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Prop efficiency, 2 versus 3 blades
Testing of the fixed-pitch, three-blade ELIPPSE prop on my Lancair shows that it is performing at 82 percent efficiency in a climb at 105 mph IAS and at least 90 percent efficiency in cruise at 200 mph TAS. Also, multi-blade, fixed-pitch propellers with correct aerodynamic shape where the blades enter the spinner have as good a cruise efficiency as a two-blade propeller, but will have better static thrust and climb performance. And because a multi-blade propeller can be made smaller in diameter than a two-blade and still pump as much mass flow, it will be quieter because of the reduced tip speed at a given rpm. Tom Aberle’s “Phantom” Reno biplane qualified at 221 mph with its 64-inch two-blade prop in 2003, 241 mph with its 59-inch diameter three-blade propeller in 2004 at 250 rpm less than in 2003, and 251 mph with its 59-inch diameter four-blade propeller in 2007 at the 2003 rpm. Many commented on how quiet his propeller was as he flew by on the home course, where his noise was mainly from the engine exhaust. This in contrast to the T-6-like scream of the other racers! From this article: http://www.eaa.org/experimenter/arti...02_elippse.asp |
RE: Prop efficiency, 2 versus 3 blades
The quote in the post above was written by Paul Lipps in his article about propeller design for one of his record setting airplanes. In the same article is a little description about him:
Paul Lipps spent many years in the aerospace industry, 28 of which were with General Electric (GE), where he worked on the Atlas Space Launch Vehicle radar/computer guidance system at Vandenberg Air Force Base. While with GE, Paul developed high-accuracy refraction-correction equations and a tropospheric radar noise model for use in the Kalman filter guidance equations. He also designed a computer-driven radar simulator, with phase- and amplitude-modulated X-band signals that were injected into the radar’s antennas to produce a high-fidelity, interactive radar simulation of an ATLAS flight. This allowed radar checkout, training of radar and computer operators, and simulation of radar and computer problems and gave accurate ATLAS flight simulation for checkout of the computer guidance program under non-nominal trajectory, booster performance, and high tropospheric noise conditions. Prior to these accomplishments, Paul worked for Bell Telephone of Pennsylvania for six years and then spent five years working for Burroughs Corp., on the guidance computer for the ATLAS D ICBM at Vandenberg Air Force Base. Since his retirement, Paul has developed equations and the computer program for the design of high-efficiency propellers. In addition, Paul has worked with Klaus Savier of Light Speed Engineering in the design of the Plasma series of electronic ignitions and is now working on an electronic fuel injection system. At 17, Paul’s flying passion came to life while he worked at a seaplane base in his native Pittsburgh. Although at the time he logged 14 hours on floats, his flying had to take a back seat to life. He spent some time with a J-3 flying club and in 1989 earned his private pilot certificate in a Cessna 172. He has since logged more than 780 hours single-engine land, with 577 of that being in his (and another’s) Lancair 235. - Pat |
RE: Prop efficiency, 2 versus 3 blades
The most efficient speed occurred with 4 blader prop.
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RE: Prop efficiency, 2 versus 3 blades
To whom
Do not propellers suffer from tip vortices and their inherent inefficiences the same as wings? What do they do to overcome, washout? Is it true that some racers use only one blade? What would their thinking be? I notice the article says "proper aerodynamic shape near the spinner". Is that not incorperated in model size props? It seems whomever I talk to, three or four blade props are discouraged. That they are only for looks,and if you want maximum preformance in every area use the two blade. What say you? |
RE: Prop efficiency, 2 versus 3 blades
ORIGINAL: skyjockey To whom Do not propellers suffer from tip vortices and their inherent inefficiences the same as wings? What do they do to overcome, washout? Is it true that some racers use only one blade? What would their thinking be? I notice the article says "proper aerodynamic shape near the spinner". Is that not incorperated in model size props? It seems whomever I talk to, three or four blade props are discouraged. That they are only for looks,and if you want maximum preformance in every area use the two blade. What say you? As with soooo many things in life, it appears we've been wronged !!!!!!!! |
RE: Prop efficiency, 2 versus 3 blades
ORIGINAL: skyjockey Do not propellers suffer from tip vortices and their inherent inefficiences the same as wings? But when the demand for blade area sufficient to absorb the engine power is considered, additional blades are often the answer. And if you read the guys article, you'll see that after designing tips to be adequately efficient, the word "suffer" was dropped from the problem. What do they do to overcome, washout? Is it true that some racers use only one blade? What would their thinking be? I notice the article says "proper aerodynamic shape near the spinner". Is that not incorperated in model size props? It seems whomever I talk to, three or four blade props are discouraged. The range of performance from our props is so wide, that we often have really lousy props and really good ones, and that with props of identical pitch/diameter markings. The only thing limiting 3 blades from being chosen more often as the best suited for an individual plane/engine combination is the limitation of choice of 3 bladers in the desired sizes. We got lots to choose from in 2-bladers and very, very few to choose from in 3-bladers when the average modeler is looking for the best prop for his plane/engine. That affects the outcome more often than some imagined difference in efficiency. That they are only for looks,and if you want maximum preformance in every area use the two blade. What say you? What say I? Don't let the turkeys get you down. |
RE: Prop efficiency, 2 versus 3 blades
One blade props are usually a bit more efficient, and two blade props are slightly. usually one or two percent more efficient than three blade units. Full scale airplanes usually go to three or more blades in order to absorb more power with a given diameter, thus keeping the landing gear from becoming inconveniently long and heavy, and keeping tip speed below about Mach 0.85, where efficiency starts to suffer because of shock wave generation at the highest velocity part of the airflow around the airfoil.
However, wind tunnel tests revealed that model-sized propellers of under 24 inch diameter started losing efficiency at tip speeds over Mach 0.6 or so. For a given horsepower, three or more blades also tend to be quieter. Control line speed flyers commonly used props with pitch/diameter ratio of up to about 1.4, such as a 9-13 for .60 engines, obtaining optimum efficiency for model propellers. Propellers with pitch/diameter ratios of 1.0 or more are best for efficiency, possibly attaining up to about 85% in model sizes. At a pitch/diameter ratio of 0.5, for example, a 12-6, peak efficiency is probably down to about 70%. |
RE: Prop efficiency, 2 versus 3 blades
Multi blade props do suffer from vortices at the tip as well as the intersection at the spinner. If you notice they did better at high cruise speeds. That is because they produce a smaller prop wash stream which works better at high velocity. That is it is easier to produce a smaller high velocity airstream. With a larger prop it will have to push a smaller mass at the same or greater velocity and that doesn't work as well at high speeds.
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RE: Prop efficiency, 2 versus 3 blades
The article about the design of his 3 bladed prop by Paul Lipps puts forth a startling idea.
He designed such good tips for his blades, that having more tips didn't matter much at all. Amazing deal that....... Since we have almost nothing to go on about the efficiency of any of our props, their tip efficiency or whatever, it's amazing to think that one brand/design of prop is going to be more or less efficient than any other brand/design prop simply because one has 3 tips and the other has 2 tips. Take the props to the field and test. You'll find out that the 3-blades we've got available perform better as often as not. Which is about the same as 2 bladers do when compared to ANY other prop. |
RE: Prop efficiency, 2 versus 3 blades
ORIGINAL: topspeed The most efficient speed occurred with 4 blader prop. That's not true. The FASTEST speed occured with a 4 bladed prop but that doesn't mean it was the most efficient. To add to all that has been said above in the Zaic Yearbooks Frank had an article in one of them with a lot of math attached that showed why single bladed props were more efficient. And no, I didn't understand it or try to learn more to achieve that level of enlightenment..... :D But that was not the only place I've seen that claim and support for it. However two bladed props avoid all the harmonics and off center bearing loads and generally are a much nicer option. I've done two single bladed props for my old time rubber models and I don't think I'll be doing another one. They just shake too much....... |
RE: Prop efficiency, 2 versus 3 blades
ORIGINAL: BMatthews ORIGINAL: topspeed The most efficient speed occurred with 4 blader prop. That's not true. The FASTEST speed occured with a 4 bladed prop but that doesn't mean it was the most efficient. To add to all that has been said above in the Zaic Yearbooks Frank had an article in one of them with a lot of math attached that showed why single bladed props were more efficient. And no, I didn't understand it or try to learn more to achieve that level of enlightenment..... :D But that was not the only place I've seen that claim and support for it. However two bladed props avoid all the harmonics and off center bearing loads and generally are a much nicer option. I've done two single bladed props for my old time rubber models and I don't think I'll be doing another one. They just shake too much....... Ok I assumed it was also the most efficient. I once put a three blader on my1/12 size combat Zero and it was really cool...45 degree angle ascent from the toss to the lowest clouds...ok I had also 1.4 hp power to use. Three blader feels more solid. |
RE: Prop efficiency, 2 versus 3 blades
And in spite of the fact that it looks like we've got a lot of props to choose from, the overall differences in our props are far greater than the differences caused by the addition of a blade to the blade count.
That is why it's so silly to refuse to try 3-bladers because you heard they were less efficient. It's one of those details that's true and insignificant. If we had paired 2 and 3 bladers, where the same design criteria were present in both, then the 3 would be SLIGHTLY less efficient than the 2. But we don't come close to having that. Not even close. The rule of thumb we use most often isn't even close, that to choose a similar 3 blade you reduce the diameter by 1" and increase the pitch by 1. Amazingly simple, and not even in the ballpark most times. Try it more than once and you'll see how silly it actually is. The only real value to the pontification that "2 are more efficient than 3" is when it's used with "so you don't want to......". When you hear those two together, you've just been given an insight into the quality of that expert's advice. |
RE: Prop efficiency, 2 versus 3 blades
That's not true. The FASTEST speed occured with a 4 bladed prop but that doesn't mean it was the most efficient. |
RE: Prop efficiency, 2 versus 3 blades
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Is this even better for a pusher ?
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RE: Prop efficiency, 2 versus 3 blades
And for Sport Pilot and others who are thinking that prop force is F=Ma, ie Mass of air Accelerated, this works fine for a ducted fan or turbine, but not a prop. The prop is essentially a rotating wing, so the Thrust comes from the old Lift formula thing, L (or F) = 1/2 x Density x Vsqu x Cl x area. Which helps to explain why the pitch has to be matched to the speed of the model, to keep the prop wing within it's aoa limits at the usual speeds of operation. This also explains why the static thrust results have little relationship with the best prop for your model, essentially a static prop is mostly stalled and all you are measuring is how good a fan it is. As with full size, to extract the best performance from a prop you have to vary the pitch for the airspeed, fine pitch at slow speeds (takeoff and landing) and coarser as the airspeed increases. Otherwise simply prop for your intended operation, funflys (slow models) use fine pitch, large diameter props, speed models use much coarser pitches, matched to the high airborne speeds. The downside is with higher pitches that you loose a bit of takeoff performance from the stalled prop, and funflys can't fly fast cause the prop aoa approaches the 'zero lift' angle as speed increases (speed limited). As with the full size, it's all a compromise, and you need to fly a lot of props to get the best one for you, your model and the way you like to fly. Until some bright spark comes up with a simple, foolproof, light V.P. unit for model engines.
Evan, WB #12. |
RE: Prop efficiency, 2 versus 3 blades
Why does that matter about 2 blade VS multi blade? BTW a turbine or ducted fan works on the same principle as a prop and work on similar math. That is that both F=MA and aerodynamic calculations can be used to find the thrust. However for a system on paper you will not be able to use F=MA without working out the aerodynamics and/or hydraulics of the system.
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RE: Prop efficiency, 2 versus 3 blades
Sport pilot, I would have thought not, one really simple reason being that as a prop plane increases speed the prop thrust reduces, thrust is maximum on the ground, reducing as speed increases. Jets and fans, on the other hand, increase their thrust as speed increases. For my simple understanding of the the things, the differences are too great to make simple comparisons of the maths.
Evan. |
RE: Prop efficiency, 2 versus 3 blades
Props compared to turbojet ? rocket ?
There is a huge difference The jetcraft can never reach a speed where the turbine is being "out run" but thrust (reaction) will finally max out. On a propped craft - outrunning the prop can be done. The variable speed props cure this -except for the fact that the engine finally reaches maximum power The really fun test is a static test - the various props can be engineered to a fair thee well but ultimately the static thrust is simply a result of the power applied . big prop little prop doesn not matter . Do the test using an electric motor so yo can compare measured thrust against watt input. You can make the prop look like a wing section but it wil lwork with a symmetrical section -for most model applications It is harder to measure the pitch tho. I just reshape the blades till the engine works best at the speeds I want. After all the airframe is part of the equasion so there really is no best prop for all uses . |
RE: Prop efficiency, 2 versus 3 blades
There once was a time when planes and onboard equipment were so heavy and engines so weak that hobbyists did everything within their powers to optimize their choice of props. 3 bladed props have been out decades before I ever got involved with the hobby, but the pioneers of this sport who mainly just did C/L and F/F learned from experience that 2 bladed props outperformed 3 blade, where Nth degree performance is being pursued. There is no evil conspiracy or mass hysteria , if 3 bladed props were in fact BETTER, there would be lines wrapped around the block with eager buyers.
If you fly the scale of model where you wouldn't notice if you accidentally left a pair of pliers and a screwdriver inside the fuselage, then I'd say your choice of props is wide open. If you fly planes like I do where horsepower is fractional, then your choice of props that work best is narrowly defined. Same goes for anyone who is flying a 6.5cc and under plane against a stop watch, AMA and FAI records are held by single & two bladed props in all categories. The smallest 3 blader I have is a 5x3 and it gives decent sport type performance. I'm sure there are applications in the hobby where 3 bladers might even be superior, but there are many applications where they don't cut it. Competitors are always looking for an edge and no stones have been left unturned. |
RE: Prop efficiency, 2 versus 3 blades
Very interesting thread!!!
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RE: Prop efficiency, 2 versus 3 blades
The difference between props of the same dia/pitch is very often more than the difference in efficiency that MIGHT come from having a 3rd blade compared to an otherwise equal 2blade.
And in the real world, having enough blade to suit the engine is the bottom line. Whatever insignificant loss of effiency happens from having additional blades won't be significant with the gross differences we see in our prop selections. |
RE: Prop efficiency, 2 versus 3 blades
This discussion cannot be resolved without data. Fliers are like fisherman (it was this big) and all we have is anecdotal information about which is best. Airplane drag tests will show the in-flight force required to propel the plane at a given speed. Bench tests of engines will give us torque/power curves and the difference between engine output and aerodynamic drag is prop "efficiency".
We can chart our engines with [link=http://bj-model-engines.com/AmericanHobby/Dyna.html]this.[/link] We can record in-flight speed and engine rpm data with [link=http://www.eagletreesystems.com/Plane/plane.html]this.[/link] And after 40 years of speculation and argument we can be the generation that provides evidence one way or the other. |
RE: Prop efficiency, 2 versus 3 blades
The Number of Blades The number of blades has a small effect on the efficiency only. Usually a propeller with more blades will perform slightly better, as it distributes its power and thrust more evenly in its wake. But for a given power or thrust, more blades also mean more narrow blades with reduced chord length, so practical limits have to be considered here. The chord length can be increased while decreasing the diameter to keep the power consumption constant, but a diameter reduction is usually a bad idea in terms of efficiency, as long as the tip mach number or tip cavitation is not an issue. It gives some insight into the effect and impact of efficiency resulting from the number of blades. |
RE: Prop efficiency, 2 versus 3 blades
Cavitation of aircraft props is a misnomer.
It simply doesn’t exist in gases, but is rather a phenomena found in liquids. |
RE: Prop efficiency, 2 versus 3 blades
ORIGINAL: CrateCruncher Airplane drag tests will show the in-flight force required to propel the plane at a given speed. And after 40 years of speculation and argument we can be the generation that provides evidence one way or the other. What I am wondering at the moment is how much different is the prop in front of the plane compared to one aft of the plane. Does the laminar flow in fuse become disturbed with lesser props...I am leaning into at least 7 props fan...with high pitch and small diameter instead of two blades. Seems to me the bigger the prop is the further will it throw the wash. There is no need for that in a pusher...slipstream is smooth. The suction will affect airflow at least 3 feet in front of the 800 mm dia prop at 5 - 6 000 rpm. |
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