3 blade vs 2 blade props
08-30-2011, 05:44 PM
#51
RE: 3 blade vs 2 blade props
hi
if you can try both 2 blade and 3 blade props (as many brands as you like ) on your airframe ( do remember we are propping the airplane not the engine always ) fly the thing
which gives you the best vertical performance-and general flying characteristics-and engine performance on your AIRFRAME
YOU FOUND YOUR ANSWER done deal
can get a little expensive buying all those props
BEST REGARDS TONY
if you can try both 2 blade and 3 blade props (as many brands as you like ) on your airframe ( do remember we are propping the airplane not the engine always ) fly the thing
which gives you the best vertical performance-and general flying characteristics-and engine performance on your AIRFRAME
YOU FOUND YOUR ANSWER done deal
can get a little expensive buying all those props
BEST REGARDS TONY
08-30-2011, 05:55 PM
#52
RE: 3 blade vs 2 blade props
I have absolutely no expertise in this area, but I have to ask why the number of prop tips would not matter? I would think that 3 bladed props would be less efficient than 2 bladed simply because of tip losses. They are spinning wings, so just as a wing has its greatest drag at the tips where vortices are generated, would not the same be true of props? A biplane is less efficient than a monoplane in part because it has four wing tips instead of two. So why wouldn't three prop tips be less efficient than two?
I assume that the 3D planes have very large engines for their size and have ground clearance issues, just as WWII fighters went from 3 to 4 to even 5 blades as engines got more powerful and bigger props were needed to harness the power.
Jim
I assume that the 3D planes have very large engines for their size and have ground clearance issues, just as WWII fighters went from 3 to 4 to even 5 blades as engines got more powerful and bigger props were needed to harness the power.
Jim
08-30-2011, 06:08 PM
#53
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RE: 3 blade vs 2 blade props
I am just building a Seagull Extra 75 and plan on installing a 15 cc gas engine on this. Recommended is a 2 blade 10X7, just curious - how do I find the equivalent size in a 3 or 4 blade prop?
08-30-2011, 06:53 PM
#54
RE: 3 blade vs 2 blade props
One application that I have consistently had good results with 3 blade prop is where I use a larger engine. Ground clearance is a concern, obviously. The extra power seems to more than compensate for any loss of efficiency. Example, put a Super Tigre 90 on a 60 sixed plane. I use a 12 x 8 ( 3 blade) prop. I think a 2 blade would offer more performance, but it is a trade off for the clearance.
turbo
turbo
08-30-2011, 06:55 PM
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RE: 3 blade vs 2 blade props
I think that this says it all -
If two-blade propellers are the most efficient, then why don’t all propellers have two blades?[/b]
The short answer is because efficiency doesn’t propel the airplane, thrust does. Efficiency is the ratio of the power coming out of the propeller to the power going into it. A two-blade propeller is capable of achieving a higher efficiency than a three-blade propeller and so on, but at the same time it uses less power and produces less thrust.
If you were to operate a propeller at a lower power setting than that for which the efficiency is at its peak, you would have a lower thrust and also a lower efficiency. Likewise if you operate at a higher power setting, the thrust will be higher but the efficiency is lower there also. There is therefore an optimum power setting for each propeller where its efficiency will peak. If conditions require more thrust than is available from this optimum power setting, then the power must be increased and prop efficiency begins to fall off from its peak value. There reaches a point where a propeller operating at a power higher than that which results in peak efficiency has the same efficiency as a prop with more blades operating at less-than-optimum power. Further increases in power favor the performance of the propeller with more blades. This is because the propeller with fewer blades is no longer operating at its peak efficiency.
The short answer is because efficiency doesn’t propel the airplane, thrust does. Efficiency is the ratio of the power coming out of the propeller to the power going into it. A two-blade propeller is capable of achieving a higher efficiency than a three-blade propeller and so on, but at the same time it uses less power and produces less thrust.
If you were to operate a propeller at a lower power setting than that for which the efficiency is at its peak, you would have a lower thrust and also a lower efficiency. Likewise if you operate at a higher power setting, the thrust will be higher but the efficiency is lower there also. There is therefore an optimum power setting for each propeller where its efficiency will peak. If conditions require more thrust than is available from this optimum power setting, then the power must be increased and prop efficiency begins to fall off from its peak value. There reaches a point where a propeller operating at a power higher than that which results in peak efficiency has the same efficiency as a prop with more blades operating at less-than-optimum power. Further increases in power favor the performance of the propeller with more blades. This is because the propeller with fewer blades is no longer operating at its peak efficiency.
http://www.hartzellprop.com/service_support.php?id=87
08-30-2011, 07:00 PM
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RE: 3 blade vs 2 blade props
ORIGINAL: OliverJacob
I am just building a Seagull Extra 75 and plan on installing a 15 cc gas engine on this. Recommended is a 2 blade 10X7, just curious - how do I find the equivalent size in a 3 or 4 blade prop?
I am just building a Seagull Extra 75 and plan on installing a 15 cc gas engine on this. Recommended is a 2 blade 10X7, just curious - how do I find the equivalent size in a 3 or 4 blade prop?
http://www.mh-aerotools.de/airfoils/propuls2.htm
But basically add a blade, drop an inch in diameter if you don't want to get fancy.
08-30-2011, 07:22 PM
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RE: 3 blade vs 2 blade props
My input; and,l a lot of good and interesting stuff here. Also an aeronautical engineer or two I suspect. Anything on aerodynamics for flight Instructors or even stuff for private pilots on aerodynamics is good because they are devoted mainly to small full scale aircraft that are similar to our R/C airplanes. Couple of things, props that do not change pitch except for flexing are called fixed pitch. Props that are found on Cessna 182s and Extra 300s are constant speed. They are like gears on a car. Tremendous advantage on take off in fine or sometimes called flat pitch. Very efficiant on fuel and engines in cruise with courser or higher pitch and as the term suggests, constant pitch adjusts itself to maintain a set RPM. So, it is true that in pure aerodynamics, a one blade would be more efficiant. Yes, of course the comments about the WWII F4U having an inverted gull wing so as to place the landing gear under the lowest point allowed for shorter and stronger gear legs, and you need enough prop blades to harness the power and still clear the deck with tail up for ground operations. Did you know some Reno Racers, F-8s and may be some others had such huge engines and props that they could not allow the tail to come up on ground ops? Here is a test than shows basic thrust of a prop and engine combo in acition. Drive a stake in the ground and attach it to the tail of the plane with the fish scale in between. Yes, model props change shape at near max RPMs and they are designed to. It can be an advantage. Speed guys know a lot more than I do about this. Hope this helps. Wallace L. Tharp
08-31-2011, 04:54 AM
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RE: 3 blade vs 2 blade props
Wallace, my brother in western Pa. recently sold a Cessna 182 with a 233hp Continental 477 in it. It had a variable pitch prop. He thought seriously about installing a 250hp Lycoming, interestingly the 250hp Lycoming kit came with a three blade prop. Sometimes we think that things that require more power are always bad but that is not always the case.
Someone mentioned gears and traction above. I saw a demo of a Minneapolis Moline G 1000 Vista farm tractor once that had four wheel drive, it was pulling a huge disk and the rear tires were digging down almost to the rims due to wheel slip. The camera moved in to show this than pulled back, the operator engaged the four wheel drive and the rear tires magically walked right up on top of the dirt and the tractor visibly accelerated with no change in the throttle setting. Some might say this is unrelated but as Hugh said,effectiveness matters, and in this case lead to effeciency.
Someone mentioned gears and traction above. I saw a demo of a Minneapolis Moline G 1000 Vista farm tractor once that had four wheel drive, it was pulling a huge disk and the rear tires were digging down almost to the rims due to wheel slip. The camera moved in to show this than pulled back, the operator engaged the four wheel drive and the rear tires magically walked right up on top of the dirt and the tractor visibly accelerated with no change in the throttle setting. Some might say this is unrelated but as Hugh said,effectiveness matters, and in this case lead to effeciency.
08-31-2011, 12:24 PM
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RE: 3 blade vs 2 blade props
Effectivness isnt always efficient......
I like the 3 bladed M/A on my Skybolt.
It seems to fly loops truer. Slows down faster. And just looks scale.
Feels as tho it has more low end grunt.
13x8x3 on a Satio 1.00.
I like the 3 bladed M/A on my Skybolt.
It seems to fly loops truer. Slows down faster. And just looks scale.
Feels as tho it has more low end grunt.
13x8x3 on a Satio 1.00.
