Solved Please help me sort this motor question

[Monte.C]

I do think I understand the basics in a very general way. The first number represents the physical size of the motor - this varies among manufacturers but can be used to compare one motor to another in a relative way. The second number is a representation of RPM per rotational force applied. I guess. Can you help me translate this to a real world application?

I have two motors, both Emax. A 2213-935KV and a 2206-1500KV.
I had a stik/trainer that I ran on an 1806-2280KV A-type motor, based on comparison with the Mighty Mini series planes. It flew great on 3S, seemed just right for me, just learning to fly. The plane I'm setting up now is comparable to that first plane but a little bigger. 34" wingspan and pretty light, certainly not heavy. It has lots of wing.

Since this is just a little bit more plane, and based on reports here on the forum, I'm thinking it won't hurt to go up a size. (My throttle thumb isn't in the habit of getting stuck in the full-on position.)

The first number for these motors is very similar, but they have very different dimensions. The KV ratings are different. For real: Are these overkill for my plane? Do I need to step down to a 20XX-size motor? Which might be more suitable? Is it like... one is "hotter" and one is more torque-y? The 2206 runs faster, the 2213 is more brutish, like a lower gear. This doesn't give me a solid answer. I'm not looking to go real fast. Can anybody help me understand better?

 

[Hondo76251]

I try to find the spec sheets for any given motor i plan to use. I generally go off of what the estimated thrust with a given prop is.

I also tend to like (tall?) Motors. They always seem to make more power in a compact size for the builds i like to do. Ive really liked my 1308s and my 1808s but they are harder to find.

 

[Monte.C]

I try to find the spec sheets for any given motor i plan to use. I generally go off of what the estimated thrust with a given prop is.

I also tend to like (tall?) Motors. They always seem to make more power in a compact size for the builds i like to do. Ive really liked my 1308s and my 1808s but they are harder to find.

Oh yeah, spec sheets. Do I have some sort of brain blockage? Gotta check the charts.

 

[FlamingRCAirplanes]

Oh yeah, spec sheets. Do I have some sort of brain blockage? Gotta check the charts.

The taller motor will provide more low speed thrust, so you could put a 10x4.7 slowfly prop on there and it would work fine, just not very fast, now the smaller motor would take a 7-8 inch prop and wouldn’t make as much thrust, but would have a lot higher max speed, so use it for a racer, or, put a 9x4 slowfly prop on there and it will act somewhat similar to the bigger one

 

[Hondo76251]

Easier to find on some motors,

Had to find a blog of some guy testing stuff to get stats for the 1808 2400kv's i like. They're sweet on the smaller FT stuff. About 700 grams of thrust with a 5 or 6" lower pitch prop. That puts them at about the same power as the Chineseum 2212 2200kv's you see everywhere with quite a bit smaller/lighter package and very quiet, especially when i use them with the avan long range props wich i think are 6x3?

 

[Hondo76251]

I generally dont fly too aggressively so i like the middle ground between slow fly and acrobat. That said, so long as you can get the motor amd battery to balance in the airframe there's no such thing as too much power! Lol

 

[Monte.C]

I generally dont fly too aggressively so i like the middle ground between slow fly and acrobat. That said, so long as you can get the motor amd battery to balance in the airframe there's no such thing as too much power! Lol

That's the kind of advice I've been reading - I think mostly from @SSgt Duramax. Can't argue with that. And I'm on the slower half of the spectrum too.
I don't even know if I have props @9"+. We'll see, but to maiden I might just have to underprop the plane.
I guess I won't stress about 22XX motors. No need to be scared and try to stretch an 18XX to it's top end.

 

[SSgt Duramax]

That's the kind of advice I've been reading - I think mostly from @SSgt Duramax. Can't argue with that. And I'm on the slower half of the spectrum too.
I don't even know if I have props @9"+. We'll see, but to maiden I might just have to underprop the plane.
I guess I won't stress about 22XX motors. No need to be scared and try to stretch an 18XX to it's top end.

Yeah, but I do try to pick the right motor for what I am trying to achieve. I could throw twin 3536s on a couple of my planes but I don't, because it doesn't need them. I kind of just go a step up from the size recommended and match my KV to desired prop size.

The way I like to talk about KV, is kind of the way I like to talk about horsepower in car motors. It is kind how RPM and horsepower relate in car motors. You can have two motors with the same horsepower (wattage) that do completely different things.

A 2207 2100kv makes 350 watts and a 2217 880kv makes only 320 watts. However, the 2217 will happily spin a 11 inch prop on 3s, which would destroy the 2207 motor.

Kind of like the 3.0L supercharged V6 is my car makes somewhere around 500hp and a Cummins X-15 about the same horsepower with it's 15.0 I6. So these two engines are the same right? No, one is good at high revs and making my car go fast, and one is good at pulling 100,000lbs of junk down the road. You wouldn't want the car engine in the semi truck, and you wouldn't want the semi engine in my car.

When I say stuff like this, I always have to allow for the fact that someone will say something stupid like:

If your geared your car engine really low, it would technically be able to pull 100,000lbs

A semi truck engine would work great in a F1 car if not for the weight

Technically if you did a gear reduction/belt drive on the F pack motor it would be able to spin a 11 inch prop

NO! STOP WITH THE LUNACY! When someone asks what a good motor to pull their 50 inch wingspan trainer with a 11 inch prop you don't recommend a freaking F Pack motor with a gear reduction set up do you? You recommend an adequately sized motor I bet.

If I were to ask you what motor I should swap into my 1980 Peterbuilt which had 250hp, you would recommend a F20C out of a S2000 would you?

Use some common sense! /rant




Seriously if you just remember big motor low RPM for "towing" a big plane slow and small motor high RPM for little plan fast you are good.

 

[Monte.C]

Yeah, but I do try to pick the right motor for what I am trying to achieve. I could throw twin 3536s on a couple of my planes but I don't, because it doesn't need them. I kind of just go a step up from the size recommended and match my KV to desired prop size.

The way I like to talk about KV, is kind of the way I like to talk about horsepower in car motors. It is kind how RPM and horsepower relate in car motors. You can have two motors with the same horsepower (wattage) that do completely different things.

A 2207 2100kv makes 350 watts and a 2217 880kv makes only 320 watts. However, the 2217 will happily spin a 11 inch prop on 3s, which would destroy the 2207 motor.

Kind of like the 3.0L supercharged V6 is my car makes somewhere around 500hp and a Cummins X-15 about the same horsepower with it's 15.0 I6. So these two engines are the same right? No, one is good at high revs and making my car go fast, and one is good at pulling 100,000lbs of junk down the road. You wouldn't want the car engine in the semi truck, and you wouldn't want the semi engine in my car.

When I say stuff like this, I always have to allow for the fact that someone will say something stupid like:







NO! STOP WITH THE LUNACY! When someone asks what a good motor to pull their 50 inch wingspan trainer with a 11 inch prop you don't recommend a freaking F Pack motor with a gear reduction set up do you? You recommend an adequately sized motor I bet.

If I were to ask you what motor I should swap into my 1980 Peterbuilt which had 250hp, you would recommend a F20C out of a S2000 would you?

Use some common sense! /rant




Seriously if you just remember big motor low RPM for "towing" a big plane slow and small motor high RPM for little plan fast you are good.

You're hurting my ears.

Well technically...

 

[Monte.C]

Yeah, but I do try to pick the right motor for what I am trying to achieve. I could throw twin 3536s on a couple of my planes but I don't, because it doesn't need them. I kind of just go a step up from the size recommended and match my KV to desired prop size.

The way I like to talk about KV, is kind of the way I like to talk about horsepower in car motors. It is kind how RPM and horsepower relate in car motors. You can have two motors with the same horsepower (wattage) that do completely different things.

A 2207 2100kv makes 350 watts and a 2217 880kv makes only 320 watts. However, the 2217 will happily spin a 11 inch prop on 3s, which would destroy the 2207 motor.

Kind of like the 3.0L supercharged V6 is my car makes somewhere around 500hp and a Cummins X-15 about the same horsepower with it's 15.0 I6. So these two engines are the same right? No, one is good at high revs and making my car go fast, and one is good at pulling 100,000lbs of junk down the road. You wouldn't want the car engine in the semi truck, and you wouldn't want the semi engine in my car.

When I say stuff like this, I always have to allow for the fact that someone will say something stupid like:







NO! STOP WITH THE LUNACY! When someone asks what a good motor to pull their 50 inch wingspan trainer with a 11 inch prop you don't recommend a freaking F Pack motor with a gear reduction set up do you? You recommend an adequately sized motor I bet.

If I were to ask you what motor I should swap into my 1980 Peterbuilt which had 250hp, you would recommend a F20C out of a S2000 would you?

Use some common sense! /rant




Seriously if you just remember big motor low RPM for "towing" a big plane slow and small motor high RPM for little plan fast you are good.

I think that might be part of the difference between being "smart" because you memorized a lot of stuff (ahem... chemistry) and understanding & applying (engineering). Well technically, with a gear reduction... I can SGR you so hard.

 

[SSgt Duramax]

I think that might be part of the difference between being "smart" because you memorized a lot of stuff (ahem... chemistry) and understanding & applying (engineering). Well technically, with a gear reduction... I can SGR you so hard.

So to answer your question... the 2212 would be good for a 9-10" prop and the 2206 would be good for a 7-8 inch prop. Sorry for the rant, it had been bottled up in there for a while.

Of course you could get a watt meter and sit there and agonize about finding the perfect prop and agonize over the last percentage left on the table, but whats the point. High KV is high revs, low torque, and low KV is low revs and high torque.

90% of the time my guiding principle is to ish get the motor size right, then look at what the years of experience running said motor have resulted in, which gives me a range of props to work with, and do that.

I turbo'd my miata and built a megasquirt for it when I decided I wanted insane amounts of power instead of trying to put some fancy engine in it. For the specific reason that thousands of others had done it successfully. A lot of the time, trying to do something because it is "different" and "it will technically work" is a good recipe for a pile of broken parts and an unfinished project.

My dragon has two of those 2306 1500kv motors like you have, and everyone runs a 7x6 or 8x5 on them, so that is what props I am going to go with once I find out whether I want more speed/efficiency or static thrust. Right now it came with 7x4s which are underpropped.

 

[SSgt Duramax]

Oh, and another thing, the RC car world talks about "turns" in the motor, which I think is probably more relevant than KV (although it results in the same).

More turns equals more torque and lower rpm (lower KV motor) and less turns equals a lower torque and higher rpm motor. That to me spells out the difference in about 10 seconds. Put a 540 sized high KV motor (low turn) in a rock crawler and you are smoking something. Put a low KV in and it does just fine. Put that same low KV motor in my rustler it will pop a wheely or spin it's tires, and go 20 mph.

 

[Hondo76251]

Yeah, would sure be nice if there was an easier way to compare motors but it is what it is i guess. Kind of why i like to just look stats and get a ballpark of what to expect. I have no desire to set up the stand and watt meter myself but for most common items someone has already posted data on it.

I do like to ponder the theoretical side occasionally, not that it matters with my style of flying but fun to think about. As i said, i prefer tall moters. I generally get the tallest motor for a given bell diameter i can find. More of a fitment thing for me but i see now that the opposite seems to be more popular. I figure its the drone guys doing that. Maybe they fly hard enough to feel a difference? Who knows

 

[Hondo76251]

Funny, its really more the prop in our world that makes the difference. Any given prop spun faster will make more thrust (up to a point) and any given prop will require (X) amount of power to make (Y) amount of thrust and somewhere in there is a "sweet spot" of max thrust with minimum power required to achieve a given speed. I'd almost prefer to see a chart built like that for props so i could pick the prop size and performance level required for a build then look for a motor to match rather than vice versa 🤔

 

[Monte.C]

Guys I really do appreciate the discussion. Selecting motors (and props) isn't my favorite part of this, but I'm getting a better feel for what's going on. You've reassured me that there IS some guesswork involved. Try it and see, according to some guidelines. One thing I know for sure: if the motor's getting too hot I have to change something.

 

[chrisvdv]

@Monte.C have a look at this. Maybe of some help.

 

[chrisvdv]

This as well

 

[Hondo76251]

Once you start getting an idea what props work, then start throwing 3 blades into the mix lol!

 

[Monte.C]

Once you start getting an idea what props work, then start throwing 3 blades into the mix lol!
View attachment 224641

I want to! I've been in the habit of running 3 blade props on my smaller planes, like 26"-32" and my Widgeon (high motors), but my planes are approaching a more reasonable size, the last couple are 34" wingspan, low motors, and 3 blades would guarantee the full force of the landing on a blade.

I like the smaller diameter of a 3 blade prop, and the looks of it too.

 

[Hondo76251]

Same, mostly because they're more available on the smaller stuff.

But 2 blades always seems to be more efficient at any size. I dont need the low end power, the grip as they say, that the 3 blades give the quads.