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Author Topic: Stupid question time  (Read 4061 times)

Offline Randy Powell

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Stupid question time
« on: October 24, 2016, 11:28:58 AM »
What is the benefit, if any, of a 35mm motor over a 28mm motor given the same Kv?
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Offline Mark Scarborough

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Re: Stupid question time
« Reply #1 on: October 24, 2016, 11:51:49 AM »
I think you will find that typically the 35 mm motor will have a higher listed current rating for continuos use.

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Offline Randy Powell

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Re: Stupid question time
« Reply #2 on: October 24, 2016, 11:58:21 AM »
Yea. I was thinking in terms of load. I have a 28mm motor and a 35mm motor. The 35 is quite a bit heavier and I was trying to decide if the 28mm had enough juice to carry the plane with a 5s battery. The thing is going to be nose heavy as it is. But I'd sorta like the additional power. The plane is a bit porky. Either with fit.
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Online Crist Rigotti

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Re: Stupid question time
« Reply #3 on: October 24, 2016, 12:47:16 PM »
The 35 will run cooler.
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Offline Tim Wescott

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Re: Stupid question time
« Reply #4 on: October 24, 2016, 01:12:00 PM »
The 35 will run cooler.

A smart-assed engineer could build a 28 that would refute that claim, but it would have to be longer, and probably heavier, than the 35 motor it competes with.  I doubt there are any such critters on the market.

Randy, it's not a question of "does it have the juice?" or "can it make the power?"  -- the answer to both is "yes".  The question is "how hot will it get while doing so?", and the answer is "more than a bigger, heavier motor."

I really wish that we had a herd of science-minded teenagers on tap here, because I think that another set of questions to ask (and get the answer to) is "how much advantage is there to more aggressive cooling measures?" and "what's the most efficient way to cool an electric motor in a model going 55 miles per hour?".  But -- I don't have time.  It'd be a good science fair entry, or extra-credit project for a high-school physics class, or possibly even a good chunk of a senior-year project if you happen to be doing aeronautics or mechanical engineering.
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Offline Igor Burger

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Re: Stupid question time
« Reply #5 on: October 24, 2016, 01:17:26 PM »
Depends on length, it can be that short larger motor will withstand less current then long 28mm. Weight gives usually better orientation. Heavier will give more power.

IF the length is the same, then that larger will have lower internal resistance, better radiation and therefore will withstand higher current, probably will have also higher no load current.

Offline Randy Powell

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Re: Stupid question time
« Reply #6 on: October 24, 2016, 03:36:14 PM »
I originally thought this 28mm I have would be a good motor, but then I thought that it was likely with the size of the plane and the weight that I would probably burn it up. Thus the thought of putting a 35mm motor in. There's room. The plane is about 610 squares and will likely be about 60oz ready to fly or so. Both motors are about 770kv (well one is 760 and the other 780).
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Offline Mark Scarborough

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Re: Stupid question time
« Reply #7 on: October 24, 2016, 03:53:45 PM »
I originally thought this 28mm I have would be a good motor, but then I thought that it was likely with the size of the plane and the weight that I would probably burn it up. Thus the thought of putting a 35mm motor in. There's room. The plane is about 610 squares and will likely be about 60oz ready to fly or so. Both motors are about 770kv (well one is 760 and the other 780).
My 109 stunter flew on an Eflight 25 and 4 S, it was 650 squares and 63ounces as I recall.

It was 870 KV though, and I seem to recall Mike H saying the 25 did not like a 5S setup,, so it might not be a good way to go, but the 35  mm at that KV should be ok I would think. Whats the rated power continuos ( watts or amps)
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Offline Larry Renger

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Re: Stupid question time
« Reply #8 on: October 24, 2016, 09:20:11 PM »
Oh, no, here I go with one of those abhorred "well I really don't know" things.  :X

 I suspect that the bigger diameter gives more torque at any given power setting. So it requires less current to drive the prop at a given rpm. Thus lower heat.

Be free to shoot me down on this!  HB~>
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Offline Randy Powell

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Re: Stupid question time
« Reply #9 on: November 01, 2016, 12:04:47 PM »
Well, I picked up a 35mm motor to put in - and it doesn't fit. Too long. So, I will modify the mount to accept an EFlite 32 as it is a bit shorter and will fit in the hole. Fun things, these toy planes.
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Offline William DeMauro

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Re: Stupid question time
« Reply #10 on: November 01, 2016, 05:11:29 PM »
Randy,
  Not a stupid question and I'm jumping in here late. When you said 35 mm, when I started reading this I thought you meant width not length, as did most of the other respondents it seems. The first number in the size of the motor usually means width. The second number means length. BUT then different manufactures measure from different places, For example, a          Turnigy 35-48 is roughly the same size as an axi 2826 or Cobra 2826 or an E flite Power 25. Now a Cobra 3520 is a shorter but wider motor with approximately the same outside dimensions as the Power 32  that you mentioned. Now the more I write the more confusing this can get, so I feel to make sure that everyone stays on the same page, It may be best to always try to remember to mention the Exact brand and motor size when asking questions.
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Online Crist Rigotti

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Re: Stupid question time
« Reply #11 on: November 01, 2016, 05:35:09 PM »
What was the question about Randy?  Length or Diameter?  I still stand behind my statement that a cobra 35 series will run cooler than the 28 series.  Trust me I know.
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Offline William DeMauro

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Re: Stupid question time
« Reply #12 on: November 02, 2016, 08:57:23 AM »
I agree with Crist.
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Offline Randy Powell

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Re: Stupid question time
« Reply #13 on: November 02, 2016, 02:26:10 PM »
I was initially looking at a Cobra 2826/12 or a Cobra 3520/12. I set it up for the 3520/12 figuring either would fit then. Thus the question. I also picked up a 3525/10 mostly because it was a bit lower kv (820 versus 780) but he 3525 is about 3/8" longer and weighs another 80g compared to the 3520/12. It doesn't fit and is too heavy. Fine. Could go with the 3520/12 or just modify the mount slightly (elongating the holes a bit) and use an E-Flite 32 (I have a box of those). It's the same length as the 3520 and about the same weight. And I know them a lot better.

The question here was more is there any benefit to going with a larger diameter motor - what are the benefits and drawbacks. I had a 2826 that I've never used and thought it might be a better choice to keep the weight down - but only if it would work. You guys convinced me that a largely diameter motor would be a better choice so I started looking at what I could use. Decided that going with a set up I have experience with is a better choice.

I'm still learning this stuff and can use all the help I can get.
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Offline Joseph Daly

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Re: Stupid question time
« Reply #14 on: November 02, 2016, 05:42:17 PM »
I have used both 28 and 35 series motors and the 35 series motors run cooler for what ever reason. I have used both 3515 and 3520 motors both with 4s and 6s batteries. The 3520 with a 700sq in, 72 to 74 oz plane with no power issues what so ever. I also have used the 3515 with a 600 sq in, 60oz plane with no power issues what so ever. And I have even used it to pull around a 64 oz plane with no issues.

Online Mike Alimov

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Re: Stupid question time
« Reply #15 on: November 02, 2016, 07:01:55 PM »
Randy, i have used Cobra 2826/12 to pull my testbed - TF Score (680 sq in, 65 oz), close to max length lines at 5.3 sec lap, and measured temp upon landing using laser thermometer-  its within acceptable range.  610 in/60 oz should be a piece of cake for that motor.  5s battery, start with APC 12x6 prop.

Offline pmackenzie

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Re: Stupid question time
« Reply #16 on: November 02, 2016, 07:29:59 PM »
Oh, no, here I go with one of those abhorred "well I really don't know" things.  :X

 I suspect that the bigger diameter gives more torque at any given power setting. So it requires less current to drive the prop at a given rpm. Thus lower heat.

Be free to shoot me down on this!  HB~>

Torque constant ( torque output versus current) and the Kv are inversely proportional, so two motors with the same Kv will have the same torque per amp of current.

This does not take into account any second order effects like losses and any non-linearity, but should be fairly close since in F2B we don't operate at the extremes of motor performance.

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