3blade vs. 2 blade

[WaltG]

I currently have a three blade 11 x 6 on my Cessna 182 and I heard it would be more efficient to have a 2 blade any thoughts

[Trobinson99]

As a general rule the fewer the propeller blades the more efficient the power system is.

unless a plane comes with multi blade props, I always tend to go with 2 blade.

[rifleman_btx]

It's like this, less blades equal less rotational weight, less rotational drag, or less total frontal surface area. That equates to less stress on the motor. That will translate in more useable torque and power for pulling (or pushing) the plane around the sky. It's much the same concept as putting wider tires on a car. You will get alot of initial punch out of the hole, but you will loose the top end and acceleration after you pull though the engine/ motors top torque rpm due to drag and rotational resistance since there all that works against the power train. And this on an electric motor will drain your batteries faster as you will need to use more throttle to obtain the same speed the more blades you go.

[fhhuber]

The real issue of more blades is the tips.

one of the largest contributors drag for an airplane or a propeller is the tips (wingtips, tips of the props, tips of the tail-planes...)

The purpose of multiple blades is to pack more blade area into a smaller circle. More blade area = more useful thrust.

A great example is the Corsair. the whole airplane was designed around the engine, with intent to get high speed and have a rugged aircraft capable of handing the abusive landings on a carrier.
They would have loved to use a 2 blade prop, but they needed to keep the landing gear short and to limit the speed of the tips of the props (prop tips nearing Mach is noisy and damages the blades) Early versions used 3 lades. Later versions used 4 because they improved the engines, giving more horsepower and thus they needed more blade area to make use of that extra horsepower.

************

We rarely use a prop that is actually scale diameter. Our 2 blade props on the Korean War Corsairs are generally 80% to 90% of scale diameter. Go to a 4 blade and you have to cut the diameter to limit tip drag (Tip drag is proportional to RPM squared * diameter * blades) Or go to a much larger motor...

I'm putting way oversize motors in a B-25 to turn scale 3 blade props. It can fly on 11X5 3 blade props (normal for glow power with this model), scale is 15.5 inch diameter. I'll limit airspeed by selecting the pitch. (ARF is not meant for the speed the large motors could pull it)

[Parky]

First of all, I am an aero engineer, and one of the things I do is design propellers. Most recently, I did a 7 blade, low noise prop for an electric VTOL

The basic answer here is that it really does not matter much, as long as you have equivalent 2 or 3 (or 6) blade props. By equivalent, that means the same diameter, the same pitch AND the same TOTAL blade area. The motor load, and efficiency is going to be very close. However, most multi blade model props have about the same area per blade.. so you add more blade area and the motor load goes up, and more power in the same diameter means lower efficiency. The APC 3 and 4 blade props do this pretty well.. the 4 blade has really skinny blades so the total blade area is about the same as the equivalent 2 blade props. (note, you need to be careful here.. skinny blades often have very thick % airfoils, and those do not work well at model sizes)

Now, when you go look at WW2 fighters, the engines got more powerful in later versions of a given plane, but they could not put on a larger diameter prop, because it was too hard to put on longer landing gear, so the solution was to add more blade area, normally by adding more blades.The Spitfire went from 2 blades to 5, for example (or a pair of counter rotating 3 blades.. 6 blades total). Yes, the efficiency of the prop got a bit worse due to more power and the same diameter, but not too much at high speed, and there was so much more power that the thrust at low speed went up in spite of lower efficiency.

Yes, a 3 blade prop has more tips than a 2 blade, BUT for the same total power and thrust, it is lower load per blade which means lower tip drag per blade. The net result is not much change.

A lot of model props have pretty poor blade airfoils. If you are doing that, then more blades is more bad airfoils and that is not good.

Anyway, for models.. The key thing is to get a prop that matches the motor and the airplane. If you want a prop that looks scale, then you just have to find a motor that matches it.

FWIW, if all else is equal, (RPM, power, diameter) more blades means less noise... sometimes a lot less noise.

Bob

[themudduck]

First of all, I am an aero engineer, and one of the things I do is design propellers. Most recently, I did a 7 blade, low noise prop for an electric VTOL

The basic answer here is that it really does not matter much, as long as you have equivalent 2 or 3 (or 6) blade props. By equivalent, that means the same diameter, the same pitch AND the same TOTAL blade area. The motor load, and efficiency is going to be very close. However, most multi blade model props have about the same area per blade.. so you add more blade area and the motor load goes up, and more power in the same diameter means lower efficiency. The APC 3 and 4 blade props do this pretty well.. the 4 blade has really skinny blades so the total blade area is about the same as the equivalent 2 blade props. (note, you need to be careful here.. skinny blades often have very thick % airfoils, and those do not work well at model sizes)

Now, when you go look at WW2 fighters, the engines got more powerful in later versions of a given plane, but they could not put on a larger diameter prop, because it was too hard to put on longer landing gear, so the solution was to add more blade area, normally by adding more blades.The Spitfire went from 2 blades to 5, for example (or a pair of counter rotating 3 blades.. 6 blades total). Yes, the efficiency of the prop got a bit worse due to more power and the same diameter, but not too much at high speed, and there was so much more power that the thrust at low speed went up in spite of lower efficiency.

Yes, a 3 blade prop has more tips than a 2 blade, BUT for the same total power and thrust, it is lower load per blade which means lower tip drag per blade. The net result is not much change.

A lot of model props have pretty poor blade airfoils. If you are doing that, then more blades is more bad airfoils and that is not good.

Anyway, for models.. The key thing is to get a prop that matches the motor and the airplane. If you want a prop that looks scale, then you just have to find a motor that matches it.

FWIW, if all else is equal, (RPM, power, diameter) more blades means less noise... sometimes a lot less noise.

Bob

Parky, it was totally fascinating for me, reading your contribution. You obviously know your stuff. Thanks for the post. I've always been curious regarding the difference that the number of blades makes and now I feel smarter! (not joking, I appreciate it!)

[AZFlyer]

Parky, it was totally fascinating for me, reading your contribution. You obviously know your stuff. Thanks for the post. I've always been curious regarding the difference that the number of blades makes and now I feel smarter! (not joking, I appreciate it!)

I really appreciate these posts, because I've always wondered about this myself.

Really big question here, then:

Why does a 2 blade prop make the plane faster? Can you give me a simple answer? I understand efficiency, but given less tip drag and everything else you said, bottom line: why does going down to a 2 blade prop make the plan faster?

thanks

[rifleman_btx]

I really appreciate these posts, because I've always wondered about this myself.

Really big question here, then:

Why does a 2 blade prop make the plane faster? Can you give me a simple answer? I understand efficiency, but given less tip drag and everything else you said, bottom line: why does going down to a 2 blade prop make the plan faster?

thanks

From his previous answer what I can deduce from it seems to me that because the blade area on each blade (atleast for our model props) are the same on the 3-4 blade vs 2 blades. Of course that means more drag on the motor with the extra blade area. And then when we go up a blade count remember we are also having to drop in pitch and diameter. So that will indeed make it slower, however this is just my understanding, I will like to hear from him on this as well.