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If I currently have a 2 blade 14x6 prop, is a 3 blade 14x6 the same?

Because of the extra blade, do I need to lower the size or pitch?

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figured it out

The general rule for converting a 2 blade prop to a 3 blade is subtract 1" from Diameter OR subtract 1 from the Pitch.

carrellh

You found the "rule of thumb" which should get you somewhat 'in the ballpark' to start testing. The true answer will come when you actually try the prop on your engine/plane combination and decide how you like it.

The selection of three and four blade props is pretty limited from most manufacturers. I assume demand is relatively low compared to two blade models.

Zinger has a large selection of three and four blade wood props.
http://www.zingerpropeller.com

Gray Beard

My Feedback: (-1)

Carr, my LHS has a pretty good selection of three blades in stock. What I can't find in stock is a three blade spinner!!! I can order them but the shipping charges take all the fun out of that.

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Thanks for the advice, but I am certainly NOT going to pay $30 for a 13" prop.

da Rock

You don't have to.

I use 3-blades on about half, maybe more than half, my planes. Master Airscrew 3's are around $10 for 13"ers, if I remember correctly. Check out Tower's prices and while you're there, look at the Carl Goldberg spinners. They're cut for MAs and that's a good thing. They're also priced like the MAs are versus the Zingers, if you compare their price to TruTurns (which don't seem to be cut for any prop on the market). But don't get me wrong, I got TruTurns on more models than not. They're good, but you pay for it. I got the CG's just now because I'm doing a bit of prop testing on two planes and they'll both look better with black spinners. The CG's come in white or black. TTurns come in aluminum.

about the 2 versus 3 blade deal.................

da Rock

The rule of thumb is way too simple. Truth is, it's basically BS when the 3-blades we have available are factored into the equation.

The blades on the 3's are way different than on the 2s, even when comparing MA 2s to MA3s. The blade area is the big difference. It appears that MA might have tried to match total blade area. That pretty much throws out the comparison by efficiency and any other that might have been the basis of the rule of thumb. Matter of fact, MA's suggested engine to prop chart isn't even close to what my models/engines have told me they want.

What engine/plane are you propping?

da Rock

A 14x6 would be on a 91 glow?

I'm using MA 14x7(3)s on my H9 Corsair, H9 Thunderbolt, and H9 Spitfire. Wanted the look a 4blade would give, but ain't none of them puppies. All three fly great but fly like they are overpowered, which they are. At the other end of the performance envelope for 91s, I'm flying a Sukhoi doing IMAC patterns and have that 14x7(3) MA on it because it's the best performing prop on that engine on that airplane. I think that prop is $12 something.

And if I remember correctly, Master Airscrew recommends that prop for 1.5 to 1.8 glow engines. Makes no sense in light of what actually works at my flying field with my model airplanes. Got no clue when or how they came up with the list of recommendations.

I personally think one reason 3blades aren't more popular could be the people marketing them.

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Ya, I have been looking at the MA props.

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60 size EP which asks for a 14x7(it'll spin up to a 20) 2 blade. Most of the 1:1 RV-8s have a 3 blade prop and I wanted to do that for years.

Hangar 9 RV-8 to be exact.

To be honest, I have no clue what size 3 blade to get.

Jetdesign

My Feedback: (8)

I have read on many prop sites that the general rule is to decrease the diameter AND increase the pitch when converting from 2 to 3 blade propellers.

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I was going to try a 13x8

Sandmann_AU

3 blade props lose something in efficiency over 2 blade props, and the rule of thumb is exactly that - just something to get you close to the mark. I've heard people say reduce either the pitch or diameter by an inch, I've heard others say decrease both. The truth is you're only going to get a real answer by putting the prop on the engine and seeing what happens.

I'd have to ask you WHY you're looking for a 3 blade prop? There's really only two reasons for it - scale appearance (in which case you're probably better looking for a static 3 blade and using a 2 blade for flight) or if your engine's so powerful that you can't get ground clearance while absorbing enough power (which is why corsairs had 3 or 4 blade props AND bent wings!). If your plane's flying and landing fine with a 2 blade prop you're better off leaving it alone - the extra blade will only cause you losses.

As an example, I have a GP Super Skybolt that's designed for a .90 4 stroke engine as a maximum, but I'm running a 1.60 twin cylinder OS engine in it instead. To keep the engine from over-revving I needed a 16" 2 bladed prop, but with that I was barely able to take off, and landings ALWAYS caused a prop strike - Putting a 14x9 3 blade prop on solved my problems. FYI it flies, looks, and sounds great but is really too heavy, and I had to do some major mods to keep it from snapping horizontal stabs when pulling out of dives.

Jetdesign

My Feedback: (8)

Also noise reduction. The slower spinning prop will make less noise, which is why a lot of them are used in competition with noise limits.

Sandmann_AU

Noise limits? Huh? You obviously live in the wrong country.

da Rock

Actually, the idea that one is less efficient than the other is, how can I say this...... about as applicable as saying that a 12x6 is less efficient than a 12x10. So we should all used 12x10s? Nah...... For there to be a realistic reason to believe the efficiency matters, the blades would have to have similar area, shape, pitch distribution and a few others. Even if you compared blades on Zinger 2-bladers to Zinger 3-bladers, you'd discover they don't compare in area, etc. You've got to actually have all those details close for the efficiency to acutally matter.

There is a perfectly good reason to use a 3-blader other than clearance and noise reduction. They're often better suited to your engine/model and do the flying job you want better. Since there are usually many, many 2-bladers to test on your engine/model while there are usually only a couple of 3s that might work, it's not a given you'll find a 3 that suits better. However, if you actually do try a few 3s, there's a good chance you'll see the efficiency deal for what it is, a sound byte that sounds better than it bites.

Precision pattern and IMAC have proven just how lame the efficiency deal is. After they started pushing 3s for the reduced sound they make the prop industry saw they needed to pull their thumb out and make a decent selection of 3s for the demand. And darned if they didn't discover that appropriate dia/pitch 3s in a decent range worked very, very well. A couple of guys at the flying field were pressured into quietening down their IMAC planes after they started competing. You never heard such whining until they actually flew the new 3s. You should hear them now extoll the virtues. Unfortunately, they've as bad as the "efficiency" expounders. They now think 3s have some magic to them and are "usually better for a number of reasons". They may be right about the reasons but truth is, the best 3 won't be any more magic than the best 2. Test props with any effort and you'll discover the biggest difference in performance between them is the availability of test props to try.

I've got a 3 on my Sukhoi because it's be best prop for vertical on that engine/plane, especially in the summer. I've got a 3 on my Skybolt because it's the best all round AND there is no worries about clearance. No 2blader on that airplane gives the same pull over the top of maneuvers the 3 gives. I only tried two 3s on the Skybolt because there weren't any others to try. One is excellent, the other not so good at all. It ain't the blade count that matters any more than it's the absolute efficiency numbers.

Gray Beard

My Feedback: (-1)

What Joe is stating is how three blade selection was always done, today on some prop sites I see the manufactures stating to just go down in size? As Rock stated, only way to find out is testing to see what the plane will fly like with different props. That goes with any prop/engine/plane combo though. As for noise, the loudest prop I have ever heard was a three blade carbon fiber on a friends 40%. Just ear shattering!! As to the rule of thumb being BS, well, BS, it's just a starting point though like any prop and it gets you in the ball park.
I keep seeing Master Air Screw mentioned, there is a better prop in three blade out there for small glow engines. Look up GRAUPNER props. Only one of my LHS stock them and size selection is sort of hit and miss, in stock there are more of the MAS props. The Graps are not the best looking prop on the market, they are gray and about the same color as APC.
As to extra power or torque between the two and three blades, I have yet to see it, what I do see is a much better/faster breaking action, cut power and my planes come to a sudden halt. Sometimes this is a good thing. Some of my little pattern planes come in a bit too hot and the three blades slow them down better for landing. Knowing this you can understand why a lot of IMAC and pattern guys like them.