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swap a 4 blade prop for a 3 or 2 blade prop?

Old 12-14-2021, 10:22 AM
  #1  
Rhondas
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Angry swap a 4 blade prop for a 3 or 2 blade prop?

a plane with a 4 blade E-flite prop 4 Blade, 10.5x8: and I want to swap it out for a 3 Blade or a 2 blade . so what are my size choices in length and pitch ? anyone know helps
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Old 12-14-2021, 12:15 PM
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solentlife
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A 4 blader is purely a scale looking item and will not actually give better performance - in fact it could lead to less - due to blade cavitation at high rpm. On ground - the thrust will suffer until model attains speed ...

Generally a 2 blade gives better performance - but of course doesn't look the part.

10.5x3.8 is a weird size and is hard to find a direct 2 / 3 blade replacement. I would suggest in 2 blade form either 11x4 or 10x5 ..... similar in 3 blade. See later post after Q caught the mistake !!

There's an old myth of up 1 / down 1 when changing blade count .... its generally ok ... but not actually correct. Pitch gives speed ... diameter gives thrust / torque ... so care needs to be taken not to compromise one or other.

Last edited by solentlife; 12-14-2021 at 07:32 PM.
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Old 12-14-2021, 02:07 PM
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ron_van_sommeren
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Keep in mind that a plane propeller cannot cavitate, it needs a liquid to cavitate:
wikipedia: Cavitation.

Like a wing the propeller stalls:
wikipedia: Stall_(fluid_dynamics)
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Old 12-14-2021, 02:25 PM
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solentlife
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Originally Posted by ron_van_sommeren View Post
Keep in mind that a plane propeller cannot cavitate, it needs a liquid to cavitate:
wikipedia: Cavitation.

Like a wing the propeller stalls:
wikipedia: Stall_(fluid_dynamics)

Pedantics Ron ... the effect is similar. Prop stalls and thrust is lost.
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Old 12-14-2021, 02:59 PM
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quorneng
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Did not the OP say the 4 blade was a 10.5x8 (eight) not 3.8!
Surely the replacement should have at least an equal pitch. Say 12x9 two blade or 12x8 three blade.
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Old 12-14-2021, 04:02 PM
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ron_van_sommeren
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Originally Posted by solentlife View Post
Pedantics Ron ... the effect is similar. Prop stalls and thrust is lost.
I don't like making errors, I don't like criticism, I don't like it when I spread errors/mistake/misconceptions.

Last edited by ron_van_sommeren; 12-14-2021 at 04:36 PM.
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Old 12-14-2021, 07:31 PM
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Originally Posted by quorneng View Post
Did not the OP say the 4 blade was a 10.5x8 (eight) not 3.8!
Surely the replacement should have at least an equal pitch. Say 12x9 two blade or 12x8 three blade.
OOP's .... !!! Well spotted Q !!

"10.5x8 is a weird size and is hard to find a direct 2 / 3 blade replacement. I would suggest in 2 blade form either 10x8 or 11x8 ..... similar in 3 blade."

I don't think it necessary to go up 2 in diameter as a 4 blader is not that efficient in my experience.
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