Prop question

Joined
Oct 30, 2003
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192
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Conway, Ar
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Highly modified KB-3
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950 Gyro - 200 FW
Guys, I have a question I can’t seem to get an answer for. If you have a four blade prop and the chord at the widest point is 3 inches for each prop, you would have a total of 12 inches of prop chord. Then if you have a three blade prop with each prop having 5 inches of chord, you would have a total of 15 inches of prop chord. Would the 3 prop setup create more thrust than the 4 prop arrangement?
I realize there are numerous variables that would affect the outcome but assume all things are equal except for the number of props. What do you think?
 

JDB

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Oct 21, 2019
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Sasolgurg, South Africa
Hi the 4 blade will give you more thrust. We have put quite a few 4 blade props on Gyro's here in South Africa and the thrust is definitely improved with 4 blades. Most guys here also do the piston convex at the same time which also gives them more horses.
 

XXavier

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Prop blades are essentially rotating wings. Hence, for the same pitch, diameter, and revs, the thrust is proportional to the blade area. If you have four blades and 3 inches chord in one case, and three blades and 5 inches in the other, the latter prop will give (3 x 5)/(4 x 3) = 1,25 => 25% more thrust.
At least to a first approximation, since propellers are very complex things...
 

Smack

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We swapped a 3-blade prop with a 2-blade prop on the race plane and picked up top-speed mph.
Brian
 
Joined
Oct 30, 2003
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192
Location
Conway, Ar
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Highly modified KB-3
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950 Gyro - 200 FW
Guys, thanks for your responses, but can't say I'm convinced with any answer. XXavier's response is the most logical.
 

PW_Plack

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West Valley City, Utah, USA
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As additional blades are added, the efficiency of each decreases because they're operating in each others' turbulent wakes, which worsens the closer together they are. Fewer, longer blades are usually the way to improved thrust. Four-bladed props at typical gyro power levels are usually used only where diameter is limited by frame clearance, or when intentionally trying to limit top speed using drag, as in early Air Command ultralights.

We had a local with a CLT Air Command who got a significant increase in thrust after replacing his Warp Drive 68" three-blade with a Warp Drive 70" two-blade. Not everyone has enough clearance to the keel and rotor to do that.
 

Wolverine

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Oct 6, 2016
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Florida
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Wolverine Gyroplane
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What's the best way to balance a three blade prop without a dyna vibe, etc.?
 

Jean Claude

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I piloted gliders C800, Bijave, C 310, airplanes Piper J3 , PA 28, Jodel D117, DR 220, Cessna 150, C
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Guys, thanks for your responses, but can't say I'm convinced with any answer. XXavier's response is the most logical.
Xavier's answer is logic but the required torque will then be greater, which your engine not gives. So, the rpm decreases until to balance it and the thrust also.
Yes, the things are more complex.
 
Last edited:
Joined
Oct 30, 2003
Messages
192
Location
Conway, Ar
Aircraft
Highly modified KB-3
Total Flight Time
950 Gyro - 200 FW
Wolverine, what brand prop are you trying to balance? If you purchased them new as a set they should be balanced by the manufacturer. You should be able to use a cooking scale that measures in grams and sand on the two heaviest props until all three are withing a couple grams of each other.
 
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