Wondering how much thrust a drone propeller generates

[Nick Nowaczyk]

TL;DR

Basically I’m wondering how to calculate how much fast a drone propeller or fan spinning affects the amount of energy that it releases. I can’t find any information and I’m sorry if this is in the wrong spot I’m new to the forums.

I was wondering how fast you would need to spin a drone propeller to lift x amount of force and mostly how the rpm of the propeller affects the force of the airflow it releases. I’m incredibly new to physics in general and you can take everything I say with a grain of salt.
I have no actual device but for a hypothetical the question would be something like:
“I have a fan/drone propeller that is .8m in diameter and the propeller is made of carbon fiber. How fast would it or how many at what speed would it take to release a force of 1000N?”
Again, sorry if I’m going about this all wrong, I’ve never taken a physics course and may have no idea what I’m talking about.

 

[Baluncore]

Welcome to PF.
If you look at the blades of a propeller, you will see they have an airfoil profile like a wing. It is that wing moving through the air that lifts the drone propeller upwards, pulling the drone up by the motor mounts. The air that goes downwards is a reaction to the lift of the blade section.

The lift force will depend on the RPM and the number of blades on the propeller. It will also depend on the blade profile and the twist along the blade. That makes it complicated.

A rotor of 0.8 m diameter will have an area of 0.5 m².
The rotor disc loading will be 1000 / 0.5 = 2000 N per square meter.
I think that is a bit high, and you may need a bigger prop or less weight. Look for specification data on a similar size drone. Find the rotor diameter and calculate the swept area. Find the total drone weight in kg, then multiply by 9.8 to get the hovering force in newtons. Divide force by area to compute a realistic rotor disc loading. Compare that with your example.

 

[A.T.]

“I have a fan/drone propeller that is .8m in diameter and the propeller is made of carbon fiber. How fast would it or how many at what speed would it take to release a force of 1000N?”

Estimating the thrust just from propeller geometry and RPM is not a simple computation. One can use numerical methods, but it's much more reliable to look at the datasheets from the propeller, which are based on experimental measurements.

 

[russ_watters]

From the other direction, though, is fairly straightforward: force (thrust) is rate of change of momentum of the air. The propeller just accelerates a disk of air. Larger propellers mean lower velocity to generate the same force -- and less power.

 

[Arjan82]

Maybe this document helps, it's a good explainer (unfortunately imperial units...):
https://nar-associates.com/technical-flying/propeller/cruise_propeller_efficiency_screen.pdf

With some good guestimates you can come a long way. But you would need some more details of the propeller, the most important ones are diameter and pitch. You've already mentioned diameter, for most propeller performance data this is also just a scaling parameter. This means you can say something about a 0.5m diameter propeller if you have data of a 1.0m diameter propeller, as long as the propellers are mostly geometrically similar.

But pitch is the other important parameter, you can not scale performance data for that. I don't know what the typical pitch is for the propellers you are talking about. If you have the pitch you need to find a performance curve for a propeller roughly of similar shape but most importantly of similar pitch.

The rest are 'second order' effects (I mean, based on the level of detail you are asking for). Yes, the number of blades is overrated, it hasn't got that much of an influence on the performance... (the difference between 2 and 3 bladed propeller is largest, and diminishes rapidly after that, but by large I mean <10% thrust difference at equal RPM or so)

 

[A.T.]

Maybe this document helps, it's a good explainer (unfortunately imperial units...):
https://nar-associates.com/technical-flying/propeller/cruise_propeller_efficiency_screen.pdf

The OP sounds like it's about a copter-drone. Static propeller thrust is more relevant while hovering than cruising efficiency for fixed wing aircraft propellers.

 

[Lnewqban]

I was wondering how fast you would need to spin a drone propeller to lift x amount of force and mostly how the rpm of the propeller affects the force of the airflow it releases.

Welcome, Nick!

The pitch of the propeller is important as well.

Please, see:
https://rcplanes.online/calc_thrust.htm