bob62

My Feedback: (28)

I run a Saito 150 with a 16x8 or a 15x7 three blade, I am not that smart but the difference I see is that the engine sounds better on the three blade. The single blade does have a better vertical.

da Rock

Most times when the 3 blade is suited to the engine/plane, it gives better vertical because it's usually got better thrust from having 3 blades instead of 2, and it's turning slower. Slower rpm usually works more efficiently. But that's not the only way it happens. Higher aspect ratio blades are significantly more efficient and when splitting up the same amount of blade area into 3 parts instead of 2 often results in the 3 blades all having higher aspect ration. But that's not the only way it happens........... The comparison isn't anywhere close to simple if you're looking for "the reason".

The real problem with comparing 3 bladers to 2 bladers is that absolute lack of available 3 bladers to test. When you've usually got only one, sometimes 2 props to test it takes a lot of luck for that one or one of those 2 three bladers to suit your engine/plane. And you're comparing one or two three bladers to how many 2 bladers? Twenty or thirty? Or more?

airraptor

My Feedback: (66)

so why does a three blade give better thrust? because it has more blade area? Then the the 2 blade wasnt correct for the engine. you cant compair full scale planes to our models either as they use more prop blades for different reasons.

larger diameter props make more thrust since they have a larger "disk" area. if more blades make more thrust then why dont 12 blade ducted fans make more thrust?

for all props of the same exact pitch "measured" the Bolly Clubman II props make the most thrust on the same engine. next is the APC props and these are two blade props.

da Rock

When one prop is better than another it is basically true that the lesser prop "wasn't correct for the engine", or at least not as correct.

Trying to get across that there are so many variables why one prop is better than another is really tough. Blade count is only one variable.

If you were able to find a 2 blade with the same blade area as a 3 blade and they both had the same pitch (another detail that won't really be the same since so many other variables affect the real pitch of any prop), then the two props aspect ratio will differ and THAT affects the trust, as does the overall efficiency that results from rpm. Since the 3 blade will undoubtedly turn slower, and slower is more efficient THAT affects the comparison and on and on and on.

In our model world, it is seldom that we find any 3 blade to fly, much less a couple to try to see which is better suited. That is the major reason our modeling history came up with idea that 3s aren't going to fly as good as 2s. It's a real shame our history didn't stimulate 3 blade retailing and we'd have a lot more choices for our next model.

Truth is that 3 blades usually do give more thrust than 2 when the two props fit the airplane and suit the engine. And it's even more true when the engine is more powerful than necessary. Truth is that tip drag is more for 2 tips than 3. Truth is that tip drag effects prop efficiency so little it's often ignored. Truth is blade count seldom is ignored in full scale.

Why do all of my WWII fighters fly better with 3 bladed props than with 2? Because all of them are powered by the same power motors and those motors can exploit the 3 blade better than the largest diameter 2 the Spit, Thunderbolt, Corsair, and Warhawk can carry.

We can definitely compare 2 to 3. Diameter is just another detail like # of blades.

da Rock

When I discovered Master Airscrew had some 3 blade props that would look good on my warbirds I bought the one they suggested for the engine on most of my warbirds. Big surprise.... That sucker wasn't close to suitable. It was TOO SMALL. So I bought the next two "larger" 3-blade props. The largest flew the plane best of the 3. So I bought the next step up. Amazing...

I got tired of hearing the same old "you'd get better pull with a 2 blade" so I went back and ran some more testing with 2 bladers. Turns out I'd already gotten good tests with the bunch I had. They included enough higher pitch props to prove that simply filling in with more and more pitch simply pushed the prop envelope into "inefficiency". When I put on 2 bladers that finally had enough pitch to make up for the restricted diameter, the rpm and pitch were out of the envelope.

Turns out that there aren't any magic props, no magic manufacturers, and no magic at all. 3 can work better than 2 and we really can't point out which detail is the reason. Probably because there are way more than one or two that are.

vertical grimmace

My Feedback: (1)

On full scale, number of blades has been more because of prop/ ground clearance. Not because they created more thrust.

It seems the main reason to use multi blade props on models is so many just think they look cool. Any other benefit seems to be a justification for their use. Rarely have I seen it because of ground clearance, but this would be justified.

All you have to do is look at the real performance events, and you will not see multi blade props. I am talking about racing and speed. There may be some use in Aerobatics.

From my experience, and in my opinion, 2 blades are just a better prop choice overall in general, than a 3 or more bladed prop.

acerc

I have read a lot on this subject being a scale builder and many of my builds require a 3 blade to fit the model. I hold to the idea that a 2 blade and a 3 blade are of equal if one takes into account as many of the variables that pertain to our application. HP and rpm in my experience is the main factors that must be accounted for., i.e. what size prop and what speed it needs to turn to be appropriate for the given model. HP and RPM will dictate the diameter and pitch that best serves the airframe, carbon fiber would be best. One question that has came to my mind in my quest to understand the differences and how they apply to my models is this, " If a 2 blade is more efficient then why is it that boats almost never have 2 blades".
The only way I see to fairly test the 2 and 3 blade would be to have a company make both blades with the same exact airfoils. And they need to be ran on an engine or motor that can maintain a constant given rpm with any given prop. It would also require being done on a thrust table that could give the results of the props thrust at given rpm settings.

vertical grimmace

My Feedback: (1)

I think a true test would be to have a somewhat neutral airframe, something like a Kaos. Then with a particular engine, try multiple types sizes of props and then you can feel the difference in flight. A static test on a bench would not tell the whole story without flying.

It seems to me that these questions only arise when someone WANTS to use a multi blade prop and wants to be reassured that it is the better option, when it generally is not.

I think the question with the boat props is that they are of such a small diameter. Even with airplanes using smaller props, you see higher pitch, like in racing. So the small diameter, needs more pitch and blades to utilize the HP correctly.

acerc

Vertical there is one factor that also needs to be taken into account with the racing side of things. And that would be the rpm they are turning and the ability to make a three blade of that size with the strength to withstand the rpm. Also with the larger blades of the larger planes one also has to take into account the blade tip speed reaching mach speed. Look at full scale air racing, most are using three blades.

acerc

That is true for the feel. But for the actual thrust difference it would have to be in a controlled measureable manner. A thrust table would give a thrust measurement of equal diameter, equal pitch props, at a given rpm. That seems to me the only way to definitively say a 34x10 2 blade is any different than a 34x10 3 blade. And with the measurements one could also say what size 3 blade would be equivalent to a given 2 blade.

But until someone is willing to go through the exspence of build a Dyno for props we'll just keep on debating it.

pimmnz

Of course if you really want to know just how many blades to use, ask the C/L speed guys...they spend lots of time, money, wind tunnel testing...and they use a single blade, with a counterweight on the other side...
Can't trust a damn thing, can ya...!
Evan, WB #12.

vertical grimmace

My Feedback: (1)

The reason why I state this is that a Dyno of some sort, does not seem like it would be able to tell you about the varying load you would get. Going vertical, coming out of turns etc. You just have to fly it to get this.
Again, on the full scale planes, they throw the biggest engine they can fit (in the racers and warbirds). Then they have to figure out how to load the engine properly to get it into the power band, without have the prop hit the ground. I bet if they could have used a 2 bladed prop on the Corsair they would have, but the engine had too much power. In fact, they bent the wings just to give ground clearance, and to keep the struts short. Full scale and models are really not analogous.

da Rock

Zinger happens to make 3 blade props that come close to what you describe. They make a metal hub that 3 individual blades bolt into. The individual blades look like 2 bladed props with one blade sawn off, leaving a blade with a hub. They would be the basis for a comparison as you describe.

At first glance you might assume you could simply find a 2 blade Zinger with the "correct" pitch and diameter for a suitable comparison with a 3 blade and run your comparisons. However, there really are way too many variables that blow away correctly matching 2s and 3s. And when you start trying to make the perfect match you often realize why we've been so wrong over the years with our idea that 3s will never be as good because of efficiency (or any other reason). You discover how impossible it is to come up with identical pairs.

I've got 2 different prop pitch gauges I've used over the years. You need one if you carve your own props. You really need one if you modify manufactured props. They are a real eye opener if you do nothing more than measure the manufactured props our hobby offers. They really are a good tool to judge the quality of each prop you buy but better still, to see just how good each mfg is. One of our most popular "magic prop" mfgs, one that has a rabid following, produces millions of props that have different blades. I'm guessing they simply run their molds too fast. They obviously discovered early on that it's close to impossible for modelers to recognize when one blade differs from another. It really is for a simple reason. Your engine can't tell the difference, so how would we. It's been proven over and over you can run blades that differ from each other in radius without any obvious negatives. Get the balance right and go. so......

So anyway, consider matching up the Zinger setup. Start by making a Zinger plan hub to create a 2 blade. That way you can use the blades from one test setup for the companion setup. First thing that is obvious is there is already a huge disparity between blade areas. We need both the 2 and 3 to have identical blade area, right? Failure? Not any worse than trying to find a stock Zinger 2 blader of appropriate diameter having what basically is 1.5X the blade area of Zinger's normal production.

But what would happen if we carved our own 2 blade "Zinger" having 1.5 the "normal" blade area? We'd definitely wind up with an invalid aspect ratio that wouldn't come close to the aspect ratio of the 3 blade. We're trying to prove efficiency based on number of blades, right. Aspect ratio greatly affects efficiency. It affects almost everything. It's why gliders have such high aspect ratio wings.

If we work up a 2 blade with the same aspect ratio as the blades of the 3 blade, we've suddenly got a prop that isn't much like what Zinger sells. Could it out perform the 3? Of course, but it could also not outperform it. And we'd be out of luck telling what "efficiency" made the difference. And we wouldn't have Zingers to buy like the one that won the comparison, that is if it had.

Did we start with the best 2 blade for that engine/plane? We better had or a comparison test that any 3 blade won wouldn't be worth spit, since all that would have proven was the winner suited the engine/plane better, not that it was inherently superior to all 2 blades.

In our modeling world today, we best not try to start the comparison with best 3 blader because there aren't anywhere enough of them to select the best suited. And if we don't start with either the best suited 2 or the best suited 3, we aren't going to prove anything of value about 2s versus 3s.

See the unending loop? We really have no way to prove anything other than one prop is better than another. We certainly aren't going to prove if 2s are more anything than 3s. But luckily we can prove if the 2s we've got in our kit are better than the 3s we have. Unluckily, there won't be any 3s to speak of. And that almost certainly dooms the 3s to lose in the hands of most of us.

da Rock

You're spot on about having to simply try what props we have.

As for the Corsair (Hellcat and Thunderbolt since they all used the same engine), they might have used a 1 blader if the only basis was efficiency. They could have also added "P47 struts" to get more blade off the ground. When the Thunderbolt got the more powerful engine, they went to paddle blades and turned an airplane that lagged a Spit in climb into one that outclimbed that beautiful thoroughbred. Funny thing was the paddles were even more "inefficient" than the previous retrofit to 4 blades had been.

When I discovered the correct Master Airscrew 3 bladers for my warbirds, I had already tried props on my first warbird (of that series) for almost 2 years. I'm a compulsive prop buyer/tester. I was pretty sure the best available prop (obviously a 2 blader) was on that sucker. When I tried the 1st MA 3 blade, it looked like the old saying had been proven, at least to the guys watching the testing. When I tached the sucker it was pretty clear why that 3 blader was losing to a 2 blader. But it was what MA recommended for a .90 to 1.08 glow. The tach showed otherwise. So I ordered the next step up as recommended by MA. It was for 1.08 to 1.5 glows.

It did better in the air at least. The tach showed it wasn't suited. So I ordered what MA recommends for 1.5 to 1.8 glows. In fact, to save myself weeks of waiting, I ordered the next 3 "bigger" ones.

I now have a number of warbirds that climb like angels on fire. Their OS90s are running like gangbusters swinging props that are way too much for them according to the manufacturer, and they love it. And so do I. I'm guessing that today's OS90s are significantly different in their power curves than previous.

Oh yeah, I recently retrofitted my Thunderbolt to electric with an outrunner that was advertised as a "90". It has the same Scalded Angel climb as the glows. Of course, when I'm flying those warbirds scale I don't use all the throttle with the glows nor the electric. BTW, the outrunner mfg has a 60 that has proven to outperform that 90. Today's model industry really doesn't have all their ducks in a row. It pays to figure out a lot of this stuff for yourself.

vertical grimmace

My Feedback: (1)

Being a very avid competitor in many different disciplines of model aviation in the past (and present), primarily Control line, I have a very large box full of props. It is amazing the difference props make in how an airplane performs. Control line really allows you to appreciate this. Line tension was always a big factor with those. All of my experience with CL, has me biased toward larger diameter, lower pitch if possible. It is just better thrust. I see guys fighting to slow down their warbirds at my RC field, but they run such high pitch. There is a sweet spot for props, and that can only be found through testing.
I am going to admit my bias, I am too cheap to buy a bunch of 3 blade props to test them! lol

aspeed

I have to agree there. Control line gives an accurate measurement of speed, and acceleration if you choose to time it. Measuring speed on an rc plane can be fudged a lot by making a dive to make things look better. A pylon course for example is good, but there are also differences in flying errors. An Extra hanging on the prop has much different needs than a faster plane. It would be mostly static thrust that would be desired. The cars are very subjective to feelings coming out of the corners etc.

byrne1157

Wow! I guess I don't have as much experience as some of the rest of the guys on here, but I wanted to post a few observations and/or questions that could be food for thought.
1. I notice that most, if not all of the props I see are what I would call "Graduated Pitch" profiles. I am going to guess that is because of the need to reduce pitch the further you get from center due to what I will call "Leverage Loading" of the prop next to the hub.
2. Considering that fuel economy is probably the best way to measure prop efficiency, aircraft speed could be the best indicator of this. (Least time in the air at a given altitude to cover a given distance, while burning the least amount of fuel.
3. It seems a little hard to compare full-scale aircraft propping to model aircraft propping, since most full-scale aircraft utilize variable-pitch props. Kinda like apples and oranges.
4. This is quite the dilemma, it makes me think all sorts of things. Lots of fun to be had here...

da Rock

And the good thing about not wanting to buy a bunch is there is no temptation to do that as there aren't any to speak of tempting any of us to buy them.

vertical grimmace

My Feedback: (1)

I always thought it was fun testing props. My problem now is that I fly big stuff, and I already do not like spending $25 for a prop I will most likely break. If I found a 3 blade that worked better, I still would not use it as it would be cost prohibitive to me. Once it is spinning, nobody can see how many blades are up front anyway.

MTK

As I mentioned before, post 13, the experiment is not that tough to determine the facts...You need to measure the variables in a controlled experiment

Prop..........get Xoar or Falcon electric 2 blade woodies. Ask Xoar or Falcon to convert 2 bladers to 3 bladers to ensure same foil, diameter, thickness, pitch, blade area and area distribution
Motor.........easiest to get useful numbers from a motor. Use a larger capacity motor than needed to keep heat effects to a minimum. Knowing voltage in and amps in will determine the power being consumed
Thrust........Get that from thrust table (fish scale??)
Environment.....Large enough room with fixed temp and RH like a school auditorium or gym.......any effects from obstructive interferences become negligible

Analyze which prop will give the most thrust for a given wattage and do it for a full throttle range from say, 1000 rpm to 8000 rpm, in maybe 1000 rpm increments. Then you can figure out which format is more efficient...... QED

da Rock

The theory most often used attempts to match the AOA from root to tip to the speed that section of prop is moving through the air. The prop at the root will have an airspeed less than the tip which has the greatest airspeed. However, when you start measuring the AOIs and move from mfg to mfg, you discover many of them either don't embrace that theory or simply ignore it. You'll also encounter mfg's who have their own theories. One of the most popular actually flattens out the blades toward the tip. They also flatten out the airfoil as well. They also advertise their props can be proven as better by taching them on the ground as they increase idle speed measurably. Apparently they think that's a realistic proof of "better".

Of course, fuel economy is easily the least important aspect of performance modelers care about. Which might also be why almost no modelers pick props by that measurement. The real problem modelers have is lack of any accurate and reliable tools to measure just about anything. With today's technology that might improve with any luck. It's also one reason we have so many modelers around who believe one brand is magic. And so many who believe one dia/pitch is the best for specific engine sizes.

We actually have the same problems as full-scale, fitting the prop to available clearance, engine power, and desired operating envelope. But it can really be fun for sure if you're willing to try more than one brand of prop, and more than one diameter, and more than one pitch, and sometimes when there are 3 blades available and they suit your desired envelope, more than just 2 blades.

We will probably never really have 3 bladers in quantity enough to provide a large enough statistically reliable sample to prove anything about them. How often have you been asked what prop is the best for my model? He wanted to hear which ONE so he could buy it and fly, didn't he.

OliverJacob

My Feedback: (12)

I have no scientific data to prove, but I have heard that a one blade prop would be the most efficient way. A counterweight helps with the balance but they are impractical to produce and come with a some safety concerns.
So the two blade prop is a good compromise. Multi blade props initially have been introduced to gain ground clearance, our WW2 planes had huge engines and couldn't stow away a huge landing gear you would need for a large prop.
Now the help keeping the noise level low.

In order to find out - I'd rig up an electric motor with a power meter and a scale to measure the thrust. Now use a 2 and a 3 blade prop while feeding the same amount of power to the motor and see how much thrust you get.

aspeed

One blade props have more problems on larger motors, so they likely won't catch on. If you break a blade on takeoff or whatever, there is a good chance of your plane turning to powder too. Two blades are pretty good as you say. Your tests for thrust are just static thrust on the electric. Air tests of actual flying are much more effective. A high pitch prop will unload a lot more in the air and the rpm's will go up, as well as less load on the motor etc. On a two stroke motor there is a powerband that the motor needs to be in to be happy, and not overheat. I am sure the original poster didn't know what he was getting into.

da Rock

The scientific data has been published for years. The scientists proved one blade beats two and two beat three etc, and have papers on what the probable reason is. They usually blame it on prop tip efficiency. Unfortunately nobody in modeling seems to have noted HOW MUCH efficiency difference we'd see in modeling, and nobody seems to have done anything more than make an assumption that difference means, "2s are better than 3s" with no qualifications. And a bunch have morphed the efficiency difference fact into "there will always be a 2 that will beat any 3".
An electric motor on a meter has been used a lot to measure the power the motor draws, but won't tell you anything about the thrust. You need to measure the thrust. Building a rig that allows the powerplant to pull and that measures the pull being generated is a good way. A couple of guys around these forums have done just that. The rigs have scales built into them or attachable to them that measure pull.

Unfortunately, those rigs don't measure anything but static thrust. As aspeed just mentioned, the thrust that matters is the thrust generated in the air when the model is flying. Until that's measured, we still aren't proving much at all, no matter what we assume.

This puzzle really needs to start with some scientific proof that we actually have a pair of "equal" props, one 2 and one 3 blade. Unless we do, then it's just as sensible to try out the very few 3s that might work while we're trying out more than just one 2 blade to find what prop flies our model best the way we want.

Sport_Pilot

The dirty air theory is wrong. No matter how many blades, even if one is right behind the other as on a turbine fan, the blades go through clean air. That is because the prop is pulling the air toward the prop and the dirty air is behind the prop.