Most Efficient Number of Prop Blades?

[Halo]

Always been curious about the most efficient number of propeller blades.  Early war planes seemed to favor three blades, later birds four, and a few even more.  

On a recent drive saw a row of large contemporary windmills (or whatever they're now called) on a ridge.  They all had three blades.  Assuming they are the latest thinking, are three blades the most efficient propellers?

[jodgi]

There are numerous factors that go into that equation. I doubt there is a simple answer to your question.

One example:
Prop blades (size and numbers) increased as engine power increased. On old planes with "small" engines, 2 blades often was the most effective solution. To get the most out of powerful engines designers use(d) more blades.

Just to start us off.

[SELECTOR]

you want the most out of the engine 2 solutions longer blades or more blades.
longer blades.. will hit ground and tips go supersonic and thats not efficiant
more blades is the solution

[BlkKnit]

the most efficient number is...1, according to what I have been told.  However this is not actually best.  Where the first blade bites into the wind fully, the next is biting less because it is following the first.  The amount of bite decreases as more blades are added.  It really is more of a theoretical argument, since really, more blades are better if you have the power to move them efficiently.

I dont know about other planes, but the King Air series by Beech is one which originally has 3 blade props, but can be (and often is) easily upgraded to 4 blade props.

The Shorts 330 had 4 balde props, the 360 has 5 blades (or is it six? cant remember).  Saab 340 =4, Saab 2000 = 6.  There are probably other examples, and i am sure my 1 blade statement will be refuted with real evidence instead of the hearsay I have provided.  I hope someone does provide some real answer.....it is an interesting question.

[jodgi]

Isn't it the Saab 2000 that has "bent" propblades to prevent supersonic airflow over the bladetips?

[VWE]

The current trend on turbo props with the increased number of blades has more to do with noise reduction than improved efficiency. The more blades you have, the lower the rpm for the same thrust at a lower decibel.

On the water, the unlimited turbine powered hydrofoils all use 2 bladed screws...

[BlkKnit]

hmmm....I didn't notice any "bend" to them.

[jodgi]

Maybe I saw a pic of a test bird then.

[GScholz]

I believe counter-rotating props are the most effective. Two counter-rotating 4 bladed props are surly more effective than a single 8 bladed prop. The world's fastest prop driven plane uses counter-rotating 4 blade props.

[Jasta]

Ive been to an air force base somwhere in the US... probably cant say because of security, but they have been experimenting with 8-bladed props on some aircraft. Very interesting sight.

[hitech]

The question of how many blades is most effiecent is a miss leading question. You need to typicly qualify that statement.

Given that length and rpm range are not a factor A one bladed prop will have the best peek effieciancy of turning torque into thrust.

Adding more blades is done simply because you can not make the prop longer because of stress forces or simply will not clear the ground.

Blade area also efects things by making the prop more efficent at low speeds but less effecient at higher speeds. You change this by making the prop wider, length is much preferible do the increased induced drag from a change in aspect ratio.


HiTech

[beet1e]

[Halo]

Theoretically I would think there would be one optimum propeller design, but apparently not since there are so many variants, from two-bladed to eight-bladed (e.g., the Russian Mi-26 Halo helicopter) as well as counter-rotating three-blade and four-blade combos.  

Question remains, however, since wind generating towers presumably have the option of any propeller design, why do the only ones I've seen in the eastern and western United States have "only" three blades?  

Surely if two blades or four blades or any other number were more efficient, they would be used on these towers.  

Seems like pictures of old windmills, e.g., in Holland, have four blades.  And we've all seen the classic farm windmills with 18 blades.  

To further add to the confusion, the controversial Osprey tilt-wing uses a three-bladed prop on each engine, but the latest C-130J turboprop has six-bladed props with a scimitar shape.  

So there you are -- big latest technology windmill uses three-bladed props.  Why not two or four or more blades?  And does this principle, whatever it is, transfer to all other prop uses, including aircraft, helicopters, and even boats?  

Okay, I'll do a quick Google and see what that turns up.

Okay, I did the quick Google search (both propeller efficiency and windmill efficiency) and am more confused than ever.  

Always knew propellers were among the most complicated devices on any airplane.  Tradeoff between length, pitch, thickness, engine power, and other variables.  

Apparently the search will always be on for the most efficient propellers or screws or fans.  Doesn't look as if there is any such thing as an optimum number of blades.  

But fascinating question anyway.  If more light can be shed on this subject, please flick the switch.

GScholz, excellent point about the Bear being the world's fastest propeller plane.  I thought surely some fighter like the Sea Fury would be.  Great top money question for an aviation quiz show.

[NUKE]

how do counter rotating props work? I always wonder how they are geared to counter rotate.

Also, I didn't know that the bear was that fast.....pretty amazing.

[Hades55]

I strongly believe propellers are not dead even for the today batlefield. Ofcourse not like the propellers we know.
I also strongly believe that a propeller can pass the limit of 700something and even tuch transonic speeds.
Im sure someone working on it