Feb 17, 2013, 03:46 PM Registered User Joined Jan 2013 31 Posts Discussion 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.
 Feb 17, 2013, 05:05 PM Registered User Staffs, UK Joined Nov 2003 10,379 Posts 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
Feb 17, 2013, 05:41 PM
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Quote:
 Originally Posted by mashp39 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
Last edited by Zor; Feb 17, 2013 at 06:41 PM. Reason: correcting typos where noticed.
 Feb 17, 2013, 09:14 PM Registered User United States, CO, Dacono Joined Nov 2010 695 Posts Great chart Zor, thanks a bunch!
 Feb 18, 2013, 03:19 PM 2 Fast 2 Low & 2 Loud United States, CA, Winchester Joined Nov 2011 3,060 Posts Typically the plane will slow some but thrust or lift will increase.
Feb 18, 2013, 04:06 PM
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Quote:
 Originally Posted by scootrb4 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
Feb 18, 2013, 09:59 PM
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United States, CA, Winchester
Joined Nov 2011
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Quote:
 Originally Posted by slipstick 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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 Feb 19, 2013, 03:13 AM Registered User Staffs, UK Joined Nov 2003 10,379 Posts 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
Feb 19, 2013, 09:10 AM
2 Fast 2 Low & 2 Loud
United States, CA, Winchester
Joined Nov 2011
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Quote:
 Originally Posted by slipstick 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.
 Feb 19, 2013, 09:33 AM Jesus Freak Southern CA Joined Oct 2008 3,209 Posts So if my model was using a 3 blade 12x6 could I just use a 2 blade 12x6?
 Feb 19, 2013, 10:33 AM Registered User United States, NM, Holloman AFB Joined Oct 2007 86 Posts 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.
Feb 19, 2013, 07:45 PM
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Quote:
 Originally Posted by scootrb4 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