Three bladed r/c prop?

Matt- There are several prop mfgrs that make three-bladed props. APC, Windsor (Master Airscrew) and Graupner are a few. Your P-40 seems to be about typical for .60 power. You'll be in the right ball park sizing the 3-blade prop by reducing the diameter from the recommended

2-blade prop by one inch, and leaving pitch the same. It will work just fine. Only disadvantage will be having to listen to all the experts tell you it won't work due to gross inefficiency, etc., even as they watch it fly.

Abel

3 bladed props are quite common. Used mostly with 4 strokes. Available from any hobby dealer. Gord Schindler MAAC6694

60" isn't exactly "large", Matt. As for looking authentic... No one will see how many blades there are when it's in the air. You'll do better to spend your money and time on scale details like panels, rivet lines, cockpit and pilot, landing gear struts, the proper finish, and scale-type flying techniques. 3 blade props are much less efficient than two blades, and they really load the engine. You'll need a much smaller diameter that might not adequately fly your model (read "crash").

Check previous threads on prop pitch vs diameter for a particular application. There are also lots of web sites with good info that will help you pick the proper prop for your engine and plane.

Oh, to answer your question: yes, they do make 3 blade props for R/C. Dr.1 Driver "There's a Hun in the sun!"

I believe I'd put a .90 two stroke on it, or a 4 stroke .72-.90. Top Flite's F4U I'm building is 62" (same scale), and the recommendations are .60-.80 two stroke. Never put the small end of the power range on a model.

I never said it wouldn't fly. However, it might not fly well or handle in a scale-like manner. Dr.1 Driver "There's a Hun in the sun!"

Dr.1 Driver- Agreed on using plenty of power, especially on a scale warbird. Don't want to sacrifice the details that make it special due to concern over marginal pwr/wt. Now then, as you know the driving factor behind hallmark wing configuration of the F4U was the prop. Will you really put a non-scale two bladed prop on it?

Abel

Hmmm, your comments included: "You'll need a much smaller diameter that might not adequately fly your model (read "crash")".

First, it's pretty typical that the recommended 3 blade prop is just one inch smaller than a 2 blade prop. Considering the typical prop diameter of a 60 two stroke, or 90 four stroke, I'd say a 1 inch reduction is not "much smaller".

Next is the question of "scale-like". From what I've seen, many models are so over powered that their flying is in no way "scale-like". The reduction in efficiency by going to a 3 blade prop would bring such planes closer to "scale-like" flying. Perhaps you really meant to say the model would no longer be way over powered, so one would need to actually fly it on the wing?

And finally, if I were new to this topic, I certainly would interpret your "read crash" comment to mean you think it won't work.

I may use a static display 4-blade prop, but the flying prop will ABSOLUTELY be the properly-sized 2-blader for the engine/plane combo. Don't EVER make suggestions like that. My wife (the plane is for her) already wants scale folding wings!!! :) Dr.1 Driver "There's a Hun in the sun!"

As anyone who's been reading my posts for any length of time knows, I'm a big proponent of larger diameter-low pitch props. I simply believe they fly better, fast sport and racing models excluded. I certianly believe they fly scale planes better.

Now my 1/4 Fokker Dr.1 is powered by a Zenoah G-23 with an 18-6 prop. No one who's seen it fly could call it over-powered or say it doesn't fly in a scale-like manner.

Just because a plane is "overpowered" (in your words) doesn't mean it can't be flown "on the wing" or in a scale-like fashion. All it requies is a little throttle management. Also, the extra "power" of a large engine is easily absorbed by a larger diameter prop.

With a heavy or draggy plane, a small diameter prop will often thrash around through the air, producing barely enough thrust to accelerate the model to takeoff speed. In the air, it's full throttle all the way, with no reserve for manuvers or emergencies. Oh, excuse me, that's over powering again. :) Pitch equals speed, and diameter equals thrust. When you trade one for the other, like when you add blades, you often compromise the flying capabilities. Let's just say my F4U have a large prop. Dr.1 Driver "There's a Hun in the sun!"

Dr1Driver pontificated:

Once again, from the part of my post you chose to delete, 1 inch is a small difference. Your post implied the person asking the question would crash if they went with a 3 blade prop.

Well, that's different then.......you didn't say before that you were building it for a woman., For a man's airplane, you would want to boost the power up to where the only way to absorb it is to go with a big hungus multi-blade prop, just like the rider-scale original.

Abel

Between a 10" 3 blade prop, and an 11" 2 blade prop, a common size for the originally suggested engine (.60), a 1" difference in diameter is a difference of almost 66 sq.in. of swept area. That's almost a 20% loss. One fifth. To me, that's NOT a "small difference".

As for the possibility of the plane flying poorly or even crashing, if it were my plane, I'd stick with the larger 2 blade prop, AND the larger engine.

I'll also stick to my opinion that it will not fly as well with the smaller 3 blade prop or engine originally suggested.

Subject ended, I'm outta here.

Dr.1 Driver "There's a Hun in the sun!"

Aw c'mon, Gerald. There's a near 20% loss in swept area as you said, but a 50% increase in the number of blades generating thrust, which you failed to mention. The designers of the WWII warbirds weren't stupid. How many of them had 2-bladed props? Efficiency is a factor, but certainly not the only one. If it were, the only sensible prop for at least our smaller engines would have one blade. One-bladed props, though, are used only in very specialized competition applications by a small minority of participants in the competition. There are good reasons why that is so, just as there are good reasons for using multi-blade props, If you're into scale, then scale fidelity is often reason enough.

Abel

Interesting that immediately after a post saying you choose larger diameter, lower pitch you come back just the opposite. Prop charts go to 12 or 13 for .60 size engines. Why did you decide on the smallest diameter? Also interesting that you said you'd put a .90 2 stroke in it (max diam 16"), but then quoted .60 prop sizes. Seems you can't make up your mind. Any chance you're a politician?

, a 1" difference in diameter is a difference

It's a 20% loss in swept area, not thrust. There's much more to calculating the actual thrust than just swept area. I'm wondering even more if you are a politician.

Yes, I am sure you would. And you'd probably do your best to convince anyone willing to listen to you that they should do as you say. You'd probably even continue doing this as you were watching a plane with a 3 blade prop fly perfectly well. Abel certainly had you pegged.

The difference between props will most likely be negligable, especially with a .90 two stroke. The noise would be lower on the 3 bladed due to the reduced tip speed. It would also give more ground clearance.

See ya, don't let the door hit your butt on the way out.

There is a 50% increase in the number of blades, but not in the thrust. The more blades you have, the less efficient the prop is, since each blade travels in the wash of the preceeding one. Can you say cavitation? Turbulent air?

WWII birds had multiple blades to absorb the tremendous power of the large engins they put in the planes. A P-51 couldn't have a 30' diameter two blade prop, so it had a smaller 4 blader. A perfect example is the F4U, which even had the wing planform conformed to allow for a giant prop without the landing gear being inpossibly long.

As for one blade props, control line speed planes have run them for years, with great success.

I'm all for scale fidelity, but not at the expense of flying qualities. I repeat, in the air, who can count the blades? Use a static display multiblade prop and fly with what works best. Dr.1 Driver "There's a Hun in the sun!"

Yes there is more to it then simply swept area as Dr1 well knows as past posts have demonstrated. In fact the power absorbed is proportional to the fouth power as long as pitch is held constant.

Thus an 11 inch two blade prop will absorb about 46% more energy then a 10 inch two blade prop, both at the same pitch and turning the same rpms. Now, add a third blade to the smaller diameter prop and you will get equal energy absorbtion at constant pitch and rpms. But no place close to equal thrust because the extra blade is much closer to the turbulance of the blade in front of it and thus is an inefficient thrust generator.

Full scale planes used multiblade props for the simple reason they had to. It was the only way to get to the energy absorbtion required for the engines used. A two blade prop sized to absorb the engine energy would either hit the deck each rev or have such a high pitch it was useless. Besides, at the much lower rpms that a real plane turns the effiency loss of going to a three or even four blader is not as bad as it is in a model.

Bottom line is neither Dr1 or the original full scale designers are wrong. Both are correct as you would be well able to see if you knew what you were talking about.

Well, aren't you just full of yourself?

Dr1 implied that using a 3 bladed prop will cause poor flying, and hinted that it would lead to a crash. This is the point in dispute, not whether a 3 blade is as efficient as a 2 blade prop. If you had bothered to read what I was saying, instead of looking for a chance to show off, you would have realized this. Neither Dr1 nor you, with your arrogant attitude, have said anything to prove that a 3 bladed prop will lead to an unflyable or poorly flying plane. You are arguing about a point that is not in dispute, 3 blade props are less efficient than 2 blade props. In spite of this, many people ARE using 3 bladed props and getting perfectly good results.

The fact is that the OP probably just wanted a simple answer, can he use a 3 bladed prop? The answer is yes, he can. He will get somewhat less thrust, but assuming he uses an adequate engine, such as the .90 2 stroke that Dr1 recommended, a 3 blade prop will not lead to an unflyable plane. There's plenty of ACTUAL EXPERIENCE to back this up.

You appear to be one who likes to have long winded discussions and who enjoys trying to impress, or perhaps baffle, people by spouting theories and complicated formulas. I prefer to live in the real world and rely on actual experience rather than theories and formulas. Real world experiences say 3 blade props will work just fine. I can deal with the math and the physics, but I choose not to. I'm not impressed with, or baffled by, your arguments. Your kind of discussion is just not interesting. So, take your snooty attitude somewhere else. Oh, and you too, take care to not let the door hit your butt on the way out.

Sure, I can say cavitation, but we're talkin' airplanes here, not yer bass boat. You were doing better when you inferred a relationship between prop diameter (hence the area of the stream tube entering the prop) and efficiency. For a given thrust, propulsive efficiency increases as the area of the stream tube ingested by the prop (or jet turbine , etc. - same applies for all air breathing propulsion systems) increases. It's not a linear relationship, though, and it varies with speed. At 400 kts, the area of the stream tube for an F4U is upwards of 100 times that of an F86, and the propulsive efficiency is approximately twice as great. How much difference in efficiency would there be if the area differed by 20%?

Abel

Dr1Driver, could you please explain, so I can understand, the 66 sq. in. calculation? I tried to come up with the 66 figure, but got nowhere. I used the basic formula for the area of a circle (pi r squared) resulting with 94.9 sq. in. for the 11 inch prop and 78.5 in. sq. for the 10 inch prop. A difference of 16.4 sq. in.

I also agree with your view. This topic, and its contributors, are increasing my knowledge. Thanks!!

It's because in his rush to awe everyone with his knowledge, he squared the diameter, not the radius.