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How do I 'calculate' the correct prop load for the motor?

Old 03-12-2009, 12:35 PM
  #1  
DarZeelon
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Default How do I 'calculate' the correct prop load for the motor?

So far, with brushless setups I used and recommended to others, I took suggestions from other people and from other fora, regarding the prop selection...

I would like to know how to calculate the 'target RPM' from the known facts; such as the motor's kV and the input voltage.


Assuming the kV specifications of the brushless motor is correct (925) and that the 3S Li-Po pack supplies under maximum load (25C) is ~10V, the calculated RPM is 9,250...

The motor actually spins 7,500-8,300 RPM; depending on the prop selected.

It does not seem to overheat (it is warm just after running; not scalding hot).


What is the importance of the calculated RPM and what actual RPM should I be aiming for?

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Old 03-12-2009, 01:04 PM
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slipstick
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I've never heard of anyone looking for a "target RPM". We're normally much more interested in motor current and how the plane flies.

Assuming the current is o.k. for all your props then you need to consider what type of performance you want which will tell you if you want a large diameter, low pitch prop or a smaller diameter, higher pitch, both of which might have exactly the same loading and produce exactly the same RPM but very different flying characteristics.

In my experience you generally see around 80-90% of the notional RPM for most useful props. Since all yours are within that range any of them should work. Much below about 80% on a good motor sounds like an overload but the current is usually getting too high for comfort anyway and that's what I'll be watching not the percentage of notional RPM.

Steve
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Old 03-12-2009, 01:45 PM
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DarZeelon
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Default Prop selection

Thank you, Steve.

This is in fact what I have done in practice, but what I am asking for is a more 'formal', recommended MO for this...



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Old 03-12-2009, 04:08 PM
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slipstick
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Don't think I can help you I'm afraid, I largely gave up hoping for a "calculation" that will tell me everything years ago .

But if you're serious you first need to decide exactly what it is that you are trying to optimise. There are a lot of possibilities and some of them are mutually exclusive. E.g. you will not get maximum power at the propeller and maximum system efficiency at the same time.

Good luck, it could be an interesting discussion to follow.

Steve
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Old 03-12-2009, 05:08 PM
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DarZeelon
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Default Calculating...

Thank you anyway, Steve.

It is just that many fliers; especially those running small motors, that have a rather high kV, don't set them up to turn anything even close to 80% of the 'notional RPM', yet none are running into any type of problem...

As if ignoring the kV is harmless...

This may not be true, as far as huge brushless motors go.


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Old 03-12-2009, 08:39 PM
  #6  
eflight-ray
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The 'kv' number quoted for electric motors is the no-load figure, i.e. no propeller fitted.

Fliers can fit all sort of different propellers to exactly the same motor depending on what flight characteristic they require from the plane.
Large diameter fine pitch or small diameter, coarse pitch props can draw the same current, but give totally different flights.

In the same way a lot of fliers use inrunner motors with gearboxes, different ratios and different size propellers.

Then of course there is the different number of cells that fliers wants to use, (voltage).

All these means it is very difficult to lay down design rules. Prop rpm allied to pitch can give an indication of expected flight speed, but then again it depend on the type of model.
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Old 03-12-2009, 08:57 PM
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LectricPlane
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lots of good info here:
http://www.wattflyer.com/forums/showthread.php?t=27019
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Old 03-13-2009, 02:09 AM
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Dr Kiwi
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If your motor can spin a prop at 70% or better of Kv x v... then you are in the right ballpark... if you are down at 50-60%... you are over-loading the motor and generating heat rather than useful rpm... if you are into the high 80's or 90's you are either flying a high speed pylon plane, or you need a bigger prop to make the most use of your motor's capability.
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Old 03-13-2009, 08:47 PM
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ron_van_sommeren
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Mabye Prop Calculator can help you a bit Dar:
"Prop Calculator computes the performance data of propellers with a given geometry, notably in-flight thrust and power drain across the utilizable airspeed range."
www.drivecalc.de
-> Prop calculator (bottom of page, English&German)

The man behind Prop Calculator is Helmut Schenk, who's also the man behind Drive Calculator. More e-flight calculators.

How's MVVS electric doing in Israel?

Prettig weekend Ron
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Old 03-13-2009, 11:18 PM
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DarZeelon
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Default Calculating...

Thank you, Dr. Kiwi.

Ron, I expected to see you respond here.

Thank you for the links. They do express the idea slightly better than before.

The '75% rule' give the express solution.



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