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Another prop question.


fuel9m
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Hi

Can someone confirm that a 4 blade prop provides more thrust than a 2 blade of same dimensions? My 2 blade is trashed after falling off yesterday. The previous owner of the motor gave me a repaired 4 blade prop which is in 2 sections. I just wonder why he fitted a new 2 blade prop if the 4 blade set up provides more thrust? I assume the pitch of the blades affects thrust as well but the broken 2 blade prop looks virtually identical to the repaired 4 blade - same dimensions and pitch.

Im going to fit the four blade and see how it works before I fork out for a brand new prop - probably 2 blade as it will be cheaper and the last one worked fine.

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  • 1 month later...

After falling off!? :oops:

If your four blade(s) is/are the same diameter, pitch, chord as your two blade prop, then you will likely get more thrust out of it - simply more acting surface. Chances are good that you'll have lower rpms too.

As a rule, just using the number of blades does not a better prop make. Ideal props are only ideal for what the pilot intends to use them for when properly matched to an engine redrive.

So if your four blader runs your engine slower for the same climb rate, expect better economy, but also expect that you will not get top rpms as with your other prop and use the full range of your engine.

That is (for my hypothetical example): 4 blades @ 5krpm = x thrust, while 2 blades @ 6.5k rpm = x thrust

Why bother with a prop that you've got to spin up 1.5k rpms more for the same thrust?

1) less torque steer / inertia

2) cheaper to replace

3) often quieter

Why bother with a prop that has twice the blades?

1) lower rpm @ cruise thrust

2) better fuel economy

3) Higher TBO for engine @ lower average throttle

Manufacturers did a lot of the testing to pick a prop, but if you've done any power mods - intake/exhaust silencer addition/removal, tuned pipe addition / removal then revisiting a prop choice is in order.

Have you tried fitting a single of the two prop (four blade) system to see if it's sufficient? (assuming it's possible with your configuration)

If you try this, do keep an eye on overspeed - tach required along with your engine max rpm spec.

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If your engine does not over rev with a two blade then stick with this, you will not get anything more out of it with a three blade.

Three blades are used if you cannot stop the engine over revving with only two blades.

Pete b

GSC two blade

GSC three blade and many hours plying with a rev counter and thrust tester

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blade is trashed after falling off yesterday

Jeeez, you don't have much luck do you?!? Please don't say you didn't tighten the bolts?!?

No Ben, this thread was started in November when it happened - the first and ONLY time that my prop fell off. The new one is fine, all torqued up, marked and checked after and before every flight!

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