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Old May 16, 2014, 10:50 AM
Registered User
Calgary Alberta
Joined Apr 2004
166 Posts
Quote:
Originally Posted by jackerbes View Post
Yeah, we kind gotten focused on a different aspect of the conversation, haven't we? You want to spin those 55mm fans with less current and can settle for less than the thrust they are getting out of them at some higher input levels right?

Do you have a link to any details on how the fans are mounted? The shaft sizes that will fit them the like?

And any info on the thrust to RPM curve they get out of them now? And the minimum amount of thrust you think you can fly with?

With props the input and RPM/thrust are exponential so that last portion of the thrust comes at a high price as far as the amount of input power it consumes. If fans are the same way, backing off on the throttle will give you an appreciable drop in the input power.

If you know or can estimate lower RPM/thrust level that you can get away with it might be easier to answer the question. And I have a fair assortment of motor laying around, I may be able to either loan you a motor to try or even test a fan for you to figure out that RPM/thrust thing.

Jack
I figured it would cause a stir and get way over my head just by the question

I'll grab pics and measurements later today - this part number is for their ill fated arrow - but those appeared to be 3 blade edf's from research... So wondering if its a revision or...

Really most 50mm edf's are good for 10+oz on 3s - I'm looking for say 4oz (now more weight) on preferably 2s...
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Old May 16, 2014, 12:00 PM
Jack
USA, ME, Ellsworth
Joined May 2008
17,215 Posts
OK, but thrust comes with RPM with any kind of prop, including impellers, so having some data on the thrust to RPM curve is invaluable. Any motor that will get it to the same RPM as any other motor does, will produce about the same thrust.

The Thrust Estimator at flybrushless.com has only two impeller in it now as far as I can tell, if you go there and choose the Wemotech 48mm EDF 1.89 x 3.3mm you can see some of the kind of data that I'm talking about.

Looking at the curve plotted for that EDF fan (4 oz. = 114g) it looks like you need 39,000 RPM or so to get that thrust.

Then, if you look at the test data for the fan here:

http://www.flybrushless.com/prop/view/323

You can see it took about 50W or a little less to get it up to about 29,000 RPM. And you can use that to choose a motor that will top out with a full throttle no load RPM that is a little over that, maybe 32,000 or so, and that might put you in the ballpark

A 50W motor, by the 3W per gram rule, would be a 17g motor. And to get 32,000 RPM out of it on 2S (8.2V) you would need to have a Kv of 32,000 / 8.2V or 3900. So the answer would be a 17g 4000 Kv motor. If there is such a thing...

Jack
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Old May 16, 2014, 05:53 PM
Life-abstract=conformity
S.F. Bay Area
Joined Aug 2002
1,890 Posts
Interesting project but why not just fit your existing,airframe purposed smaller dia. fans to the larger nacelles?
Carve,turn or otherwise fabricate some foam bushings (w/appropriate reinforcement as req'd) to center your Df into the nacelle. You could even mount your DF eccentric if that gives you any advantage.
Skies
Jay
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Old May 16, 2014, 07:15 PM
Registered User
Calgary Alberta
Joined Apr 2004
166 Posts
Because the cartoon a10s nacelles are cartoonish they're intentionally oversized and frigging huge...

So a 30mm fan stuffed in a 3 hole looks really funny - 50mm looks closer to 'right'...
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Old May 16, 2014, 07:49 PM
Life-abstract=conformity
S.F. Bay Area
Joined Aug 2002
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Quote:
Originally Posted by AndyCold View Post
So a 30mm fan stuffed in a 3 hole looks really funny
You mean like cartoonish?
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Old May 17, 2014, 09:46 AM
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Calgary Alberta
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Induces a lot of drag too
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Old May 19, 2014, 11:22 AM
MigLovin'
Sanford, FL
Joined Dec 2008
71 Posts
Jack, our claims are in opposition to each other, so naturally someone will be wrong. However, in this case I think the point I was trying to make is that there is no definitive answer (each can be more efficient in different scenarios), so for the sake of this person's thread who is new to all of this, it isn't going to do any good to post that Wye is more efficient than Delta. (Uncle Si's voice from Duck Dynasty): That is clearly your opinion, and not a FACT, Jack!

The thread you requested that I somehow failed to post earlier is here: http://www.rcgroups.com/forums/showt...=372669&page=5

I linked you to page 5 because it has the poster's conclusions, but I enjoyed reading it from the first page.

I did read your thread about that 750 motor that you changed from Delta to Star and got way less amps. The only problem I have is that you did not follow the scientific method very well. You compared your results to the STOCK setup, which is often an incorrectly stated kV thanks to the manufacturer, but more importantly is made using low-quality wire. They can even be off with the number of turns on some poles, and more commonly have sloppy windings. This is especially the case when the supplier is Hobby King, and not someone like Scorpion or MicroDAN. You can almost always re-wind those cheap motors with high quality copper in the same Turns/Termination configuration and get more efficiency from them.

Also, if you've dealt with these little things and the way they come from the factory, you will know that it isn't an easy task to take apart a stock motor and "change the termination" from Delta to Wye. A complete re-wind is almost easier, and is much more worth the time because of the change in quality of copper, as I mentioned a moment ago.

Yes this thread got a little bit off-topic, but we all just come on here to give our two cents, anyway

I hope OP Andy finds this information useful.
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