winglift

My Feedback: (10)

When and why would you want to use a 3-bladed prop? Is it better than a 2-bladed prop? What is the rule of thumb in choosing which one to use?

beppeVRCS

My 2 cents is that you use a three blade prop (apart from look and scale requirements) when you do not have enough clearance for a 2 blade prop... increasing the number of blades decreases efficiency. Sophisticated high efficiency systems have one blade only!
Beppe

vbortone

My Feedback: (2)

Downline braking. It was very good in 2 strokes engines.

hurrejola

my rule of thumb for sizing a 3 blade prop, is, one inch les pitch and 1 inch less diameter, and it has worked all right,

Hector

Stuart D

If noise is an issue a 3 blade does tend to be quieter and as stated ground clearance is also improved .

Two blade props are more efficient as a rule but they also require more right thrust and have a larger prop wash .
This can be good or bad just depends on the airframe .

As for sizing a three blade the larger the engine the greater the difference in diam .
eg 23cc = 1 inch decrease in diam
30 cc = 2 inch decrease in diam
50 cc = 2 inch decrease in diam
100cc= 3 inch decrease in diam
150cc= 3 inch decrease in diam
200cc = 3 to 4 inch decrease in diam

Pitch is another story but that relates more to airframe + usage = required pitch

Stu

MTK

Efficiency is measured how?

The only time the blades are turning in the previous blades' wake is when the plane is on the ground. True for single blades too.

Even at moderate speed of 60 mph, at the typical rpm we turn, 3 bladed props work well and each blade bites new air. Agree on the clearance and quietness. 3 blades will load the engine more and that load will delay acceleration a bit

Jetdesign

My Feedback: (8)

I thought the decrease in efficiency was referring to (increased angular momentum + drag of 3rd blade) / (thrust gain of 3-blade prop), so basically the ratio of energy input to performance gain isn't worth it. Never tried it myself though, so could just be a big RCU rumor, lol.

I've been contemplating a 3-blade lately; seem to have the extra power to use, don't want a larger diameter, would like some extra grunt. I do have a wide blade to try, and maybe just leaving the system conservative and switching to lighter packs and lighter (carbon) prop will be the right choice, if I can get throttle curve to fix the feel of the power system.

winglift

My Feedback: (10)

Okay, for me it makes sense to use the 3-blade prop to reduce the diameter because of the low clearance. Has anyone use the Zinger 3-blade props which has replaceable blades? Also, they have both the 11x8 3-blade and the 11x8W 3-blade. Is the wide version better for pattern?

Dean Pappas

Hello Winglift,
When it comes right down to it, you only want to run a 3-blader when you want to run an engine whose RPM sweet-spot and max horsepower output, when combined with a "low" pitch, say 10 or 11 inches for good downline braking, would force you to use too big a diameter, just to absorb the available horsepower. The big diameter, combined with the higher RPM, would then cause the tip velocities to be too high, making lots of prop noise (howl). The 3-blader simply lets you absorb that max power level at the same RPM and pitch, and still keep tip velocities down.
The price you pay in decreased prop efficiency (a handful of percent) is more than offset by running the engine in the sweet-spot, rather than lugged-down. If not, stick with the two-blader.

I hope that helps,
Dean Pappas

N429EM

There's an awful lot of small aircraft at a nearby airport, running 3-blade props. They must be doing something right.

winglift

My Feedback: (10)

That helps a great deal! I apologize for this question, but how do you know what the sweet-spot is on an engine? In my case I use about 1/3 to 1/2 throttle and the FZ70 pulls the BeatOn50 quite well, so is the sweet-spot where I set the engine for most maneuvers or is there more to it than that?

Dean Pappas

Hi again,
I don't know where the RPM sweet-spot for an FZ70 is, but it cannot be more than 10,000 or so.
What that means is that you can run up to a 13" diameter prop without problem.
My guess (and this is only a guess) is a 13-7 prop. Try it for a start.
Hello those of you with experience with the FZ70 ... what size prop are you running and what is your ground RPM?
If you could tell us whether you think you are loading the engine heavily with your setup, or not, please add that information as well.
I did see my buddy Matt run a YS 63 on a 13-7 and it was just a teeny bit lugged down, but handled it well. No, I don't remember the RMP.

Regards,
Dean

Roberto B.

Of course, because otherwise thir propellers would hit the ground!
Or would turn their props at + Mach1 speed at the tips.

thailazer

I've used a lot of 3 blade RC props for scale effect but never felt they were as efficient as a 2 blade. That is opposite my experience in full scale aircraft where a 3 blade would always put you back in your seat at take-off more than a 2 blade in the same aircraft. One thing about the McCauley 3 blade on my old Cessna 182 was that it had a lot more drag than the original 2 blade when you throttled back. Descents of 4500 feet a minute were possible and it was one of the things I always demonstrated as it could bite you if you did not realize your vertical speed. I imagine a 3 blade on an RC aircraft would have more drag at low power as well which would give you steeper approaches.

MTK

Dino,

Around 9500 on 22% nitro/19% oil. A very sweet running little engine.

The DLE 55 experiment has hit full gear. The break-in prop was my 22x12 hybrid. Just not enough prop in my set-up.

I re-drilled one of your 3 bladers to fit the DLE but it was no go. Decided not to run it....felt a bit weak in the hub area and I felt it wasn't safe to even try. Just got a new Mejlik 20x12 3 blader in service. The engine has never been happier. Extremely strong and quiet set-up. The guys at field I flew at today commented how electric like this set-up is in terms of quietness. I guess they are used to flying mufflers with hard mounted engines so their frame of refernce is skewed. Truth is that when anyone else was flying at the same time, the only time I heard my engine was at full revs

After break in is completed I think the engine will pull a 21x12 3 blader

najary

Here why use 4 3-bladed prop:

Post 399, http://www.rcuniverse.com/forum/m_93...16/key_/tm.htm

Best regards,

Isaac Najary

MTK

What about post 339 in your thread? What does it have to do with 3 blade discussion? If I understand it correctly, you are using a lower pitch 2 blade electric prop on your model and are reporting a reduction in power consumption.....nothing new there

najary

Sorry, see post 399.

Best regards,

Isaac Najary

MTK

Yes exactly.... I have found the same thing with larger 3 blade props on petrol engines. Currently using a 20x12 Mezjlik on the DLE55cc with terrific results. The braking of this prop is really something of note. And it is definitely a more constant speed set-up. This engine will turn a 21x12 I believe after it is fully broken in. I will change the prop to increase diameter next Spring

RoyR

My question, is EVERYONE concerned about the last little gram of thrust or efficiency? Unless you are entering contests all the time I don't think there will be a whit of difference between using a 2,3 or 4 bladed prop. Many (note I didn't say all) of us just like to get out and fly. If my airplane goes a tad bit slower on one prop I probably won't notice. SO I think the factors in using multi-bladed props should be, 1. Do you need to increase your ground clearance? and 2. Do you like the looks?
The rest is all fluff.

MTK

Spoken like a guy who doesn't compete. Sport guys don't have to understand what the competition guys seek. And that's fine............there's room for everyone

NJRCFLYER2

My Feedback: (42)

Is it "fluff" to drastically reduce the noise that your airplane makes? Unless you have a guaranteed lock on your flying site and don't have to worry about that kind of thing, you might want to reconsider buying the proper 3 blade prop. If you're not sure how to figure out what the proper 3 blade prop is, consider asking some competition guy.

RoyR

[/quote]
Spoken like a guy who doesn't compete. Sport guys don't have to understand what the competition guys seek. And that's fine............there's room for everyone
[/quote]

Yes I said that Not everyone competes. I read posts like this all over RC Universe. I meant there are people that Need to know all the fine points, but many are just every day sport fliers. I've known some new people that were afraid to do something a little different until they had a dozen experts tell them it wouldn't kill their plane. My point was if you aren't in competition of one sort or another and fly for fun, to go ahead and enjoy all the aspects of building, changing your plane, trying different maneuvers, etc. Someone had to be the first to try a 3 bladed prop without expert advice.

cmoulder

Roy,

With all due respect, this is the pattern sub-forum and there was a specific question about when and why one might want to use a 3-bladed prop. The original poster presumably asked it in the pattern (and not sport flying) sub-forum because he wanted to know the finer points of the topic.

The original poster has gotten advice and succinct explanations from some of the most knowledgeable people in the hobby concerning his question.

Why are you so concerned that somebody wants to learn something new? Why do you presume that the original poster is a noob sport flier and not a progressing competitive flier?

RoyR

Ok, I did a search on 3 blade props for other information and began reading this post. I didn’t pay attention to the area it was in, so I probably overstepped my bounds by commenting here.

The reason I did is I see some new people get so bogged down in the technicalities of flying they never learn to just enjoy flying. I have been flying RC since the 70s but I’m new to gas. When I went looking for information on RC Universe there were so many posts from Experts saying how you had to modify the engine, drill out things, replace this or that. Some posters even claiming they were even more knowledgable than the manufacturer. For someone new to the hobby this would be overwhelming advice that is not necessary for a sport flyer. Many people can get discouraged before they even begin.

I was just assuming there were new flyers here being fed too much information and I threw in my two cents. I was wrong.
That’s it.