Da Rock Master Airscrew props have way less pitch then they state. measure them and compair to APC props.
you say three blade props are more efficient than two blade props. LOL dude you are way off on a three blade prop you have one more tip producing lift/drag. You talk a good game but your are worng. In fact we are all worng in this forum in that we all have read snippets of information from the net and base our own theory on those readings.

Also who cares if one prop make the most thrust on a test bench. lots of thrust and zero airspeed means nothing. We are flying airplanes they need to move thru the air to fly. So the prop with the most thrust while flying " for our desired performance" is the prop that provides the most thrust. sure your 3 blade props make those planes climb well but in no way are the best props for speed.

I will always agree that 2 blades are best but that doesnt always mean I will fly with a 2 blade set up also.
before you state why Warbirds went with 3 or more props you need to see how they were designed and what they were designed for. We can all agree to the fact that most props going past .85 mach isnt very effiecient right?

warbird fighters need lots of power to climb and to have high speeds. doesnt matter on the prop per say in that its "power" than gets climb and speed. now this added power to reach the design goals of these fighters had some restrictions. firts and foremost is prop tip speed. they needed to keep this close the .85 mach or less. a two blade prop on 2,000 hp would need to be around 16 foot in diameter to asorb that power. to keep the tip speed with in the desired range you would need a gear box added to the engine. Well this would add weight to fighter. This would take away from the climb and speed. now another reason for not running a larger 2 blade prop was that you would need much longer landing gear struts. you may think that would be easy but far from it. the longer struts would need 3-4 times the strength in the wing to handle them. this is because the struts would need to be placed farther out from the fuselage so they could retract. so they would need stronger spars because more of the weight was placed farther out from the center of the wing and would need stronger main spar.

Ok now another area with the longer struts was you would have more stress put into the main spar because of the longer struts placed more torsion on the spar. think or a 3/8 drive ratchet on a stuck bolt. its tight and you can budge it, but if you use a breaker bar ( longer strut) you apply more torque/torsion to it. So now you would need a much stronger wing spar again to hold this extra torsion placed o nthe spar which again would add more weight from the center of the wing. all this added weight reduced the speed and climb of the fighter.

So in comes a shorter prop but with more blades to asorb the power. now they reached their goal with the fighters. even larger planes that had now ground clearence props used more than two blade props to reduce weight and keep the tip speed down.

now on our models we dont care to much about noise, how much fuel they burn but care how well they fly. if a three blade prop works well for you and you like they way it flys then good its your plane and your hobby. I like two blades as i find the APC props to be the best for the price. sure carbon props maybe a bit more accurate from prop to prop but that is because they have only a few molds for each prop. APC has many molds for one size and needs that to meet the demands and to keep the price where its at. the molds are machined out of aluminium and sure one maybe off some but most of us sport guys it doesnt matter as our planes are way over powered.

by the way APC make three blade props and they use the same airfoil.

also for those wondering most props make 90% of all their thrust with on 2/3rds of the prop.

The twist on the propeller is actually to ensure it maintains better consistency in angle of attack along the blade length at normal rotating speed and in forward flight.

There are many diagrams to represent this here https://www.google.com.au/search?q=p...&bih=576&dpr=1

Basically.. in any fixed pitch propeller (full size included.. eg Cessna 152) the inner part of the propeller is stalled at full power with zero airspeed.. as your airspeed is increased the angle of attack of the air striking the propeller blade reduces and at flying speed most of the propeller should be working efficiently.

The speed of the blade is faster at the tip and therefore the blade angle is reduced by the graduated twist you mentioned. Aerodynamically, this twist maintains efficient angle of attack along the length of the propeller blades at forward flying speeds.

A constant speed propeller widens the speed range at which the propeller can operate at more efficient angles of attack.

As for the original argument.. it is quite simple.

One Blade is very efficient, but causes asymmetrical loads on the engine shaft.
Two Blade is the next most efficient and has balanced loads on the engine

The propeller has to absorb the power output of the engine. So when using 2 blades, in theory, the more powerful the engine, the larger the propeller must be.

The limiting factors become.

a, Propeller tip speed (Noise and efficiency losses if tip speeds approach the speed of sound)
b, Physical size

So if you have too much power for a practically sized 2 blade propeller, then use 3 or more blades, and accept a slight efficiency loss.

There are a couple of books where he can get the info he needs.

Model Aircraft Aerodynamics by Martin Simons. Mine is the 4th edition and may still be published.

Aircraft Propeller Design by Fred E. Weick has been out of print for decades but still considered a valuable source. It is usually very expensive if you can find a used copy.

There are some simple formulas used to calculate efficiency. Basically, you are measuring the velocity of air produced through the propeller disk due to Bernoulli's theory. The difference in pressure causes air to rush towards the propeller. This is the first type of air velocity (V). Static thrust only gives you half the data. The air causes the propeller to move forward. Airspeed is the second type of velocity (v).

BLW that book ay be good but Bernoullis theory is a very small part of how a wing or a rotating wing makes lift/thrust.

Agreed, I just posted a bunch of videos in the Aerodynamics forum on this very subject..

http://www.youtube.com/watch?v=aFO4PBolwFg

Believing lift is produced only by faster moving air over the top of an airfoil would mean a wing / propeller / helicopter rotor can produce lift with no downwash / prop wash...

Try standing under a hovering helicopter.... try standing behind a real prop aircraft at full power... Propwash / downwash is VERY real and it is the reaction to this deflection of air that produces thrust / lift,

Bernoulli does factor into the equations but his theorem is often applied incorrectly in this context as the videos show.

http://www.youtube.com/watch?v=zV06PgxarW0

Again, full scale and our models are apples and oranges. Really do not belong in the same conversation. Our models are running certain engines, with specific power abilities. Also, there are very few choices of props for our models. Unlike full scale, where they can specifically design a prop around a specific engines abilities and needs.
So maybe full scale engineers have studied and researched extensively enough to create a better multi blade prop for a specific engine (turbo prop), while the model airplane industry does not benefit from such a large research budget.

Here is a link to a suggested process for testing model props (with minimal equipment, and focus on results, not theory) - Its been around for several years and may have some relevant data reported.

http://www.sciencebuddies.org/scienc...ero_p018.shtml

How many blades on the prop in your picture? 8 or 9? But this plane was happy with 4? I don't think anyone has mentioned one of the main reasons they put more blades on the prop of most full scale for high altitude high speed flight. When flying fast and especially in thin air a smaller higher velocity air stream wins every time. In fact with a large diameter the engine will not have enough power for the air velocity required. If anybody said this I missed it .

That doesn't mean the theory isn't relevant or valid. I fully understand lift and thrust as well as the newer ideas that are popular among enthusiasts. I think too much was read into my brief comments.

What do you mean newer idea's that are popular? These means you dont belive it?

sport pilot I would say that you need to read more about props and read stuff way back. dont type in a search then say the firsat thing you read here.

also this is to talk about 2 blade VS 3 blade on model airplanes.

I didn't mean to start an argument.

If the blade is well designed and tests as highly efficient by something like the Froude formula, then an extra third blade will make a smoother rotation in flight.

You said that book only covered a small part of how lift is produced. The model airplane book covers a lot. The book by Weick is from years of research and wind tunnel testing. I have a background in teaching basic rotary wing aerodynamics, and neither you nor I would ever make it thru all of that book. My answer is what contained a small part of info.

I did read way back and I have reread and don't see where this was mentioned. I mentioned this because one of the major reasons for multiblade props for full scale does not apply for our models. You can't go real fast when a prop will not pull against the air and increase the velocity. So you have to decrease the diameter and increase the blades.

sport what you are saying doesnt make any sense. so if an engine will not pull hard enough with a 2 blade prop it will pull harder with a smaller diameter and more blades. LOL The engine only makes so much HP. by what your saying I can put a 12 blade prop that is super small on my plane and go real fast???

Yes that is correct, at very high speeds that is. With increased diameter a set HP will only move the air at a certain velocity. Your plane cannot exceed that velocity at straight and level flight. So with the same HP you have to decrease the diameter increase pitch and install more propellor blades. This is exactly what a jet engine does BTW.

However they usually increase the power at the same time, but the above is correct.

Sport lolfirst off these are full scale planes and this forum is about model airplanes.

Second you’re still off base with your statements. Once you reach a certainlevel of HP/torque a two blade prop will not work. You run into tip speedissues. Hence the reason for more blades and even wider blades. You are partlycorrect in that you need to reduce diameter some and increase pitch to gofaster but adding another blade will draw even more power from the engine. Thiswill lower the max rpm that engine will swing that prop and then will be slowerthan the two blade set up.

Also comparing a jet engine fan to an engine driven propeller is just wrong. That’slike saying a tomatoes tastes the same as an orange since they are both fruits.
I am not trying to mean or argue to much but you should do more research on propellers and make sure your research is from a legitimate source.

Again full scale "real" airplanes use mor than two bladed props to cut noise and asorb the HP the engine puts to the prop with out exceeding high mach numbers at the tips. The C-130 models b-I models have engines that make around 10,000 shaft HP what size two blade prop would that need? It would be so large that it would have to spin at less than 800 rpm. So more blades were designed to use that HP.

Anyway sport please do some research.

BLW yeah I am not trying to argue either just rhetoric.

Sport I am in the Air Foce so I know how a jet engine works and I have my A&P.

Which is why I said that we do not have that issue, but others try to make claim to full scale issues that do not pertain to our models.



No I am right on with my statements. The new 5, 6, 7, and up bladed props are way smaller than required to prevent tip speed issues. In fact they are often replacing much bigger props which did not have tip stall or ground clearence issues. The smaller diameter reduces HP requirments much more than the higher pitch will make up for it, so the extra blades are required for the HP, as well as to get bite in thin air.

See item #6 here http://www.mccauley.textron.com/von_...erformance.pdf.

sport here is item six from your link for everyone

6. What is the importance
of propeller diameter?
Ideally, the propeller diameter should be
greater for efficient low airspeed operation
and smaller for high airspeeds. A propeller
with a variable diameter would solve the
problem, but the structural and control
problems involved would increase the cost
and weight to a point where the advantage
is not practical. The diameter of a fixed pitch
propeller is generally reasonably large to
favor low airspeed operation. At the same
time, the blade size is kept small to favor
higher airspeeds and permit faster turning at
low airspeeds so that higher power is
available from the engine. The diameter and
blade size of a constant speed propeller can
generally be greater because of the variable
blade angle provision.

like I said you read one thing and then read the first sentance and then base everything on that. Anyway when you make a claim on here and "post" a link the OP asking a question might beleive that and then they are stating only partial information like you are. what you are forgetting is that the prop and engine need to be matched to the desired performance you are looking for. I am done arguing with you sport.

OP my opinion is that two blades for modle airplanes are best. now if your going for a certain look then sure go for a 3 or more bladed prop. I run 3 and 4 blade props on some of my planes. sure I take a performance loss on the same engine but I have more power than needed anyway. all of the pylon and glider planes in competion use a 2 blade prop. Now some of the 3D and pattern planes use 3 or 4 blade props but again its for their need to be quiter and or provide more braking action on a down line that the 3 or 4 blade gives them.

I'd have to think the size of the propeller and pitch makes a difference, If you have a propeller that's 10 inches long at 5,000 rpms the two blade will have less area and time to "Bite" into the air. A blade 48 inches long will have more area and be more efficient. The blade I assume would have to be able to create a wind tunnel. So if you take a 3 bladed prop that's 10" 8 and spin it too fast, It won't have the area or time to bite into the air. Maybe a prop 10" 4 would do a better job of it

[QUOTE=oldbassard;11646398]I'd have to think the size of the propeller and pitch makes a difference, If you have a propeller that's 10 inches long at 5,000 rpms the two blade will have less area and time to "Bite" into the air. A blade 48 inches long will have more area and be more efficient. The blade I assume would have to be able to create a wind tunnel. So if you take a 3 bladed prop that's 10" 8 and spin it too fast, It won't have the area or time to bite into the air. Maybe a prop 10" 4 would do a better job of it[/Q

The real question is, what do you consider to be efficient? There is a big factor of subjectivity in this problem (and a bit of nonsense in this thread, LOL)

At first it sounds simple: the goal is to get the most thrust for the least amount of fuel burn. The most efficient prop would seemingly be very big and spin very slow. But maybe this thrust monster doesn't move air fast enough for your airplane's wing to generate lift. It's not because the prop is inefficient.

So now the goal becomes: to get the most amount of thrust, at appropriate speed, for the least amount of fuel burn. So now the prop is a little smaller and spins a little faster. On a calm, comfortable day, flying a long straight path at constant altitude, the fuel burn is incredibly low. The flight back is on a hot, muggy day with choppy winds at altitude, and requires some travel over a mountainous region. The wind gusts change the load on the prop and the engine seems to need more fuel to keep RPM high enough for comfortable speeds. It is difficult to gain altitude to clear the mountains. A smaller prop with variable pitch would seemingly allow better management of rpm and air speed.

Now the goal is: most thrust, at appropriate speed, reduced size, variable pitch, for the least amount of fuel burn. So the most 'efficient' prop becomes something very different from: which has more thrust for a given amount of fuel burn, or which burns less fuel for a given amount of thrust.

I can't say I've read every post here, but I would bet there are many reasons behind the number of blades on any airplane's propeller that aren't mentioned. But it all boils down to: what does the customer want? If everyone's main focus was fuel burn, we all would be flying in hot air balloons. The choice of size, pitch, and number of blades on a full-scale airplane comes down to the purpose of the aircraft (how high, how fast, how heavy, how comfortable, and in what conditions), the expectations of the customer, and a million other factors related to the power plant, airframe, and the hundreds of other systems involved in making airplanes fly (fuel temperature/limits, fuel metering, oil temperature/limits, exhaust gas temperature/material capability, air conditioning/electrical system demand, etc, etc, etc.)

It is basically the same for our models. Our engines operate at certain RPM, have certain power bands, and can handle certain amounts of heat. Some of us fly fast, some slow, some hover and some glide around. Some airplanes are slippery, some are dirty. Some take-off on grass, some on paved runways, some on water, and some are hand launched.

So given 2 propellers, one 2-blade and one 3-blade, of equal diameter/pitch/material, allowed to spin slowly in ideal steady-state (constant speed) conditions, and focused solely on which produces more thrust/energy input, my money is on the 3-blade.

2 blades or 3 blades ???

No real question for the vast majority of glow fuel models. There really aren't any 3 blades readily available, and almost none that take an effort to get.

But look at IMAC. They have a very good choice of 3s and after the initial development process that happened when they were basically forced to try 3s, they discovered that 3s worked as good or better than 2s had. Look at the history of pattern flying. Same story.

In both stories, there was a learning curve. It's still going on, but basically there was a discovery that with some choice of props, there was more performance out of 3s than most expected. And they discovered that some engines were better with 3s than others had been with 2s. That's not surprising because engines had been refined over time to better suit 2s simply because that's all there were.

When there is little or no competition, there is no real proof.

I heard an IMAC guy at the field awhile back griping about how poorly his plane was flying. Turns out he'd busted his expensive (and only) 3 and had dug out his last, best 2. He was advised to try a 3, that 3s were more efficient. Say what!!!! Our hobby is really humorous sometimes.

Moving a larger mass of air at a slower speed is usually more efficient. Some of the bigger resistive forces are bearing friction and wind sheer. Both of these go up with blade velocity. Two good examples: wind turbines and the 'open rotor' turbine engine, which may make a comeback.

Wind turbines use the largest blades they can to minimize frictional losses (other reasons too).

The open rotor turbine is a very efficient engine. The blades are exposed, rather than inside a duct. There is a huge boost in efficiency because the sheer forces at the blade tips are significantly reduced in open air. In layman's terms, this is because of the 'no slip' rule, where the air touching the inside of the duct is not moving, and the air touching the blade tip is moving at the same speed as the blade tip. The smaller space you have between the duct and the blade, the larger the sheer forces due to viscosity. Open rotor eliminates that boundary layer.

With any design, you have to manage tip speed as airfoil effectiveness changes drastically when the air flow gets close to sonic speeds (air starts to compress).

Another thing I don't think I've seen mentioned is the size of the prop stream with respect to the airplane. Obviously the goal is to keep air moving over the wings and stabs. With a larger diameter prop, the stabs have a better chance of being bathed in the prop stream. This allows the control surfaces to be effective in more adverse conditions than with a smaller prop stream. I would imagine this is important to full-scale design, and if so, it would make sense that a larger diameter prop is desirable. If power is limited, you would have to sacrifice the third blade to get a larger diameter prop.

Just because the 3-blade of equal diameter 'needs more power', it doesn't mean less efficient. It means the engine is not sized to accelerate and maintain propeller RPM in varying flight conditions for that prop. There is a trade of efficiency for overall performance (instantaneous thrust, acceleration, etc).

The single blade propeller has always been the more efficient type.
There was a reason for free-flight competitors to use it:




Read page 73:

http://books.google.com/books?id=guUsAAAAYAAJ&dq=air%20screw%20two%20versu s%20three%20blades%20efficiency&pg=PA73#v=onepage& q=air%20screw%20two%20versus%20three%20blades%20ef ficiency&f=false

Page 73 explains how to find the horsepower required for a different number of blades if it is known for a certain number of blades. It gives a couple of formulas for you to figure out the new diameter and is based of your changing the blade area by a constant.


The page appears to be within a paper on boat "screw propellers" and it's highly doubtful that constants would be identical for both air and water. Has there ever been a boat that ran just one bladed props? There certainly haven't been any production aircraft during the golden age of props. There was certainly good reason for fuel efficiency in the 60s.

Efficiency really is only one very insignificant detail within the definition "better".

"2 is more efficient than 3" is most certainly nowhere close to "2 is better in every way than 3" and never will be "2 is always better than 3".
Yet modelers seem to have decided that 2 is always better in every way. Fortunately, the difference in efficiency is not significant, otherwise all the IMAC, pattern, and scale planes would be struggling to fly with their 3 bladers.

Screw propellers are totally different.
Water's viscosity is totally different.
There is only one statement from p73 on that discusses 'propulsive efficiency' - which is at the very end and has actual numbers to support.