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Old Sep 01, 2008, 10:51 PM
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Why is 2 blade prop more efficient then 3 blade prop?

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!
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Old Sep 01, 2008, 11:06 PM
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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
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Old Sep 02, 2008, 01:03 AM
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Like Norm sez and remember each blade has a tip vortex that accounts for the induced drag. Bill
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Old Sep 02, 2008, 04:48 AM
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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
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Old Sep 02, 2008, 05:06 AM
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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.
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Old Sep 02, 2008, 06:05 AM
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Quote:
Originally Posted by 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.
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.

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Old Sep 02, 2008, 06:39 AM
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Yes

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
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Old Sep 02, 2008, 09:03 AM
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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?
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Old Sep 02, 2008, 11:33 AM
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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...
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Old Sep 02, 2008, 11:56 AM
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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.

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Old Sep 02, 2008, 01:00 PM
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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?
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Old Sep 02, 2008, 01:43 PM
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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
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Old Sep 02, 2008, 01:57 PM
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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?
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Old Sep 02, 2008, 04:02 PM
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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.
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Old Sep 02, 2008, 09:14 PM
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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)
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