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What is optimum number of propeller blades

jpblomberg (Mechanical) (OP)
11 Feb 08 16:17
I have been thinking about how to optimize number of blades for a given propeller application.  I realize there are many applications for them so let me narrow this down somewhat.

Case 1:
Most of the large electrical wind turbine generators have three blades.   This number is arrived at probably by balancing economics and structural concerns.  If the economics were not a factor (price of the blade, hub, tower etc.), then how many blades would make sense?  It seems to me that with 3 blades, a majority of the air goes between the blades and doesn't have a significant effect on them.  My thought is that more blades would produce higher torque but the maximum rotational speed would be reduced due to the extra blade drag.  My thought is an optimum number of blades would extract the maximum amount of energy from the wind.

Case 2:
An airplane propeller.  So far as I know nobody makes a 1 blade prop (which could easily be done with a counter weight).   Of course and infinite number of blades, or I suppose even a hundred or thousand would just make a solid disk, which doesn't produce thrust.  For the case of airplane at take off, is 1 blade best? How about 2, 3, 4, 5, 25, 100 blades?  In the case if a large engine, where the rotational speed is not is not limited, but the diameter of the propeller is,  what would be the optimum number of blades given a particular diameter and angular speed?  What number produce the most thrust?

I realize that there are other factors that affect performance.  What are your thoughts?

John

"Credo ut Intelligam"  I believe in order to understand -St. Augustine

btrueblood (Mechanical)
11 Feb 08 16:49
The optimum blade, aerodynamically, is the biggest one possible (more swept area = more thrust).  More blades do not necessarily produce more thrust, as the wake of each blade can interfere with trailing blades (well, there's more to it than that, but you get the idea).  The limits on the bigger is better rule are:  the engine rpm for best fuel consumption is usually a narrow range, which fixes the blade rpm at some value, drag, which increases nonlinearly as the diameter and number of blades increases, and structural limits for the blade material.  Other limits then come into play, such as clearance for the prop. provided by the landing gear...
MikeHalloran (Mechanical)
11 Feb 08 18:56
The last time I messed around with model airplanes, the fastest ones were driven by pulsejets.  Second fastest used counterbalanced one-blade propellers.

Mike Halloran
Pembroke Pines, FL, USA

rb1957 (Aerospace)
11 Feb 08 19:24
i thin it Way to much of a simplification to say more blades are more efficient, it has to do with the "activity factor".  Prop design is very complex, it involves conflicting design requirements ... the power being transferred wants a big disc, the tip speed wants a small one.  maybe, as a simplification, you want as few blades as you can to absorb the power, on the longest blade.

as far as an airplane goes, there are a very few five blade props (on dash8 S400, and H.S. Fury if i remember right).  there have been contra-rotating props (the russian "Bear" i think, and a version of the gannet).  that tells me that as far as planes go 4 is pretty optimal, 6 is the practicalupper bound

personally, i've looked at new wind turbines and wondered ... 1st, "how the heck do they come up with that design?", and 2nd "is that really the most efficient balde shape ??"
GregLocock (Automotive)
11 Feb 08 22:07
I think wind turbines are a tricky case - the most cost effective design (kwH per $) may not be the most efficient (kWh/year).

In ship propellers it is more /efficient/ to run fewer, smaller width, blades than more, higher width blades. But, you can put more power into a given diameter of prop if it has more area.

The ratio of the total projected blade area to the disc area is called the BAR. The data I have seen is not very commital on the efficiency tradeoff is for number of blades for a given BAR, but for airscrews fewer is definitely better.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

thruthefence (Aerospace)
12 Feb 08 19:04
One bladed prop? Here's one:

http://en.wikipedia.org/wiki/Single-blade_propeller

saw a picture of a similar one on a Cub once, streamlined counter weight opposite the blade.
jpblomberg (Mechanical) (OP)
19 Feb 08 14:02
Thanks for all of your responses,  I guess I think thinking of the question more in terms of energy balance.

For the case of a wind mill:  The propeller disk sweeps out a certain circular area A.  The air moving past it has density and speed.  Given these, one can estimate the maximum potential power a propeller could make.  What percentage of this maximum wind energy can a windmill actually (or theoretically) extract?  Does adding more blades extract more of this energy (at least to a certain point)?  Would the speed of the wind (I'm thinking of Reynolds number and not supersonic) affect the number of blades and/or energy extraction.  I guess this is how an ME thinks about AE problems.

I had not seen the one bladed propeller, thanks for th link.  I suppose the thrust asymmetry puts and additional load on the thrust bearings.  Another case I have not seen is a multi blade prop with the blades on one side and a counter weight on the other.  There doesn't seem to me to be any advantage to this configuration.

"Credo ut Intelligam"  I believe in order to understand -St. Augustine

rb1957 (Aerospace)
19 Feb 08 15:08
i'd invest in a propeller design textbook.  i covered this more years ago than i care to remember in my final year at uni, and still have the notes (somewhere).  i remember it as being a pretty straightforward calc (to determine the pwoer absorbed by a propeller.

btw, IMHO belief is on the road to mis-understanding a problem, but then i guess that's just me and not St Augustine !
mohr (Aeronautics)
22 Mar 08 7:16
Hi jpblomberg :

Case 2:

"For the case of airplane at take off, is 1 blade best?"

It depend on the power you need to transmit. But the reduction unit or crankshaft will be affected by the cyclic aerodyamic load from the blade.

"How about 2, 3, 4, 5, 25, 100 blades?"

Again it depend on power to transmit but between limits. If
you increase the blade number, also increase the aerodynamic interference between blades and the prop hub design. Also
a more blade number the blade will be more "thin" affecting the blade strength and flutter rigidity.

"In the case if a large engine, where the rotational speed is not limited, but the diameter of the propeller is,  what would be the optimum number of blades given a particular diameter and angular speed?"

The engine rotational speed and prop diameter are connected in order to maintain the propeller tip velocity under or near 65/70 % of a mach due to a strong decrement of airfoil performance affecting blade efficience. (if you pass this limit the tip section is entering in transonic regime.)
Also you must consider the propeller noise due authority regulations about this matter.

"I realize that there are other factors that affect performance."

Yes jpblomberg, there are other factors and I can be seid a
lot of strongly connected factors. If you "touch" one them
you will affect to all others. And this is the art of prop.
design.

Best regards  


dalcazar (Bioengineer)
8 Apr 08 2:02
The theoretical maximum for wind turbine power extraction, called the Betz limit, is 59% of the wind's total power, the biggest and most efficient turbines these days can extract up to about 35% from last I remember reading.

I believe one of the main reasons for keeping blade count low on wind turbines is to present as small a frontal section as possible to the wind while still maximizing swept area and torque. If you start adding more the amount of force exterted on the tower increases and you end up breaking something. the rotors have to be able to withstand hurricane force winds and adding even a single extra blade increases the force exerted on the tower by 33%.

Theoretically speaking if you could add 2 or 3 blades to a wind turbine and you were 100% certain you would never experience more than the optimal wind speed then you could do it no problem, but that is beyond the realm of reality until we have materials that can withstand that kind of power.

Sacrificing blade speed for more torque isn't a problem since the only thing you're interested in making from the wind is torque, the more torque, the bigger sized generator you can run. Slower blade speed should minimize the effects of interference between blades.

In the case of propellers you have the classic case of an open prop vs a ducted fan. Ducted fans create more thrust at 0 airspeed than a prop of the same size does because the blades don't interfere with eachother as much, however once you exceed about 80 knots the ducting creates more drag than the prop loses from the tip loses. Ducted fans can and do have more blades: 5, 7 or even 9 (the number must remain odd to prevent resonance problems) and turbofans have many many more.

While high bypass turbofans have large ducts which might seem counterproductive, they need the low-speed thrust to get off the ground with huge takeoff weights and to deal with possible engine loss. The average cruise altitude of a jetliner makes for a low enough atmospheric pressure to offset the extra drag.

If you're looking at efficiency though you have to stick with open props, nothing beats a turboprop when you count effciency in pounds per dollars per mile. And in that case, blade count pretty much depends on how powerful your engine is and how much power it can succesfully use. I saw some of the newest C-130s with 6 bladed ultra thin props the other day at the naval air base in oxnard.

And just as fun engineering trivia.. the Tu-95 and it's civil airliner spin-off the Tu-114 are powered by 4 turbines producing 15,000 HP each driving their counter-rotating 4 bladed props that are 18ft wide. At maximum speed the tip speeds exceed mach 1 (in the case of the Tu-95 only) and make it one of the loudest aircraft ever made, to the point that most crews experienced permanent hearing loss. The Tu-114 holds the absolute speed record for a prop driven plane to this day.

I guess the moral of that story is that if you have unlimited power you can get away with a lot :)
tbuelna (Aerospace)
26 Apr 08 2:02
jpblomberg,

It's my understanding that large (utility scale) wind turbines commonly use 3 blade rotors due to issues with the passing frequencies of the blades past the tower.  Since utility scale  wind turbines are a commercial product, the manufacturers would naturally want to use the minimum number of blades possible to limit production cost.  Even though they're constructed of inexpensive FRP, the blades can still account for up to 25% of the cost of a large wind turbine.

The common rotor configuration for large wind turbines is a 3-blade rigid rotor with the blades facing upwind.  However, I've seen a recent design proposal for a 2-bladed, down wind facing, hinged rotor.  Similar to a helicopter rotor.  It was claimed to be less expensive to manufacture, produce less loads on the tower, and be able to operate in higher winds than the conventional rigid rotor.

http://www.windturbinecompany.com/
Intermesher (Aeronautics)
29 Apr 08 23:25
The following might be of some interest;

It is said that for helicopters, the fewer the number of blades the better.

Here is some information on helicopters with single-blade rotors.  http://www.unicopter.com/B472.html


I looked into a single-rotor helicopter a while back. This is it's lead page http://www.unicopter.com/ElectroRotor.html. The single-rotor appears to be attractive in hover and in vertical climb. However in forward flight, with the disk flying edge on to the airflow, there were problems.

 
dalcazar (Bioengineer)
7 May 08 2:16
Downwind hinged rotors are OLD technology, there are a few in the wind farm close to the bay area of san fransisco.

The downwind rotors have had the interference in wind flow from the tower quoted as a disadvantage, they do allow for an easier furling ability for high winds tho.
nveneno (Aeronautics)
2 Jun 08 23:03
The blade tips produce the greatest amount of power loss, but longer blades are affected by the mach number, so there must be a balance between many short blades and few long blades. Additionally, it must be consider that the induced flow in the blades disturb each others.

Now with the use of CFD software, the 3D aerodynamic design produce convergent results. For instance, the Airbus A400M aircraft increase the number of blade of the propeller to 8 to improve SFC; while General Electric reduce the number of blades of the fan in their new GEnX engine to 18 from the 22 blades in the previous GE90.

In the case of helicopters, with cyclic movements, it can be demonstrated that greater number of blades reduce the vibrations.

veneno
dalcazar (Bioengineer)
23 Jun 08 21:48
Something I've discovered while doing research that should be added here, the addition of extra blades to a wind turbine affects the disturbances experienced by a discreet point in the turbine blade. This much is obvious and already stated, but in a wind turbine that increased disturbance leads to a loss of quality of the power delivered by the generator.
darksat (Aeronautics)
20 Aug 08 7:04
Ok, on a somewhat related topic.
Im looking at propeller design for a paramotor (paraglider with motor) the speed is going to be low, the engine power is going to be only a few horsepower and it needs to generate at least 30KG+ of thrust (preferably 50KG)
Now as i am trying to get the weight to be significantly lower than a standard paramotor I was thinking about using a lot more blades (4-6) instead of the standard 2-3 blade design.
main rational is that with more blades I can have a smaller prop safety cage and keep the weight down.
so if I can generate the required thrust with a 2 bade propeller how much smaller can I make it with a 6 blade propeller.
 
GregLocock (Automotive)
20 Aug 08 7:22
OK, can you give us some idea of cage weight vs diameter?

It is a good question.

How many hp do you have, what rpm?



 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

rb1957 (Aerospace)
20 Aug 08 7:35
on a simplistic level i'd start with Dnewprop = Doldprop/sqrt(newnumblades/oldnumblades) = Doldprop/sqrt(3).

adding blades increases the Activity Factor of the prop and i think this reduces slightly the thrust.

consider the tip speed ... the plane's airspeed might be slow, but you want to keep the tip speed subsonic, otherwise it'll be way noisy and inefficient.

maybe you can reduce the weight of the safety cage some other way ... composite materials ?  is the cage intended to prevent the blades hitting important things (people, bits of the plane) if it breaks off or "merely" to stop unintended contact with the moving blades ?  if it is the former, smaller blades might give you a rational reason to reduce the cage structure.  how often have blades been shed ?
darksat (Aeronautics)
20 Aug 08 10:27
The cage is intended for a number of safety factors, the most important being to ensure that the propellers dont slice through the lines of the paraglider (kind of important) and to a lesser extent to protect the pilots arms & legs and ensure the prop wont hit the ground if there is a rough landing or the pilot slips up at takeoff.

An example of the lighest mass produced paraglider on the market is here. http://www.paramoteur.com/us/info_r2.htm
and its engine details here (swedish but tech info at bottom in english).
http://www.radne.se/store/Product/ProductDetails.aspx?PartNo=3201


Now im looking for something a lot more lightweight and have  spotted this engine. http://bcmaengines.com/MT57.htm
and have been chatting with the guy from http://eggmotor.wordpress.com/ about possible development.

His engine generates 6 horsepower (7.5 with nitrous)
which gives 28kg thrust at 8330rpm and 37kg of thrust at 9240rpm (nitrous boost)
He is using a 1.8 Metre 2 blade propeller and his rig also weighs 16KG empty.

The cages generally weight between 8-20KG (and also generate drag). There is a lightweight one here at 5.5KG (1.3metre prop) http://www.paramotorkits.com/main.asp

Now as you can see, im trying to figure out a way to make a superlightweight paramotor that is ultra portable.
Something that will compliment an ozone ultralight 19m2 paraglider.


 

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