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Author Topic: Motor choices...  (Read 326 times)
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kaintuck
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« on: February 15, 2021, 08:58:17 AM »

I'm STILL electric ignorant  Huh...so I'm in the planning stages on the power package for my 52" Hudson bomber....here's my choices, I have a matched set of geared 370's....and I have brushless....the geared would be cool in sound...but what should I plan for?
The ESC is another 15gm....
https://ibb.co/r3sSr4L

Please tell me what's best?...I'll be turning 3blade props, like 6" x 3.5 or  6" x 4...the plane should weigh under 3lb...I HOPE!,!
Marc
Motor choices...
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Robmoff
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« Reply #1 on: February 15, 2021, 12:49:02 PM »

That is not far off Twinstar size (remember them, I do because I have one waiting to maiden). 370s might be OK for that, it was designed for ungeared 400's, so you should be in the same ballpark. Though it was redesigned for twin brushless for more performance, I for one don't really see the point of a scale airliner (or WW2 era bomber for that matter) doing unlimited verticals.
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lincoln
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« Reply #2 on: February 15, 2021, 10:26:48 PM »

A lot depends on how much the model will weigh and what performance you want. I'm sure some people could build this light enough for a pair of those old IPS motors. 3 lbs sounds very heavy if you're building it stick and tissue style. I seem to recall people saying that the old Comet 54 inch models could be built down to 7 ounces or so, which would probably be ok with just one IPS. However, you're talking about much smaller props, more weight, and I'd guess less wing area. That means more power, even if the setup is as efficient as you can get at that prop size. Note that gear ratios and kv are important here. If you mounted as many IPS motors as you could fit, all geared for 12 inch props, you could probably taxi up a 45 degree runway, but never take off from a flat one. ;-)



I suspect, if it's anywhere near 3 lbs., you may need 70 watts per lb.  The old GWS models, like the Pico Stick, as I recall, flew fine, if slowly, at something like 30 watts per lb.

I used to have an electrified glider that climbed at a fairly steep angle at 45 watts per lb., but it was low drag,  weighed 11 ounces and had a 50 inch span.

There was an on line calculator for electric setups that I used to use, but it's gone.

Keep in mind that I've never had a model similar to yours. My electrics have been gliders or light and slow.
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kaintuck
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« Reply #3 on: February 16, 2021, 06:51:20 AM »

70 watts per lb, I guess the canned motors are nice...maybe for a cub type build....and in my ignorance, I thought canned motors didn't need a controller. They are already adding 20more grams with less power....
I have 4 brushless, I will see which has more power and get a mate...then see who make lightest esc's, and get a matched set.
I haven't even 'laid her keel' yet....so i still like to talk and you fellows educate me....
https://ibb.co/k1GWXjN

I'm trying to keep her light....and having fun plannng the build. I don't want to shed a wing in flight Shocked....but we be thinking 1/2 comet talyorcraft and 1/2 electric type building!
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kaintuck
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« Reply #4 on: February 18, 2021, 03:48:23 PM »

I have one motor and esc....I tested it and she runs fine. Some reviews are saying this should be ok at lower props than listed, which should be fine for my build. I want to run 3blade props, 6" dia max....
https://ibb.co/vzbZ1Qw https://ibb.co/qF56MVY
I will just get another motor/esc....and I may be in biz!
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lincoln
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« Reply #5 on: February 19, 2021, 03:54:39 AM »

If you use the 2822 with a 6 inch prop, you will get MUCH less thrust than is shown on the chart unless you go to 4s or 5s, but in that case I wouldn't count on the motor surviving. The 2812 wouldn't lose as much thrust when dropping from a 7 inch to a 6 inch prop,  so maybe that would be ok. But in that case, you'd be buying 2 motors and might as well get something that has data for a 6 inch prop.

I'm guessing that the geared 370's are meant for much larger props. It seems possible using them direct drive might work if you kept everything light, and possibly with 2s, but this is the kind of thing I'd use that defunct on-line calculator for. Brushed motors use a different sort of ESC than brushless ones do, but you can also use a switch instead. Years ago, cheapskates would use a servo and a toggle switch.I would only use a switch on a modestly powered model, though power can be reduced by using a smaller prop.

People manage to build similar designs at wildly different weights. The lighter they are, the less strength and stiffness they need. I suspect that, if you use heavy, brushed ferrite motors like those 370's you will need extra strength in the wing between the motors and the fuselage. Then again, the brushless motor you show isn't much lighter.
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kaintuck
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« Reply #6 on: February 19, 2021, 01:30:30 PM »

You say this motor wouldn't survive long pulling a 6x3 three blade prop? I don't understand...is there a better choice to use? I have one cf-2812/14 and a a2212/13t motors also....
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TimWescott
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« Reply #7 on: February 19, 2021, 02:15:49 PM »

You say this motor wouldn't survive long pulling a 6x3 three blade prop? I don't understand...is there a better choice to use? I have one cf-2812/14 and a a2212/13t motors also....

He's saying that with three cells, it won't turn a 6-3 fast enough to blow a butterfly off of a teflon pan.  And if you do use a 6-3, you need at least one more cell.  You may be able to get by with more pitch and three cells, but see below.

I disagree that you'll get into trouble that way -- the motor is rated for a max current of 15A, and the basic construction will clearly sustain higher RPM (look at the ratings for the CF2812).  You should be able to get about 10k RPM at full throttle with four cells; you'll just need to get an ESC that'll work at that voltage.

The smaller diameter prop will be less efficient, so you'll need more power for the same thrust.  If you really end up at a 3-pound build weight, I think I'd give the CF2822 a try, but be mentally prepared to need more -- and you should count on experimenting with props.  It sounds like your diameter is constrained -- if I were you, and a 7" prop were just barely too big and a 6" way too small, I'd seriously considering reworking 7" props down to whatever diameter does fit.

I'd also make sure I had a way of measuring current into the ESC on the ground.  (A Watt Meter is probably best).  At home, find a propeller that makes it pull close to 15A (or whatever your motor's top rated current is) at full throttle, then go see how it flies.  Be sure to bring lower-pitch props to the field.  If it's a dog with that prop, then you're already at maximum capability and you just need a different motor or more cells.  If you're afraid you'll rip the wings off going above half throttle, then reduce the prop pitch.

If you do change the number of cells, be aware that that baseline "maximum current" prop changes, too -- DC motors "want" to go at a speed determined by the motor characteristics and the battery voltage (that's the "Kv" rating).  They'll do that even if it means pulling enough current to burn up themselves (or the ESC or the battery).  So if you bump up a cell, do that ground test again, starting with less pitch.

I'd definitely go brushless, BTW.  Brushed motors are just less efficient than brushless ones (really, motors with cheap ceramic magnets are less efficient than ones with rare-earth magnets, but that's basically the same thing as brushed vs. brushless these days).  I'd also go a bit big on the ESC, especially if you're going with a cheap brand.  If you're expecting to pull 15A on a regular basis, get an ESC that's good for at least 20A.
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lincoln
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« Reply #8 on: February 20, 2021, 05:47:39 AM »

Just to make things complicated, last time I checked, you could still find cobalt, brushed motors on eBay. Also, I don't know if they're common, but I once saw a listing for a pair of rare earth magnets intended to replace ferrite ones in an electric motor, possibly Speed 400 size. I don't know what this does to the kv or other specs.

---------
Using a 6-3, you'd need much higher voltage and rpm, beyond the specs shown. So I wouldn't count on them holding up. I don't know if thin wire is better or worse than thick for withstanding the forces of high rpm's. I suspect it's worse. If I recall correctly, a higher kv winding needs more turns and will probably need finer wire for those turns to fit.
« Last Edit: February 20, 2021, 05:58:33 AM by lincoln » Logged
dputt7
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« Reply #9 on: February 20, 2021, 06:14:32 AM »

  Not wanting to get into your Motor / Prop discussion, why don't you just move the nacelles out a 1/2" or even 3/4" depending on how it looks, then you could run some decent size props.  Just a thought.
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Yak 52
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« Reply #10 on: February 20, 2021, 07:08:28 AM »

Some thrust test data for your A2212/13:

https://www.flybrushless.com/motor/view/206

(This is a great site for motor comparison)

I wouldn't personally choose to under prop by too much - you'd just be carrying weight you don't need. If you want to go smaller on props then you can go larger on Kv. The 2212/6's take a 6" prop if I recall but they are better suited to fast 'parkjet' style flying.

I think there's some scope for brushed geared motors like the old GWS type on a model like this - the advantage is you can use a single ESC (DC current) which might be lighter. But the best would probably be brushless motors that are well matched for weight/power/Kv.

Everything depends on the wingloading!
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TimWescott
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« Reply #11 on: February 20, 2021, 03:10:09 PM »

Using a 6-3, you'd need much higher voltage and rpm, beyond the specs shown.

Likely true, although you can push the specs (more on that below).  I suspect that the correct 6" diameter 3-blade prop, for those motors, would have a higher pitch than 3".

I know some control line guys from the Internet who fly the classic 1/2-A designs on the classic 1/2-A prop sizes (6-3 or 5.5-4), spinning at 15,000 RPM or more.  If I had a concern with that particular motor design shown, it'd be the cheezy spigot-and-adaptor motor mount that those use.  There's certainly stouter motors in the same size that I'd trust 100%.

I don't know if thin wire is better or worse than thick for withstanding the forces of high rpm's. I suspect it's worse.

The wires aren't in the part that's turning in a brushless motor.  The worry at high RPM is the bearings, and if you go to insane lengths to solve that, the bell either bursting or deforming enough to rub the magnets against the stator.  Or perhaps, with the motors pictured, the mount falling apart.  None of those is going to happen going from 7500 RPM to 10000 RPM.

If I recall correctly, a higher kv winding needs more turns and will probably need finer wire for those turns to fit.

It's the opposite.  With an ideal, no-loss motor, the motor spins until the back-EMF of the winding opposes the voltage applied by the ESC, which, in turn, is roughly equal to the battery voltage.  The voltage per turn per RPM is constant, so more turns mean a lower Kv (and a higher torque for the current applied, because the torque per amp per turn is roughly constant).

Decent motor designs will fill the available space with wire, so if you go to fewer turns you use thicker wire, and visa-versa.

If you look in a serious technical motor catalog each data sheet will be for a series of motors that all have exactly the same mechanical layout, just with more or fewer turns.  As the motor's rated voltage goes up, its rated current goes down proportionally, because if you fill the available space with wire more turns does, indeed, mean smaller wire.  It turns out that for any given mechanical design the practical power you can get out of the motor is constant; the only thing you can vary by rewinding it is the voltage at which you'll achieve that practical power.

This is enough of a thing (from a pro source, not hobby motors) that it's a common figure of merit to rate a motor's mechanical design by torque out per watts dissipated.

Pretty much the two limiting factors on a small DC motor are the amount of current you can put through it without it burning up and how fast you can spin it before it breaks.  It's really that simple.  With bigger motors you get into limitations on how high a voltage you can apply to it before you get spark-over, but that's not an issue unless you're dealing with motors in factory machinery.
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kaintuck
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« Reply #12 on: February 24, 2021, 04:59:40 PM »

dputt, I will look at that...haven't thought about doing that!....yak and other guru's~ for some reason I cannot retain electrical info in my pea-brain......grew up the age of cox motors I guess........if im to understand: a certain dia of 3blade prop turns to fast, it burns the motor up. if I get a 3blade with small diameter but higher pitch, I will make the motor happy with lower rpm's, but may make the plane fly to fast?

My diameter will be 7"- 8" max...(I think if I move the engine nicel some) master airscrew have 3 blades: 9x7 8x6 or 7x4 or 6x4. the larger dia could be cut down~ so what should I try???

I can make a test box...mount the prop/motor/batt combo and see how it pulls, and if she burns out~

im still in the planning stage..........and if I may ask~ what motor prop would you guys get if you were building this bird?
marc
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lincoln
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« Reply #13 on: February 24, 2021, 11:54:06 PM »

I forgot that the wires in a brushless motor don't turn, and I got kv backwards.

For bench testing, make sure the motor has enough air blowing past it.
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TimWescott
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« Reply #14 on: February 26, 2021, 12:51:02 PM »

...if im to understand: a certain dia of 3blade prop turns to fast, it burns the motor up. if I get a 3blade with small diameter but higher pitch, I will make the motor happy with lower rpm's, but may make the plane fly to fast?

If you put any prop on an electric motor that's too large, it'll burn the motor up.  Electric motors are like the Little Engine that Could with abusive parents.  Give them too large of a job, and they'll kill themselves trying.

Using a 3-blade prop that'll fit, with a pitch that'll make enough thrust to take off won't make an RC plane fly too fast -- it'll just mean that you have to throttle down after takeoff, just like a scale plane.  The only problem with solving your issues with more pitch is that at some point the propeller will stall on the ground, and then you won't ever get off the ground.

You can always get more blade area by getting a much larger diameter prop and cutting it down -- that'll increase your ground thrust without increasing the pitch.  It may be inefficient in flight, but you're not going to be setting up a profit-making airline with this thing.

Personally, I'd stick with scale nacelle locations, but I'm a glutton for punishment.

You can:

  • Use a 3-blade prop (I strongly recommend APC, but if Master Airscrew floats you boat, do that.  Get an electric prop if you can)
  • Adjust the pitch (by showing up at the field with a variety of props) for more ground thrust
  • Adjust the blade width for more ground thrust ("how?" you ask -- by getting a much larger diameter prop and cutting it down -- voila! paddle blades
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