Prop Speed Calculation
08-23-2007, 04:59 AM
#1
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Prop Speed Calculation
Does anyone know how to calculate prop speed.Now a stupid question.Why when using a 1.27 ratio lower compared to a 1.5 ratio why do we have to drop prop pitch.With the 1.27 ratio the prop shaft is actually turning slower so i would think a bigger pitch prop would be required.
08-23-2007, 06:14 AM
#2
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The gear ratio refers to the number of engine revolutions for each prop shaft revolution.
If the engine turns 5000 rpm with a 1.5 gear ratio the prop shaft rpm is 3,333.
With a 1.27 ratio at 5000 engine rpm the prop shaft is turning 3,937 rpm.
So in theory you will go faster with a higher (numerically lower) gear ratio with the same prop at the same engine rpm.
If the engine turns 5000 rpm with a 1.5 gear ratio the prop shaft rpm is 3,333.
With a 1.27 ratio at 5000 engine rpm the prop shaft is turning 3,937 rpm.
So in theory you will go faster with a higher (numerically lower) gear ratio with the same prop at the same engine rpm.
08-23-2007, 12:34 PM
#4
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If you go to a higher gear ratio (lower numbers), it will take more power to turn the same pitch as previously used at the same rpm as previously turned with the lower gears (higher number).
If you can do these things, turn the same rpm with the same pitch with the taller gears (lower number), you will go faster.
Just changing the gears dose not do this. You change the gears because you ran out of prop pitch and you are on the rev limiter or over reving the engine. (or other reasons we won't go into)
If you can do these things, turn the same rpm with the same pitch with the taller gears (lower number), you will go faster.
Just changing the gears dose not do this. You change the gears because you ran out of prop pitch and you are on the rev limiter or over reving the engine. (or other reasons we won't go into)
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Brett Anderson / BBLADES Professional Propellers
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Brett Anderson / BBLADES Professional Propellers
920-295-4435 http://www.bblades.com/
[email protected]
08-24-2007, 01:47 PM
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In theory which will go faster if you even out the numbers say. A 1.5 with a 28p or a 1.36 with a 26p. I was always told a 1.5 with a 28 would be faster than a 1.36 with a 26.
Last edited by z.zuperboat; 10-17-2007 at 11:59 AM.
08-24-2007, 03:29 PM
#7
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In theory which will go faster if you even out the numbers say. A 1.5 with a 28p or a 1.36 with a 26p. I was always told a 1.5 with a 28 would be faster than a 1.36 with a 26. I know the 1.5 would accelerate faster. So the only reason to go down in gear from a 1.5 to a 1.36 say would be because you run out of prop pitch ? In my case I have a 1.68 turning a 35p, kind of running out of prop pitch. I know rpm and motor power band are to be considered !
The negatives: The bigger the prop, the more torque it will generate, and acceleration suffers.
Finding that perfect delicate balance is Throttle Up's Specialty!
08-24-2007, 03:59 PM
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When you get into it it's a delicate balance.How many people would be out there that are searching for every little bit but are missing out due to a simple factor like gear ratio or pitch.Take a reasonable size vee,mid weight.It would need torque more so than a comparably size cat i guess.Remember the post on here about Fountain using 1:65 ratio Bravo's.Were they achieving something by turning a big prop slower?
