challenged to say the least, when I do math, I might come up with some crazy stuff. So I ask many questions.
I know one fellow that only runs Scorpion motors and speed controls.
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Old Aug 01, 2009, 12:58 AM
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Morgan City/Patterson, Louisiana, United States
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Ok, I dug through Innovative's site and found the performance calculator - it's in the download section. So it looks like a Scorpion 4035-460 on 8s, right at 100 amps and an APC 11 X 8.5E prop will give me 16,485 rpms which translates into a pitch speed of 132 mph and just under 14 pounds of thrust. APC 10 X 10E at just over 100 amps is 155 mph and just under 13 pounds of thrust. I know rpms and thrust are theoretical but this will get me in the ball park.
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Old Aug 01, 2009, 01:53 AM
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In the case of electrics, theoretical means 'within about 10%'. Well good enough for practical purposes, in other words. That's going to be rather impressive. Make sure and use big enough batteries to get a reasonable flight time.
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Old Aug 01, 2009, 03:27 AM
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Morgan City/Patterson, Louisiana, United States
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Quote:
Originally Posted by Andrew McGregor
In the case of electrics, theoretical means 'within about 10%'. Well good enough for practical purposes, in other words. That's going to be rather impressive. Make sure and use big enough batteries to get a reasonable flight time.
Yep it should be impressive. Both the 10 X 10E and 11 X 8.5E should spin in the neighborhood of 16,000 rpms and that should make some impressive prop noise. Within 10% is more than adequate for what I was looking for. I forgot to mention that I'll be using Scorpion's 130 amp high voltage ESC and a pair of 5,000 mah 30C 4S packs. I should have several minutes in the air, depending on right stick discipline.
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