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Author Topic: Question about motor Kv, batteries and props  (Read 1230 times)

Teodorico Terry

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Question about motor Kv, batteries and props
« on: October 15, 2019, 07:12:30 PM »
Hello,

A few years ago the R/C club decided to add a C/L field to our location.  Surprisingly, we have a few club members who have become regular C/L flyers once again.  I flew C/L as a kid and even after graduating from college I continued to fly for a few years and even participated in a few contests.  I currently fly R/C with mostly a focus on pattern which now happens to be just about 100% electric.  I went sober back in 2007 (i.e. gave up glow engines and glow fuel) and I have been flying exclusively electric since then.

I am currently putting together a modified SIG fazer  (moved the stab 2.5 inches back, greater span stab and elevators but of narrower cord, different rudder, right wing panel shortened by 1.5 inches and narrower flaps and adding tips to the wing) as my first C/L build in a long time.

I am planning on going electric and I was surprised to see relatively high Kv motors (ie. the Cobra 2820/12 970Kv) as being fairly common along with relatively small diameter props (i.e. 11" or less) and low pitch (say 4 or 5).  I was thinking about going with a 13" prop pitched at 6.5 on a Hacker A30XL-12 running on 4S or something similar.  In my experience, a larger diameter prop spinning at lower RPM's is more efficient than a smaller prop at higher RPM's for the same input power.  I have not run the numbers yet but I will be shooting for a pitch speed of about 55-60 mph which will probably translate to around 50 mph when you add the drag of the lines and model.  I am guessing that the current draw will be in the 25 amp range or so.

The question is, am I way off the mark since I am taking a somewhat different tack?

Thanks,

Teo
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Offline Ken Culbertson

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Re: Question about motor Kv, batteries and props
« Reply #1 on: October 15, 2019, 07:45:42 PM »
Quote from: Teodorico Terry on October 15, 2019, 07:12:30 PM
Hello,

A few years ago the R/C club decided to add a C/L field to our location.  Surprisingly, we have a few club members who have become regular C/L flyers once again.  I flew C/L as a kid and even after graduating from college I continued to fly for a few years and even participated in a few contests.  I currently fly R/C with mostly a focus on pattern which now happens to be just about 100% electric.  I went sober back in 2007 (i.e. gave up glow engines and glow fuel) and I have been flying exclusively electric since then.

I am currently putting together a modified SIG fazer  (moved the stab 2.5 inches back, greater span stab and elevators but of narrower cord, different rudder, right wing panel shortened by 1.5 inches and narrower flaps and adding tips to the wing) as my first C/L build in a long time.

I am planning on going electric and I was surprised to see relatively high Kv motors (ie. the Cobra 2820/12 970Kv) as being fairly common along with relatively small diameter props (i.e. 11" or less) and low pitch (say 4 or 5).  I was thinking about going with a 13" prop pitched at 6.5 on a Hacker A30XL-12 running on 4S or something similar.  In my experience, a larger diameter prop spinning at lower RPM's is more efficient than a smaller prop at higher RPM's for the same input power.  I have not run the numbers yet but I will be shooting for a pitch speed of about 55-60 mph which will probably translate to around 50 mph when you add the drag of the lines and model.  I am guessing that the current draw will be in the 25 amp range or so.

The question is, am I way off the mark since I am taking a somewhat different tack?

Thanks,

Teo
You mentioned the Cobra 2820/12 which is my weapon of choice for a .46 sized ship.  In CL that equates to a mid 500 sq" ship around 48-52 oz ready to fly with battery.  I run mine around 8,600-8,800 rpm on a 4s 2800 battery.  11-6 two or three blade prop.  I have tried larger props and lower ROM's and smaller props and higher rpm's.  Settled on the 11-6.

Glad to have you back in CL.

Ken
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Offline Brent Williams

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Re: Question about motor Kv, batteries and props
« Reply #2 on: October 15, 2019, 10:19:14 PM »
The Cobra 2820/12 970Kv gets used in so many planes because it is very capable and of good quality.  It has a good record of success in planes ranging from 500-620+ square inches.

As an example of the upper side of the 2820 application window, Martin Quartim (Brazil) uses one in his 51oz (battery installed) Pathfinder ARF.  4s2700, KR timer, APC 11x5.5, ~65ft lines (handle to centerline) 

I used the same motor / prop combo in a 45oz plane and it was very strong.  For most medium sized/weight planes it is a great unit.

Martin's Pathfinder.

« Last Edit: October 16, 2019, 10:37:26 AM by Brent Williams »
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Teodorico Terry

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Re: Question about motor Kv, batteries and props
« Reply #3 on: October 16, 2019, 05:58:36 AM »
Hi -

Thanks for the info, I did look up the motors online and they are relatively in-expensive so they might be worth a try.  I was thinking of setting up a simple test bench for the motor,  ESC and timer so that I can learn to operate them together.  I have a Hacker A30-10XL wich has a Kv of 900 so I might play with that one first and then decide where to take it from there.  The intent is to use a Castle ESC set-up in governor mode which will be a first for me. 

Thanks,

Teo
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Offline Mike Alimov

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Re: Question about motor Kv, batteries and props
« Reply #4 on: October 16, 2019, 07:12:06 AM »
Quote from: Teodorico Terry on October 16, 2019, 05:58:36 AM
I was thinking of setting up a simple test bench for the motor,  ESC and timer so that I can learn to operate them together. 

Teo
A word of caution: when running your setup on a test stand, do NOT use the same size prop as you plan to use for flying; doing so will overload your system in terms of current, and may severely damage motor, ESC, and even battery. For example, I found that for powertrains that normally swing APC 12-6 in the air, the test stand equivalent is APC 10-6.
To make your tests especially insightful, use the Castle Edge Lite line of ESCs, and take advantage of their data logging capability. It can tell you battery voltage, current, rpm, governor gain, mAh consumption, and many other things, at up to 10 Hz sampling rate.
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Teodorico Terry

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Re: Question about motor Kv, batteries and props
« Reply #5 on: October 17, 2019, 10:58:43 AM »
Hi -

Thanks for the reminder.  I normally use a watt meter for testing as well as tach so I will be measuring the current and power.  You are correct that the static readings will be higher than those in flight, as the motor unloads in flight (slightly higher RPM, lower current draw).  The motor will only be running for a few seconds at a time.  I just want to learn how to set-up the ESC correctly and understand the operation of the timer.

Teo
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Offline John Rist

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Re: Question about motor Kv, batteries and props
« Reply #6 on: October 17, 2019, 02:25:08 PM »
Quote from: Teodorico Terry on October 17, 2019, 10:58:43 AM
Hi -

Thanks for the reminder.  I normally use a watt meter for testing as well as tach so I will be measuring the current and power.  You are correct that the static readings will be higher than those in flight, as the motor unloads in flight (slightly higher RPM, lower current draw).  The motor will only be running for a few seconds at a time.  I just want to learn how to set-up the ESC correctly and understand the operation of the timer.

Teo
To run setup (except RPM) I just remove the prop. Doesn't hurt electric to run no load.  The biggest problem (for me) setting up a KR timer is getting the speed controller's low throttle set point in the right place.  If the set point is too high you won't get the burp (letting you know all is ok) when you flip the ON switch.  However this is a function of the ESC not the KR timer.  I use a servo tester to  set up the speed controller.  The one I have can measure the pulse width out of the KR timer and also output the pulse width required to run the ESC.  Looking on eBay I no longer see one that will measure the KR timer output. But it looks like all will drive the ESC. Most ESC setup procedures talk in terms of RC transmitter throttle stick position. The servo tester can take the place of the transmitter.  For the speed controller I simply read the ESC setup procedure and fiddle with it till I get the startup burp.   As it turns out most new ESC factory settings are good to go.  Although I have had some Trunigy Plush ESCs that needed calibrating.
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