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Old Jan 19, 2004, 07:36 PM
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3 or 4-blade prop vs. 2-blade prop

If I switch to a 3-blade or 4-blade prop, what are the benefits?

More thrust, power, speed?

I have a Superstar EP with the factory prop right now. I would like to try a different prop with more blades if there is any benefit to doing so.

I already have the GP 600 gearbox and larger 2-blade prop(not installed), but would like to try a 3 or 4-blade prop for curiosity.

Could someone please explain the benefits/drawbacks to this approach?
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Old Jan 19, 2004, 11:29 PM
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The advantage of the multi-blade props is only that they can absorb more power for a given ground clearance... or other structure clearance.....(and they look kinda cool.......mostly because they are associated with high powered airplanes)
Each blade that rotates through the air disturbs the air that the next one encounters...the most efficient props were the single bladed ones,,,,but there are then rotating balancing to contend with. It was done in the thirties. One of my insructors in A&P school told the story of a friend of his that had one installed on a Piper Cub....he would carry the broken half of some other prop in the plane, and when he saw a group of folks on the ground, he would throw it out the window......then throttle back so his mono-prop could be seen spinning and then fly all goofy and gyrating-like barely making it down alive, just to see the look on thier faces and get a chuckle...... do a google and you'll probably find a picture.


Jim
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Old Jan 20, 2004, 08:17 AM
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Many myths abound regarding multiblade props versus two; one normally stated early in most threads about the subject is that more blades affect the airflow to others. This is simply not true as shown by several photos in the thread referenced below.

Multiblade props when properly matched to a motor will provide increased performance as compared to a two blade prop!

Sail n Soar and I had quite a conversation about this subject; he the opponent I a proponent. We hung in there together and in the end he actually found a well respected engineering reference and did his own MotoCalc analysis supporting my position. Hereís the link to the summary findings, look at the other pages as well: http://www.rcgroups.com/forums/showt...5&pagenumber=3

Back to your case specifically, the variable ratio gearbox (GD600) gets you close. Then youíve got to find a production prop if you donít want to make one yourself; this maybe problematic.

Keep the pitch the same but reduce the 2 bladesí diameter to 92% for a 3 blader & 87% for a 4.

Check out my Mustang (photos & videos) in my gallery; it flies very nice (looks good too) with its 4 blader: http://rcgroups.com/gallery/showgall...stype=2&si=jrb
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Old Jan 20, 2004, 11:23 AM
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Nobody has ever said that extra blades didn't increase performance. Of course a 3-blade propeller is going to give you more thrust than two, and a 4-blade more than a 3-blade.

The issue is EFFICIENCY. Three blades will not give you 1.5 times the thrust of two. Four blades will not give you 2 times the thrust of two. As you add blades, the losses increase exponentially; the prop becomes less efficient overall, but it does increase performance.

Performance and efficiency are two completely different issues. You're confusing them.
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Old Jan 20, 2004, 11:27 AM
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Multiblade props are more efficient at making thrust from rpm than a two blader.

Several form of efficiency are discussed in the reference thread; doubt seriously Iím confused.
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Old Jan 20, 2004, 03:32 PM
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Semantics at work!

I think that mkirsch1 is on the right track here...


If you were to compare thrust generated per unit of power absorbed by the prop, you would see that a 1-bladed prop is the most efficient.

The efficiency -will- drop as you increase the number of blades, but it's a fairly minor amount, and in most cases it's simply impractical to stick with a 2-bladed prop when power levels get high.


The comment: "Multiblade props are more efficient at making thrust from rpm than a two blader." is again, misleading, and odd.

Sure the multiblade prop produces more thrust at a given RPM, but how is that relevant to power and therefore efficiency?

Clearly, a multiblade prop will require more torque to turn, so the power going to the prop will be higher for a given RPM.

Even if you adjust the pitch of the multiblade to match the torque of the two-blader, you now are comparing apples to oranges as the thrust curves are completely different!



All through World War II props were getting bigger and bigger with more and more blades, as the power level of the engines increased, but ground clearance wouldn't allow for larger props!

They even got to the point of contra-rotating props to absorb the power available!

They would have used two-bladed props if the landing gear could have been designed to be 20' tall! (and the tips didn't go transonic)



It's far too easy to make what is apparently a "logical" leap of faith!
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Old Jan 20, 2004, 03:45 PM
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As was described in the referenced tread there is only 1 very special case where a single blade prop is used Ė 1/2A control line speed. Can you state either other modeling application or full scale for that matter?

1st note that the revs are quite high; as high as an EDF Ė 30krpm.

Also, the prop is specifically aligned based on the position pf the crank and the single low mass piston.

They use a single blade, because they need a larger prop disk for static thrust as the models must ROG.
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Old Jan 20, 2004, 07:49 PM
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Quote:
Originally posted by jrb
Can you state either other modeling application or full scale for that matter?
I can. There are self launching sailplanes (full scale) with one blade props.

There's usually a good reason why designers go with more than two blades. It's either a tip speed issue (which is also a noise issue) or ground clearance or something.

I'd be surprised if a 4 bladed prop was more efficient than a two. If it is, then NASA has missed it and is building planes with inefficient props!

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Old Jan 20, 2004, 11:17 PM
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Some do as can be seen in the photo; not all though.

And like the 1/2A CL speed for a reason other than propeller efficiency -- simplified stowage!
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Old Aug 11, 2006, 07:07 AM
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Hi, may I join your forum with regards to two blade props versus multiblade propeller. Right now I have being experiment multiple blade propeller for use in Rc Electric ducted fan.
I found if I cut down or modified a 6 bladed prop to 3 blade the EDF runs more efficiently and it works , more thrust at lower power.
However when I cut down to two blade I encounter sever resonance /vibration at a certain rpm(infact several rpm setting) .

Although the props(or impeller is well balance) and runs outside the ducted shroud.
Anyone has any idea how to address this problem or looks like I have to use at least 3 blade.

Thanks in advance for your discussion.

Cheers
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Old Aug 11, 2006, 08:01 AM
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You're getting resonance with the fan shroud. Probably you have to stick with 3 blades.
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Old Aug 11, 2006, 09:10 AM
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Quote:
Originally Posted by Andrew McGregor
You're getting resonance with the fan shroud. Probably you have to stick with 3 blades.
Thanks Andrew, for your feedback. Will go back to 3 or more blade as your advise.

Cheers
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