HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Dec 20, 2003, 01:05 AM
Registered User
Joined Dec 2002
273 Posts
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?
austinflyer is offline Find More Posts by austinflyer
Reply With Quote
Sign up now
to remove ads between posts
Old Dec 20, 2003, 02:25 AM
space for sale
plane stupid's Avatar
Detroit City, Michigan, United States
Joined Sep 2003
181 Posts
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%
plane stupid is offline Find More Posts by plane stupid
Reply With Quote
Old Dec 20, 2003, 09:10 AM
Registered User
Ralph Weaver's Avatar
Indianapolis, IN USA
Joined Nov 2000
2,305 Posts
Check out http://www.djaerotech.com/dj_askjd/d...bladeprop.html

written by a guy who designs full scale props for a living.
Ralph Weaver is offline Find More Posts by Ralph Weaver
Reply With Quote
Old Dec 20, 2003, 12:36 PM
1,1,2,3,5,8,13,21,34
Shortman's Avatar
United States, OR, Portland
Joined Nov 2003
2,116 Posts
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.



Steven
Shortman is offline Find More Posts by Shortman
Reply With Quote
Old Dec 20, 2003, 12:51 PM
RIP Ric
Andy W's Avatar
Marietta, GA
Joined Jun 1999
43,312 Posts
.. 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)..
..a
Andy W is offline Find More Posts by Andy W
Reply With Quote
Old Dec 20, 2003, 01:05 PM
jrb
Member
jrb's Avatar
Edina, MN, USA
Joined Oct 1999
11,433 Posts
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.
jrb is offline Find More Posts by jrb
Reply With Quote
Old Dec 20, 2003, 01:50 PM
Go get them Meg!
lrsudog's Avatar
Cabin 21...
Joined Jan 2001
2,118 Posts
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".
lrsudog is offline Find More Posts by lrsudog
Reply With Quote
Old Dec 20, 2003, 03:10 PM
jrb
Member
jrb's Avatar
Edina, MN, USA
Joined Oct 1999
11,433 Posts
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.
jrb is offline Find More Posts by jrb
Reply With Quote
Old Dec 20, 2003, 05:57 PM
Go get them Meg!
lrsudog's Avatar
Cabin 21...
Joined Jan 2001
2,118 Posts
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.
lrsudog is offline Find More Posts by lrsudog
Reply With Quote
Old Dec 21, 2003, 08:03 AM
Registered User
RonJ's Avatar
Sparta,WI,USA
Joined Nov 2000
1,610 Posts
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.

RonJ
RonJ is offline Find More Posts by RonJ
Reply With Quote
Old Dec 21, 2003, 04:13 PM
Registered User
Rotten Robbie's Avatar
Kent, Wa.
Joined Feb 2001
642 Posts
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.

Robbie
Kent, Wa.
Rotten Robbie is offline Find More Posts by Rotten Robbie
Reply With Quote
Old Sep 23, 2010, 11:02 PM
Segelfliegen bedeutet Freiheit
SkyCadet's Avatar
Canada, MB, Winnipeg
Joined May 2010
1,083 Posts
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!

skycadet
SkyCadet is offline Find More Posts by SkyCadet
RCG Plus Member
Latest blog entry: New plane - Evader!!!
Reply With Quote
Old Sep 24, 2010, 02:42 AM
Registered User
Threshold's Avatar
London UK
Joined Jul 2004
2,530 Posts
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.
Threshold is offline Find More Posts by Threshold
Reply With Quote
Old Sep 24, 2010, 07:39 AM
Canadian Bacon
flypaper 2's Avatar
Kingston, Canada
Joined Jun 2004
13,062 Posts
You would have to measure the thrust with the same brand of props. APC measures the pitch differently than MAS.

Gord.
flypaper 2 is online now Find More Posts by flypaper 2
Reply With Quote
Old Sep 24, 2010, 08:17 AM
DX5e fatal flaw- PM me!!!!
United States, NY, Cortland
Joined Sep 2010
2,839 Posts
Now windmills- 3 blades is optimum.


Dave
CNY_Dave is offline Find More Posts by CNY_Dave
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
What about 2 or 3 blade props? Mattydiah Beginner Training Area (Aircraft-Electric) 3 Oct 27, 2004 06:39 PM
Why brushless motors are more efficient than brushed ones? JLSalas Power Systems 41 Nov 16, 2003 08:47 AM
2 or 3 blade props jsomers1234 Power Systems 42 Nov 30, 2002 06:56 PM