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Two vs three-blade props

How do they compare?

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Tim Kearsley09/11/2009 13:03:49
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Hi all,
 
Does anyone know approximately what the equivalence is between two-blade prop size and three-blade?  For example, if the appropriate two-blade prop for an engine is, say, 14 X 8, what would be the approximate equivalent in a three-blader?
 
Cheers,
 
Tim.
 
David perry 109/11/2009 16:53:07
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IIRC leave the pitch the same but knock an inch (actually, c 10%) off the dia.
 
If its leccy though, use a meter.
 
Dont do it for performance though will you, 3 bladers are worse than 2.  They werent used for performance reasons (unless you class not hitting the ground during rotation as performance, in which case they were!!!)
 
D
Simon Chaddock09/11/2009 17:00:34
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Tim
This is a common question and the full answer is not that simple. A good starting point would be to reduce the diam or the pitch by 1 inch for each extra blade but they would have different thrust characteristics.
Taking off the diameter but keeping pitch would retain similar thrust/speed characteristics whereas keeping diameter but reducing pitch would produce more static thrust but that thrust would fall off faster with increasing  airspeed.
Note a multi blade prop is always a bit less efficient than the equivalent 2 blader running at the same speed, so you pays your money and you takes your choice, but experimentation is the only real answer.

In full size aircraft up to 8 blades may be used to absorb the available engine power within a particular overall diameter but this is set either by the need to keep the prop tips below supersonic or simply for ground clearance.
The loss in efficiency is accepted as inevitable.
I hope this helps.
Myron Beaumont09/11/2009 18:08:30
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I seem to remember folding one blade props with a counterbalance used on rubber models many moons ago ( circa: Ken Anderson)  Am I right ? Probably not to be recommended on IC setups but maybe quite efficient due to the small diameter of our props ie Better grip on clean air ?
David perry 109/11/2009 18:57:53
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You are correct re the single bladed props...I never did get to grips with the thrust / drag, static vs dynamic idea of them at all!
 
D
Tim Kearsley09/11/2009 20:04:03
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Thank you all very much for your replies chaps.  I was toying with the idea of fitting a three-blader to my new Hangar 9 Spitfire (maiden flight last Saturday - beautiful!).  I'd like to make it look a bit more scale and a Spit with a two-blade prop don't seem quite right somehow!
 
Cheers,
 
Tim.
 
Simon Chaddock09/11/2009 21:34:02
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Tim
2 blade Spitfires? or yes there were. Like the prototype, the first 73 used Watts fixed pitch wooden props, although they were soon replaced by variable pitch 3 bladers..

Tim Kearsley09/11/2009 22:02:23
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Well there you go!  Just shows how wrong you can be!
 
Many thanks for that Simon - most interesting.
 
Tim.
 
Aslan10/11/2009 00:28:56
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Brilliant Tim........ Brilliant.
Tim Hooper10/11/2009 07:33:16
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Posted by Myron Beaumont on 09/11/2009 18:08:30:
I seem to remember folding one blade props with a counterbalance used on rubber models many moons ago ( circa: Ken Anderson)  Am I right ? Probably not to be recommended on IC setups but maybe quite efficient due to the small diameter of our props ie Better grip on clean air ?
 
 
 Myron,
 
I think that the ic-powered freeflight brigade have experimented with single-bladers over the years.
 
From what I recall they needed very accurate balancing and substantial construction, not to mention the wear on the crankshaft bearings due to the (essentially) assymetric thrust produced.
 
 I could be wrong.....
 
tim
Phil Wood.10/11/2009 07:57:20
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I've wondered about that as well Tim.........you can static balance the prop 'till the cows come home but the thrust is always going to be off centre and causing vibration.
 
That's why I fly slopers.

Polycan'taffordfuel.
andy watson10/11/2009 09:10:35
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The problem with translating 2 bladers to 3 is that there is not such a range of 3 blade props available to make such a simple switch.
 
I am currently procrastinating over which 3 blade prop for my Saito 1.20 powered stuka.  The Saito manual recommends (2 blade) props of 16*6, 15*8 or 14*10. 
 
Since I am not looking to get max speed I want to keep the diameter as big as possible- reducing pitch, and from memory (though I haven't checked yet) our props are smaller than scale.  This would mean my ideal prop would be 16*4.  Not found one available- the smallest pich 16" prop supplied by Just engines is 16*8- would that cause me a problem?  Otherwise I am down to a 14*9 which should be OK, except I would prefer the big diameter small pitch option for the reasons above.
 
Other shops sell some inbetween these 2 options, but none fit for the 15*6-7 or 16*4-5 range I should be looking at.
John Privett10/11/2009 14:01:48
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Single-bladed props are sommonly used in control-line speed.
 
The pic below is Paul Eisner's model that set the 2.5cc world record in 2001 at 208.28mph (335km/h) though I believe a Hungarian has recently submitted a claim  for a new record.
 

The Irvine 15 revs at around 39,000 rpm - and sounds amazing! The prop is made of carbon fibre with a counter-balance in the hub. 
 
One reason for using single-blade props here is that the pitch can be fine-tuned by inserting a thin piece of mylar film behind one side of the hub.  That wouldn't be possible with a 2-blade prop as it would increase the pitch of one blade and decrease the other one!

Edited By John Privett on 10/11/2009 14:03:47

Myron Beaumont10/11/2009 20:19:25
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John
Now, that is what you might call "fine tuning" isn't it !
John Privett11/11/2009 01:33:50
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It is indeed Myron.  But that's the sort of tweak you need to be able to make to get the last km/h or two out of the model...
Martin McIntosh22/11/2009 22:35:19
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I used to fly my TN 62" Spit on a 12x7 APC with a YS63 but the landing speed was too high so I changed to a YS53 on a 13x5 wood which solved the problem but the power was well down. I changed back to the 63 but this time on a 11.5x6 narrow three blader. This gives way less revs than the 12x7, surprisingly, but it is enough and it now looks right and allows a slow landing.
About time that the prop manufacturers made a much greater range of three bladers.
Simon Chaddock23/11/2009 15:57:36
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Of course if you make your own.........
True scale flight props however just like the real thing you have to be so careful landing.

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