Need some help comparing (and understanding) brushless motors

[SkyEye]

Hi all,

I'm in search of a "C pack" compatible motor to to use on my C pack swappable. For reference, I fly the FT P40, FT Bloody Baron, and FT Simple scout, and I'll be adding an FT Mig-3 to the collection soon.

I have been using FliteTest's 2218/1180kv Radial for years and am thoroughly impressed with its performance, however I need an alternative solution that I can source on Amazon, and hopefully for cheaper (budget ~$20). I am coming to understand that I won't be able to get the same performance of the 1180kv Radial, but I would like to get as close as I can. You can't really beat the $30 Radial, but I wanna see how close I can get.

Weight isn't exactly the biggest constraint for me, but I would like the motor's output and draw be at least comparable to the Radial. I want to be able to swap the motors around my 'fleet' with little-to-no reconfiguring.

Here are the motors I am looking at so far. I understand how to interpret stater size, KV, amp and volt ratings but it gets messy for me when I'm looking at it all together. Also, an unfortunate reality is that Amazon vendors don't often give the most detailed specs, but this is what I have found. Guidance and/or a recommendation would be much appreciated.

1. Goolsky Original Surpass High Performance 2216 (2834) 1120KV - https://www.amazon.com/dp/B0799HRLGZ/?tag=lstir-20
Watt: 380W
Voltage range: 7-15V
Max amps: 26A
NOTE: There is an 1180kv variant of this motor as well, which adds to my confusion

2. Diamond Dynamics 2835(2216) 1200KV motor - https://www.amazon.com/dp/B09BYXHJLG/?tag=lstir-20
Watt: 295W
Voltage range: 7.4-14.8V
Recommended ESC 30A

3. Diamond Dynamics 3530 1100KV - https://www.amazon.com/dp/B09BZ75KC7/?tag=lstir-20
Watt: 313W
Voltage range: 7.4-14.8V
Recommended ESC 50A

4. FlashHobby D2836 1120KV - https://www.amazon.com/dp/B08M9K3VS8/?tag=lstir-20
Watt: 336W
Voltage: 11.1

Final comment. I understand the 3530 is much beefier (larger stater size) but why does its specs seem similar?

 

[Merv]

...Final comment. I understand the 3530 is much beefier (larger stater size) but why does its specs seem similar?

A lot of manufacturers cheet, the 3530 motor is likely measured from the outside of the can not from the stator as it should be. It likely has a 2830 stator.

Any motor with similar volts, watts & kv will give similar performance, atleast for us mere mortals. The high performance racing guys demand high performance motors. If you are one of them, go for it.

 

[SkyEye]

Thanks for the tip. I won't be needing ultra high performance. Ultimately, I want this motor to be decent, but easily replaceable. I'm not sure if you would consider the Radials high performance.

Follow up question. I am sort of leaning towards the Goolsky 2216/1120KV, but it has a specification that says its max amperage of 26A. This is low comparatively to the 1180KV Radial, which tops out at 35A. I'm confused on how to interpret this.

 

[Merv]

...but it has a specification that says its max amperage of 26A. This is low comparatively to the 1180KV Radial, which tops out at 35A. I'm confused on how to interpret this.

Volts × amps = watts
It would appear to be a lower watt motor.

In liquid fuel engines power is measured in horse power. In electric motors, power is measured in watts.
The engineers tell me it takes about 746 watts to equal 1 HP.

 

[Piotrsko]

Yup close enough on the watts/hp. Two thoughts: watts DOES NOT equal thrust. They help spin the prop, but don't determine efficiency particularly if the tips go sonic.

How about an extra cell in the battery? Watts are volts times amps so more volts is more watts. Most of my ESCs tolerate an extra cell, but they are cheap enough anymore to just replace. And I tend to not fly at full power always so heat isn't generally an issue

 

[Merv]

...I'm not sure if you would consider the Radials high performance...

The radials are fine motors. When I mentioned high performance motors, I was mostly referring to quads guys.

Here is an example of a guy who uses high performance motors. If you want to get deep in the weeds motor performance, this is your guy.

 

[Merv]

...Two thoughts: watts DOES NOT equal thrust...

I totally agree, when you are discussing thrust. The first thing to ask is at what speed do you want the thrust.

Take 2 motors that have the same wattage, one low kv & one high kv, both using suitable props for maximum performance.
The low kv motor will always produce more thrust on a test stand & the high kv motor will always fly faster. Both are producing the same output, just at different speeds.

This is similar to a tractor & a race car, both with the same horse power. The tractor will always win pulling a heavy load & the race car will always go faster. The speed at which you want the thrust matters.

 

[Bricks]

If one really wants to go down a rabbit hole lets talk prop tip speed.

 

[SkyEye]

Thank you all for your responses.

Yup close enough on the watts/hp. Two thoughts: watts DOES NOT equal thrust. They help spin the prop, but don't determine efficiency particularly if the tips go sonic.

How about an extra cell in the battery? Watts are volts times amps so more volts is more watts. Most of my ESCs tolerate an extra cell, but they are cheap enough anymore to just replace. And I tend to not fly at full power always so heat isn't generally an issue

I am definitely open to adding a cell. I'm looking at a motor that is rated for 15V, so it should take a 4S. I will run it first through the power analyzer and make the motor isn't getting hot.

The radials are fine motors. When I mentioned high performance motors, I was mostly referring to quads guys.

Here is an example of a guy who uses high performance motors. If you want to get deep in the weeds motor performance, this is your guy.

I totally agree, when you are discussing thrust. The first thing to ask is at what speed do you want the thrust.

Take 2 motors that have the same wattage, one low kv & one high kv, both using suitable props for maximum performance.
The low kv motor will always produce more thrust on a test stand & the high kv motor will always fly faster. Both are producing the same output, just at different speeds.

This is similar to a tractor & a race car, both with the same horse power. The tractor will always win pulling a heavy load & the race car will always go faster. The speed at which you want the thrust matters.

I'll check him out. I am only starting to understand the low/high KV difference that you're explaining, but doesn't high thrust at low speed turn into high speed? I get so confused thinking about this stuff.

If one really wants to go down a rabbit hole lets talk prop tip speed.

I am a graduating Aerospace Engineer and have had my struggles with calculating prop tip speed and all the power and thrust coefficients. For the life of me I can't even understand/explain the 'cookie cutter' RC calculations, so it'll be some time before I can fathom the real physics . Any guidance, is appreciated.

Ideally, I'd like a motor with a thrust chart, which proves to me that I can use a certain propeller to achieve a certain thrust that can lift my aircraft.

Since I am going with Amazon motors and am not getting such a thrust chart, how can I guarantee that I am getting a powerful enough motor? To complicate things, we don't have drag coefficients for these planes...

 

[leaded50]

this stuff aint easy to see or understand. I dont know, but relate to my experience.
Kw isnt a main for whats best.
High Kw - "less torq" - higher speed , cant take as high pitch propeller. OR
Low Kw - "more torq" - can take higher pich propeller, and then get the higher speed.
Thats if both motors are similar size, both use similar battery, just Kw shanged. If size of engine is different, or battery voltage, then the end effect will also be different, whatever Kw motors have.

I found that at a plane at eg. 900g - 1200g ready flying with battery, electronics eg. i feel at least 300W is what i want. lower, can get trouble take off from ground. 1500g - 2000g = needs approx 500W + , All depending on type of plane. I dont see reason to make a overpowered "screamer" in eg. a Cessna-style plane, but in a fast style plane - yes!.
Close to every motor im using is also rec. for min. 4s use, even if using just 3s battery in some. Then i can regulate eg. max power by what V im using.

 

[Merv]

...but doesn't high thrust at low speed turn into high speed? ...

Not for the same amount of energy. A large volume of air at low speed produces more thrust per unit of energy than a smaller volume of air at high speed. If you input enough energy you can make a high thrust high speed system, but it will always cost you in efficiency.

With kv, think car transmission. Consider 2 motors with the same watts (power). A low kv motor with a large diameter low pitch prop is going to move a large volume of air slowly. Lot's of thrust but limited top speed. Similar to a low gear in a car, lots of lugging power, limited top speed.

A high kv motor will have a smaller diameter prop & likely a higher pitch prop. It will move a smaller volume of air much faster. Limited thrust but higher top speed. Similar to high gear in a transmission. Less lugging power but higher top speed.

 

[Piotrsko]

Motor 101 (kinda) Tip speed: diameter of prop in inches divided by 12 or feet times pi times rpm times forward speed. At around 750 mph you generally break the sound barrier or on a 10" diameter prop above 10,000 rpm. Influenced by altitude, and temperature. You'll know, the prop gets very much louder. Pitch is the theoretical distance a prop should move forward with every rotation same as screw pitch. A low rpm motor tends to swing a longer prop which obviously makes more thrust. High rpm motors turn faster so more distance in a given time period but don't have as much torque so need a shorter prop but goes faster.

Double Hmm just figured out why 737 max jet engines don't don't exceed the speed of sound in the intake.

 

[SkyEye]

this stuff aint easy to see or understand. I dont know, but relate to my experience.
Kw isnt a main for whats best.
High Kw - "less torq" - higher speed , cant take as high pitch propeller. OR
Low Kw - "more torq" - can take higher pich propeller, and then get the higher speed.
Thats if both motors are similar size, both use similar battery, just Kw shanged. If size of engine is different, or battery voltage, then the end effect will also be different, whatever Kw motors have.

I found that at a plane at eg. 900g - 1200g ready flying with battery, electronics eg. i feel at least 300W is what i want. lower, can get trouble take off from ground. 1500g - 2000g = needs approx 500W + , All depending on type of plane. I dont see reason to make a overpowered "screamer" in eg. a Cessna-style plane, but in a fast style plane - yes!.
Close to every motor im using is also rec. for min. 4s use, even if using just 3s battery in some. Then i can regulate eg. max power by what V im using.

Thanks for this. I feel like you all have given foundational working knowledge, and now I can make relatively educated decisions.

Not for the same amount of energy. A large volume of air at low speed produces more thrust per unit of energy than a smaller volume of air at high speed. If you input enough energy you can make a high thrust high speed system, but it will always cost you in efficiency.

With kv, think car transmission. Consider 2 motors with the same watts (power). A low kv motor with a large diameter low pitch prop is going to move a large volume of air slowly. Lot's of thrust but limited top speed. Similar to a low gear in a car, lots of lugging power, limited top speed.

A high kv motor will have a smaller diameter prop & likely a higher pitch prop. It will move a smaller volume of air much faster. Limited thrust but higher top speed. Similar to high gear in a transmission. Less lugging power but higher top speed.

That certainly helps me understand that a lot bit better.

Motor 101 (kinda) Tip speed: diameter of prop in inches divided by 12 or feet times pi times rpm times forward speed. At around 750 mph you generally break the sound barrier or on a 10" diameter prop above 10,000 rpm. Influenced by altitude, and temperature. You'll know, the prop gets very much louder. Pitch is the theoretical distance a prop should move forward with every rotation same as screw pitch. A low rpm motor tends to swing a longer prop which obviously makes more thrust. High rpm motors turn faster so more distance in a given time period but don't have as much torque so need a shorter prop but goes faster.

Double Hmm just figured out why 737 max jet engines don't don't exceed the speed of sound in the intake.

Good to know. I never would've thought that RC prop tip speeds got transonic but that makes a ton of sense.

To start I'm going to go with the Goolsky 2216 (2834) 1120KV that I mentioned above. Its rated for max 380W, but only 26A, which is a little concerning. My FT P-40s takeoff weight is ~900g. I'll run it through my power analyzer first to check the amperage, fly it and check for excessive heat. For now.

I'm guessing that higher drag like flaps/airbrakes/external payload would add to the power draw on the motor? Is there a way I can estimate what my max power draw is going to be without flying?

 

[DamoRC]

I'm guessing that higher drag like flaps/airbrakes/external payload would add to the power draw on the motor? Is there a way I can estimate what my max power draw is going to be without flying?

Imagine you added so much drag and weight to the plane that it flew like a workbench. Meaning that if you test the motor/prop/esc/battery combo safely attached to your bench you will likely see the max power the motor will draw.

If you look at one of my old posts on speed testing you’ll see that the motor/prop combo drew slightly more current on the bench than it did in the air.

 

[SkyEye]

Imagine you added so much drag and weight to the plane that it flew like a workbench. Meaning that if you test the motor/prop/esc/battery combo safely attached to your bench you will likely see the max power the motor will draw.

If you look at one of my old posts on speed testing you’ll see that the motor/prop combo drew slightly more current on the bench than it did in the air.

I will give that a shot.

Will running my aircraft at full throttle on a start-up bench suffice?

 

[Piotrsko]

for testing, where you mount it doesn't matter: bolted to a bench or the nose of a plane. The only advantage to the bench is: pretty sure you dont have enough thrust to move the bench very far. I have had planes get away from me doing a thrust test, gets really ugly really fast in the house, sometimes just messes up foamboard outside. Otherwise, static thrust, voltage drop or maximum current won't change a whole lot. An airplane will never have the drag of a workbench which is a good thing although I have seen some come close