The benefit of 3-blade props --> anyone with similar experience?

Hi, when flying smaller (5" to 10") copters in fast forward flight, three-blade propellers are almost standard, because they produce less vibration. The most commonly accepted reason seems to be that with three blades, the oscillation of angle of attack of the blades in forward flight is more balanced than with two blades (where always one blade has a high angle of attack, and the other has a small angle of attack, producing vibration).

But on larger copters, I still mainly see two-bladed props.

I recently built a custom 3-blade propeller hub for the T-motor MF1806 propellers that we are using on our 5 kg, 18" quadrotor. I noticed some vibrations while flying at higher speeds in a large wind tunnel (video), and I thought that the 2-bladed props might be the reason.

And today I tested the new 3-blade setup, and I like it much better. I was flying the same auto mission with both setups. The mission is to oscillate several times between two waypoints at 30 meters distance with 5 m/s and with 7 m/s.

Vibrations decrease by a factor of 3, the power consumption is almost identical, the mean throttle drops. What is the reason that 2-blade props are so popular on larger drones…?

Edit: I accidentally flipped the labelling in the vibration table… Now it is correct. 3-blade has of course a much lower vibration level in forward flight.

1 Like

You have raised an interesting topic because our props on multi rotors are operating as props were never intended to operate.

From a fixed wing perspective it is well tested that more blades is less efficient, where the prop directly faces the oncoming air flow.
The most efficient in this case is actually a single blade prop, and yes, they were a thing, although you don’t see them much anymore.

But considering the direction of airflow on multi rotors its a whole different story.

It would be interesting to hear if people have actually tested the characteristics of props in horizontal for ward flight.

Im just going down the same path. My copter has max power and hover time with 11"-2.
But i noticed with 10"-2 it flies much more sporty and can do a flip with half the height loss (in spite of the lower max power).

Now if a 10"-3 gives me the max power of the 11"-2 and the handling of 10"-2 that would be great.

For efficiency ecalc predicts a loss of 3 minutes out of my 34 minutes hower time, which is not too bad if the performance gains are what i expect.

It seems the bigger the prop, the smaller the efficiency losses because the blades are further apart and the lower rpms mean more time until one blade reaches the turbulence of the previous one. Also the hover RPM of 3 blades is lower helping here.

So it is clear that efficiency suffers more on small, high kv builds, but even there the handling gains seem worth it. So why dont we see 3-blades on larger general purpose copters? While hover time may decrease a bit, in dynamic flight the drag decreases and max flight distance improves again. I’m excited to try soon, I could be wrong.

If anybody has experience with 3-blade influence on filter parameters and thrust expo i would be very interested.

I would like to test 3 bladed props on my 500mm test drone. Where can I find 10 or 11 inch 3 blade props for multirotors? Everything I see is for planes.

Master Airscrew have 9x4.5 3 blade:

and others:

I think the “efficiency benefit” of 1- or 2-blade props vs. 3-blade is a misunderstanding. I had the same opinion for a long time, but now I think differently.

2 or 3-blade props with the same diameter do not have a noticeable difference in efficiency. But, if your aim is to generate a specified maximum static thrust, then you will use a smaller 3-blade prop and a larger 2-blade prop (as these will generate the same amount of static thrust at a certain input power).

Naturally, the larger 2-blade prop will have a higher efficiency (due to the decreased disc loading related to Froude efficiency).

To conclude:
At same diameters, there is hardly a difference in efficiency, but using 2-blade props instead of 3-blade allows to use larger diameters (at the same power input), and that gives you more efficiency.

Dr. Martin Hepperle, a pretty renowned aerodynamicist states the following:

[…] The Number of Blades has a small effect on the efficiency only. Usually a propeller with more blades will perform slightly better, as it distributes its power and thrust more evenly in its wake. But for a given power or thrust, more blades also mean more narrow blades with reduced chord length, so practical limits have to be considered here. The chord length can be increased while decreasing the diameter to keep the power consumption constant, but a diameter reduction is usually a bad idea in terms of efficiency, as long as the tip mach number or tip cavitation is not an issue. […]

Keep in mind that the propellers need to be as light as possible for motors with a small diameter, otherwise the motors will not have enough torque to accelerate them. Some propellers from MasterAirscrew are awfully heavy. I had the best experience with very cheap 3-blade props from ABS, but they are not available anymore. And you can convert every folding prop to 3-blade when you mill your own hub from carbon.

You’ll also need to increase the D-term quite a lot On my .
Here are more props:

1 Like