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Author Topic: What is the right size motor  (Read 576 times)

Offline Ken Culbertson

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What is the right size motor
« on: November 11, 2020, 01:29:15 PM »
Getting ready to start replenishing my motor inventory having lost all of them in the fire.  My collection came from word of mouth and trial and error, not from knowing what I was doing.  This time around I want a stable that is properly fitted to the planes I will have.  The only line I am familiar with is the Cobra.  I had three 2820/12s, one 2826/12 and two 3520/14 in my inventory.  All of these seem to be popular models but are they the right models.  I am uncertain which characteristics I should be looking for.  Every one of these motors is capable of powering a full size PA.  The 2820/12 would need a huge 4s battery but it would pull the ship.  The characteristic I look for first when flying a new motor is how fast it recovers when a load is applied followed next by how much battery it uses.  We  use "rule of thumb" figures of about 700-900 RPM@Volt which I assume is because of our rather low RPM requirements.  What if you go higher or lower?

I know that everything in electric is related, even more so than IC and I don't want to get off on tangents, I just want to get in the right ballpark.  Assuming I stay with Cobra (I am leaning towards a Bad Ass) then I plan for now to go with a 3520/14 - Castle 75 amp ESC for my PA and a 3515/18 - Castle 50 amp ESC for my profile.  Timers are still an unknown.

If there is something already written that answers this point me to it - Ken



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Offline Dennis Adamisin

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Re: What is the right size motor
« Reply #1 on: November 12, 2020, 05:21:36 PM »
Hi Ken
Start with the prop the airplane will need to fly on.  Larger diameters are "thrusty" but smaller diameters fly better.

I usually pick a prop (lean toward smallish diameter) that flies the airplane then pick a motor that balances the airplane.  I usually end up with oversize motors that way, but then I can try to minimize the battery.  In a perfect universe this results in a light power system...

Current bird (720 squares, 67 oz) uses a Brodak 3520 with 4Sx3500 turning a 12x6EP.  Also a Brodak F2B Hornet 60A & Hubin FM-9.  It has WAY to much motor for that prop.  Thus, have option (if I wanted to bother trying) for lighter motor and moving pack forward up to 2" to compensate...

Brodak site has a chart showing their products matched to props.  Can use as a guide for "brand X" products too...
Denny Adamisin
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Offline Dennis Toth

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Re: What is the right size motor
« Reply #2 on: November 14, 2020, 09:41:53 AM »
I like Dennis's approach to sizing the motor by picking the prop. I like to use the smallest diameter prop that will do the job. This approach is not new but was used back when flaps first came out and the ships had shorter tail moments. Large diameter props slow the turn rate and also add lots of load (for e power that's amps). A small well designed prop can give lots of pulling power at a lower amp draw and allows a smaller battery pack.

Best,    DennisT

Offline Ken Culbertson

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Re: What is the right size motor
« Reply #3 on: November 14, 2020, 02:11:49 PM »
I like Dennis's approach to sizing the motor by picking the prop. I like to use the smallest diameter prop that will do the job. This approach is not new but was used back when flaps first came out and the ships had shorter tail moments. Large diameter props slow the turn rate and also add lots of load (for e power that's amps). A small well designed prop can give lots of pulling power at a lower amp draw and allows a smaller battery pack.

Best,    DennisT
I have been planning on an 11 x 5.5 three blade.  Should I start there?

Ken
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Offline Dennis Toth

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Re: What is the right size motor
« Reply #4 on: November 14, 2020, 05:54:55 PM »
Ken,
You might want to check the List Your Setup for a similar size ship. I would think that something around a AXI 2826 -10 at 920kV or similar would be a starting place on a 4S - 3500 -  3800 mah pack.

Best,    DennisT

Offline Ken Culbertson

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Re: What is the right size motor
« Reply #5 on: November 14, 2020, 08:04:49 PM »
Ken,
You might want to check the List Your Setup for a similar size ship. I would think that something around a AXI 2826 -10 at 920kV or similar would be a starting place on a 4S - 3500 -  3800 mah pack.

Best,    DennisT
That will probably work for the profile but it seems a bit small for the PA which will be about 700 sq" and 65oz.  Will the AXI take a 5s?  The Cobra 2826 will.  I have had luck with a 5s 3000 mah in a 3520/14.  My new PA will be about the same as a MaxBee with a slightly larger wing. (my Avatar is the one I am going to build).  What prop would you select to pull such a beast?

Ken 
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Offline Dennis Toth

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Re: What is the right size motor
« Reply #6 on: November 14, 2020, 08:26:37 PM »
Yes, the AXI will take 5S packs. You should trim the prop diameter for a ground amps of around 33 amps with the 4S pack should be good with a 3500 mah. For the larger ship go with the motor size (could be a different brand) recommended by Dennis.

Best,    DennisT

Offline ericrule

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Re: What is the right size motor
« Reply #7 on: November 17, 2020, 11:27:03 AM »
Years ago I had a similar conversation with the folks who used to distribute Hacker motors. Larry Renger was also involved since he knows a whole lot about model airplanes. After listening to a lot of technical stuff Larry and I went for a coke to try to come up with something reasonable that non technical people like me could use in real life. We determined that in order to fly well we needed to have 100 watts of power for every 1 pound of weight. After a number of field tests we upped that to 150 watts of power for each pound of model weight to give us "contest level" power. That number seems to have worked very well.

A simple way of determining prop size and pitch is to use the prop chart Cobra has developed. That chart gives you a prop size and pitch that is very close to perfect. Of course it is only close but at least it gives you a good place to start. The only way to make sure is to flight test it.

Another simple table to use is motor KV to battery. 3 cell batteries need a motor kv of 1100 - 1300, 4 cells need kv of 850-950 while 5 cells need kv of 600 - 700. 3 and 4 cell motors use about 450 mAh per minute of flight while 5 cells use about 368 mAh assuming proper prop loading and leaving 25% battery capacity after the flight.

All of the above data are approximations. Like everything else in life theory dissolves when it comes head to head with reality. Get as close as possible then flight test and adjust.

Offline Ken Culbertson

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Re: What is the right size motor
« Reply #8 on: November 17, 2020, 12:33:51 PM »
Years ago I had a similar conversation with the folks who used to distribute Hacker motors. Larry Renger was also involved since he knows a whole lot about model airplanes. After listening to a lot of technical stuff Larry and I went for a coke to try to come up with something reasonable that non technical people like me could use in real life. We determined that in order to fly well we needed to have 100 watts of power for every 1 pound of weight. After a number of field tests we upped that to 150 watts of power for each pound of model weight to give us "contest level" power. That number seems to have worked very well.

A simple way of determining prop size and pitch is to use the prop chart Cobra has developed. That chart gives you a prop size and pitch that is very close to perfect. Of course it is only close but at least it gives you a good place to start. The only way to make sure is to flight test it.

Another simple table to use is motor KV to battery. 3 cell batteries need a motor kv of 1100 - 1300, 4 cells need kv of 850-950 while 5 cells need kv of 600 - 700. 3 and 4 cell motors use about 450 mAh per minute of flight while 5 cells use about 368 mAh assuming proper prop loading and leaving 25% battery capacity after the flight.

All of the above data are approximations. Like everything else in life theory dissolves when it comes head to head with reality. Get as close as possible then flight test and adjust.
This was VERY helpful and exactly what I was looking for.  The technical jargon puts me to sleep even if I do understand it!

Ken
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Offline GerryG

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Re: What is the right size motor
« Reply #9 on: November 20, 2020, 11:13:39 AM »
My personal experience would support the numbers that Eric has put forward. These numbers (the Mah used) actually explain why my battery drainage is higher than I expected. Out of this I have a question. If I were to go to the flying field with 2 4S batteries and 2 5S batteries, can I use both types on the same plane (ignoring the slight weight difference) and assuming that the ESC and the motor can handle 5S? Or is there something else that I need to know?

Thanks

Gerry

Offline Ken Culbertson

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Re: What is the right size motor
« Reply #10 on: November 20, 2020, 12:23:47 PM »
My personal experience would support the numbers that Eric has put forward. These numbers (the Mah used) actually explain why my battery drainage is higher than I expected. Out of this I have a question. If I were to go to the flying field with 2 4S batteries and 2 5S batteries, can I use both types on the same plane (ignoring the slight weight difference) and assuming that the ESC and the motor can handle 5S? Or is there something else that I need to know?

Thanks

Gerry
Since I have done this my answer is a "Yes But".  It all depends on what your ESC is expecting and if the motor is rated for the higher voltage.   I have, by accident put a 5s battery on a Cobra 2820 thinking it was a 2826 and it did not damage the motor.  The ESC may have stepped the voltage down.

Ken
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Offline John Rist

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Re: What is the right size motor
« Reply #11 on: November 20, 2020, 01:08:05 PM »
My personal experience would support the numbers that Eric has put forward. These numbers (the Mah used) actually explain why my battery drainage is higher than I expected. Out of this I have a question. If I were to go to the flying field with 2 4S batteries and 2 5S batteries, can I use both types on the same plane (ignoring the slight weight difference) and assuming that the ESC and the motor can handle 5S? Or is there something else that I need to know?

Thanks

Gerry


If you are running a constant RPM setup You probably can't tell the difference.  For a given motor and prop running at a fixed rpm it takes a fixed amount of power.  As the voltage goes up current goes down and power level is the same.  This is assuming that the ESC can handle the higher cell count.  The motor doesn't care because the power input to the motor is the same.  All bets are off if you are not running some king of a RPM governing mode.  Another issue is efficacy.  Not smart enough to know the answer to that one.  It has something to do with the motors KV.  As you add cells to fixed setup you are getting more head room.  (Max RPMs vs set RPMs)   Don't know how plays out.
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Offline GerryG

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Re: What is the right size motor
« Reply #12 on: November 20, 2020, 02:17:41 PM »
Thanks for the replies guys. I needed to know about this before the Thunder Power Black Friday specials (assuming there will be some).

Gerry

Offline ericrule

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Re: What is the right size motor
« Reply #13 on: November 21, 2020, 12:33:34 PM »
The fellows are correct that 4S vs 5S should not damage the motor or ESC (assuming the ESC can handle the higher amps). Biggest problem will be due to the difference in the KV on the motor. KV is defined as the maximum rpm per volt of battery. If you have a motor with a KV of 950 (typical 4S) that means 950 x 14.8 = 14,800 maximum potential rpm with no load. If you put the 5S battery on that motor the math becomes 950 X 18.5 = 17,575. You lap times are going to be very fast.

To get a bit technical these numbers are with no load (prop) on the motor. Of course using a prop will add load so you can reduce the "maximum potential rpm" by somewhere in the 16-20% range. If you take into account the manufacturer's efficiency for that motor you can reduce the figure by another 18 - 25 %.

Electric motors are very similar to glow engines in that they "want" to run within a certain rpm range. The best example of this is the OS FP engines. If you try to run an unmodified OS40FP at 9200 rpm with a 10X6 prop your lap speeds will be fast and you are unlikely to get a 4-2-4 break. If you drop down to a 10x4 prop and set the rpm in the 10,000 rpm range where the engine "wants" to run you will get decent lap speeds and constant rpms. Electric motors are the same put a prop on that allows them to run at the proper amp draw with the right rpm setting and plug in the proper size battery and you are good to go.

I have found that putting a Watt Meter between the battery and ESC is the easiest way to determine what size and pitch prop to use. Check the motor's specs to see what the normal amp draw is, set the rpm to the normal range. If you draw the right amount of amps your prop is probably the right diameter and pitch. Too high means you're loading the engine too much. Too low means you're trying to run below the engine's efficiency range.

I apologize for the technical babble but the questions could not be answered any other way.


Offline John Rist

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Re: What is the right size motor
« Reply #14 on: November 22, 2020, 10:58:52 PM »
I have found that putting a Watt Meter between the battery and ESC is the easiest way to determine what size and pitch prop to use. Check the motor's specs to see what the normal amp draw is, set the rpm to the normal range. If you draw the right amount of amps your prop is probably the right diameter and pitch. Too high means you're loading the engine too much. Too low means you're trying to run below the engine's efficiency range.

"set the rpm to the normal range"  I assume this means that you are running a setup that controls RPMs.  How close to the amp draw speck limit is safe and most efficient when running less than full RPMs?   When setting lap time is it reasonable to adjust RPMs as well as prop size?  Also it is my understanding that the motor unloads some during flight.  Is this a lot or a little?  Last question.  Does a 3 blades prop improve efficiency over a 2 bladed prop"  IE. for a fixed lap time will the 3 blade prop draw less current?
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Offline ericrule

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Re: What is the right size motor
« Reply #15 on: Yesterday at 05:00:40 PM »
"set the rpm to the normal range"  I assume this means that you are running a setup that controls RPMs.  How close to the amp draw speck limit is safe and most efficient when running less than full RPMs?   When setting lap time is it reasonable to adjust RPMs as well as prop size?  Also it is my understanding that the motor unloads some during flight.  Is this a lot or a little?  Last question.  Does a 3 blades prop improve efficiency over a 2 bladed prop"  IE. for a fixed lap time will the 3 blade prop draw less current?

I am running a KR Governor which controls the entire flight. All the ESC does is transfer energy from the battery to the motor.

Amp draw on the RSM Black Tiger 3548C motor I am using is 23 amps when the motor is at it's most efficient level of 83%. That equates to a rpm setting of 10,200 with the APC 11x 5.5 reverse pitch prop. Lap time is 5.2 sec using .015 stainless steel 7 strand lines of 62 feet length. I would rather lengthen or shorten the lines to achieve preferred lap times instead of moving the rpm up or down and changing props. Once I have the motor running at it's most efficient level I do not want to change things thereby reducing motor efficiency and using more battery energy.

You are correct that the electric motor "unloads" during flight just like the glow engines do. Best I can estimate is that it increases between 300 -400 rpm (only way to check this estimate for accuracy would be to run an ESC like the Castle Creations that has an internal report feature). Since the KR Governor does a whole lot of nice things like protect the motor from burning up in event of a prop strike and give me what feels and sounds like a 4-2-4 break I do not what to do this.

As far as a 3 blade prop goes having the additional blade means you obtain 1/3rd more pull on the prop compared to a 2 blade prop of the same diameter. Not sure that is going to improve on my current motor runs. It would certainly draw more amps and use more battery energy so that would necessitate a total change in my rpm, prop and line length set up. Not worth the trouble in my considered opinion. Good "Rule of Thumb" is a 2 blade 11" diameter prop is roughly equal to a 3 blade 10" prop. I will use a 3 blade prop if I need a larger diameter than the 2 blade I currently run but I don't have enough ground clearance. John, I freely admit that I am not an expert in the area of props. Perhaps one of our aeronautical engineer members should be consulted. What I have outlined above is based upon my own flight experiences not scientific fact.

Offline GerryG

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Re: What is the right size motor
« Reply #16 on: Yesterday at 08:16:36 PM »
More good info Eric. Thanks. I am currently running my Black Tiger 3548 in a RSM P-51 (sort of a B not a D). I say sort of a B because of problems getting a proper B canopy. How big is the airplane that you have your setup on? I am currently using an APC 11 1/2 x 6 at 9500 RPM. From your info it seems that I can go to a smaller diameter prop to increase the RPM a little and gain efficiency, even on that size of an airplane. I say that size because the 11 x 5.5 is what I use on my Vectors (and at 10,200 RPM). The main difference is that I fly the Vectors on 60 ' lines and the P-51 on 62' lines.
Gerry


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