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Old Oct 23, 2012, 09:31 PM
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2 Blade v. 3 or 4 Blade Prop

Hello All,

Just picked up a Thunder Tiger Pro 46. Following manufacturer recommendations, it says I should start with a 10x6 for break in, and a 10x7 for operation.

Does anyone have any suggestions as how to decide the number of blades? (I was thinking starting with 2) And how to account for an increase in number of blades?

I've also looked around and tried most of the static thrust calculators out there, but none seem to take into account blade numbers, does anyone know of any formulas (with true scientific background) that may help?

Thank you!
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Old Oct 23, 2012, 10:10 PM
-insert witty saying here-
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2 blade 10x7 will be optimum. Three and four blade props aren't very efficient at the RPMs a 2 stroke .46 will turn.
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Old Oct 23, 2012, 10:10 PM
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Two blades.

1. Easier to find
2. More choices of size
3. Easier to balance
4. I think 3 blade props sound "funny".
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Old Oct 23, 2012, 10:20 PM
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4 blades are very uncommon. 2 blades are cheap and everywhere. For beginners I suggest a Master Airscrew black or Scimitar prop for durability. Everybody is going to tell you how APC props are the schnitt but I have seen a lot of beginners spend a lot of money on APC props after a simple runway strike that a Master Airscrew will shrug off. Depending on your application I would use an 11X6 prop after your break in, it seems to works really well on every .46 size engine I have used it on.
However I also tell people to grab a hand full of props in the approximate size and fly them, then you will know first hand. Here is my Thunder Tiger 46 Pro on my sons trainer during initial flights.
Nexstar takeoff with Thunder Tiger 46Pro (0 min 32 sec)

Ignore my crappy crosswind takeoff please.
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Old Oct 24, 2012, 01:44 PM
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Isn't it better for your engine to have the prop break as a result of a runway strike, rather than imparting all of the force back to the crank, rod, wristpin, etc.?

And is anyone really comfortable starting an engine (or even being around someone else) knowing the prop's integrity has been reduced from a runway strike?

How much are props?

Kurt
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Old Oct 24, 2012, 06:22 PM
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OH come on.
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Old Oct 24, 2012, 11:37 PM
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United States, Mt, Bozeman
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Quote:
Originally Posted by jeffie8696 View Post
OH come on.
I like your opinion here Jeff.

Two blade will easily get you the most thrust, three blades are mostly for looks/ground clearance.

Runway strikes (should) occur at a low throttle setting, which, will just shut the engine down with no damage to the engine and often leave a MA prop doing just fine.
Also, if you are flying anything but a speed demon, you probably won't notice the difference between a MA and an APC.
I ride the MA 10x7 3 blade on a 46 ax, and it pulls the trainer like crazy. It really pulls off the line and gets it some pretty good speed too!

*The only reason I have it rather than a 11x6 is due to clearance.
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Old Oct 25, 2012, 02:31 AM
use the 4s luke, use the 4s...
scrub monkey's Avatar
Australia, QLD, Booyal
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For toughness get a ram prop. Damn near indestructible. And fairly cheap to. Cheaper than an apc for sure.
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Old Oct 25, 2012, 04:03 AM
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From the Zinger FAQ

http://www.zingerpropeller.com/_2_blade_.htm
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Old Oct 25, 2012, 04:07 AM
use the 4s luke, use the 4s...
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If 3 and 4 blade props are supposed to give more thrust how come they arent a craze in 3d?
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Old Oct 25, 2012, 04:31 AM
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G'day gr_yamms,

Back to your original question.

Manufacturers recommendations usually refer to two blade props. 10X7 or 11X6 are about right for most applications on your engine.

When choosing your prop you are looking at pitch to match the expected speed range of your model (high pitch equals faster speed, lower pitch equals less speed but better climb). You then look at the diameter that your engine is capable of turning and staying within its power band. The bigger the diameter your engine can manage the more thrust you are going to get out of your engine. As others have mentioned, be prepared to experiment with sizes and brands of prop if you want to get the best possible performance.

A two blade prop allows you to have the maximum diameter for a given pitch, so will normally give the best performance. A three or four blade prop will require a reduction in either diameter or pitch to keep rpm the same.

So why would you go to a three or four blade prop?

Ground clearance problems (or fuselage clearance in a twin)
Noise reduction ( normally only a problem in bigger engines)
Scale model of a three bladed original.
It looks cool.

The usual rule of thumb (not scientific but it's close) is to reduce either pitch or diameter by one inch for every extra blade. So your 10X7 becomes a 9x7 or 10X6 three blade, and your 11X6 becomes a 11X5 or a 10X6.

Xoar props suggest multiplying the diameter by .904 (I think from memory) when you go up by one blade, which matches quite well.

Hope that helps

Dave H
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Old Oct 26, 2012, 12:21 AM
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Quote:
Originally Posted by hutchinstuff View Post
I like your opinion here Jeff.

Two blade will easily get you the most thrust, three blades are mostly for looks/ground clearance.

Runway strikes (should) occur at a low throttle setting, which, will just shut the engine down with no damage to the engine and often leave a MA prop doing just fine.
Also, if you are flying anything but a speed demon, you probably won't notice the difference between a MA and an APC.
I ride the MA 10x7 3 blade on a 46 ax, and it pulls the trainer like crazy. It really pulls off the line and gets it some pretty good speed too!

*The only reason I have it rather than a 11x6 is due to clearance.
We use 3 blades on our big warbirds because they reduce decibels. We are talking Mejzlik props here, not wood and the carbons are already less noisy.
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Old Oct 26, 2012, 11:31 PM
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I dont believe just decreasing the pitch of your prop will increase thrust, you need to increase diameter when reducing pitch to pick up thrust. I think it is a common misconception.
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Old Oct 26, 2012, 11:48 PM
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Birmingham, Alabama
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Quote:
Originally Posted by kbozarth View Post
Isn't it better for your engine to have the prop break as a result of a runway strike, rather than imparting all of the force back to the crank, rod, wristpin, etc.?

And is anyone really comfortable starting an engine (or even being around someone else) knowing the prop's integrity has been reduced from a runway strike?

How much are props?

Kurt
no, unlike pissant electrics, fuel engines are typically over-built and take no damage from issues like spinner first lawn darts into terra-r. maybe knock off a cooling fin, or warp a carb barrel till you wear it back in-round lol
again, like engines, props are far stronger. wood props can split, yes. non wood props either suffer light marring, the tips are "wore off". you simply clip the the same length & continue on. (or prop strike your next landing and itll wear it about true again)


all things are relative to what what RPM you are turning, the power curve of the engine VS the rpm you're turning, the prop & the flight speed.

a pair of classic examples with common .45 class engines:
when tached for max ground RPM, a 10x6 will have a faster pitch speed calculation against a 10x7. Yet in the air, the 10x7's will usually unload more. (depending on engine powerband)
an 11x5 will often times produce more thrust than an 11x6, simply because it doesnt bog sport engines down as hard. (being relative on engine powerband and flight speed)





the fewer the number of blades the BETTER and more efficient it will be. 1>2>3>4>5>6>7. as modelers, we do not typically come near supersonic tip speeds that full scale aircraft avoid. 1 blade props must be impeccably balanced & aero. 2 blade props are easiest to make.
3 blades are normally only used in applications where you have prop-clearance issues. to convert power requirement from 2 blade to 3 you drop one inch in dia/pitch.
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Old Oct 27, 2012, 12:49 AM
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Israel, Ramat HaSharon
Joined Nov 2003
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Yamms,


In the past, it was assumed that turbulence produced by one blade would hamper the effectiveness of other blades; that the less blades a prop has, the more efficient it would be.

This went as far as racers using a one-blade counter-weighted prop.
See this photo.

But recently this opinion no longer seems valid; at least as far as it pertains to full-size aircraft, as you can see in the Airbus M400 props...

The only disadvantage of a multi-blade prop is a smaller disk-area, which mostly affects static thrust.

Within reason, the more blades - the merrier.
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Last edited by DarZeelon; Oct 27, 2012 at 05:59 AM.
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