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Props-Duration, Performance, Stability Props-Duration, Performance, Stability


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Old 01-16-2016, 04:01 AM   #1 (permalink)
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Default How to increase propeller lift: Pitch vs Diameter vs RPM

I need to change propellers for a quad which is significantly heavier than the previous.

What is the general formula which relates Thrust (lift) with RPM, Pitch and Diameter?

In this article here https://quadcopterproject.wordpress....t-calculation/ I found this formula:


where P = power in watts ...

"Solving for mass is useful for quadrotor helicopters because it can be directly related to the mass of the aircraft. In particular, a thrust (mass) that equals the mass of the aircraft is needed for hovering."

However I'm missing pitch of propeller in the equation.
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Old 01-16-2016, 04:47 AM   #2 (permalink)
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That formula is incomplete because it does not consider propeller efficiency (into which pitch would be factored for a given advance ratio). Looks like whoever put the page together may have borrowed formulas from elsewhere without really understanding it.

You maybe want to rean Martin Hepperle's web site: http://www.mh-aerotools.de/airfoils/propuls4.htm

In a nutshell thrust should increase roughly linearly with pitch, assuming the RPM of the prop does not change (in reality it will slow a bit so you will get a bit less of an increase). Increase pitch very far though and hovering thrust will start to drop as the prop begins to stall.
The more efficient way to increase thrust is by increasing diameter.
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Old 01-17-2016, 04:43 AM   #3 (permalink)
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Ok neither your page formulates a formula taking into into account the pitch, however they say:

"The momentum theory does neither take the planform of the blade into account nor the characteristics of the airfoil sections. For the design or the analysis of a propeller more sophisticated models are necessary, but the momentum theory always gives a good estimate for the maximum efficiency which we can expect."

so basically you say it increases linearly with pitch and to the square with diameter.

Now tell me if this makes sense (taken from http://www.djiguys.com/Forum/viewtopic.php?f=7&t=5479): someone indicates for a DJI F450 that

8" props - 4S Battery
10" props - 3S battery

shouldn't bigger in diameter props require more power and thus bigger batteries???

thanks
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Old 01-22-2016, 01:09 PM   #4 (permalink)
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Quote:
Originally Posted by hubschraub View Post
8" props - 4S Battery
10" props - 3S battery

shouldn't bigger in diameter props require more power and thus bigger batteries???

thanks
No, Increasing propeller diameter along with reducing battery weight such as in this scenario may yield the same amount or better the amount of thrust in which a multirotor needs to fly.

However the other determining factors also not mentioned as this probably pertains to a DJI F45, but was silently considered is the motors and esc's limitations.

10" props with a 4s battery in this case may generate more drag and thus generate more heat than what both the motor or ESC's may be able to safely handle.
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Old 01-22-2016, 01:17 PM   #5 (permalink)
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The most efficient setup would be enormous propellers turning very slowly.. Basically you want to capture the largest volume of air possible and move it as little as possible.

This is the principal behind human power multi-rotor helicopters that lift hundreds of pounds on only a few hundred watts of power:


Of course there are practical limits to how far you can go with the giant prop idea, structural strength and/or weight becomes and issue, also you would need reduction drives as electric motors cant run slow enough with any efficiency.
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Old 01-22-2016, 01:21 PM   #6 (permalink)
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Quote:
Originally Posted by hubschraub View Post
Now tell me if this makes sense (taken from http://www.djiguys.com/Forum/viewtopic.php?f=7&t=5479): someone indicates for a DJI F450 that

8" props - 4S Battery
10" props - 3S battery

shouldn't bigger in diameter props require more power and thus bigger batteries???

thanks
That makes perfect sense. The bigger props load up the motor more so you need to reduce cell count in order to slow the RPM and prevent over-amping the motors. The 3s setup with the bigger props would be more efficient but the 4s setup would have more power and more ultimate lifting potential. Theoretically the ideal solution would be to use the 4s battery and the large props but have a reduction drive on the motors... but added weight and complexity are the real world drawbacks.
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Old 01-22-2016, 02:12 PM   #7 (permalink)
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Quote:
Originally Posted by Grumpy_Old_Man View Post
That makes perfect sense. The bigger props load up the motor more so you need to reduce cell count in order to slow the RPM and prevent over-amping the motors. The 3s setup with the bigger props would be more efficient but the 4s setup would have more power and more ultimate lifting potential. Theoretically the ideal solution would be to use the 4s battery and the large props but have a reduction drive on the motors... but added weight and complexity are the real world drawbacks.
When using motors based on RPMs achieved by Kv's another way to achieve gear reduction is to use a smaller 3S battery, but increase the batteries capacity. You may find that you can run two smaller 3s batteries with more capacity to run in parallel that may actually weigh less than the one 4s battery.

One thing a brushless motor is inherently good at making without gears is torque.
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