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All Forums >> RC Airplanes >> Questions and Answers >> Three Blade Prop
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Three Blade Prop - 6/21/2008 10:57:33 PM   
twowings53


 

Posts: 19
Joined: 11/20/2007
From: Morrison, IL, USA
Status: offline 		Good day to all. I was wondering if there was a rule of thumb when using a three blade prop...Jim
       Post #: 1

RE: Three Blade Prop - 6/21/2008 11:27:24 PM   
Campy



Posts: 3396
Joined: 1/2/2002
From: Baltic, CT, USA
Status: offline 		When converting from a 2 blade prop to a 3 blade prop, either drop 1" in diameter OR 1" in pitch. For instance, going from a 12x6 2 blade to a 3 blade prop, you would want EITHER a 12x5 3 blade OR a 11x6 3 blade prop.

Unless you need the ground clearance stay with a 2 blade prop. The more blades you have on a prop, the more INEFFICIENT it becomes (at least for us modelers {:-) )

(in reply to twowings53)
       Post #: 2

RE: Three Blade Prop - 6/22/2008 1:34:55 AM   
da Rock



Posts: 6711
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		Only problem with 3 bladed props is that there aren't any. Well, actually, there is almost no selection.

Very few LHSs have any in stock. And if they did, there won't be but best case a couple of pitches for each diameter.

My 55AX liked a MA 12x6(3) cut down to be an 11.5x6

The OS61FX liked a MA 12x6(3) It would pull the 12x8(3) but wasn't giving full power.

The OS75AX runs a 13x8(3) MA. It the engine's best prop, yet the engine runs in the lower end of it's rpm range.

The OS91FXs all do very well with the 14x7(3) MA which is good luck because I've not found any other 3blade 1 step up or 1 step down. Closest are 13x8 and 14x9. Would like to try a 14x8, 15x6, 15x5 but they either aren't produced or aren't advertised where I can find them.

(in reply to Campy)
       Post #: 3

RE: Three Blade Prop - 6/22/2008 1:50:29 AM   
khodges


 

Posts: 5094
Joined: 7/3/2003
From: newton, NC, USA
Status: offline
quote:

ORIGINAL: twowings53

a rule of thumb when using a three blade prop...



One more chance per rev to cut your finger (or thumb).

I know the argument about the blades travelling in the turbulence of the one before it decreasing the efficiency, etc, but it still seems to me the extra thrust of the additional blade would outweigh the loss of efficiency. I once flew a Sportsman Waco using a Saito 1.00 and a 15-6 two blade prop. I went to a 14-7 three blade and was amazed at how much better my vertical performance was, even with an overall decrease of almost 1000 rpm at WFO.

_____________________________

Club Saito #2, WACO Brotherhood #20. What other trouble can I get into?

(in reply to twowings53)
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RE: Three Blade Prop - 6/22/2008 2:23:18 AM   
stang


 

Posts: 251
Joined: 6/7/2002
From: Edmonton, AB, CANADA
Status: offline 		The best "rule of thumb" is don't use one.

_____________________________

Happily turning expensive fuel into a loud noise.

(in reply to khodges)
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RE: Three Blade Prop - 6/22/2008 6:13:58 AM   
JohnBuckner



Posts: 3933
Joined: 12/23/2001
From: Kingman, AZ, USA
Status: offline 		Now this is an efficient prop no rule of thumb needed:

Attachments
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_____________________________

"Keep your controllines tight"

(in reply to stang)
       Post #: 6

RE: Three Blade Prop - 6/22/2008 6:46:27 AM   
twowings53


 

Posts: 19
Joined: 11/20/2007
From: Morrison, IL, USA
Status: offline 		Hey johnbeckner it's nice to see you see circle turners. Before i was in r/c i did C/L for 25 years sport and was competitive NATS 1973 (AMA43008) Keep it going....Jim

(in reply to JohnBuckner)
       Post #: 7

RE: Three Blade Prop - 6/22/2008 11:13:01 AM   
da Rock



Posts: 6711
Joined: 10/11/2005
From: western, NC, USA
Status: offline
quote:

ORIGINAL: khodges


quote:

ORIGINAL: twowings53

a rule of thumb when using a three blade prop...



One more chance per rev to cut your finger (or thumb).

I know the argument about the blades travelling in the turbulence of the one before it decreasing the efficiency, etc, but it still seems to me the extra thrust of the additional blade would outweigh the loss of efficiency. I once flew a Sportsman Waco using a Saito 1.00 and a 15-6 two blade prop. I went to a 14-7 three blade and was amazed at how much better my vertical performance was, even with an overall decrease of almost 1000 rpm at WFO.



Lots of theory actually doesn't apply. OK, "apply" isn't a good choice of word.

The "efficiency" theory is one that misses by a wide margin for models.

We really don't have prop choices enough that efficiency makes a difference. I've got a number of models that fly as good or better on 3-bladers as they do on 2-bladers. How is that possible with "less efficient" props? One major possibility is that the efficiency difference is so minor as to be of no significance. I'll vote for that after seeing almost every model I have fly as good or better on the 1 or 2 three bladers that are available. After all, there are 10-20 two blade props that fit the engine. How can just a choice between just 2 wind up giving as good results as the best from 10 or more?

What makes a difference in models is what works.

And my TigerII, Calmato Sport, Tiger60, P47, F4U, and Skybolt all fly as good or better on three bladed props as they do on the best two blades I've tried. Why? Luck and testing.

And I just tested speed and vertical and uniformity of run (subjective). I didn't check fuel mileage. Is fuel mileage what "efficiency" means? Could be. Never read anyone quoting the efficiency theory who clearly explained it. Or how they'd come to believe it.

(in reply to khodges)
       Post #: 8

RE: Three Blade Prop - 6/22/2008 2:56:19 PM   
Sarges_heroes2003


 

Posts: 505
Joined: 7/2/2004
From: Springfield, MO, USA
Status: offline 		for any scale 2 bladed is the most efficient (I.E. translates the engine torque to thrust W/ minimal loss) the only reason full scale planes used 3 or more blades is ground clearance. plain and simple. now a single blade would be THE most efficient save for the counter weight you would have to add to ballance out the blade which is why 2 bladed props are the most common. we could go into the full physics &wind tunnel tests to prove it or just go with it.

P.S. I run 2 bladed models.

_____________________________

I believe the cripple stool is the Cadillac of the poopin stools!
Larry

(in reply to da Rock)
       Post #: 9

RE: Three Blade Prop - 6/23/2008 11:57:24 AM   
jetmech05


 

Posts: 1654
Joined: 7/12/2005
From: Jacksonville, FL, USA
Status: offline 		here we go again.....I've seen some models fly better both speed and vertical on 3 blades than 2..
Sarges heroes...question please sir....If your statement were correct why then is the J model 130 coming out with a 5 blade prop? there was plenty of clearence with the 4 blade prop..and the first A models had 3 blade props...
Now Robert Johnson in his book "Thunderbolt" said something to the effect of keeping up with or almost keeping up with a 109 going vertical once they went to the 4 blade 16 ft prop...finally had a prop that the massive R2800 could throw around....
Besides in flight the prop blade due to forward momentum is always in stable air... the turbulent air from the previous blade is now behind the advancing prop blade.

(in reply to Sarges_heroes2003)
       Post #: 10

RE: Three Blade Prop - 6/23/2008 12:04:07 PM   
jetmech05


 

Posts: 1654
Joined: 7/12/2005
From: Jacksonville, FL, USA
Status: offline 		Now on the other had a 3 blade prop on the Mustang and the Raptor are terrible and I advise a student to switch to a 2 blade prop before we fly...but look at the thing it has a very wide prop blade....
Seems that Master Airscrew..yea yea I know Master Airscrew is putting out very good scale looking 3 blade props

(in reply to jetmech05)
       Post #: 11

RE: Three Blade Prop - 6/27/2008 3:14:51 AM   
saramos



Posts: 2247
Joined: 12/6/2003
From: Northridge, CA, USA
Status: offline 		Want a three blade prop? Take a look at the SP series of engines from RCV.

Scott

(in reply to jetmech05)
       Post #: 12

RE: Three Blade Prop - 6/27/2008 9:18:04 PM   
ecmiller


 

Posts: 134
Joined: 12/22/2004
From: Norwalk, OH, USA
Status: offline 		The power to weight ratio is skewed so much toward power that any inefficiencies attributed to 3 or 4 blade props is overshadowed.

Apply the same power to weight ratio to full scale aircraft and you will basically have.....well...a really fast aircraft.

(in reply to saramos)
       Post #: 13

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