Whelp. Gonna be lazy here

batjac Wed Apr 25, 2018 8:15 pm

I searched the forum for an answer about two vs. three bladed props, but didn't find anything conclusive. I have a new multi-engine design that's been kicking around in my head, and I might start it next week. I want to use three bladed props for it, and was wondering about the thrust differences between a Cox 5x3 two bladed prop and a Cox 5x3 three bladed prop. Also, the difference between the left handed Cox three bladed vs. the right handed Cox three bladed prop. They have different blade profiles, so I was wondering how the two three bladers compare.

Doing a search only found the thumb rule that prop loading on an engine for a three bladed prop is about equal to a two bladed prop of 1" more diameter or pitch. That's somewhat useful, but not. I was going to build a little test stand to measure relative difference between props. Not an accurate thrust scale, just something to give an idea about the thrust differences between props. But with the project back-up I have, I don't want to take a couple of days to build the test rig.

So, I'll just be lazy and ask if anyone has a good idea about the thrust difference between the two and three bladed props, and between the two different types of three bladed props. Anyone do any testing in the past on this issue?

The Backed-Up Mark

**Cribbs74** Wed Apr 25, 2018 11:35 pm

I don’t have the foggiest idea about the thrust differences. I can say it probably doesn’t matter as they either do what you want or don’t. If it were me I’d bypass the math and science and just go for it. I’d rather see/feel the difference in action than think about it beforehand.

That said, there is probably someone on here that is smart enough or experienced enough with 3 blade props that can give an educated answer.

That’s my 2 cents, and you didn’t even have to ask! Smile

Mark Boesen Wed Apr 25, 2018 11:49 pm

i don't think a three blade is as efficient as a two blade, especially at high rpm. You'll notice how many speed guys, free flight, racing, combat, etc. use a three blade prop, maybe a one blade. I think when your talking 8"-12" dia. props, 1" diameter less for three blade is close, but on a 5" three blade it's probably less. I just don't know of any three blade props out there that can compete with a good two blade for our 1/2a engines.

coxaddict Thu Apr 26, 2018 3:01 am

My son and I had a marine propeller repair service a decade ago. Boats use 2, 3, and four bladed props. The only thing I remember is allowing the right sized propeller to be free and clear of the hull, basically in uninterrupted water . Two bladed props were more efficient but size would be the limiting factor. Thus 3 and 4 bladed propellers are used.
I think using 3 bladed props in our hobby allows the plane to sit lower to the runway. The Cosairs had bent wings to raise the fuselage higher as they ran with very large props. RC Plane

**RknRusty** Thu Apr 26, 2018 7:43 am

Mark, the only thing I can add to that rule of thumb is to add an inch of pitch to the 3 blade prop while reducing the diameter an inch. And my knowledge only applies as far as I know to the 10" through 14" props. For the little ones, a half inch might be better increments. For dissimilar shapes,I doubt if even calculus will get you a useful estimate. I'd follow Ron into this fight, but I'd like to watch you do a thrust test for the entertainment factor. And bring it to show-and-tell one evening for us on the show. You could sure open a hilarious can-o-worms on Stunthangar with it too, Lol.
Rusty

getback Thu Apr 26, 2018 10:57 am

Mark , all the 3 bld. props. i have are 5Dx3P . I don't think you will get the RPMs out of that prop to be competitive against a 4or 5 inc. with say 4P there for the trust will bee down , I measured the 3 bl and 2 1/4 " per bld as the radius is less than 5" . But may be enough for what you want it to do in the air ? and they do look COOL ! As Ron said bee easier to mount and fly each . Babe Bee .049

roddie Thu Apr 26, 2018 4:42 pm

batjac wrote:I searched the forum for an answer about two vs. three bladed props, but didn't find anything conclusive. I have a new multi-engine design that's been kicking around in my head, and I might start it next week. I want to use three bladed props for it, and was wondering about the thrust differences between a Cox 5x3 two bladed prop and a Cox 5x3 three bladed prop. Also, the difference between the left handed Cox three bladed vs. the right handed Cox three bladed prop. They have different blade profiles, so I was wondering how the two three bladers compare.

Hi Mark, These are interesting questions. Is there a particular reason why you wish to use three-bladed props? I have some Cox/Thimble-Drome "black" 3-blade/5" diameter props.. but the right-hand ones are 5"d x 3"p.. and the left-hand are 5"d x 3.5"p. I can't make a direct comparison there. I'm interested in left-hand propellers particularly for their use in the CEF 1/2A reed-engine class speed competition.. and have one "single-engine" entry that I'm working on.

I've recently scratch-built a twin-engine model; a 1/2A C/L P38 Lightning profile/sheet-wing airplane to be powered by Cox .049 reed-valve product-engines. I understand your queries on propeller-choice. I wish I could offer some advice based on experience.. but I've only recently tested 5"d. x 4"P. two-bladed propellers sold by "DAL" under the part#5040. These props are "experimental".. because they are primarily designed for "drones" having brushless/electric motors. They are sold in packages of "four" propellers; two "right-hand".. and two "left-hand". These propellers have "5040" and "5040R" designations molded into their blades. The "5040R" is a left-hand propeller.. which I didn't initially understand. The "R" I believe is for "Reverse-rotation".. when applied to a "quad-copter" for which the props were designed.

Regarding "thrust"......... I made a 1/2 hearted attempt at a "dynomometer" a few years ago. I know that you remember.. because you commented "favorably" on it.. Smile

It wasn't my original design.. but an adaptation that utilized a "buoyant" fixture" for the engine.. to float in a small tub.. with a tether attached to a digital-scale. Do you remember? I should really re-visit this.. as it was a fun experiment. I called it the "tub-dyno"..

The first vid.. engine starts and runs backwards.....

The second vid.. engine starts; spinning the propeller in required "tractor-mode"....

It was discussed (back then..) whether this was an accurate measurement of the engine's available performance/thrust-output.. "static". I haven't performed any further-testing using different propellers.. but I think that there's a lot more room to explore there. The aero-engine "unwinds".. once its vehicle is "in the air". This is what complicates the equation.

Marleysky Thu Apr 26, 2018 5:01 pm

Roddie had a excellent “Tub-Dyno” going on there. Searched “Benchtop Dyno” and found this from our fellow hobbits* on the quad Copter forums. I haven’t search for any existing test reports on any brand or blade style,yet.

* hobbits are spellchecker version of hobbyists!!

https://oscarliang.com/diy-thrust-station-measure-motor/

batjac Fri Apr 27, 2018 4:30 am

The reason I want to use the three bladed props is that I'm building a sorta-scale-but-not-really WWII bomber. I just thought the three bladed props would look better on it. I'll just have to try the props and engines I have and see what happens. I was going to make a scale like in the second picture of the above link. Just an arm resting on a cheap scale. I don't need accurate thrust values, just differential values for the props. If I don't like what I get, I guess I'll just go to four APC 6x2 props.

Roddie, I have two of those scales that I ordered after reading your "tub dyno" thread back then. I can't believe it's been four years already.

The Aging Mark

GallopingGhostler Fri Apr 27, 2018 1:51 pm

batjac wrote:Doing a search only found the thumb rule that prop loading on an engine for a three bladed prop is about equal to a two bladed prop of 1" more diameter or pitch.

I don't know if this may be a clue, but for the Cox .020 Pee Wee and Tee Dee, props by Cox are 2 blade: 4.5x2 (standard) and 4x2.5 (racing); 3 blade: 3.125x2.5. Peter Chinn observed that the 3 blade was optimal for the Tee Dee.

As Rusty alluded to, inasmuch as there are decent folk out there, I'm sure the barracks lawyers in other forums will be willing to advise on the perfect 3 blade. Doh!