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Old Feb 17, 2013, 03:46 PM
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3- blade

What is a good rule for using a 3 blade instead of 2 blade prop? What will be the affect on plane speed, pulling power, etc?Does a 3 blade affect a 2 stroke different than a 4 stroke? All input will be considered.
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Old Feb 17, 2013, 05:05 PM
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From 2-blade prop reduce diameter by 10% and leave pitch the same. E.g. 10x6 to 9x6 3-blade.

Performance will then be very similar. Slightly less thrust from 3-blade, pitch speed about the same.

Steve
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Old Feb 17, 2013, 05:41 PM
Zor
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Quote:
Originally Posted by mashp39 View Post

What is a good rule for using a 3 blade instead of 2 blade prop? What will be the affect on plane speed, pulling power, etc?Does a 3 blade affect a 2 stroke different than a 4 stroke? All input will be considered.
mashp39,

Slipstick is giving you good advice but it is not easy to find prop 10% smaller in diameter.
5 x .9 = 4.5
6 x .9 = 5.4
7 x .9 = 6.3
8 x .9 = 7.2
9 x .9 = 8.1
10 x .9 = 9.0
11 x .9 = 9.9
12 x .9 = 10.8
13 x .9 = 11.7
14 x .9 = 12.6
15 x .9 = 13.5
16 x .9 = 14.4
17 x .9 = 15.3
18 x .9 = 16.2
19 x .9 = 17.1
20 x .9 = 18.0

We have to select a diamiter available on the market.
A rule of thumb is to reduce the diameter and/or pitch or both by 1/2" for the smaller props and by 1" for the larger props.

A three blade is often selected to improve ground clearance.
Note that the prop does not know if the engine is a 2 strokes or a 4 strokes.
Also 4 stroke engnes are limited to lower RPMs and will use larger diameter props.

Zor
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Last edited by Zor; Feb 17, 2013 at 06:41 PM. Reason: correcting typos where noticed.
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Old Feb 17, 2013, 09:14 PM
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Great chart Zor, thanks a bunch!
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Old Feb 18, 2013, 03:19 PM
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Typically the plane will slow some but thrust or lift will increase.
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Old Feb 18, 2013, 04:06 PM
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but thrust or lift will increase.
Usually the opposite. You need to use a smaller diameter so the prop disc is smaller and the 3-blade is slightly less efficient than 2 blades. The net result is LESS thrust.

Steve
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Old Feb 18, 2013, 09:59 PM
2 Fast 2 Low & 2 Loud
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Originally Posted by slipstick View Post
Usually the opposite. You need to use a smaller diameter so the prop disc is smaller and the 3-blade is slightly less efficient than 2 blades. The net result is LESS thrust.

Steve
Going by these prop charts for a Cobra motor I had purchased. I see what I though I experienced in flight.
Example 11x8x2 = 1740g thrust
11x7x3 =1886g while pulling the same 377 watts as the 11x8
But speed dropped considerably

Of course if you dropped down in diameter the thrust # would drop
Looks like there is a lot more to prop selection than a rule of thumb, I'm just learning....
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Old Feb 19, 2013, 03:13 AM
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Interesting chart but note that the two props you're talking about are completely different makes and types of prop. Plus we're in the fuel section....unfortunately IC motors don't react in the same way as electric motors to different loadings.

OTOH I haven't tested the theory very often so it's always possible there are exceptions. Maybe if the original prop was too small for the motor you could use a 3-blade the same diameter and end up with a better match .

Steve
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Old Feb 19, 2013, 09:10 AM
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Quote:
Originally Posted by slipstick View Post
Interesting chart but note that the two props you're talking about are completely different makes and types of prop. Plus we're in the fuel section....unfortunately IC motors don't react in the same way as electric motors to different loadings.

OTOH I haven't tested the theory very often so it's always possible there are exceptions. Maybe if the original prop was too small for the motor you could use a 3-blade the same diameter and end up with a better match .

Steve
I understand, & yeah, I have never seen a chart like that for a gas engine.
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Old Feb 19, 2013, 09:33 AM
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So if my model was using a 3 blade 12x6 could I just use a 2 blade 12x6?
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Old Feb 19, 2013, 10:33 AM
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Yes and no. You could use a 12x6 2 blade IF your engine has extra available RPM's to give you. Otherwise you need to increase either pitch diameter or both to put the same amount of load on the engine. eg. 12x6x3 turning 16,000 could mean 12x6x2 turning 17,500. So you could possibly grenade your engine if it unloads too much.
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Old Feb 19, 2013, 07:45 PM
Zor
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Quote:
Originally Posted by scootrb4 View Post
Going by these prop charts for a Cobra motor I had purchased. I see what I though I experienced in flight.
Example 11x8x2 = 1740g thrust
11x7x3 =1886g while pulling the same 377 watts as the 11x8
But speed dropped considerably

Of course if you dropped down in diameter the thrust # would drop
Looks like there is a lot more to prop selection than a rule of thumb, I'm just learning....
Yes, a rule of thumb is only a starting point.

I have to assume that the chart is showing values measured in a static test without any forward speeds.

It means that most of the blades are stalled and the values not realistic.

Also like slipstick mentioned the different manufacturer is likely to have a differnt blade shape as well as a different airfoil of the blades.

Still the chart can be useful to some modelers but it does not reflect the in flight performance.

Zor
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