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Old Dec 20, 2003, 01:05 AM
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Are 3 blade props more efficient than 2 blades

Are three bladed props more efficient than 2 blades? Is so, what sort of thrust gains can I expect...say on a 10x10 prop 2 blade vs 3 blade?
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Old Dec 20, 2003, 02:25 AM
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plane stupid's Avatar
Detroit City, Michigan, United States
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none as a general rule more blades are bad but some planes need them to look right. as i don't have first hand kowledge i am less then 100% sure only 95%
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Old Dec 20, 2003, 09:10 AM
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Ralph Weaver's Avatar
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Check out http://www.djaerotech.com/dj_askjd/d...bladeprop.html

written by a guy who designs full scale props for a living.
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Old Dec 20, 2003, 12:36 PM
Shortman's Avatar
United States, OR, Portland
Joined Nov 2003
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3 Blade props are a little more common now in the gas area from IMAC noise regulations. The 3 blade props quiet the engine while giving more thrust then a 2 blade prop would.

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Old Dec 20, 2003, 12:51 PM
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Marietta, GA
Joined Jun 1999
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.. but are generally less efficient, as they have less 'clean air' to bite into. There used to be a lot of single-blades in use in pylon (using a counterweight).. you see this in self-launching sailplanes a lot too, but that's for convenience (so the motor/prop can be folded back into the fuselage)..
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Old Dec 20, 2003, 01:05 PM
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Edina, MN, USA
Joined Oct 1999
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Actually austinflyer you can get better performance with multiple blades.

Here’s a thread where Sail n Soar and I had a stimulating conversation about multi blade props: http://www.rcgroups.com/forums/showt...5&pagenumber=3 .

It was very good that we hung in there during the discussion and that SnS found an Engineering reference which put to rest the old multi blade prop myth.
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Old Dec 20, 2003, 01:50 PM
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The original posting is actually asking two very different questions. To Paraphrase; A) Are three bladed props more efficient then two bladed? B) Will a Three bladed prop give better performance then a two bladed?

One engineering reference does not answer both questions. I have to agree with Andy here. Nearly one hundred years of empirical and anecdotal evidence seems to weigh in favor of one bladed propellers being [i]More efficient[i], due to the increased likelyhood of operation in an undisturbed fluid.

That being said, real world compromises make one bladed props incredibly impractical, and efficiency does not necesarily translate to more thrust.

Add to the problem a need to provide maximum thrust at the most effective pitch speed for the particular aircraft an at widely divergent airspeeds, means that "The best" prop is usually a multi blade.

But for a fixed airspeed, fixed power output application (Such as a free flight model) a single blade would appear to remain the "Most efficient".
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Old Dec 20, 2003, 03:10 PM
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What is meant by efficient?

Changing rpm into velocity (thrust really)? Multiblades are clearly more efficient.

Watts Out to Watts In? Again multiblades are more efficient.

Lrsudog, did you read the referenced thread or just react to the posting here?

Should be noted that the engineering reference was found by a opponent of mutliblades! He was also surprised by his MotoCalc analysis which also showed a mutliblade to be better. Again, these findings were reported by an original opponent to multiblades.

Old myths die hard! Many still quote the “one blade affects the other’s air” when both photographic and acoustic data clearly indicate otherwise.

I the proponent did not go back to my Propulsion Test and BSAE & MSAE references; it was an opponent and he found a refernce contrary to his pinion.

Without the ability to match a prop’s load effectively to drive systems power output keeps the myth going and going. Its quite easy to hang a prop on a system that will not give you better performance when there’s no way to alter the system.

A variable ratio drive will allow you to find a better prop, yes which might even be a multiblader, as compared to having a fixed ratio between the motor and prop!!!

Here’s a link where a glow guy went to a variable ratio drive to get away from the inherent limitation without: http://www.rcgroups.com/forums/showt...hreadid=178549 .

We’d be in hurting status if we could only go direct drive.
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Old Dec 20, 2003, 05:57 PM
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Yes, I did read it.

It remains, (and the comments you posted from Hartzell evidence this) that for a fixed given thrust and fixed desire airspeed, the efficiency of a propeller climbs as the number of blades drops. Presumably this advantage disappears when blade count drops below "1".

I would think that efficiency would normally be defined as the ratio of power applied to the propeller vs. the power the propeller imparts to the medium it is moving through.

I assume that when we are speaking of "More efficient", we meant in absolutes, and not across an un-named spectrum of design requirements. Clearly a multi-blade propeller will be more efficient across a wide range of RPMs and airspeeds.
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Old Dec 21, 2003, 08:03 AM
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My GWS Beaver loves 3 blades. After breaking my last stock GWS prop all I had in the box was a few 3 bladers so popped one on. Best flight I ever had on the ol' Beav. Hand launches with a gentle push and delivers 13 solid minutes on 8-750 nimh's (Ray-O-Vacs). Looks good to.

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Old Dec 21, 2003, 04:13 PM
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Kent, Wa.
Joined Feb 2001
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The simple answer is that if the maximum diamater 2 bladed prop does not produce enough thrust than move up to a 3 bladed prop.

But a 3 bladed prop will not produce 1.5X the thrust of the same diameter and pictch 2 bladed prop. But it will almost be 1.5X. But you also need to have to have at least 1.5X the power to drive it.

Real airplane that use props with more than 2 blades do it because of the limited diameter. Why shouldn't model airplanes do the same.

Kent, Wa.
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Old Sep 23, 2010, 11:02 PM
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Canada, MB, Winnipeg
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Old thread, great info!

Hey gang!

The 3-blades up the thrust by ~1.4x [sqrt of 2], however, depending on a LOT of factors [blade area/aspect ratio, size, mass distribution, etc...] there can be situations where a 3-blade may be more efficient.

My DriveCalc shows 188-190W for a 2-blade 10x7 APC prop, the current draw is the same as a MAS 9x7x3 blade, except that the effective thrust with the latter is 25% higher! Same watts, close RPM, more thrust!

Glad to hear that "myth busting" is active!

BlueSkies to all!

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Old Sep 24, 2010, 02:42 AM
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Threshold's Avatar
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What about a bigger diameter giving more thrust? If you fit a 3 blade to a motor you will have to go down in diameter that must give less thrust.
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Old Sep 24, 2010, 07:39 AM
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Kingston, Canada
Joined Jun 2004
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You would have to measure the thrust with the same brand of props. APC measures the pitch differently than MAS.

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Old Sep 24, 2010, 08:17 AM
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Now windmills- 3 blades is optimum.

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