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Brushless Motor Help

Old 03-17-2009, 04:37 AM
  #1  
Hutchie
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Default Brushless Motor Help

Hi all,

I have been out of the Model RC game for a few years and have decided to get back into it once again. In this time some new technology has taken off and left me behind. I have done considerable reading and feel like im up with the whole brushless motor scene but i still have one query with regards to inrunners and outrunners.

I am building a model which will come in at around 550grams and only has a small 6.5 X 4 prop running direct drive. A speed 400 brushless inrunner (4000Kv) was the first option that came up and seemed like a good idea. But then i noticed the huge range of brushless outrunners out there, and started to wonder. Although outrunners have higher torque and can direct drive larger props, would an equivilent speed 400 outrunner work well with only a small 6.5" prop? If i were to go with an outrunner over the inrunner would i be potentially losing top-end RPM?

I liked the option of the outrunner because they seem to be taking over a little and are the recomended motors on many new models(it would be good if the motor could stay usefull into the future), they also seem a little cheaper. But as i am staying away from a gearbox i dont want to lose out on power.

Any thoughts or suggestions would be much appreciated.

Andrew.
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Old 03-17-2009, 05:44 AM
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gyrocptr
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Could provide some details regarding the type of flight performance that you're targeting? High speed, jet-like? Aerobatic? 3D aerobatic? Gentle-cruiser? Do you have an idea of what your wing-loading will be (oz per sq. ft.)?
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Old 03-17-2009, 06:12 AM
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Hutchie
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Cheers for the reply.

Required performance would be good park flyer with moderate aerobatics, but certainly not Racing or 3D. Wing loading i think will be about 13 oz per square ft.

*When i did that wing loading calculation i regreted being an Aussie who works in grams and centimeters not ounces and feet. Grrrr.

Andrew.
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Old 03-17-2009, 11:06 AM
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Sky Sharkster
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Default Innie Or Outie?

Hello Andrew, Welcome to Wattflyer!
Here's a link to a similar question from last year;
http://www.wattflyer.com/forums/showthread.php?t=21355
In addition to my simplified answer there, the links provide a much more in-depth analysis to your question.
For the type of model you've described, I would suggest a much-lower Kv outrunner, 1000 Kv or thereabouts.
A 4000 Kv inrunner (without gearing) will only be happy at high RPM and with a very small prop. This, in turn, will mean a high prop speed, which translates to high airspeed. Basically, it would be best suited to a small, light racing-type plane.
For mild aerobatics and "Sport" flying, you would (normally) use a larger diameter prop at lower RPM. Either a low Kv outrunner or higher-Kv geared inrunner.
Perhaps it may help to think of a non-geared 4000 Kv inrunner as being like 4th gear in your car. Lots of top-end speed, but little acceleration and no low-speed torque.
Hope this helps, Good Luck!
Ron
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Old 03-17-2009, 11:38 AM
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Hutchie
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Thanks very much for the info. The outrunner sounds like a good option. Just one clarification though, when you say use a lower Kv outrunner with a larger prop, are you refering to a prop larger than i specified that the model recomends(6X4). Many of the outrunners ive looked at talk about much larger prop sizes. Would a 6X4 still be suited to around a 1000Kv outrunner?

The knowledge has been much appreciated and glad to be back online, this is a great site.

Andrew.
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Old 03-17-2009, 12:24 PM
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Sky Sharkster
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Default Prop Size?

Hello Andrew,
I didn't realize there was a specific prop size needed for the model. If this is the case, you will need to check the specs for the available motors.
The two main factors in determining prop size are Kv and input voltage; disregarding gearing for the moment. If, for example, you're using a 2s (2 cell) LiPO, the voltage would be 7.4v (3.7v per cell) and a certain size prop (for a given Kv motor) would be recommended.
But, if you decide to use a 3s (3 cell, 11.1v) LiPo on the same motor, the recommended prop would be smaller. Higher voltage, smaller prop.
So, yes, I would guess the motor would have a higher Kv than 1000, but it also depends on the particular motor and your intended battery voltage.
Here is an example of a motor that would use a small prop (direct drive) and have enough power for a 550-600 gram model.
This is a racing-style motor, an inrunner of 2000 Kv. The amp draw will be high and require a high capacity battery + ESC.
http://www.e-fliterc.com/Products/Te...rodID=EFLM2000
Since you have a small prop requirement, you will have to work back from the prop and adjust the Kv and input voltage to accomodate this. High Kv (smaller prop, high RPM) and high voltage (more cells) each reduce the useable prop size.
But the trade-off is higher rpm, more speed and greater amp draw (higher capacity and "C" rating for the battery pack).
Personally, I believe you would be better off with a larger prop (if there's a way to fit it) and lower Kv motor. Lower RPM/airspeeds, more torque, less amp draw.
It's all a big balancing act, Kv, RPM, Amp Draw, prop size, Input voltage. With the small prop, you're going towards the "racing, high rpm, high amp-draw" end of the spectrum.
Here's a free online motor calc. If you input a certain size motor and adjust the prop size and voltage, it may help to clear all these variables up. http://brantuas.com/ezcalc/dma1.asp
Ron
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Old 03-17-2009, 12:30 PM
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No you would not want to use a very small prop like a 6x4 on an outrunner at 1000Kv. That would result in very low power.

Can we be clear here ? Are you actually physically restricted to using a small prop like that ? If so you will need a higher Kv, probably nearer 2500. Exactly what diameter can you actually get in ?

But if it is physically possible to use a larger prop then larger props are generally more efficient so that's what is normally recommended.

Steve
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Old 03-17-2009, 07:59 PM
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kyleservicetech
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What ever you do, try to obtain, beg or borrow a meter capable of measuring the current pulled out of the battery by your battery pack, ESC and motor. In the USA, the Astroflight Whattmeter is a good one, but more exist world wide.

In electric power, it is quite easy to overload these motors with to big of a propeller for the battery voltage and motor used. That will result in short motor/battery life, or even much smoke. Or if you use to small of a prop, the models flight may be disappointing.

With measuring the actual current pulled, you can select between propellers to give the performance you need. Just a note, buy good stuff in these electric models, cheap motors/ESC's/Batteries may cause disappointment.

Check out "motocalc" on the internet for software that allows you to pick a motor and battery pack and propeller size, based on your model airplane. This software is free for the first 30 days or so, then to keep using it, you need your credit card. A number of different software programs are available, to make decisions somewhat easier on this electric stuff.

I've been using the Hacker series of motors, all work very well, and all are very well made. Two of my models use the Hacker A50-12S motor, which runs on six series, two parallel A123 packs, that pulls 57 Amperes at 18 Volts. This is just about 1 1/4 horsepower, I'm on the second season with these motors, they all work just like brand new. Hacker also makes much smaller outrunner motors, in the "A" class.

Brush type motors are far less expensive, but they have a much lower efficiency, resulting in much shorter flying time, like 1/2 or so.
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