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If you had two of the same propellers layered on top of each other going at the same RPM, with some space in between, would the thrust be double the amount of a single propeller at the same RPM?

If not, what factors affect this?

The propellers are pushing down on the ground by the way.

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At least at the small/model scale, the answer is no. There is significant efficiency loss due to the stacked propellers not receiving the same clean air as they would if they were horizontally separated.

This is demonstrated experimentally in a Youtube video by rctestflight which compares the efficiency/thrust production of three different propeller configurations sharing the same propeller type and motor type:

  • Coaxial — What you're talking about where two props are stacked vertically so that they share the same rotational axis.
  • Overlapping — Where two props' discs overlap each other by roughly half a prop diameter
  • Normal — Where two props are positioned side-by-side without any overlap

enter image description here

To quote Daniel from rctestflight in his video:

Any prop overlap is significant in efficiency loss. Traditional side-by-side motors are 25% more efficient than coaxial and 8.4% more efficient than overlapping props.

To me, losing 25% efficiency is not worth the small form-factor and increased ease of transportability that a coaxial system produces.

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    $\begingroup$ Note that this is measuring efficiency. If you were to measure thrust you would find it worse. 2 props = twice the max thrust but coaxial props will give you only around 10%-20% gain. That's a difference between 100% and 20% which is a lot $\endgroup$
    – slebetman
    May 20, 2020 at 9:06
  • $\begingroup$ @slebetman The important thing would be the thrust to weight ratio, I should think. For which your observation does not bode well. $\endgroup$
    – Peter Wone
    May 21, 2020 at 8:25
  • $\begingroup$ @PeterWone Yes, especially since efficiency suffers at double thrust leading to requiring bigger, heavier batteries. Weight of motors would normally be offset by the airframe weight not changing so the total weight would less than double but the battery requirement might be the killer $\endgroup$
    – slebetman
    May 21, 2020 at 9:55
  • $\begingroup$ @PeterWone Oops, I thought you were commenting on my answer. See my answer for some experimental data that you may be able to get almost double thrust but at the expense of efficiency $\endgroup$
    – slebetman
    May 21, 2020 at 9:56
  • $\begingroup$ @ifconfig This video by members of JPL, NASA talks about how 2 rotors on top of each other are more efficient!!! Are they wrong? $\endgroup$
    – Atharva
    Dec 10, 2022 at 5:19
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TLDR: The thrust would not double. You can get close but it won't be easy.

There are a lot of factors affecting this but in my experience the distance between props dominates.

I once built a push-pull RC airplane and got close to 180% the thrust of a single prop. The configuration is similar to the Dornier Do335:

enter image description here

Dornier Do335

So you can get quite close to double. But look at the distance between the props. That's at least 5 times prop width.

I've also tried building a coaxial two motor EDF unit. The result was I got at best around 106% thrust - that's right, the second motor takes twice the power but only adds 6% of additional thrust.

I've also tried coaxial configurations outside of a ducted system and got around 120% thrust. So a second motor takes twice the power but added 20% thrust. This was OK because at the time I was depending on the coaxial rotation to counter torque.

But this is all anecdotal from my personal experience. So I wanted to find any research on this and it turns out that someone at Rutgers University have already done the experiments: https://rucore.libraries.rutgers.edu/rutgers-lib/55491/

I'll quote the relevant conclusions:

With a pitch of 4.4 inches, contrarotating second, third, and fourth rotors roduced 71%, 53%, and 41% of the first rotor’s thrust, respectively. With a pitch of 4.4 inches, co-rotating second, third, and fourth rotors produced 61%, 42%, and 40% of the first rotor’s thrust, respectively. This confirmed that contra-rotating coaxial rotors generate more thrust than co-rotating coaxial rotors by effectively recovering the energy put into the swirl of the leading propeller’s wake. With a pitch of 10 inches, co-rotating second, third, and fourth rotors produced 97%, 67%, and 54% of the first rotor’s thrust, respectively. This demonstrated that downstream propellers are capable of generating more thrust when operating at higher pitches. The thrust losses of the downstream propellers do not have to be as high as they typically are.

Note that they managed to get almost double the thrust of a single propeller: 197%. But for this to work the second propeller had to have a much higher pitch than the first.

But do note that in the section of the report about thrust measurements (page 46) the author cautiously noted:

Although a multirotor system could benefit greatly from a coaxial arrangements, some can be an exercise in futility.

If you really want to know if your idea would work I'd suggest building a test jig and measure the thrust. Use the Rutgers paper as a guide for how to tweak the configuration (eg: making the second prop have higher pitch)

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    $\begingroup$ The prop pitch thing getting almost double thrust shouldn't be surprising because this is how an inline turbojet compressor works (only optimized for pressure instead of thrust - but if you can get pressure you can get thrust at the tail end). But actual data on turbojet engines are hard to come by and tend to be trade secrets. So it's nice to find a paper like this publicly available $\endgroup$
    – slebetman
    May 20, 2020 at 9:33
  • $\begingroup$ Pusher props are more efficient at speed, and pull props give more control surface airflow at low speeds. I wonder why contrarotation is more efficient. $\endgroup$
    – Peter Wone
    May 21, 2020 at 12:23
  • $\begingroup$ @PeterWone That's quite simplistic and not true at all. The proof can be seen from the history of propeller driven speed record which has never been held by a pusher prop. But in RC pylon motor glider racing the world championship was won by a pusher prop flying wing a few years back (note that this is not normal). They did it by having a very long fuselage putting the prop very far away from the wings and paying close attention to the aerodynamics of the join between the body and the wings. Blocking air behind your props is not as bad as turbulence in front of your props $\endgroup$
    – slebetman
    May 21, 2020 at 14:08
  • $\begingroup$ Your last sentence is very plausible and clearly you've done research, I stand corrected. $\endgroup$
    – Peter Wone
    May 21, 2020 at 22:38

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