warbird kev

My question is whats the difference, i've heard that the third blade on a three bladed prop doesnt do anything, that a three bladed prop diameter is shorter, i heard that the three bladed props will pull tee stumps out the ground, then i heard that the two bladed props put out just as much as the three bladed, then that the three bladed is just for scale looks only yesit flys but its just the same, a lot of things i've heard, i really want touse one of these three bladed props,but is it worth the 14 or 15 beans if it puts out the same thing as a two bladed. all info is appreciated thanks guys.

soarrich

My Feedback: (98)

The reason for a 3 bladed prop IS ground clearance.

A two blade prop is the way to go if you have the ground clearance, the three blade props are more expensive, harder to balance, have less selection and knowledge of which engine airplane combos work best.

da Rock

The only real differences that matter for modelers is there are almost no 3 blades to try.

Matching a prop, any type, to an engine/plane combination really is mostly about blade area after you make a diameter choice and have a rough idea of pitch.

There are a number of really out of date things you will hear about 3 bladers.

"They are less efficient" is an example. The difference in efficiency is so insignificant it's a joke. In fact, a buddy just tried a 3 that was the same diameter and pitch of a 2 that was his last "good" test prop. He got a blast for doing something that "everyone knew was stupid". I wasn't the one blasting him, as I was the one who loaned him the prop and suggested he ignore conventional wisdom. It turned out to nearly pull the firewall out of the airplane. Why? Because you can't count on the pitch numbers on any model prop beyond a rough suggestion how that prop might work. And because all the BS about 3 blades is wrong, mostly because of the former. Why did that happen? Don't know for sure, but it looked like that engine on that plane simply needed more prop blade area. Fact is, it got more with the 3.

My AW Extra300's favorite two props are a 2 blade and a 3 blade. They both pull the sucker over 80mph if I want. And both pull straight up until I'm bored.

My warbirds all have 3 blades for two reasons. They look scale and after doing prop testing that was more than just trying a 2nd prop, those 3 bladers worked better. The only warbird that's been on radar was over 90mph. Efficiency? yeah right

Sport_Pilot

The only reason a three bladed prop may be less efficient is that there is an extra prop tip, but usually the tip speed is less so often there may be no loss in efficiency.  The reasons to use one in our models is that it should lower prop noise, more ground clearance, better high speed thurst (if you use a higer pitch), and on most models they look cool or scale like.

exeter_acres

My Feedback: (2)

depends on the size... on larger props, the 3 blade can be quieter... ie on larger gas planes.

radius1x1

I've been moving to 3bladed props and see no difference. In fact, the last plane I bought came with a Scale 4 blade. I love the scale looks and to me that's half the fun.

eddieC

My Feedback: (2)

For our models, there's little difference.

From a design & efficiency standpoint, the fewer the blades, the better. The primary reasons for number & diameter of props on full-scale is prop clearance and keeping prop tip speed below the speed of sound (big loss of efficiency).

da Rock

There is another reason for multi-bladed props for full-scale. Matching the blade area to engine power. It is also a clearance consideration sometimes. However, it's primary reason is needing more blade area. The 7 blades on some turbo-props is a perfect example. Imagine the diameter of a 2 bladed prop with the same blade area. And of course, as you mentioned, imagine the scream from those 2 tips.

In WWII, most of the fighters got more blades as the war dragged on. They were getting more powerful engines usually. The P47 was one that wound up getting 4 blades and then getting wider 4 blades. They called them "paddle blades".

Model speed flyers have flown 1 bladed props off and on. They read that fewer tips were more efficient. The popularity of 1 bladed props shows how important that efficiency isn't.

eddieC

My Feedback: (2)

Sorry, Rock, you're beatin around it but not followin ya. You're close tho.

Blade area and clearance:
I agree on matching blade area to HP. But my point is the designer has to do it (1) with the blade tips below Mach 1 and (2) with as few blades as possible.
The Corsair was not conceived as a gull-wing originally, but the design was altered to allow for the large 13'4" prop diameter needed to absorb 2,800 HP put out by the P&W 18-cyl. Double Wasp. They could have used 4 (or more) blades at a smaller diameter turning higher RPM and left the wing alone, but they would lose (1) speed and (2) efficiency. The tip speed is limiting (Mach), so the bigger diameter, using fewer blades turning slower, is more efficient. Those tips are near or at Mach 1 at takeoff power on the F4U, as they are on many full-size prop planes.
As for the 5- & 7- blade modern turboprops, the reason for that is each blade can be thinner (and weigh less) and each absorbs a smaller percent of horsepower. Advances in blade aerodynamics and metallurgy allow for those thin scimitar or deeply swept blades, but the designer will tell you they give up some efficiency to blade count, as each blade that's added is increasingly operating in more disturbed air. The newer, thin blades are more efficient than the older, fat, flat blades, but the net gain is still just a few percent. The 90-series King Air has a 5-blade aftermarket blade available called the Blackhawk.

Re-read what I posted. It's not that more blades are more efficient, the opposite is true. Fewer blades, turning slowly, are more efficient. And yes, those speed guys are still onto something as single-blade props are still used at CL speed events. They are also used by free-flighters, as I saw indoor here last year. In full-scale, some self-launching gliders use 1 blade, and 1-blade props were common for early low-powered planes (Aeroncas) and for engine break-in at engine shops.

Them durned fellers at MIT an sech been spreadin' them rumors agin! Durn their hides!

http://www.flyinggiants.com/forums/s...ad.php?t=10595

One blade may be odd & rare, but it is most efficient.


(Zziiiipp) OK. Suit's on.

[8D]

TimBle

fewer blades, higher efficiency.

more blades, more thrust


assuming you have matched the prop area to the engine horsepower

jay1st

Talking about prop blades, any reason there is no "turbofan blades" like props ?

EDIT: I mean like this : http://history.nasa.gov/SP-4219/4219-322.jpg

da Rock

There have been and are now. On full scale turboprop engines.

BTW, notice also the ground clearance and distance between tips and fuselage.

Sport_Pilot

Then why does a large plane like a C-130 with absolutely no ground clearance problem go from a four blade prop to a 6 blade prop? Reason because the 6 blade prop has lower tip speeds, is more efficient (see lower tip speeds),bites thin air better, and has higher high speed thrust.

jay1st

I meant for us, RC guys, not full scale.

Sport_Pilot

You can look at this another way, but it is really the same thing. A lower speed but larger diameter airstream and a higher speed but smaller airsteam on the ground may have the same thrust. But at speed the prop can only accelerate the air to the same speed and thus has less thrust at speed than the smaller but faster airstream.

Sport_Pilot

Well if you are trying to set speed records then it may only be possible with a multi blade prop with a high prop pitch. But probably not a factor at the lower speeds of pylon racing. Some pattern planes use three bladed props to pass the maximum noise rule. The only factor that is not a benifit to our models is that a mulit bladed prop performs better at high altitiude.

da Rock

You can bet the props chosen for a C-130 have the same list of requirements that all aircraft have. The list applies to the aircraft first when you're talking about full scale, btw. And the list for the props for that engine on that airplane will read pretty much the same as well.

They design or choose a package that matches the engine operating on that airplane in it's envelope. If sound is in the list, it's very probably not the 1st thing for sure.

Not so amazingly, our models are very similar.

If the prop we're testing doesn't pull what's behind it the way we would like, some of us try other props. We don't have variable pitch to help. And often we don't have a wide enough selection to help.

But as far as 3 blades versus 2...... There is often a 3 that works as good as most of the 2s. With today's selection of 3x, that's almost amazing. It could be that there really aren't many wide bladed 2s marketed nowadays.

We often start out with a recommendation of a specific prop that is said to be "the" prop for our engine. Sometimes we get advice to only use a magic brand as they're so much better than everything else. It often happens that if the one we try works good enough to fly the plane, the majority of us just shuffle on.

Hopefully, some of us will at least test a few props after we've maidened a new plane. The questions to ask are, is it as fast as I'd like? Probably more important would be, is it as slow as it needs to be and still be reliable (flying and engine response) at that rpm? Is top speed and landing speed what we want? and need? Does it have the vertical and over the top power it should? Does it idle low enough? (That's a question that's actually part of an earlier one.) And do I need to find a quieter prop?

Three blades can be better than two. A number of my planes have proven that. And they proved the old rules and sayings about 3 bladers are way out of date.

Barry Cazier

My Feedback: (17)

So....with all this inmormation/misinformation....out there....I'd be interested in trying a 3 blade prop, if you guys are saying they'd give equal or better thrust than a 2 blade.
What would be a good starting point for a DLE55. I'm at 5000' elevation and I run a tuned pipe...(MTW RV2 I think). I'm flying a EF Yak 88" I use a NX 23x8 2 blade right now. What do you guys suggest for a "test mule"/
Thanks
Barry

BEAR-AvHistory

With the selection available I run 3 blades on all the models that should have more then 2 blades in full scale & 2 blade wooden props on my WWI & other planes that had 2 blades.

I fly electric & use a watts-up to dial them in to the motor/ESC specs with about a 15/20% saftey factor. Biggest 3 blade I have is a 14X8X3 in a SR Telemaster & the biggest 2 blade is a 20X8 in a 1/4 scale PA-12 Piper Super Cruiser.

da Rock

I would suggest you ask over on the IMAC forum. The last couple of IMAC contests I worked at one of my clubs, there seemed to be a TON of the bigger planes were equipped with 3 bladers. I think most of the upper classes were nothing but 3s.

Now those guys actually do have a decent selection. They've been using three blades a whole lot more than the rest of our population and have a couple of prop mfg's who really stepped up with the goods. Of course, when a prop costs what the average modeler pays for his 60 size ARF, what'da'ya expect. But jeez, the spinner prices.....

eddieC

My Feedback: (2)

That is the UDF (UnDucted Fan) NASA worked with back in the late '70's IIRC. They created a hybrid between a propeller and ducted fan on a jet. It was meant to go on a STOL inter-city commuter. They were looking at better fuel economy (where a prop is best), low noise (that whole tip speed thingy), high thrust from a jet engine. This was back when ducted-fan jets were just coming out. Probably no application in RC. Lots of engineering.

The ultralight guys play with props much like we do. They find that a 2-blade is more efficient, i.e.: same cruise speed but better fuel economy with the 2-blade over the 3.

I'm talking most efficient, not most thrust from a given engine. Our RC stuff is quite overpowered, I'd say our primary limit is ground clearance. Also, all but the recent turboprops are fixed pitch.

straitnickel

i have a Biella 20"x10" on my DLE55 , the vertical exceleration is defenitly increased

straitnickel

see first post above, the two blade was a 22"x8"

hyflyer9

I have always been taught that a 2 blade prop with a longer blade and less pitch is better for speed but gets less thrust . But when it comes to a 3 blade prop you just go with a shorter blade and more pitch . {example} a 18"x8" 2 blade prop is good for a 26cc gas engine but the same engine with a 3 blade would use a 16x10 and still get the same performance and you can see that you would gain two more inches of clearance . Now for me I am trying to find a good quality wood 4 blade that will work good with my 26cc gas KMP P47.
well that's my 2 cents worth .!
hyflyer9.

skillet92

Yes but the C-130 has Variable props and can do any pitch it wants to! I have seen them turn like a tank. I promise it is a cool sight!

Comparing a Turbo fan to our RC Motors is not practical because the blades on most full scale aircraft stay the same RPM they just change pitch.