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Does propeller blades' airflow interfere with each other blades and decrease overall thrust? Should I consider high pitch propeller for this purpose?

My purpose: I'm looking for a 20-inch four-blade propeller for a tail-sitter Drone. But commercially I don't find anything like that. So started looking at making a 4-blade/6-blade propeller with 2-blade propellers. Thrust density is my first priority compared to efficiency.

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  • $\begingroup$ aviation.stackexchange.com/questions/53034/… $\endgroup$ – Pilothead Feb 9 at 22:56
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    $\begingroup$ Increasing number of blades does interfere with each other but it does not decrease thrust. Quite the opposite it increases thrust (which is why you see multi bladed props). What it does decrease is efficiency (due to the interference). Meaning you burn more fuel for same amount of thrust. But you do get higher max thrust $\endgroup$ – slebetman Feb 10 at 7:16
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Sure, this is not only possible, but even has some advantages over single-plane propellers. The catch is you ideally want the two to spin in opposite directions to cancel out torque and vibration.

You're building a contra-rotating propeller:

enter image description here

Pictured is a Sun Flightcraft contra-rotating gearbox kit for Rotax 503 and Rotax 582 engines.

Although more efficient (on the order of 10%) than single-plane propellers, they are also much more complicated to design. The gearbox will be heavier and harder to maintain. And they are also often much louder, although the Rotax kit pictured above is claimed to be quieter.

They're not just for small engines. Here's a Tu-95 strategic bomber:

enter image description here

On helicopters, the same idea is called coaxial rotors:

enter image description here

"Co-rotating" propellers are also possible, but far less common, as the key advantages (efficiency and cancellation of torque and vibration) are reduced or lost. I could only find some academic papers on it. Here is another.

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    $\begingroup$ Contra-rotating needs additional hardware. Just don't wanna do it because of complexity and cost. What about just literally sticking together? $\endgroup$ – SRD Feb 9 at 6:33
  • $\begingroup$ @SRD Possible, sure. I don't know about efficiency. I was able to find a short paper on it but not much practical data. My gut feeling is as a hobbyist you'd be better off with a single-plane prop. I'm not an aerodynamicist though. $\endgroup$ – TypeIA Feb 9 at 6:41
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    $\begingroup$ @Therac yes... OTOH, a counter-rotating system for a drone isn't anywhere near as difficult to build as one for manned aircraft. Could be done basically with two plastic gears and a rubber band. $\endgroup$ – leftaroundabout Feb 9 at 15:18
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    $\begingroup$ Alternatively, I wonder how well it would work to use the motor to only drive the differential rotation of the props, i.e. have the motor fixed to one of the props and drive the other one relatively to it, but both spinning freely against the fuselage. This would of course require brush contacts to supply the motor, but that's definitely doable. $\endgroup$ – leftaroundabout Feb 9 at 15:40
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    $\begingroup$ Tu-95 uses some very cunning planetary gearbox, a very funny read, but I have displaced the link at the moment. Probably it's not relevant for a small drone. $\endgroup$ – Oleg Lobachev Feb 10 at 16:50
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It's done sometimes, for ultralight aircraft and paramotors. Like this:

enter image description here

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    $\begingroup$ Have also seen two 3-blade units joined in the same way to make a 6-blade unit. $\endgroup$ – quiet flyer Feb 9 at 13:56
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    $\begingroup$ That one's going to be awkward as the bolt circle doesn't align at 90 degrees, you'd really want an 8 hole bolt circle, or make a 6 blade prop. $\endgroup$ – user_1818839 Feb 9 at 21:25
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    $\begingroup$ @BrianDrummond: Would a 90-degree angle be optimal for blades that aren't coplanar? I would think that a smaller angle would be better, since the propeller that's in front would take less time to clear the prop wash of the one in back than the one in back would take to clear the prop wash of the one in front. $\endgroup$ – supercat Feb 9 at 22:20
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    $\begingroup$ @supercat You are correct. In a helix, putting a blade more forward requires it to advance to get closer to its original tip path. $\endgroup$ – Pilothead Feb 9 at 22:50
  • $\begingroup$ @Brian Drummond The finished two-blade units are first adhesive-mounted at 90º, and then drilled. $\endgroup$ – xxavier Feb 10 at 13:20
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Any propeller 'unit' has been designed for a certain set of conditions: rotational-speed/air-speed/power/etc.

If you then double-up in any way, then you can be assured that for the same conditions, the new unit will be less efficient.

Also, the most efficient unit, all things being equal is a single-bladed propeller - some small model airplanes use this setup because the tip speed is relatively low (supersonic top-speeds are "A Bad Thing") so designing a unit with extra blades for no good reason is also very likely to be less efficient.

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    $\begingroup$ A single-blade propeller has also been used on at least one manned ultralight-- I knew a guy who has flown such, but am unable to provide more details. On another note, in the examples mentioned in the related answer aviation.stackexchange.com/a/84188/34686 , it's possible that the units were designed specifically for use in this manner, for ease of manufacture or for economy in the event that one blade is damaged needs to be replaced. $\endgroup$ – quiet flyer Feb 9 at 18:39
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    $\begingroup$ The downside of a low number of blades is that you get less thrust for your diameter. Doubling up the number of blades on the same diameter WILL be less efficient, but it will give you more thrust for the same diameter and speed. It's a tradeoff: a large efficient propeller, or a compact less efficient one? It all comes down to the fact that momentum is mv and energy is proportional to mv^2. Reduce the area of the propeller and you end up needing to impart double the velocity to half the mass of air, which uses twice as much energy. $\endgroup$ – Level River St Feb 9 at 23:15

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