jeepindog, you are telling us that all of the aeronautical engineering development on propellers since the Wright Bros. has been wrong? Both props had the same static RPM?

The 13-8 three blade has a greater power load than the 14-6 two-blade. If the engine was not constrained by valve train limits, breathing restrictions, or something else, it would have to turn faster with the 14-6. Maybe your engine is at its rev limits -- if that is the case, a steeper pitch 2-b will give more thrust.

For a given diameter & pitch, a three blade has 50% more blade area and drag than a two-blade & can transmit proportionately more power, but conversely, requires similarly more power to turn at the same rpm. However, since prop blades run in the wake turbulence of the preceding blade & since the blades of the 3-b prop are closer together, there is more loss of efficiency from wake turbulence. Additionally, there is more "mess" in the root transition zone, resulting in a shorter effective blade length -- again lower efficiency. If the 14-6 & 13-8 are turning at the same rpm, the discharge velocity of the 13-8 is 1/3 greater than for the 14-6 and has significantly greater slipstream shear losses -- another loss of efficiency, whereas the lager diameter prop is exerting the same work over a larger area at a lower discharge velocity -- more efficient (which is why helicopters have large disc areas). Finally, in the case of jeepindog's pitch & diameter change, the pitch increase more than offsets the reduction in diameter, requiring even more power --- so either there is a lot of stuff that isn't being revealed (like the engine at it's rpm limit), or the post is a troll.

BTW I've been in the modelling business for well over 50 yrs, flown thousands of hrs in FS aircraft & managed a fluid dynamics engineering research lab for 20 years. The physics hasn't changed.
[/quote]

I guess I forgot to mention that I am a liar. Merry Christmas, britbrat. I hope you feel superior with all of your guessing about the reality of what I wrote. Both scenarios that I outlined were run with the same fuel on the same airplane with the same engine on the same days. It was tried and tried again. The only thing that I did not reveal was the actual reading that I got with my tachometer. Both runs yielded rpm figures of 9,600. I do not run my fourstroke engines at redline, as I do not need to. Although it was impossible to measure unloaded rpm during flight, the exhaust sound was similar, however, this means little. The three-blade prop indeed outperformed the two-blade prop. If someone made a 13x6 that I could have tried, I would have, but I could find none at any local shop. 13x8 only.

Lachlan

so in the end, a three blade prop spinning at the same RPM as a two blade prop indeed has more drag(theres one extra blade catching air) and therefore requires more power to do so. Correct? But more drag means more pull for the plane to use in thrust right? So doesnt that, in itself,(this is me trying to comprehend, so be patient)explain the simple rule of supply and demand...in the sense that in order to have more thrust, you obviously need more power or higher revs out of your engine anyway?...meh...if i make any sense cool...if not, kill me now

Being a full scale pilot as well as a beginner in R/C, I can tell you this... ALL of the fastest and most maneuverable full scale single engine aircraft run three bladed props: V-35 and A-36 Bonanza, Cessna Centurion, Piper Malibu Mirage, Piper Lance, not to mention all the higher end Aerobatic craft like the Sukhoi, Edge, Extra, most of the Pitts, CAP, the list goes on. i've flown a tad over 3,000 hours, and around 1,000 hours of that in 450 and 600hp B model Ag Cats, and the ones I've flown with three blade prop conversions ALWAYS performed better... higher cruise speed, lower fuel burn, less noise, and a shorter take off roll with a heavy load.

But no one will believe it... Or they will tell you that you obviously had more power available, so the manufacturer should have spec'd that airframe with a bigger two-blade prop in the first place, or they had the wrong pitch...

Lachlan

Yeah, I know... some people just cant be convinced without taking them up and making them spit their lunch all over the instrument panel!![:'(]

one of the fun things about RC is that lots of things don't scale. the size of molecules, time, and wind all remain constants throughout different sizes of RC.

in particular, just because all of the high performance craft use three bladed props doesn't imply that 3 blades are better. what if they all are tail draggers? does that automatically mean that is more efficient? the 3 blades in those situations is probably a design choice to get a prop to match the airframe, where a 3 blade fits and a 2 blade won't.

Generally, props with fewer blades tend to be more efficient.
Like all rules, though, this one needs to be applied with an
understanding of the underlying assumptions. A big assumption,
which is really significant when comparing models to full scale
aircraft, is that the tips of the blades are not operating too close
to supersonic. This essentially limits the prop diameter, which
means that you need to get the necessary thrust out of a fixed
size propellor 'disc'. One good way to do that is to add more
blades. A three blade prop with subsonic tips is probably much
more efficient than a two blade prop with supersonic tips.

On most models, the two blade and three blade props you would
use would both operate with subsonic tips, and the two blade prop
would probably be more efficient, partly because it would have
larger diameter, which in itself is more efficient, all other things
being equal. On a full scale plane, with higher airspeed and much
larger absolute diameter, the prop tips are much more likely to
approach supersonic or transonic speeds, so the best prop choice
needs to take that into account.

There may be other significant differences between model props
and full scale props, but I'm guessing this is the big one.

banktoturn

ANd to complicate things....

If your comparing differnt types of blades, that throws it all off.
Like an apc 2blade may give better performance than a MA paddle blade.
But in a hover the paddle blade has better mid range.
and most multi blades are paddle in design.


As for models use what best fits you needs....
speed
torque
or just mutli for scale or clerance.
and for some price.

It can be costly to find the right prop if your trying to squeeze all you can from a motor.
good luck.

and of course when you fing the perfect prop.. you'll break it on the first landing[:@]

Full scale AC use multi-blade props to permit the transmission of available engine power without resorting to very long diameters (no space available) & to avoid the necessary reduction gearing to keep tip speeds of a giant 2b subsonic. FS aircraft have the advantage of constand speed propellers, so the 3b with the same diameter will be at a lower pitch angle than the equivalent 2b & will consequently have greater initial acceleration & climb. however, at cruise, or maximum speed, the 2b will outperform the 3b simply because it is consuming less power for the same thrust. That is why the Schneider Cup racers of the 20's & 30's used 2b props -- to wring every last ounce of speed out of the available power. That is also why the fastest RC models use 2b props & the fastest CL models use one-blade props -- not a single 3b to be seen anywhere when performance really counts.

A very well designed & manufactured 3-blade prop can be 90% - 95% of the diameter of the reference 2b, for the same pitch & power load. If it is not well designed, or is manufactured of a material that doesn't minimize blade thickness, the 3-b diameter will be more like 80% for the same power load. In the example that jeepindog gave (14-6 2b as the reference), the closest equivalent 3b size is 12.5-6 (90% equivalency) or a 13-6 @ 95% equivalency, for the same power load. However, he used a 13-8 as the 3b, & that is directly comparable to a 14-8 2b, or a 15-6 2b @ 95% equivalency, all other factors being equal (same material, same blade configuration, & preferrably, same manufacturer). In other words, the 14-6 2b is a much "smaller" prop than the 13-8 3b as far as the engine is concerned. The difficult part comes when he claims that both props turned at the same 9700 rpm. There is clearly more to the story.

If for example the 2-b prop was a wooden type (eg. Zinger), & the 3b was a nice sharp narrow-blade APC or Brolly GRE prop, that could explain part of the observed difference (the thick 2b blade was absorbing relatively more of the engine power), but it doesn't account for all of it. Similarly, was the 2-b a low aspect ratio nylon prop & the 3-b a narrow blade GRE prop? He didn't say, althought that could account for part, but not all of the difference..

jeepindog observed 9700 rpm for both props -- is he tuning & throttling his engines for 9700 rpm because he wants a safe margin to redline, or was the engine similarly tuned & throttled & able to run freely? He didn't say.

If the engine will turn a 13-8 3b @ 9700 rpm, it will turn a similarly designed 14-8 or 15-6 2b at the same RPM -- & there is no doubt which combo will pull harder.

What information is missing?

Both props were made by MA. The two-blade was a K-Series, the three-blade was the standard MA three-blade, since there is no K-Series three-blade. Same fuel, same day, therefore same atmospheric conditions (unless you want to decide for me that the humidity and temperature changed dramatically in the time it takes to swap a couple of props...) At maximum rpm on the ground, tuning the four stroke for peak and backing off 300 rpm, they both turned 9700rpm. If you want, one of the guys in my club who was there that day is a Notary Public, and he will mail you a statement proving the rpm.

Britbrat, while I respect your knowledge of aerodynamics and theory, you have yet to demonstrate any actual testing that YOU have done to back up your statements regarding model aircraft. I agree that in theory a two-blade prop is more efficient than a three-blade, but in real-world testing it is extremely hard to discern the advantage in some situations. I gave hard facts, and you simply refute them as impossible because you think I am hiding something. Well, you're right. In the example of the two-blade prop not performing as well as the three-blade prop, I left out the detail of having a really big fat guy sit on my model as I flew it. No wonder it didn't perform as expected. Also, the engine mfg (Magnum .91RFS engine) recommends a 14x6 prop for flying that engine, (different for run-in) and a 13x8 as well. I guess we know more than the mfg...

LM

What i have learned from asking this question is this: there is no answer to my exact question because there are too many considerations to take into account. I will stick to two-bladed props unless a model/engine otherwise states to use a three-bladed prop. Thanks for all the info--Nathaniel

What works in theory on paper doesnt always work the same in real life. Some people just cannot be convinced of that. I know plenty of guys who swear by the opinion that if the book says it, it must be true. It's hard to use an example of an aircraft designed in the 20's or 30's. Two blade props were used on most all aircraft back then because there weren't many three blades around. If you want to talk about racing aircraft, well, the fastest piston engined prop driven aircraft in the world, the Reno unlimited racers, ALL run multi bladed props. The Formula One and Biplane classes run two blade fixed pitch props only because the rules dictate that they have to. The T-6 class runs two blade props because they are required to be of the type that came on the aircraft (the two bladed Hamilton Standard prop on the T-6 is one of the most inefficient props ever made, by the way. The tips are supersonic at cruise power settings, that's why they are so loud.) You are correct however in saying that there is a big difference in FS and R/C in the fact that full scale three blade props are constant speed (alot of the two blades are also). that in itself opens another can of worms when you start talking about efficency. Believe me, I know what the aerodynamic laws of theory say about propeller efficiency, but there are many many real world examples to prove that it just doesnt apply in ALL cases, even though the books preach it as gospel.

Look boys & girls, there really isn't a real-world debate about this stuff, just here in the beginner's forum. Since you don't seem to be inclined to believe me, ask Burt Rutan why he DIDN'T use a 3-b prop on Voyager -- it wasn't because he couldn't afford one. Ask the Reno racers why they can't use 2b props (they sure would like to if they could). In the modelling world, ask Chip Hyde & ALL of the TOC competitors, & the pylon racers, & the RC speed freaks, & the CL speed guys, & the IMAC pattern guys why they DON'T use 3-b props.

In the case of jeepindog's situation, there is a piece of missing information -- he obviously is unaware of it -- but it is there nonetheless. For example -- does the manufacturer of jeepindog's engine recommend the 14-6 for normal flight activities, or for break in?

BTW I have flown many hrs in T-6s (Harvards actually) & in early T-6s the tips would go supersonic shortly after liftoff if you didn't keep the nose up to maintain engine load -- it is a classic case of being underproped for the power (like jeepindogs 14-6). The decision to use a 2b prop in the case of the T-6 involved availability, weight, & economics (after all, it was only a trainer). The T-6 runs just fine at climb & cruise power -- & that's where airplanes fly. A three blade would have incurred a significant engineering design cost, production cost, & weight penalty -- and was not actually needed in the intended application for that aircraft.

Britbrat, read post #35, then reread this post. I have given you ALL of the information that you surmise is "missing." What you are unaware of, however, is when to give up. Something to consider- an airframe/engine combination can turn a 30 foot prop (yes, 30 feet) but landing gear and reality mandate having an 8 foot prop (just an example.) Do you believe that a 30 foot prop that a sliderule and some graph paper says is efficient would actuall BE efficient? It would have too much flex, and too high of a tip speed, which would yield all kinds of inefficiency. Therefore, a three- or four- or more-blade prop would indeed be more practical and efficient than it's analogous two-blade prop. Your example of the T-6 is good. Hooray!

LM

jeep -- this is becomming a marathon & I'm not going to say any more after this post -- some of the commentary & examples have been entirely irrelevant, or just plain silly. I am more than aware of the reasons why multi-blade props are used in various circumstances. I spent many years doing research in fluid dynamics & have written more than a hundred scientific publications in the field. I also have a lifetime of both FS & modelling flight behind me. Don't teach me to suck eggs -- I know how to do it very well.

If what you have claimed is true, there is a reason for it -- and it isn't because three-blade props are more efficient -- they aren't. Your story clearly indicates that your engine was significantly underproped with the 14-6 2-b. However, that doesn't explain the 9700 rpm with both props --- what is your explanation? I would have thought that you would be more curious about the "why" of your observation -- it is anomalous and you clearly don't see the significance of that anomaly. I doubt very much if you have a magic engine/prop/airframe combo. That is why I have personal reservations about it. You should also be wondering why the big guns in the modelling world don't use multi-blade props in performance applications -- they certainly are not ignorant of the dynamics of flight & the appropriate application of multi-blade propellers.

Jeepindog, you don't seem to be a friendly conversationalist. Beyond that, you're comparing a 13x8 3b to a 14x6 2b. A better comparison would be a 14x8 2b, or 13x9 2b. You're obviously under-propping the engine with the 14x6, which shouldn't pull as well as a 13x8 3b. You might as well say that 3 bladed props are superior because your 13x8 3b works better than a 10x6 2b--in other words, you're just not knowledgeable/experienced enough to know how to properly prop your plane. That's not an insult, it's just pointing out a level of ignorance. Smug ignorance such as yours isn't something to boast about.

Britbrat:

As one AE to another, you're not going to convince these guys. They don't care about the science. They have compared a 14x6 to a 13x8 and come to the conclusion that the 13x8 is more efficient because it has more blades. You and I both know that it's because of the pitch and the relative AOA of the blade. Furthermore, we both know that the only, I repeat, the ONLY reason to add blades is to reduce diameter and the cost is EFFICIENCY. But how is the guy that designs aircraft for a living going to convince a stick jockey of anything? Good job on trying to clarfiy things. I can vouch for your analysis.

I don't believe any of it..Turbo Fan is the way to go![sm=lol.gif]

The key thing here is that the engine is peaked at 10k, then backed off.

My experience with 4 strokes shows that many of them "flatten out" at around 10k on the ground. For example, my OS .61FS will turn a 12x6 APC at about 10k. But it will also turn a 13x6 APC at the exact same RPM.

Clearly the 14x6 is too small a prop for the engine in question. Try a bigger prop. No question about it.

Get back to us with some RPM readings and flight tests on larger 2-bladed props if you want to sound less like a goofball.

A couple of other notes:

I suspect CL acrobatic guys use multi bladed props for reasons that have nothing to do with pulling ability. Looking closly at some CL acro planes, some have rather short gear legs. The CL guys also do a lot of engine tuning to get the engine to pull hard on the up lines but slow down in level flight. Some guys do this by setting the mixture, pipe length and other engine parameters (like prop, glow plug, etc) so that the engine is off the pipe in level flight, but in a vertical upline, the engine leans out, goes on the pipe, and pulls harder. It's possible a multi-bladed prop makes this easier to set up right. CL guys are also appearanced judged, so a cool looking prop might be just the thing to help get in to the first row. The point being that pointing at CL acro guys as evidence that a multi-bladed prop pulls better is bogus. Too many other factors there.

In a simular way, a lot of IMAC pilots are now going to 3 bladed props. But they are doing it for one simple reason. The are now judged on noise, so having a "quieter than average" plane makes for points, enough points to decide the contest. 3 blade props are quieter due to lower prop tip speed.

It just happened that I was looking over a full scale SU-26M last weekend. I noted the BIG 3 bladed prop. I also noted how close to the ground the tips were. When that thing is up on two wheels, as during take off, the tips must be awfully close to the ground. Clearly, the 3 bladed prop is there because a 2 bladed prop of approprate size would dig a trench on take off. No question about it.

Britbrat and company are right.

Jeep, you need to get more experience with engine-prop-airframe combinations.

Several things to comment on here. First, there was an actual one (1) blade prop in the old days for a full scale aircraft. The opposite side of the hub had a counterweight to maintain balance. It was too inefficient and discontinued. Many high wing turbine aircraft such as the Dehaviland DHC-6 I used to fly use 3 blade props and ground clearance was not an issue, fuselage clearance was. Several different blade shapes have been used on multi-blade propellers. One of the critical items is to keep the blade tips from going supersonic as they turn. The tips of shorter blades are traveling slower at the same RPM as a long blade. This is also a limiting factor in full scale helicopter flight. By using more, and wider, blades the necessary thrust can be obtained. Also, as far as blade angle is concerned, remember that all advanced aircraft are going to have variable pitch blades (for a multitude of reasons beyond this forum discussion). Aerobatic full scale aircraft such as the Pitts series use shorter composite 3-blade props to reduce blade weight and flexure as this can actually (and has) break crankshafts during the violent manuevering. For a model, get a prop with enough ground clearance and experiment for the best pitch for your plane. Bye.
Bruce

I am an unfriendly conversationalist who is ignorant and needs more experience with engine-prop-airframe combinations. If only someone would actually MAKE all of the pitch/diameter combinations for me to try out, I could do just that. I have never claimed to know more than anyone here, or elsewhere, have I? I stated that I respect britbrat's knowledge of aerodynamics. I put "little laughing faces" in my posts where I was making light of reality. I even told britbrat that his example of the T-6 was good. All I was asking for was easy-to-understand explanations of why fewer blades are more efficient. Fewer blades yield less friction and require less kinetic energy of rotation, right? Is that a super-simplified explanation? I ran the mfg's recommended two-blade prop and had less performance than when I swapped on a smaller three-blade prop. I fly from a runway that is at 5550' MSL. I should not be able to get better performance at that altitude than the mfg suggests. Maybe I am lucky, (which, to quote britbrat, I am unaware of ) and I got great runs out of that engine on those "test" days. I defer to those of you who know what you are talking about, as clearly I do not. I'm just a jerk, but I blame my annoying co-workers for making me grumpy! Happy New Year everyone. Hopefully someone will read this post...

Lachlan

P.S. Why was the one-blade prop that bruce88123 mentioned too inefficient? I am asking for info, not to cause trouble.

Talk about efficiency all you want. Number one, I dont think it makes THAT much difference on our R/C aircraft. You have to have a pretty large increase in efficiency to gain a small increase in speed. Number two, efficiency is not what propels your aircraft, thrust does. All efficiency is, is the ratio of power being produced by the prop to the power going into it. A two blade prop is capable of achieving higher efficiency than a three blade, but at the same time it uses less power and produces less thrust. If you operate a prop at a lower power setting than that where the efficiency is at it's peak, you achieve lower thrust and lower efficiency. Likewise, if you operate at a higher setting than peak, thrust will be higher, but efficiency will still be lower. The optimum power setting is where efficiency is at it's peak, we all agree on that one. BUT, if conditions require more thrust than is available at this power setting, then the power must be increased, and the prop efficiency begins to fall off. You finally reach a point where a prop operating at a power higher than that which results in peak efficiency actually has the same efficiency as a prop with more blades that is operating at less than optimum power. Any further increase in power favors the performance of the prop with more blades, because the prop with fewer blades is no longer operating at peak efficiency. Now, I'm not an engineer, just a dumb ole' "Stick Jockey" who cant be taught anything, but in 3,000 hours of flight time, I've flown quite a few two and three bladed aircraft. You give me two identical aircraft, one equipped with a two blade, one equipped with a three blade, and the three blade equipped aircraft will out perform the other in everything from time to climb, right on up to higher cruise speed. But, as I said before, most of this is a mute point when you are talking about R/C aircraft, at least it seems so to this dumb, uneducated "stick jockey". Oh, and by the way, reducing diameter is not the "ONLY" reason manufacturers use three bladed props. Two blade propellers produce an inherent once per revolution vibration that shakes the airframe. A three bladed prop eliminates this vibration, therefore reducing a small amount of airframe fatigue, reducing cabin noise, and making for smoother operation. Also in a single engine aircraft, the blade wake from a two blade prop beats on the windshield producing noise. Changing from two to three blades reduces the wake intensity and increases the frequency of the beat, and is percieved to be quieter in the cabin. Hmm, I guess maybe some of us dumb ole hick "stick jockeys" do know a little something.

Lowlevlflier, you got some of it right & some of it not so right.

Multi-blade props are not used to reduce diameter -- they are used to increase blade area, thus permitting the use of more powerfull engines within the physical constraints of the airframe that limit prop diameter.

In your 3,000 hrs of flying I doubt if you have encountered "identical" aircfraft with both 2 & 3-blade props. I very strongly suspect that the 3b varients that you have flown were actually more powerfull than the 2b versions -- that is the common reason why manufacturers end up using multi-blade props -- to transmit the extra power from an uprated engine, where it is no longer practical to increase blade length. Another case is where the 2b versions were not optimally proped to begin with (like the AT-6/Harvard). If simply going to a multi-blade prop was so magical, you wouldn't see 2b props on anything.

You are quite correct about noise/vibration & the perceived effect of higher pulse repetition rates.

jeepindog -- since you actually want to know about 1b props, I will try to satisfy your curiosity-- but I'm not going to engage in further argument.

1b props are not really practical for full scale aircraft, primarily due to structural limitations, rather than lack of blade efficiency.

A 1b prop can be mass balanced, but it cannot be load balanced.

The mass required to balance a 1b prop is necessarily located relatively close to the center of rotation & therefore is much greater than the weight of a matching opposing blade. Extra weight penalties are a serious no-no in aviation applications.

The eccentric thrust from the single blade imparts a strong bending load on the prop shaft -- a load that varies with both power setting and AOA. The same eccentric thrust imparts a cyclic load to the radial thrust bearings of the prop shaft. If the prop is mounted on the crankshaft of the engine (instead of a reduction gearset) these loads bear directly on the engine & its internals. The service life of the powertrain is adversely affected & the risk of a catastrophic mechanical failure is amplified.

The same thrust loads are transmitted through the engine case & mount to the airframe, where a cyclic bending load is imparted to the fuselage/nacelle structure which must must be appropriately strengthened (more weight) & with obvious consequences for airframe life.

Finally, the eccentric thrust perturbs the flight dynamics. During its rotation, when the blade is located on the right side of the aircraft, for example, the thrust tries to turn the plane left -- etc.

The single blade lifts very well, but the entire system is not practical. This is not a problem for control line models with narrowly focused objectives -- like going very fast -- the engine & airframe are essentially disposible as long as the speed objective is achieved. Single blade props are also used on indoor models where efficiency is paramount & the drive forces are very low.

Thanks, that makes perfect sense. I had guessed that part of it may have to do with bending the crankshaft, but I didin't know the name of that type of torque.

LM