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Stupid question for clever people

Old 09-22-2008, 07:56 AM
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BrentL
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Default Stupid question for clever people

Right, so...

If I have a 4 blade 7 x 3 prop, would this be anything like a 2 blade 14 x 3 prop, or maybe a 2 blade 7 x 6 prop?

I know that it doesn't quite work like that, due to efficiencies and things (more blades, less efficiency, or so I've read), but how then doen one work out the sort of output that one can expect from a 3 / 4 blade prop?

Old 09-22-2008, 08:08 AM
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MinnFlyer
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Default RE: Stupid question for clever people

Theoretically you are correct in your assumptions.

But still, I think you will be disappointed in the performance of a 4-blade prop.
Old 09-22-2008, 08:14 AM
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Default RE: Stupid question for clever people


ORIGINAL: BrentL
but how then doen one work out the sort of output that one can expect from a 3 / 4 blade prop?

You will find lots of 'rules of thumb' but there is really only one practical way to obtain a meaningful idea and that is trial and error. In many instances folks do not like the result.

Often when people insist on three blades or worse four blades the results are not pretty with silly looking little toothpicks on the nose.

John
Old 09-22-2008, 08:27 AM
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BrentL
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Default RE: Stupid question for clever people

To give a little more background, I'm in the process of emigrating, and have packed all of my nice planes away into boxes, but I'm keeping one out that I can give away to someone when I leave at the end of the month, just to have something to fly in the meanwhile (transmitter etc. can travel in my hand luggage).

So I've kept a much tattered little wing dragon 4 pusher prop out, having replaced the bushless motor and esc with the original Speed 400 so that I can take the more expensive parts with me. The Wing Dragon setup allows a maximum 7 inch prop without eating its own tail boom, and the 7 x 6 that I had on the brushless doesn't work well with the 400 (I think it prop stalls due to insufficient revs, so only really works okay at WOT).

This is why I was contemplating a 4 blade 7 x 3 or 7 x 4.
Old 09-22-2008, 08:29 AM
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Default RE: Stupid question for clever people

That makes sense. So where are you moving to?
Old 09-22-2008, 08:45 AM
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Default RE: Stupid question for clever people

Melbourne, Australia.
Old 09-22-2008, 11:30 AM
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Default RE: Stupid question for clever people

Then give it a try that is the only way you will ever know but be carefull about pulling too many amps.

John
Old 09-22-2008, 12:31 PM
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Default RE: Stupid question for clever people

I have never bought in to the more blades less effective the prop argument.....look at our full scale brothers they're moving to even more blades on props...the C-130 started out as a 3 blade (A model) to a 4 blade now it's 5 or 6...in flight as the aircraft moves forward the prop blades are always moving into realitve calm air.....
A friend of mine had the H9 Hellcat..it flew better with a more scale looking 3 blade than it did with a 2 blade.
Now I'll be the first to tell someone with Raptor PTS to remove the 3 blade prop and get a two blade...but that 3 blade that comes with the PTS...each blade is huge....lots of drag.
I think I'm going to try a 4 blade on my 180 sized P-47...
Old 09-22-2008, 03:25 PM
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Default RE: Stupid question for clever people

FWIW - when moving from a 2 blade prop to a 3 blade prop Go 1" less in diameter OR 1" less in pitch for equivalent performance. I do not know how to calculate from a 2 blade to a 4 blade prop. You WILL BE DISAPPOINTED in the performance from a multi blade prop compared to a 2 blade prop. Multi blade props work well on full size aircraft, but not that well on models.

Multi blade props are nice for display or if you absolutely need the extra ground clearance. Most people use a 2 blade prop for flying, besides, when the engine is running you can not tell how many blades the prop has.
Old 09-22-2008, 03:50 PM
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Default RE: Stupid question for clever people

I'm too lazy to find the link, but Graupner said (if I remember correctly) to convert to a 3-blade from a two blade subtract 1" dia or 1" pitch. For a three blade to a four blade subtract another inch in dia of pitch.

FWIW.




I too do not submit to the more blades = more inefficient. There's alot of variables. I do believe a 2-blade is superior to a n-blade in 3D flight though. The 2-blade will have a larger diameter air movement over the control surfaces. They all have their place.
Old 09-22-2008, 04:37 PM
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Default RE: Stupid question for clever people


ORIGINAL: BrentL

Right, so...

If I have a 4 blade 7 x 3 prop, would this be anything like a 2 blade 14 x 3 prop, or maybe a 2 blade 7 x 6 prop?

I know that it doesn't quite work like that, due to efficiencies and things (more blades, less efficiency, or so I've read), but how then doen one work out the sort of output that one can expect from a 3 / 4 blade prop?


Nope

For the same reasons a 40' wingspan monoplane flies differently than a 20' winspan biplane. But with one huge difference. And a few minor differences.

Props are basically wings, not really much else.

The area of a circle does not come close to changing at the same rate as the diameter. Change the diameter and you get a huge difference. And the area is what the wing/prop works with.

Aspect ratio has a significant part in wing performance. The efficiency of a prop is greatly affected by it's aspect ratio. But a prop has two aspect ratios. And both matter. One is the same as a wings, the chord:span. The other is the pitch versus the radius. And it matters a lot to efficiency. And it's effect is sorta backward. High aspect ratio wings are more efficient. With the pitch:span ratio, the efficiency of a prop increases as the pitch increases up to a point and then decreases. So a 14x3 prop will have less efficiency and the 7x3 has more. If I remember correctly, about the optimum is when the pitch is about half the span.

Also, tip loss efficiency is the same as with wings. Biplanes lose efficiency because they're dragging around 4 wingtips instead of 2. That ain't all of it, but it's a big reason.

But you asked two questions..................
Old 09-22-2008, 04:53 PM
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da Rock
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Default RE: Stupid question for clever people


ORIGINAL: BrentL
how then doen one work out the sort of output that one can expect from a 3 / 4 blade prop?

Unfortunately, with the choice of props nowadays, there really isn't a good way using theory.

I'm running a 13x8(3) that is the absolute best performer on one airplane. That airplane flies really well on 2bladers also. Maybe the best 2blader is the Evolution 14x6. Landing speed at idle is decided by me. OK, it's decided by the stall. The idle rpm and pitch really don't play a significant part. Because the sucker will not stall until it's almost zero airspeed (ok, that's exaggerating, but it's SLOW, trust me) and the idle rpm/pitch work about the same with 2 or 3 blade. The 2 bladers pitch does seem to give a different slow speed, but not much. The point is that the 8" pitch on the 13x8(3) in practice does not equal theory.

What we read on the props for pitch is nice to read, but don't bet the farm on it.

At low speeds, the efficiency and blade area and fan area mean a bunch.

The most effective way to "work out the sort of" prop that will work is to try whatever props you can find to buy. I've been working with 3 bladers for a year or so now. I started because of the sillyism "3 blades are less efficient than 2" that everyone seems to think means "3 bladers won't work as well as 2 bladers". Turns out that I've found an excellent 3blader for my 61s, 75, 91s. And yet to find an excellent one for my 46s, but I've not really tried.

What I've found is that I need to try every diameter that seems appropriate and to pretty much ignore the pitch. And test fly. Our glow engines operate well over a broad range of rpms. Unless you're trying to race that is. The range has worked great for the 75 and 92s. I've had to work diameter for the 61.


Old 09-22-2008, 04:59 PM
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da Rock
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Default RE: Stupid question for clever people

Right now, I can find Master Airscrew and Graupners. The MAs have a basically narrow chord blade planform. The Graupners are fat bladed. Thin wing? Fat wing? High aspect ratio, low aspect ratio?? Yup.

That has mattered with the engines I've been working with.

I've found MAs that worked great, but not been able to get the few Graupners to fit the engine/airplanes I've got.

I have not tried any 4 bladers. There aren't any I've found readily available. I have just recently seen the Zinger website and it's 4blader series. It's a hub deal with screwed on individual blades. It's a BIG hub. Can't see it on the front of my Corsair, P47, Sukhoi, but it might work under the spinner of the Spitfire and P40.
Old 09-22-2008, 05:27 PM
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da Rock
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Default RE: Stupid question for clever people

The "3bladers are less efficient" line is true. And almost totally irrelevant. Don't let it get in your way.

It is based on the prop development for individual airplanes/powerplants. A specific power demand will often predispose the blade area required. The ground clearance often drives the problem as well. So in the fullscale world, when they started talking about "less or more efficient", they were dealing with some set values. We can't do that. (Nobody sells us matched sets of 2blade/3blade/4bladers or sets of one family of props).

They were comparing how the blade area needed to absorb the horsepower of say the Packard Merlin at 2,000hp would work through the blades they could produce if they used 3 blades of about 13' diameter versus how 4 blades with the same ground clearance would work. And they found out that yeah, more blades were less efficient. They also found that the loss in efficiency was partly made up by having a different aspect ratio of the planform, but nobody (unfortunately) today has noticed that. What they were comparing were blades of basically the same family. And yeah, they found there was some difference in efficiency. And went with MORE blades.

They even called the fat blades, paddles. And manufactured them as fast as they could and bolted them on those airplanes as fast as they could make 'em.

So do we even see any props where the "efficiency troll" would cause us a problem? Nope
We don't have families of similarly designed props that include 2 and 3 bladers of the same diameter to work for a single power range. What we have are some very few 3bladers that were made to be as efficient as possible for their diameter. And some quite different 2 bladers made as efficient as possible.

The choice in props is so narrow that the performance difference we see in any 3blade we can find has almost nothing to do with how efficient that blade design would be as a 2blader, if anyone made that 2blader.

But it ain't no effort at all to simply bolt on any new prop and see what it does. I've got a 13x8(3) that flies better than any 2blader that's been on the airplane (and there have been A LOT), and it was chosen from a test not theory.

Remember, in theory, practice and theory are the same. In practice they seldom are.
Old 09-22-2008, 05:34 PM
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da Rock
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Default RE: Stupid question for clever people

3 blades that work like GANGBUSTERS...........

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Old 09-22-2008, 05:39 PM
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da Rock
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Default RE: Stupid question for clever people

A couple more that fly great
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Old 09-23-2008, 02:20 AM
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BrentL
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Default RE: Stupid question for clever people

Thanks for the info da Rock - guess it ultimately comes down to trail and error, as I discovered last night when, within 45 seconds, my speed 400 motor turned its rusty innards into goober grape and threatened to actually burst into flames.

Guess I was pulling way to many amps for that little motor. Fortunately, I have a box of them so I did a quick replacement and am going to go back to the original 2 blade 7 x 3.

Still, a very interesting experiment. When I have a bit more time and some more robust motors at my disposal, I'm going to tinker a bit more.

Thanks again.

Brent.
Old 09-23-2008, 07:59 AM
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da Rock
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Default RE: Stupid question for clever people

Hey, if you're picking props for an electric motor, you can do a bit of the selection with a meter. Also, the property of electric motors is such that tach readings are also reliable to see when you've got a prop that allows the motor to run close to it's design rpm. When an electric is over propped, it'll burn itself up, and you can see it trying with the meter and the tach and quickly stop that test. Beats wondering why it's not performing in the air and land to discover the ashes.

If you are working with glow, keep in mind that it's the performance in the air on your airplane with your engine that matters. The rpm on the ground isn't really important.

I've worked out a flight sequence I fly when comparing. It includes some subjective decisions (what's the speed over the top of both sides of a lazy 8, for example). It also includes a number of timed vertical climbs. I stopwatch a bunch of climbs to see how the prop pulls. I usually have a buddy do the timing and recording. This takes a bunch of the guesswork out of the comparison. It's also fun if you've got some field experts telling everyone that 3bladers suck and it's a waste of time to ever use them. That happened with my everyday plane middle of last year. Everyone knew how it flew and it had it's favorite 2blader on. It would do about 10sec verticals. I had a new 3blader that I really didn't think would work. The pitch was "rule of thumb" wrong. Bad wrong by the accepted, "one inch diameter or pitch less" rule. And the sucker did 6 or 8 verticals and the worst was about 11sec. And it flew the other maneuvers great, the ones I judged subjectively.

Yeah, about all we can do is try what's available for sale. The conventional wisdom we have right now doesn't work with the numbers the mfg's are printing on their products. The rules of thumb may be right, but the numbers have to be too.
Old 09-23-2008, 10:30 AM
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Default RE: Stupid question for clever people

Well when you get here,"Welcome to Austraila". Melbourne eh? so we can now call a "bloody Victorian" Cheers....

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