Why is 2 blade prop more efficient then 3 blade prop?

[butters149]

Why is the 2 bladed prop more efficient then the 3 bladed prop? Just wondering. This will probably span to other questions I have also for airplanes, thanks!

[nmasters]

Induced drag. Each blade is a wing. For the same engine a 3 blade prop has one more tip than a 2 blade prop and each blade/wing has lower aspect ratio. Lower AR = lower L/D

--Norm

[ghoti]

Like Norm sez and remember each blade has a tip vortex that accounts for the induced drag. Bill

[biber]

It's not the number of vortices that count, but the summed up power lost in them.
It's not in all cases more efficient to have less blades.
Depending on which parametres are given as fix numbers and which ones can be chosen more or less freely.
More blades give a more even distribution of thrust over the prop disc, which is good for efficiency.
More blades usually mean more slender blades and thus more little Reynolds numbers which can hurt efficiency.
Less blades can enable you to increase prop diametre, which is generally good for efficiency.
In any case the optimum number of blades is determined by many different parametres and there won't be a golden rule that says two of them is best.
It only seems to be a popular practical compromise.

See www.mh-aerotools.de for info on props.
Not to forget: http://www.djaerotech.com/dj_askjd/askdesign.html
And maybe: http://www.supercoolprops.com/

biber

[mnowell129]

Each blade travels in the turbulent wake of the blade ahead of it reducing it's efficiency. The further behind you get, the further out of the wake of the preceeding blade. The distance between blades is greater with a two blade than a three. A single blade is most efficient.

[biber]

Each blade travels in the turbulent wake of the blade ahead of it reducing it's efficiency. The further behind you get, the further out of the wake of the preceeding blade. The distance between blades is greater with a two blade than a three. A single blade is most efficient.That's to much generalisation to be correct.
There is a wake of the preceding blade but that has nothing to do with turbulence.
In fact all blades are affecting each other blade with their induced flow fields, but that does not necessarily mean increased losses.

biber

[NX-687]

I tried a one bladed 11x4.7 in place of a 9x4.7 and liked it , didnt like the hum of the counterweight , it must give a better hover as it covers more control surface , that is what I think , Im a little scared of it until I can find a bullet proof way to keep the weight on and the balancing was tricky
Didnt have a full flight and it was windy , so I couldn't judge the hover , but seemed nice to fly

[butters149]

Ok like, I heard on the later Corsairs they had to use a 4 bladed prop to harness the extra power? Even the hawker sea fury had 5 bladed prop and took down a Mig.

So more blades increase drag, but doesnt it also increase torque?

[Brandano]

in the case of the Corsair it was either a matter of adding more blades or using longer gear struts and deepening the inverted gull wing. The prop on the Corsair is enormous, there's a limit over which you can't really go when increasing the diameter, and then the only solution is to add more blades. Incidentally, wouldn't it be nice if it was just sufficient to reduce the number of blades to increase a prop's efficiency? In that case think how efficient it would be to just leave a spinner on with no prop blades to mess the airflow... :)

[biber]

In many cases, especially with IC engines, you have a certain fixed rpm value to work with.
Diametre is limited by that and tipspeed approaching mach numbers close to 1,
or its limited by other practical requirements like e.g. ground clearance with an existing landing gear.
The design speed will determine the needed pitch.
After that there are not too many major parametres left.
Then the summ of blade chords (number of blades times chord of one blade) has a theoretical optimum that's quite independend from the actual number of blades.
The rest of the optimisation is finding a good compromise of reynolds numbers, disc load distribution and obvious practical issues.
For a model without landing gear, e.g., anything more than two blades will be modified into a two blade prop or even less, anyway.
Manufacturing costs are a big factor aswell.

biber

[butters149]

Im not following your terminologies, so a 3 bladed prop, it will have to be smaller in diameter then a 2 bladed prop to have the same amount of thrust?

[JetPlaneFlyer]

Prop blades are just like wings. They produce lift in the same way as wings do but with props it's called thrust.

The lift (or thrust) produced by a prop is dependant on blade area (like wing area on a wing) and airspeed^2 of the blades as they turn through the air, plus coefficient of lift (Cl). If diameter is limited (by landing gear length) and rotational speed is limited (by mach, stress, and/or engine RPM limits) and Cl is limited (by stalling angle of the blade airfoil) then all that remains if you want to increase thrust is to increasing blade area.
Assuming fixed diameter then there are two ways that blade area can be increased; you can make the blades wider or you can add more blades. Wider blades make the blade aspect ratio lower, just like wings low aspect ratio is less efficient, so making the blade wider is not always a good idea.. This only leaves adding more blades.
Multi blade props may generally be less efficient than two blade props (given the same blade shape and aspect ratio) but they are usually MORE efficient than a two blade prop of equivalent blade area and the same diameter.

Assuming of course you have an engine powerful enough to drive the prop to it's required speed ;)

[butters149]

ok I get it, im measuring the blades and i see the blade diameter on the 3 blade prop is smaller then the 2 blade prop. The manual says the 2 blade will have more thrust. If I up the timing on my ESC or get a more powerful engine, shouldnt the 3 blade prop be in its effiency range and propel the plane faster then the 2 blade?

[markdrela]

There is no one answer -- it depends on the case.
Biber summarized it pretty well.
The main tradeoff is profile drag vs. induced drag (or induced losses).

The most meaningful comparison is for a fixed blade area, fixed blade radius, and fixed thrust. Otherwise it's apples and oranges.

When you add more blades, and narrow the chord to maintain blade area, the result is:
1) Less induced drag due to more uniform downwash over the disk
2) More profile drag due to smaller blade Reynolds numbers

On big props at large Re, 2) is not so significant, so more blades is better.
On small props where Re is small and 2) is very significant, fewer blades is better.

[BMatthews]

There's a rather lengthy write up in one of the old Zaic Yearbooks on props for rubber models. It showed how reducing it to a single blade design can actually be even MORE efficient provided the off center forces can be balanced out with rearset counterweights and the like.

BUt having tried 4 of them over the years I found that it's pretty much impossible to make them run smooth through the entire motor run. So now I just avoid them and use two bladed props.

For models there's no need for more than 2 blades to harness the power in the same way as the old piston engine fighters had. So unless there's some OTHER issue that brings an advantage to how the model flies with more than two blades it should be a non issue. For example perhaps a 3 bladed prop gives more braking effect in the verticals to a competition pattern model. Or multi blades produces a more solid seeming prop disc for a ducted prop or maybe it's the very short landing gear of a control like precision aerobatic model (which sort of harkens back to the Corsair and the need to balance prop diameter with landing gear design)

[Aphorism's Dream]

In many cases, especially with IC engines, you have a certain fixed rpm value to work with.
Diametre is limited by that and tipspeed approaching mach numbers close to 1,
or its limited by other practical requirements like e.g. ground clearance with an existing landing gear.
The design speed will determine the needed pitch.
After that there are not too many major parametres left.
Then the summ of blade chords (number of blades times chord of one blade) has a theoretical optimum that's quite independend from the actual number of blades.
The rest of the optimisation is finding a good compromise of reynolds numbers, disc load distribution and obvious practical issues.
For a model without landing gear, e.g., anything more than two blades will be modified into a two blade prop or even less, anyway.
Manufacturing costs are a big factor aswell.

biber

Well said.
Also do not forget the resonance of the blades and prop. system and the attendant coupling through the airframe.
Anyone having experience a severe prop. harmonic problem will not forget it.
I have never forgotten the experience in a Cessna 180 (an ex-float/ski plane) as the propellor went into a resonance node on takeoff to the point of cracking the windshield, wrinkling the upper cowl, poping the oil access door open. The prop was experimental, long and wide. We later found the engine mount was cracked through the tubing and gussets, three of the
Lord mounts had shed major chunks of the rubber. Oh, yes, the propellor hub was cracked through the bearing boss on one blade. We tore the engine down to inspect the crank shaft and bearings. Contiental O-470's had a really good crank and prop. flange (as we proved). That is one takeoff and dead stick landing never forgotten.
Anway, as Dr. Drela states the defined need dictates the type and tradeoff involved.
No lunch is FREE!! :rolleyes:

[vintage1]

I've generally found that the bigger and slower the prop the better..up to around a diameter to span of 30% at which point the model gets to be hard to handle on account of torque..multi blades are generally an acceptable compromise when you need to translate more power than you can on even a 30% diameter prop..that was the WWII experience..massive power and not having the diameter, or indeed keeping the prop tips subsonic..

I haven't ever experimented, its true, with ultra slow multiblade setups..I note that most wind turbines are three bladers..I wonder why..

[Ricardo RW]

Aren't wind turbines pushed by air? If yes then more blades have more frontal area... I'm only guessing.

[Texas Buzzard]

I think what BMatthews posted is correct and the bottom line.
Experience with props on real flying models means more than conjecture.

I can tell you that since about 1965 I have seen maybe half a dozen very good builders/fliers attempt to use three-bladers on .60 sized and .40 sized scale planes. Some of them did permit flight, but the 3-bladers were inferior to the standard 2-bladed props.

Ya' gotta fit the prop to the engine/motor and of course ya' gotta fit the prop to the type of plane your flying. Back then when you'd see one-bladers on a Wakefield (that actually flew) you'd think -"Hey that guy knows something I don't".

With an Andrews "Explorer" and a Super Tiger 0.46 glow I found a wooden Top Flight 10 x 6 (2-blade) did what I wanted to do. An 11 x 6 wasn't so good - it kept that engine down on the H.P. curve...so not as much thrust. I got brave one time and bought a 9 x 6 3-blader. It wasn't as good as the
10 x 6 ,2-blader. I fly with about 15 guys who have been at it for many years and mostly fly the big stuff. I haven't seen a 3-blade prop flying since Clinton was President.

Wind Turbines: Consider the RPM of wind turbines ( they are relatively low RPM machines). Notice the very high aspect ratio of the blades - an attempt to extract energy from the air. High A.R. reduces tip losses. The interaction between the blades is negligent due to low RPM.
Twenty years ago I saw very high RPM Wind-Generators (small stuff) and they used 2 blades.

[Aphorism's Dream]

I've generally found that the bigger and slower the prop the better..up to around a diameter to span of 30% at which point the model gets to be hard to handle on account of torque..multi blades are generally an acceptable compromise when you need to translate more power than you can on even a 30% diameter prop..that was the WWII experience..massive power and not having the diameter, or indeed keeping the prop tips subsonic..

I haven't ever experimented, its true, with ultra slow multiblade setups..I note that most wind turbines are three bladers..I wonder why..

Gearbox vibration loading/dampening!

[vintage1]

Gearbox vibration loading/dampening!

Guess? or knowledge?..it DOES make some sense...

[ghoti]

Windmills turn into the everchanging wind direction and so are subject to gyroscopic torque. This is much smoother with 3 blades than with two.
Bill

[markdrela]

Windmills turn into the everchanging wind direction and so are subject to gyroscopic torque. This is much smoother with 3 blades than with two.
Yep. To be more specific, a 2-blade prop has a much smaller inertia about its "spanwise" axis than its "chordwise" axis. The consequence is that when the airplane has some yaw or pitch rate, such as when doing a loop, the rotating 2-blade prop's precession torque will fluctuate on/off, and shake the airplane at a 2/rev frequency. In contrast, a 3-or-more blade prop has the same inertia on all axes, and therefore its precession torque is constant, so it always runs more smoothly in yaw and pitch maneuvers. Note that this is purely an inertial effect, and has nothing to do with aerodynamics.

[Brandano]

I always suspected the higher vibration of 2 bladed props to be due to some form inertial coupling. Another thing that will affect vibration will be the way the air hit the blades cyclically at a largish positive AOA. This effect should be most noticeable on a single blade prop, and I think it should cause a vibration aligned with the yaw axis

[Aphorism's Dream]

Guess? or knowledge?..it DOES make some sense...
Knowledge.
I'm a vibration analyst and Reliability Engineer as well as A&P/IA.
Predidictive failure and time life studies are very much in my tool kit.

[Neil Stainton]

There's a lot of expertise here. Thanks guys or the interesting and informative read.

Neil.

[Tom Harper]

In the late 1940s Malcolm Anderson held a number of control line speed records using single bladed props. Mal was a machinist with a nice, though small, shop behind his house in Alameda, Calif. He made his own engines and prop systems.

I think the balance mechanism he used was important. The backplate of the spinner was machined aluminum and was considerably thicker than the prop hub. The back of the plate was drilled at several points and filled with lead to get perfect static balance. I helped him in the pits a couple of times and noticed that there was no unusual vibration.

I agree that it is a system design issue. For Mal, he got very high RPM with a larger diameter prop and was able to easily make small pitch adjustments for fine tuning.

[donjiskra]

Guys,
The proof is in performance.
My electric model with a 2 blade produces 73 watts per pound,
with a 3 blade 91 watts per pound,
A 4 blade = 101 watts per pound.

A multi blade prop less effidient?

[markdrela]

The proof is in performance.
My electric model with a 2 blade produces 73 watts per pound,
with a 3 blade 91 watts per pound,
A 4 blade = 101 watts per pound.
A multi blade prop less effidient?
Do all these props have the same diameter?
If you reduce the diameter as you add blades, then you're mainly seeing the effect of a smaller diameter, not of more blades.

To rationally compare different numbers of blades independent of other effects, you have to have the same
* thrust
* airspeed
* RPM
* diameter
* blade area

The last two points require that if you double the number of blades, you have to halve the blade chords. Usually this is structurally infeasible.

[Aphorism's Dream]

Do all these props have the same diameter?
If you reduce the diameter as you add blades, then you're mainly seeing the effect of a smaller diameter, not of more blades.

To rationally compare different numbers of blades independent of other effects, you have to have the same
* thrust
* airspeed
* RPM
* diameter
* blade area

The last two points require that if you double the number of blades, you have to halve the blade chords. Usually this is structurally infeasible.


Bravo!!
Finally the Bull is shown the barn yard gate!
Thank you, Dr. Drela

[BMatthews]

Guys,
The proof is in performance.
My electric model with a 2 blade produces 73 watts per pound,
with a 3 blade 91 watts per pound,
A 4 blade = 101 watts per pound.

A multi blade prop less effidient?

You're comparing apples to oranges. The prop with more area is sucking more current from the battery pack and results in the model flying with a higher watts per lb. OF COURSE the model will fly better with the higher energy levels.

Now what you need to do is prop it with a larger 2 blade prop that draws the same current as the 4 blade prop and fly the two back to back and then come back and tell us which has the better performance.

Provided you use props with equally efficient blade shapes I can tell you that the 2 bladed prop that runs at 101 watts per lb will outperform the 4 bladed prop that runs at the same power level.