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Does the increase thrust of a 3 bladed make up for the decrease in rpm?

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Does the increase thrust of a 3 bladed make up for the decrease in rpm?

Old 07-06-2015, 11:06 AM
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gregoryshock
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Default Does the increase thrust of a 3 bladed make up for the decrease in rpm?

I bought a OS 75 AX engine and a 3 bladed prop. The propeller is one inch shorter than the recommended biggest size two blade. This is the first time I've ever used a 3 bladed prop. What I noticed is the RPM reads 8,500 at max throttle. I expect that the engine would get around 11,000 with the two bladed prop. Does the increase thrust of a 3 bladed make up for the decrease in rpm?
Old 07-06-2015, 11:45 AM
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Top_Gunn
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Did you remember to set your tach for three blades? (Not suggesting that you didn't, but it's an easy thing to overlook.)
Old 07-06-2015, 12:09 PM
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gregoryshock
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Originally Posted by Top_Gunn View Post
Did you remember to set your tach for three blades? (Not suggesting that you didn't, but it's an easy thing to overlook.)
Yes. In fact I've taken many different readings with the tach.

Last edited by gregoryshock; 07-06-2015 at 12:12 PM.
Old 07-06-2015, 01:27 PM
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RBACONS
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No simple answer. First off, you said you "expect" that the RPMs would be 11,000 with a 2-blade. If you haven't actually measured it, its just a number out of context and is fairly meaningless as a comparison the 3-bladed prop you are measuring. 3-bladed props are inherently less efficient that an equivalent 2-blade. In theory, reducing the diameter on the 3-blade prop reduces the engine load to the rough equivalent of a "similar" 2-blade prop. However, that is not necessarily true if you're comparing a 2-bladed brand x to a 3-bladed brand y.

Given a particular diameter/pitch, you can get significantly different RPM readings from different brands of props. Some blade designs are efficient, others aren't. Some brands are light and will load the engine less and yield a higher RPM, other brands are heavier and won't. And RPM does not necessarily equate to in-air performance. If you test a whole range of props, a particular plane design will perform best with a particular brand, diameter, and pitch (at least to the pilot's senses) and its not necessarily the one with the highest max RPMs.
Old 07-06-2015, 01:29 PM
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Top_Gunn
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I don't think your rpms should drop. The main point of a three-blade prop (aside from things like ground clearance or scale looks) is to get more thrust from a given rpm. As I understand it, the fact that the tips of the blades don't have to move so fast with a three-blade prop (because the blades are shorter), which makes this possible in principle even though the engine is pushing more total blade surface are through the air. But this is not something I know a lot about.
Old 07-06-2015, 02:53 PM
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gregoryshock
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@RBACONS and @Top_Gunn, I understand what your saying...

I guess I should change the question a little. "If you put on a 3 bladed prop of the same name brand, pitch, etc, that is one inch shorter than the 2 bladed, should the RPM change or stay the same?"
Old 07-06-2015, 03:34 PM
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RBACONS
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Generally, yes, the RPMs should be about the same and typically the goal when changing from a 2-blade to a 3-blade would be to maintain the same max RPM. But depending on the prop blade design (i.e., how draggy or efficient it is), and what the diameter of the original 2-blade prop is, it may take more or less than a 1" change and/or some pitch change as well to maintain the same load on the engine. By adding a blade, you're adding about 50% more mass (and drag?) regardless of the diameter of the original 2-blade prop. But reducing 1" in diameter on a 10" prop is a 10% reduction in mass whereas a 1" reduction in diameter on a 20" prop is only a 5% reduction. I don't think there is a one size fits all conversion.
Old 07-06-2015, 04:06 PM
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049flyer
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A 3 blade prop does not necessarily produce more thrust than a 2 blade prop. I'm always puzzled that people assume this is true. If your engine is turning a different RPM with one prop than with another, then it is unlikely they both provide the same load to the engine. Why would you assume that one produces more thrust when the load is not equal? Kind of like assuming your car is faster in 2nd gear than in 4th gear because in 2nd gear the engine turns a higher RPM.

You would need to test different props under different flight speeds and conditions to determine which works best on a particular airframe.

Three blade props are usually less efficient due to the proximity of the blades to each other as they travel one behind the other. The turbulence created by one blade affects the air flow over the next blade.
Old 07-06-2015, 07:52 PM
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vertical grimmace
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You want your engine to stay in a certain RPM range regardless of the prop configuration to keep it in it's power band. Stroke, porting etc. are a certain way inside the engine for a reason based on typical application. So you want to prop to a certain RPM range.

Now, the debate over the efficiency of multi blade props on RC sized engines rages on. I prefer 2 blade props. Electric motors I am sure act different than engines. The biggest problem with multi blades is finding the exact prop as there are so few choices. Not much to experiment with.
Old 07-06-2015, 07:58 PM
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vertical grimmace
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The only time I really tested 3 bladers much was on my CL stunt models as we were always looking for ways to get constant speed. Same descent speed in downlines as uplines. The idea was the more blades would provide braking action because of more drag of the xtra blade. I never liked how they handled otherwise though. So I moved on.

The aircraft design, and objectives are important factors. But you will see very few multi blades in use, other than on warbirds. Performance aircraft tend to stick with 2 blades, and the speed guys use single blades.

These are just ideas to nibble on. Your milage may vary
Old 07-07-2015, 05:52 AM
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MTK
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Originally Posted by 049flyer View Post
SNIP
You would need to test different props under different flight speeds and conditions to determine which works best on a particular airframe.

Three blade props are usually less efficient due to the proximity of the blades to each other as they travel one behind the other. The turbulence created by one blade affects the air flow over the next blade.
Exactly right. Flights will tell more than all of the commentary, combined...
Generally speaking, 3 bladed props tend to produce less noise than 3 bladed but they also tend to be heavier than 2 blades producing equivalent thrust. Noise reduction is a particular interest of mine so I play with 3 blades. However a well designed 2 blader such as the APC line of props, can have similar noise footprint.

The engine couldn't care less. As long as the load is turnable, it will turn it. I let air performance tell the final tale
Old 07-07-2015, 07:29 AM
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MTK is correct on the flying thing. Matching a prop to an airframe is the key. A high wing trainer is gonna want a different prop than an F1 pylon racer.
Old 07-07-2015, 09:39 AM
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jrf
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To respond to your original post; OS manufactures and sells engines in many areas that have very low noise limits. To keep the noise down, OS recommends props that limit the engine's rpm. If you tried your engine with the largest recommended 2 blade prop, you would also see about 8500 rpm. To reach 11,000 on that engine you would need a 2 blade of a size of about 12 x 8 or 13 x 6. Or a 3 blade of about 11 x 8 or 12 x 6.

I have used 3 blades on several airplanes, either for ground/water/airframe clearance or because they are cool. Also, at higher rpms, they make an awesome "singing" noise. But no, they do not make more thrust than the equivalent 2 blade (1 inch less diameter) and they will not turn faster.

Jim

Last edited by jrf; 07-07-2015 at 09:43 AM.
Old 07-07-2015, 10:10 AM
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I broke in an OS 32SX using a 10x6 2 bladed prop. After about 12 flights it would turn 19K on the ground. I then switched it to a 10x4 3 bladed prop and got the same RPM. IIRC pitch contributes more to thrust than diameter, or the number of blades. In reality, one-bladed props are the most effiecient and provide higher thrust with a high pitch than a 4 bladed prop of the same diamater iwth a low pitch,
Old 07-07-2015, 10:17 AM
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BTW, there are free pitch vs thrust vs you name it calculators on the web. They seem to do a good job. Change one item, such as going from 2 to 3 blades and it tells you the thrust difference as well as the impact on the engine RPM.

STRC is one of them.

Last edited by rgburrill; 07-07-2015 at 10:26 AM.
Old 07-07-2015, 10:47 AM
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The rule of thumb when switching from a 2 blade to a three blade is down an inch in diameter up 1 in pitch.
When going from a 2 blade to a 3 blade the RPM will change. You gotta remember that third blade is a lot of load on the engine.
You switch to a 3 blade for a few different reasons, 1 clearence, 2 speed, 3 noise. When I say speed I'm talking slowing on down lines, not faster in level flight.
Old 07-07-2015, 11:27 AM
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gregoryshock
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When I decided that I needed to ask my question, I didn't know that this is a debated subject. I wanted to use a 3 bladed propeller because it looks right for the Corsair. I was hoping that it would work good enough for the application. I am not concerned about getting max. speed (I'm not sure what people mean by speed. RPM or How fast will the aircraft will fly?) I'm concerned that the Engine will turn enough RPMs to get the airplane off the ground and fly well. If I must, I'll switch to a 2 bladed propeller. I feel a bit mislead. When I bought the engine I looked at the propeller recommendation and followed the conversion directions to choose the right size 3 bladed propeller. Where the issue came in, is that that Tower Hobbies says that my engine can turn 15x7 2 bladed propeller. But the OS manual recommends the max size of 14x6-8. I've contacted tower hobbies and Hobbico about this conflicting information. What I'm trying to determine is if the propeller I already have will work.

* I already contacted top flite about weight specs. And they say that power ratio I'm using is fine. So the only thing in question here is the propeller size and the number of blades.
Old 07-07-2015, 11:53 AM
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3 blade props are less efficient than 2 blade props. The fewer blades, the more efficient.
Old 07-07-2015, 12:01 PM
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My advice would be to get a three blade in the range of the mfg book that came with the engine and go fly it. Then, taking how it flew, change to a different pitch or diameter and continue on until you find your perfect combo. The biggest problem with a three blade in your circumstances is due to rpm. The high rpm is what makes a three blade less effective than a two.
Now being that OS recommends the 14x6-8 I would say a 13x8x3 would be a good start.

Last edited by acerc; 07-07-2015 at 12:09 PM.
Old 07-07-2015, 12:14 PM
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Gregory-

On these forums you will receive lots of great information from many experienced modelers. We are an opinionated lot and enjoy a good modeling discussion where we can talk about our favorite subject, after beer and women of course!

To get the best info out you must provide more info in. In other words, you told us which engine you have and you later gave us a little more info on what exactly you are trying to do.

So if you want to know what prop would work on your Corsair it would be very helpful if you told us a bit about the aircraft. We need to know the wing area (or the manufacturer so we can look up the wing area for you) and the weight of the aircraft ready to fly. If you don't know the manufacturer could you tell us what the wingspan is, and maybe the chord or width of the wing about 1/2 the distance from the fuselage to the wingtip. With this information we can compute the wing loading and power loading which will help us come up with a selection of props that will most likely provide the performance you are looking for. There is even a good chance that one of the gents on this forum has the exact airplane and engine combo that you have and can provide some ACTUAL info for you to consider.

It's difficult to provide too much information when asking a technical question. Give us something to work with and we will be happy to help.
Old 07-07-2015, 12:16 PM
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Originally Posted by gregoryshock View Post
When I decided that I needed to ask my question, I didn't know that this is a debated subject. I wanted to use a 3 bladed propeller because it looks right for the Corsair. I was hoping that it would work good enough for the application. I am not concerned about getting max. speed (I'm not sure what people mean by speed. RPM or How fast will the aircraft will fly?) I'm concerned that the Engine will turn enough RPMs to get the airplane off the ground and fly well. If I must, I'll switch to a 2 bladed propeller. I feel a bit mislead. When I bought the engine I looked at the propeller recommendation and followed the conversion directions to choose the right size 3 bladed propeller. Where the issue came in, is that that Tower Hobbies says that my engine can turn 15x7 2 bladed propeller. But the OS manual recommends the max size of 14x6-8. I've contacted tower hobbies and Hobbico about this conflicting information. What I'm trying to determine is if the propeller I already have will work.

* I already contacted top flite about weight specs. And they say that power ratio I'm using is fine. So the only thing in question here is the propeller size and the number of blades.

You may search for an engine thread of the specific engine you have, and find out what people are using for props. There are generally not more than 2 or 3 that are the best. It may narrow your search, and save you some money on buying props. No doubt there will be some info on a 3 blade in there as well. Just a thought.
Old 07-07-2015, 01:23 PM
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HMM three blades are better than two eh. Someone should have told this fellow before he set a world FAI speed record few year back with a single blade.

One thing for sure when thinking three blade, and that is you better be darn sure that the is every conceivable selection you may want to try is in fact actually available lest you may have to learn how to start carving them. Selection is pitifully poor when compared to two blade.

Another thing to remember since scale fidelity seems to be an issue and that is simply for many of our airplanes an efficient three or four blade prop for most engines is in fact tiny not only in diameter but blade planform and this typically looks downright silly. A reasonable two blade actually looks far better for many models. Its precisely for this reason that most scale rules allow a flight prop and for static scale judging a non flight prop.



John
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Old 07-07-2015, 02:14 PM
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^^^^^^^^^^^ This. The aerobatic guys I hang out with say the only reason to go to a 3 blade prop is for noise abatement. I flew one a few years ago on a 33% Edge and didn't really notice and less power, but the aircraft's acceleration wasn't quite as noticeable, and speed was easier to control on a downline. Oh, and it was quiet. Too quiet.
Old 07-07-2015, 03:31 PM
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Originally Posted by gregoryshock View Post
When I decided that I needed to ask my question, I didn't know that this is a debated subject. I wanted to use a 3 bladed propeller because it looks right for the Corsair. I was hoping that it would work good enough for the application. I am not concerned about getting max. speed (I'm not sure what people mean by speed. RPM or How fast will the aircraft will fly?) I'm concerned that the Engine will turn enough RPMs to get the airplane off the ground and fly well. If I must, I'll switch to a 2 bladed propeller. I feel a bit mislead. When I bought the engine I looked at the propeller recommendation and followed the conversion directions to choose the right size 3 bladed propeller. Where the issue came in, is that that Tower Hobbies says that my engine can turn 15x7 2 bladed propeller. But the OS manual recommends the max size of 14x6-8. I've contacted tower hobbies and Hobbico about this conflicting information. What I'm trying to determine is if the propeller I already have will work.

* I already contacted top flite about weight specs. And they say that power ratio I'm using is fine. So the only thing in question here is the propeller size and the number of blades.

Go into OS support and ask Bill Baxter directly with your question. I have spoken to him on the phone a couple times when I called service and Bax is a bundle of information about OS engines.
Due to the selection or lack of I have limited time playing with the 3 blades and never did any real testing. The only thing I really noticed was when I shut down the throttle it was like putting on the breaks, the plane slowed really fast. I was using a Kaos at the time and it really slowed down the landings. I just went down an inch and up one on the pitch, didn't notice any power or speed change. I only tried two brands and both the same size and pitch Groupner and MAS. I also didn't have a spinner if that means anything to you?
Old 07-07-2015, 09:11 PM
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John, Is that Carl Dodge's FAI Controline Speed plane? Here is what to remember in this debate. If someone was fortunate enough to have the best 2 blade prop for a given application, and someone else had the best 3 blade prop for the same identical application, the 2 blade will always out perform the 3 blade. Often times we have to compromise on the choice of the number of blades. In full scale aviation more blades were added to absorb the energy from more powerful engines, or as mentioned, for ground clearance. Today multi-blade props are sometimes chosen for noise reduction on both full scale, and model airplanes.
People who argue that their model performs better with a 3 blade prop, just never found the best 2 blade prop, or there wasn't a noticeable difference in performance. The single blade prop is the most efficient, but has it's drawbacks. In full scale aviation they put huge side loads on the shaft and bearings. This is punishment that the engine was never designed for. Our model engines are built to survive most crashes, so they are very robust, and can withstand side loads. Well some of them anyway! Multi-bladed props distribute the loads on the shaft, but this is way more information then was originally sought! The best advice is ask questions, and then experiment. Choose a 3 blade that allows the engine to turn the RPM's for it's best power range, and see if you are satisfied with it.

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