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Old Nov 13, 2001, 04:07 AM
-plancks only-
Franck's Avatar
The Netherlands, UT, Benschop
Joined Oct 2001
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Prop 2, 3 or why not 4 blade.... ?

Hi,

As my "Partenvia-Star" is almost nearing its completion i ran a new thought about prop choice. The plane is to be initially mounted with 480's and the standard 6.5*4 graupner props , but why should I not use a three bladed prop, or maybe a fourbladed ??

Can anyone give me a clue on this ?

Regards,

Franck
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Old Nov 13, 2001, 04:22 AM
high-speed freak
opualuan's Avatar
San Jose, CA
Joined Sep 2001
3,863 Posts
depends on what kind of batteried/esc you are using and what kind of performance you want. The easiest way it to try different combos in motocalc. If you use too many blades on too big/pitched a prop and your motor/esc/batteries can't keep up, one of those three could fry... also, I think pitch speed is reduced as you add blades...
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Old Nov 13, 2001, 05:50 AM
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Bill Glover's Avatar
United Kingdom, Bracknell
Joined Nov 2000
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This gets raised from time to time, the general view is that 2-bladers are more efficient. I tested 2 and 3-blade props on my Twin Star (6" 2-bladers, 5.6" 3-bladers) ... both set to pull the same amps (Varios - variable pitch) ... and the 2-bladers definitely gave slightly better performance. The 3-bladers looked very cool though!

On full-size there are generally reasons for going 3 or 4 blade and reducing diameter ... like not needing such a long u/c, or being able to mount the motors closer to the fus./each other on a multi engine type. Noise may also be a factor, if you're turning a smaller diameter prop at the same rpm then blade tip speed is reduced.

But I'm sure there are many factors, and there may be situations (combination of prop rpm and airspeed maybe??) where 3 or 4 blade props can be as efficient as 2-bladers.
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Old Nov 13, 2001, 09:12 AM
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N. Staffs, UK
Joined Jan 1997
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That's about it, 2-bladed props are slightly more efficient and a lot easier to get hold of.

Probably the only good reason for more than 2 blades on our planes is clearance. An equivalent prop with more blades will be smaller. Geared motors used on a twin may need more blades to keep them clear of the fuselage.

Steve
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Old Nov 13, 2001, 12:13 PM
Master
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Weilbach, Germany
Joined May 2001
2,470 Posts
Considering that the majority of contest rubber powered models (e.g. Wakefields) have single blade folders, it's surprising that 1 blade props are apparently non existant on the electric scene.
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Old Nov 13, 2001, 12:48 PM
Hairy E-Pilot
All__talk's Avatar
Ellensburg, Wa, USA
Joined Dec 2000
554 Posts
There was a discussion some time back on one bladed props, I donít know if the thread is still around. Theoretically they are the most efficient, in practice itís an issue of dynamic balance. I personally think itís very possible and have even sketched up a simple design, but havenít done any experimenting yet.

Gary
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Old Nov 14, 2001, 04:47 AM
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United Kingdom, Bracknell
Joined Nov 2000
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Control-line speed models (IC) have used single-blade props for years and years. But plenty of very successful flyers use 2-blade props, so the net advantage can't be that clear cut. You don't see any models with 3 or 4 blade props though
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Old Nov 15, 2001, 07:08 PM
Registered User
Coimbra, Portugal
Joined Feb 2000
875 Posts
Hello,


MAYBE the extra efficiency of props with less blades comes from two factors:

1) the distance between blades is larger-> the wake generated by one blade has a less disturbing effect on the the "following" blade.

2) there are less blade (wing) tip vortices. Just like a monoplane is generally more efficient than a biplane because, among other reasons, it has two, instead of four, wing tip vortices.

The tip vortices are caused by the tendency of the higher pressure air in the lower surface of the wing to move to the lower pressure zone in the upper surface (winglets try to prevent that kind of "circulation").

The energy spent is putting the molecules in "funny" motion patterns (vortices) is energy wasted.


Regards,

Jo„o
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Old Nov 20, 2001, 08:15 AM
Registered User
Coimbra, Portugal
Joined Feb 2000
875 Posts
Hi Franck,

I am using with great sucess 3 blade props on my TJ, togheter with Permax 480 motors from MPX (2,3 mm shaft, not retimed for pusher mode).

They are Varioprops with 5,8"x (...), at the moment 4". That pitch is in the middle of the supplied scale and itīs working so fine that it will remain in use in the near future. The Varioprops can be set for pusher action.

http://www.varioprop.de/

The idea was to otain, at a reasonable price, an increase in performance from the stock setup, still using a 8 cell Sub-C pack.

The TJ has limited clearances, the hand one of "personal" importance!
You can see in the photo that I added a small dorsal fin. The fuselage form is also slightly changed...

Conclusions:

-The FUN factor has increased a lot

-It is a lot quieter

-Comparing my TJ with the TJ of a clubmate flying on 8 Sub-C cells and the Graupner Comp. Set we think mine climbs better.

The same fellow has a Catalina Flying boat, similar to the HU-16 presented in the Vario site. He his using almost the same setup I use on the TJ. The differences are the use of 6" props and less pitch.

Regards,

Jo„o
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Old Nov 20, 2001, 08:43 AM
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United Kingdom, Bracknell
Joined Nov 2000
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I like Vario props a lot, they are not cheap ... but are quiet and efficient. I used a 3 blade 5.6" one on my Pico Jet for a while for the same reason - a little extra blade area, while keeping to a diameter that still clears the ground!

Quote:
Originally posted by joao
-Comparing my TJ with the TJ of a clubmate flying on 8 Sub-C cells and the Graupner Comp. Set we think mine climbs better.
8 cells is the minimum for the Comp set (7.2V motors), it's propped to run on up to 10 cells so that's where it would perform best.
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Old Nov 20, 2001, 09:38 AM
Clu
Glavin!
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Southern NJ
Joined Aug 2001
283 Posts
Anyone using / recommend a 3 blade for a Zagi?

I'm wondering if it would be quiter than the stock spoon prop.
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Old Nov 20, 2001, 09:46 AM
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Bill Glover's Avatar
United Kingdom, Bracknell
Joined Nov 2000
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Anything that moves the prop blades back a little (away from the wing) will be quieter.
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