Motor Speed, efficiency and flight time!

[gtranquilla]

Some time ago I noticed on this forum that someone had indicated propeller speeds must be kept below 80% of the speed of sound.
It reminded me of something I read some time ago on this topic. Today I found that info and thought it would be good to post it here for other who are curious.
Turns out the original comment is 100% correct and for good reason..... see below.

View attachment 11976
http://www.aerospaceweb.org/question/aerodynamics/q0104.shtml

Propeller drag generally increases in a linear fashion as the speed increased due to aerodynamic drag.
But suddenly it increases exponentially when it gets to 80% of the speed of sound.
This not due to aerodynamic drag but due to other factors as explained in the hotlink above.

The speed on the prop is always fastest at the tip and the larger the prop radius easier it is to get the tips to reach the sound barrier, if the motors can provide the torque and speed to do it.
So in general, larger diameter propellers need to be driven by higher torque but slower speed motors, i.e., motors with more poles. :dejection:

 

[jcmonty]

Yup, this is why supersonic planes are designed differently than subsonic - and generally there is a compromise because even supersonic planes fly sub sonic a lot of times.

Drag is made up of a lot of components, profile drag, induced drag, wave drag (at close to and past the sound barrier), interference drag.. You can do the math pretty easily to determine your prop is approaching supersonic speeds. Most of our aren't and you would hear it if they were. For example: a 11in prop at sea level would need to be 20000RPM + to start worrying about wave drag. I would worry about the prop exploding before you hit supersonic!

A lot of the efficiency hits come from the fact that our props are running in stalled conditions most of the times due to hovering. Fixed pitch airplane propellers are designed to operate with a specific in flow velocity, and that's why lower pitch props are generally more efficient for our use than high pitched unless you are a FFFer.

 

[Electro 2]

Mr. Monty is dead-on here. I would add:

"in general, larger diameter propellers need to be driven by higher torque but slower speed motors, i.e., motors with more poles"

*And* a Kv wind well suited to the voltage of the battery in use. Long flight times/heavy lift capability = low current consumption = motors operated at maximum efficiency. Motors operated at maximum effciency = correct Kv rating + high pole count + big props.

 

[phantom_alex]

An old thread I know but so useful! Bookmarked

Sent from my C6903 using Tapatalk

 

[gtranquilla]

You may also notice in eCalc that you can adjust the prop thickness ratio @ bottom right. A 1.1 gives more airtime than the conventional clark airfoil at 1.3 or higher. Check out the super thin cf paddle props.