Solved Please help me sort this motor question

[Merv]

...But 2 blades always seems to be more efficient at any size...

2 blade props are more efficient in producing grams of thrust per watt.

The only reason to use more blades or a bull nose prop. You need more thrust but can't use a larger diameter prop. Like in a quad, if the prop is too big, you will hit something.

The full scale guys have the same issue. The size of the prop is limited by the length of the landing gear.

 

[Merv]

..any given prop will require (X) amount of power to make (Y) amount of thrust ....I'd almost prefer to see a chart built like that for props so i could pick the prop size and performance level ....🤔

Here is the performance data for APC props. At this RPM, the thrust a prop produced, the power it requires and the airspeed coming off the prop. The power is given in horse power, I believe the conversion is 745.7 watts for 1 horse power.

https://www.apcprop.com/technical-information/performance-data/

 

[Monte.C]

The only reason to use more blades or a bull nose prop. You need more thrust but can't use a larger diameter prop.

Not true. When I use a 3-blade prop it's usually because I like the way it looks. Full stop. Efficiency? Who cares about conserving electricity? They'll make more.

 

[Pieliker96]

The only reason to use more blades or a bull nose prop. You need more thrust but can't use a larger diameter prop.

Those are two good reasons, but hardly an exhaustive list. Smaller props for example are better for minimizing gyroscopic precession and some left-turning tendencies which can be advantageous for 3D planes and the like. A prop with more than two blades may also be necessary for scale accuracy. Higher blade counts also increase the solidity of the propeller disk which increases drag with the power off, something which may be desired for STOL applications.

 

[Mr NCT]

I read that going from a two blade to three blade dropping one size, like 9" to 8", but keeping the same pitch would provide the same thrust. I put a Master Airscrew 8 -6 on a C pack motor and got ~ 800g thrust. Put on a APC 9 - 6 and got 1000g +. So much for "rule of thumb" conversions.

 

[Hondo76251]

Here is the performance data for APC props...

Thats a handy one for sure!
I know the data is out there, I guess i was more lamenting that the system we've more or less adapted hobby wide isn't exaclty straightforward and simple.

Not true. When I use a 3-blade prop it's usually because I like the way it looks. Full stop. Efficiency? Who cares about conserving electricity? They'll make more.

I've got 3 blade props I've never flown with just for displays! They look the part for sure! 😂

I do a lot of fpv cruising so im always searching for efficiency just so i can stay in the air longer 😉

 

[Mr NCT]

I've become a big fan of Cobra motors and the prop charts that Innov8tive Designs posts for them - very complete.

 

[Hondo76251]

Since weve sort of evolved (or maybe devolved 🤪) this thread into prop theory, ill help continue this rabbit trail with a thought; like @Pieliker96 said, large props will have more leverage. They can cause the torque roll to be amplified at lower speeds. I wonder if a newbie flying, say a simple cub, might benefit from using a smaller prop and motor combo if they're having troubles...

 

[Hondo76251]

Ive really liked all the emax Avon props I've tried. The "long range" ones are very quiet and seem to work well, probably my favorite props in the 5-6" range

 

[Pieliker96]

I guess i was more lamenting that the system we've more or less adapted hobby wide isn't exaclty straightforward and simple.

I'd say it actually is - far better than going far into the weeds of propeller design and making everything more confusing than it needs to be. Blade pitch and prop diameter are about the easiest things you can measure. Need more static thrust? increase diameter or blade number. Need more static and dynamic thrust? Increase prop pitch. Easy as.

Of course, the engineering reality is far beyond what blade pitch, prop diameter, and blade count can tell you. Even the data we do have from manufacturers is limited to just static thrust cases - ideally, we'd want plots of output thrust over a variety of input power and airspeed settings, from which output power can be calculated and efficiency derived.

To not only be able to effectively use that data to compare propellers, but to do so in the specific context of the plane it'll be flying on, requires so much analysis and thought beyond the typical "yeah I'll just get this prop because it's what the motor's spec sheet says is right for my battery/ESC combo": the latter is far simpler, straightforward, and appropriate for the minimally-engineered, rule-of-thumb nature of this hobby. They are just model planes, after all.

 

[Hondo76251]

I'd say it actually is - far better than going far into the weeds of propeller design and making everything more confusing than it needs to be. Blade pitch and prop diameter are about the easiest things you can measure. Need more static thrust? increase diameter or blade number. Need more static and dynamic thrust? Increase prop pitch. Easy as.

Of course, the engineering reality is far beyond what blade pitch, prop diameter, and blade count can tell you. Even the data we do have from manufacturers is limited to just static thrust cases - ideally, we'd want plots of output thrust over a variety of input power and airspeed settings, from which output power can be calculated and efficiency derived.

To not only be able to effectively use that data to compare propellers, but to do so in the specific context of the plane it'll be flying on, requires so much analysis and thought beyond the typical "yeah I'll just get this prop because it's what the motor's spec sheet says is right for my battery/ESC combo": the latter is far simpler, straightforward, and appropriate for the minimally-engineered, rule-of-thumb nature of this hobby. They are just model planes, after all.

Spoken like a true engineer!
"You see, its all quite simple if you understand absolutely everything thats going on all at once." 😂

I like experimenting, seeing what works for me. I get how it works and i like learning the technical end of things but I run into a lot of people who would like to fly who are not that interested in having a box with 100 different props laying around like i have! (Not to mention dozens of motor and battery combinations to match)
Have to admit that its a lot to learn to just play with a toy plane! Lol

 

[Monte.C]

LOL I found an F-size motor here. Woo hoo! 2205-2300kV. I know I have 6" props. Perfect for a maiden. From here I can step up to those other motors as I wish. Sorted.

 

[Monte.C]

Why aren't the Emax bullet ESCs used more around here? I've never seen them here on the forum. Is it just the cost difference? But as far as I can see these provide everything a big clunky ESC does in a practically microscopic package. I've used the 12A ones for tiny motors in very small planes and I'm about ready to buy a couple 30A ones.
https://www.racedayquads.com/products/emax-30a-bullet-esc-dshot-600

 

[Hondo76251]

I use them quite a bit, i think theyre great!

I think not having a BEC is a dealbreaker for some but i typically run a separate one anyway so thats fine with me. They fit great into small area's which is handy on many of my builds.

 

[Hondo76251]

Used one on the FT Beaver when the original got a little upset about being dunked a few too many times. 🤪

All the components are conformally coated now. She can be a submarine all she wants! Lol

 

[Monte.C]

I use them quite a bit, i think theyre great!

I think not having a BEC is a dealbreaker for some but i typically run a separate one anyway so thats fine with me. They fit great into small area's which is handy on many of my builds.

Oh that's right, I forgot about the BEC. That adds a bunch of grams back into the equation. I still love them.

 

[Merv]

...Efficiency? Who cares about conserving electricity?...

Those who want to increase flight time. Efficiency is not about saving electricity, it's about flying longer. Just using a bigger battery has diminishing returns. At some point your battery is so large you can't take off.

 

[Monte.C]

Those who want to increase flight time. Efficiency is not about saving electricity, it's about flying longer. Just using a bigger battery has diminishing returns.

Duh. Thank you, I'm not that dense, I already knew that.
The point is for a lot of people increasing flight time by 30 seconds isn't top of their list of priorities, and not everybody is trying to go 100 mph either. I want to put a plane in the air and have fun with it without burning something. And sometimes I like a 3-blade prop. It doesn't matter either way for someone like me who isn't wanting to break records or win awards.