• This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn more.
Pumpkin drop event

Prop sizing

#1
I know that most designs include a suggested propeller size, but how do you guys size props for your resized and custom designs? Or even just to slap something on to get in the air?

My original though that led to this was "how bad of an idea would it be to put a 5x4x6 multi-rotor prop on a mini scout?" I know that this will mostly be affected by the motor, it would cause the amp draw to skyrocket, and it wouldn't be exactly 3 times the thrust of a 5x4x2 due to efficiency losses (fluid mechanics and all that jazz.)
I really like the look of the cargo planes with the 12 vane props spinning slow and ww2 fighters with the 4 vanes.
 

Wildthing

Well-known member
#2
First when testing different props make sure you have a watt meter so you can see what kind of amp and watt draw is happening. Then if you had a thrust stand you could see what thrust it also creates before you even install it into a plane. Without the thrust stand but after you tested for amp and watt draw and that is ok, with it on the plane you then can do a vertical thrust test and see if it lifts the plane with ease. If you do the vertical thrust test with a proven setup you will be able to compare the results of it's lifting power.
 

FDS

Well-known member
#3
Three bladed props are usually much more current hungry and thus less efficient than 2 blades ones.
Usually you make your design, check the weight then pick the most efficient motor and prop combo that will make the thrust to weight you want.
 

Vimana89

Well-known member
#4
I haven't found those 5x4x6 to be a whole lot worse than 6x3 on a 1806 motor. I'm no expert though. Your plane won't explode. It will look cool, and fly pretty quiet and have more than enough thrust. You might just start overheating and having to come down early, that's the worst I see happening(from experience). Give it a try, those props aren't too far out of the ballpark for what should work, though I'm really not sure they will be too efficient.
 

quorneng

Well-known member
#5
Morgajus
5x4x2 against a 5x4x6?
First it has 3 times the number of blades so with the same motor & battery voltage it will try to draw about 3 times the amps. It is unlikely the motor or ESC will be able to do that without releasing the dreaded 'magic smoke'.
Even if the motor/ESC/battery can deliver the extra amps it will not, as you surmise, deliver 3 times the thrust, maybe only twice .

Look cool maybe but an efficient way to use the power? - no!
Too much like an EDF and they are horribly inefficient.
 

Vimana89

Well-known member
#6
Morgajus
5x4x2 against a 5x4x6?
First it has 3 times the number of blades so with the same motor & battery voltage it will try to draw about 3 times the amps. It is unlikely the motor or ESC will be able to do that without releasing the dreaded 'magic smoke'.
Even if the motor/ESC/battery can deliver the extra amps it will not, as you surmise, deliver 3 times the thrust, maybe only twice .

Look cool maybe but an efficient way to use the power? - no!
Too much like an EDF and they are horribly inefficient.
So those six blade props just aren't worth it. Makes sense, I never really liked them much when testing them myself. I have one plane that does ok with them(I guess despite them would be a better way to put it), but will be switching to a two or possibly three blade for that as well. Based on what you are saying, I'm not even sure I see much logic in using the three blade either, so I've ordered me some 5x4.5 and 5x5 speed regular two blade props to play with on my 2205 and 2806.
 
Last edited:

kdobson83

Well-known member
#7
On my Mini Guinea I'm running emax 1806 motors with 20 amp ESC's on 3s. I started with 6x3x2 props and just wasn't happy with the look. Speed and power were good but what cargo plane has a 2 blade prop? So I upgraded to 5x4x4 props. MUCH cooler looking and power draw wasn't much more. Now, I didn't put it on a watt meter to test amp draw but with a lipo alarm set at 3.5volts per cell I was going from 10 minutes to about 8 minutes with a 1200mah 3s battery. And nothing was hot. Motors were a tad warm but not bad.
My suggestion, props are cheap. Buy em and slap em on. Hold onto your plane and power it up to roughly 75% throttle for 10-15 seconds. Check your temps. If they are really warm or hot then don't use em. If not, try it at 20-30 seconds.
Or by a watt meter and check em that way.
Good luck.
 

Vimana89

Well-known member
#8
On my Mini Guinea I'm running emax 1806 motors with 20 amp ESC's on 3s. I started with 6x3x2 props and just wasn't happy with the look. Speed and power were good but what cargo plane has a 2 blade prop? So I upgraded to 5x4x4 props. MUCH cooler looking and power draw wasn't much more. Now, I didn't put it on a watt meter to test amp draw but with a lipo alarm set at 3.5volts per cell I was going from 10 minutes to about 8 minutes with a 1200mah 3s battery. And nothing was hot. Motors were a tad warm but not bad.
My suggestion, props are cheap. Buy em and slap em on. Hold onto your plane and power it up to roughly 75% throttle for 10-15 seconds. Check your temps. If they are really warm or hot then don't use em. If not, try it at 20-30 seconds.
Or by a watt meter and check em that way.
Good luck.
Sounds like a good quick and easy method. I'll by buying a watt meter soon myself. I never really learned how to calculate or anticipate all that stuff and I'm not much of a math person, so I sort of just got as far as I did through trial and error using FT's recommended prop for a given motor size as a loose guideline. My V Sliver design runs great with Gemfan Hulkies 5x5.5x3, but I have no reason to want to use them if a two blade of the same dimensions would be more efficient.
 
#9
Morgajus
5x4x2 against a 5x4x6?
First it has 3 times the number of blades so with the same motor & battery voltage it will try to draw about 3 times the amps. It is unlikely the motor or ESC will be able to do that without releasing the dreaded 'magic smoke'.
Even if the motor/ESC/battery can deliver the extra amps it will not, as you surmise, deliver 3 times the thrust, maybe only twice .

Look cool maybe but an efficient way to use the power? - no!
Too much like an EDF and they are horribly inefficient.
I should add that the ESCs that I've ordered are 50 amps each. So blue smoke shouldn't be a huge issue on a small motor. Burnout might still be an issue though.
After I made a mistake and ordered 2 2212 2200kv and 1200kv motors each, I've been thinking about ways to use them.

An EDF jet is on my list of "one day" projects.
 

DamoRC

Well-known member
Mentor
#10
I know that most designs include a suggested propeller size, but how do you guys size props for your resized and custom designs? Or even just to slap something on to get in the air?
For resizing or custom designs I don't think about the prop in isolation but instead think about what power system(s) I have on hand that I could use. Once you get some experience with various motor and prop combos you can decide which is right for your application. A lot of times, I size the plane to match a motor / prop combo rather than the other way round.

My original though that led to this was "how bad of an idea would it be to put a 5x4x6 multi-rotor prop on a mini scout?" I know that this will mostly be affected by the motor, it would cause the amp draw to skyrocket, and it wouldn't be exactly 3 times the thrust of a 5x4x2 due to efficiency losses (fluid mechanics and all that jazz.)
I really like the look of the cargo planes with the 12 vane props spinning slow and ww2 fighters with the 4 vanes.
There is a calculation you can do to determine prop loading which I use quite a bit and is reasonably reliable when thinking about putting different props on motors. Its

Prop load factor = Diameter ^ 3 x Pitch x (Number of blades - 1) ^ 0.5.

Its only really useful for comparing one prop to another. So if I know the current draw for a particular motor prop combo, then I can use the equation to estimate what the current draw will be if I change the diameter or pitch or number of blades. Its not perfect by any means but its a good first step to do before actually testing the new prop on the motor with a watt meter or on the plane.

In your case, the prop load factor for the 6 x 3 x 2 = 6x6x6x3x1 = 648. For the 5 x 4 x 6 it would be 5x5x5x4x2.24 = 1120, so about double the load which would be approx double the current draw. Your 50A ESC might take it but the motor might not.
 

Merv

Well-known member
#11
how do you guys size props for your resized and custom designs
I look at the all up flying weight of the plane and the flying style you prefer.

• 50 to 70 watts per pound is the minimum level of power, good for park flyers and lightly loaded slow flyers.
•70 to 90 watts per pound is perfect for trainers and slow-flying aircraft.
•90 to 110 watts per pound is good for fast-flying scale models and some sport aerobatic aircraft.
•110 to 130 watts per pound is what you want for advanced aerobatics and high-speed aircraft.
•130 to 150 watts per pound is needed for lightly loaded 3D models and ducted fans.
•150 to 210+ watts per pound gives unlimited performance for any 3D model.

If you want to go fast, select a high Kv motor, 1,800+. If you want 3D select a low Kv motor, 1,000 +/-.. After I select the power plant, I look at the mfg recommend props at the cell count I intend to fly.
 
#12
For resizing or custom designs I don't think about the prop in isolation but instead think about what power system(s) I have on hand that I could use. Once you get some experience with various motor and prop combos you can decide which is right for your application. A lot of times, I size the plane to match a motor / prop combo rather than the other way round.



There is a calculation you can do to determine prop loading which I use quite a bit and is reasonably reliable when thinking about putting different props on motors. Its

Prop load factor = Diameter ^ 3 x Pitch x (Number of blades - 1) ^ 0.5.

Its only really useful for comparing one prop to another. So if I know the current draw for a particular motor prop combo, then I can use the equation to estimate what the current draw will be if I change the diameter or pitch or number of blades. Its not perfect by any means but its a good first step to do before actually testing the new prop on the motor with a watt meter or on the plane.

In your case, the prop load factor for the 6 x 3 x 2 = 6x6x6x3x1 = 648. For the 5 x 4 x 6 it would be 5x5x5x4x2.24 = 1120, so about double the load which would be approx double the current draw. Your 50A ESC might take it but the motor might not.
That factor has got some weird units lol. Sqrt(in^4/rev)... so... square inches per square root of rotation.
But i can see how it would be useful for an estimate.

Its also a simple enough calculation that I'm not reaching for my intro to aerospace design book to factor in the wing roughness of foamboard and packing tape...

I look at the all up flying weight of the plane and the flying style you prefer.

• 50 to 70 watts per pound is the minimum level of power, good for park flyers and lightly loaded slow flyers.
•70 to 90 watts per pound is perfect for trainers and slow-flying aircraft.
•90 to 110 watts per pound is good for fast-flying scale models and some sport aerobatic aircraft.
•110 to 130 watts per pound is what you want for advanced aerobatics and high-speed aircraft.
•130 to 150 watts per pound is needed for lightly loaded 3D models and ducted fans.
•150 to 210+ watts per pound gives unlimited performance for any 3D model.

If you want to go fast, select a high Kv motor, 1,800+. If you want 3D select a low Kv motor, 1,000 +/-.. After I select the power plant, I look at the mfg recommend props at the cell count I intend to fly.
I'd already ordered/begun receiving some cheap racestar motors before I thought about any of this. They don't have suggested props in the specs on the sale site but I haven't done extensive research on the subject either.

The crusier is estimated at 4 pounds 10 oz (I think...) so I'll need about 350 watts for it.

If i use a over sized motor (higher kv) could I not just limit the throttle to approximate the lower kv rating it requests?
 
#15
Probably would help if we could see links to the specific motors and the plane (cruiser) you are trying to power.
Sorry, i forgot the FT part. Its an FT cruiser.

[US$5.24 25% OFF]Racerstar BR2212 1400KV 2-4S Brushless Motor For RC Models RC Parts from Toys Hobbies and Robot on banggood.com
https://banggood.app.link/PVLBsxEJGY
is the url to the smaller motor I purchased. I think my dyslexia kicked in and I saw "2212" as "12~~" and assumed 1206 1400kv.

[US$6.29]XXD A2212 KV2200 Brushless Motor H365 For RC Airplane Quadcopter RC Parts from Toys Hobbies and Robot on banggood.com
https://banggood.app.link/wDwEnVSJGY
Is the url to the larger one.




They were cheap and now I'm working backwards to a plane I can learn on (FT Cruiser) that uses these size motors. But I need to source an appropriate prop(s) still.
 

DamoRC

Well-known member
Mentor
#16
Sorry, i forgot the FT part. Its an FT cruiser.

[US$5.24 25% OFF]Racerstar BR2212 1400KV 2-4S Brushless Motor For RC Models RC Parts from Toys Hobbies and Robot on banggood.com
https://banggood.app.link/PVLBsxEJGY
is the url to the smaller motor I purchased. I think my dyslexia kicked in and I saw "2212" as "12~~" and assumed 1206 1400kv.

[US$6.29]XXD A2212 KV2200 Brushless Motor H365 For RC Airplane Quadcopter RC Parts from Toys Hobbies and Robot on banggood.com
https://banggood.app.link/wDwEnVSJGY
Is the url to the larger one.




They were cheap and now I'm working backwards to a plane I can learn on (FT Cruiser) that uses these size motors. But I need to source an appropriate prop(s) still.
The FT cruiser will be just 2lbs all up, maybe a couple of ounces over depending on what battery you use.

9050 (9x5) props are recommended and should give you plenty of power.
 

Wildthing

Well-known member
#17
Sorry, i forgot the FT part. Its an FT cruiser.

[US$5.24 25% OFF]Racerstar BR2212 1400KV 2-4S Brushless Motor For RC Models RC Parts from Toys Hobbies and Robot on banggood.com
https://banggood.app.link/PVLBsxEJGY
is the url to the smaller motor I purchased. I think my dyslexia kicked in and I saw "2212" as "12~~" and assumed 1206 1400kv.

[US$6.29]XXD A2212 KV2200 Brushless Motor H365 For RC Airplane Quadcopter RC Parts from Toys Hobbies and Robot on banggood.com
https://banggood.app.link/wDwEnVSJGY
Is the url to the larger one.




They were cheap and now I'm working backwards to a plane I can learn on (FT Cruiser) that uses these size motors. But I need to source an appropriate prop(s) still.
I have a few of the racestar motors, that BR2212 1400kv I am running APC 9X6 (thin electric) props on it with a 50A esc (3S) and no issues at all. In that size of motor it's not my strongest one but it still does the trick.
The 2200kv motor start with a 6X4 prop but I think you should be able to go up to 7X4. (3S)
 
#18
The FT cruiser will be just 2lbs all up, maybe a couple of ounces over depending on what battery you use.

9050 (9x5) props are recommended and should give you plenty of power.
You are absolutely right. Idk where 4lb 10oz came from... but it was 3am when i wrote that...

I have a few of the racestar motors, that BR2212 1400kv I am running APC 9X6 (thin electric) props on it with a 50A esc (3S) and no issues at all. In that size of motor it's not my strongest one but it still does the trick.
The 2200kv motor start with a 6X4 prop but I think you should be able to go up to 7X4. (3S)
FT suggests 8x6 or 9x4.7 for the cruiser, which would be better on the 2212 1400kv? I also bought 50 amp escs.
I suppose by better I mean more powerful, not necessarily faster. I tend to diy my connectors and components, which makes them a bit heavier.
I've got 14-3 power tool cord for my battery extension/splitter. 3/16 plywood for my firewalls. And I'm planning to turn it into an fpv flyer one day.
 

DamoRC

Well-known member
Mentor
#19
You are absolutely right. Idk where 4lb 10oz came from... but it was 3am when i wrote that...



FT suggests 8x6 or 9x4.7 for the cruiser, which would be better on the 2212 1400kv? I also bought 50 amp escs.
I suppose by better I mean more powerful, not necessarily faster. I tend to diy my connectors and components, which makes them a bit heavier.
I've got 14-3 power tool cord for my battery extension/splitter. 3/16 plywood for my firewalls. And I'm planning to turn it into an fpv flyer one day.
9x4.7 for raw thrust, punch out, and unlimited vertical.
8x6 for more speed, a little less static thrust.

That being said, I can't see that you would be short of thrust if you just went straight to the 8x6 given that it's a twin.

EDIT: Looked back on some of my thrust stand data for 8x6 props on a 1200Kv motor on 3S which is ~780 grams at ~15 amps, that's 1.7 pounds of static thrust, and a twin will give you twice that so I would go with the 8x6.
 
Last edited:

Wildthing

Well-known member
#20
I would pick up one of each, get a decent quality one though. Some of those really cheap props are just junk. You can have 10 different brand of props all the same size and pitch and have 10 different results not only in thrust but in amp draw.