CafeenMan

Grandpa - Most decent web sites hang around for a while, but then you never know when one is going to disappear forever. Fortunately, you can save web pages on your computer.

If you use Internet Explorer, go to the File menu, click "Save As..." and then put it someplace where you'll remember where it is. Also, it's usually a good idea to save as "web page complete" so it looks right when you view it later.

acropilot_ty

I see how it is.... When I say multiple blades doesn't always mean less thrust everyone argues, but when Bolly says it, everyone instantly agrees... Don't worry I see this every day, I'm a flight instructor... I'll tell a student something a hundred times without them getting it.. then they'll read the same damn thing in a book and it must be true because it was published by "Someone Important" (ie. not me).

[&:] Ty

CafeenMan

That's 90% of the people in the world. When people don't know you, you're credible, but once they get to know you and realize you're not perfect, suddenly you don't know anything. People are strange.

O2man

My Feedback: (5)

You obviously do not have any kids, or yours are not old enough to train yo! LOL

Jimmbbo

Agreed... Coulda done it with one blade if they redesigned landing gear to be 40 feet high...

Jim

Jimmbbo

Cool site... Thanks!!

Jim

Crayon

Don't sweat it, you've got my respect. People seem to always believe the written text. They need to start disbelieving!

The last place I worked, I had a cow-worker who wrote a Linux usage book on request of a publishing house. He didn't know the first thing about Linux or UNIX. I ended up answering almost all of his technical questions. His book went on to be a best selling Linux book. Damn publishing industry.

The place before that, we had a guy they hired as a Senior Network R&D engineer. His qualifications for the job were:
a) He wrote several books on the subject.
b) He had their respect because of this.
I've never met an networking engineer with less knowledge on the subject! Everyone ended up referring to him as wingnut. Damn publishing industry.

My personal library has over 5000 books in it. I've read most of them, and quite a few contain information that is just plain wrong. The really bad ones end up in the trash.

The publishing industry doesn't care about providing readers with accurate information or training. It's about making money.

I don't know enough about Bolly or his book to make a judgment about the accuracy of it's content.

Just because you read it, doesn't make it true. It especially doesn't mean the author is knowledgeable. Listen to the instructor, he's the one with REAL experience.

FHHuber

LOL

You would have to have a cage on the thing to catch the blades as they popped off; don't want to plant one in the roof of the local cop's car. (or a lot of other places they wouldn't be wanted...)

And you'd want something to put the blades back on so you could have the 6 blade prop at landing approach... in case you had to go around.

acropilot_ty

Details....

Ty

PS, I'm writing a book on spins (full size airplanes), so I hope mine doesn't end up in your trash Crayon.

Crayon

Trash...No, I assure you that it would have a place of honor and dis-STINK-tion in my bathroom, er, uhm library.

Seriously though, send me a copy, I'd love to read it.

Years ago I used to instruct. I remember getting pretty cheesed at students that didn't listen or thought troubleshooting the instructor was a good thing to do. People don't realize how much work is involved in preping for a class.

Erik

Flypaper 2

Was just questioning to myself where Bolly says " Diameter for diameter for diameter,a well designed four blade prop will, in the same circumstances, perform better than the equivalent two blade prop". Sounds incomplete to me. would need a lot more horse power or a lot less pitch to do it. Someone enlighten me.

FHHuber

Can keep same diameter and same pitch... gear the motor and reduce RPM. You can even go with a higher pitch

A slower swinging prop is more efficient. (less tip vortice at the blade end... less wasted power.) but I doubt that is what Bolly was refering to.


There is more blade area with the 4-blade than the 2-blade.. More blade area means that the prop CAN develop more thrust. (not that it always will... Static thrust may be the same but thats another complete subject)

Mech_gadget

With all of this background information could any of you kind people make a recommendation for the following engines:


Saito 150 4s- three blade (future project)

Magnum 120 2S- 3 & 4 Blade (for P51)

Thanks,

Jetison

Would a four bladed prop give more thrust than a two or three bladed prop?

BMatthews

Not without a larger engine to spin it...

The Reader's digest of the whole thread is that the less the number of blades the more efficient the prop is. The only reason we have other types is for various reasons why the engines can't use two blade props due to size and tip Mach speeds and such.

Any given engine will produce more thrust with a two blade prop than a multi bladed prop assuming the prop is sized to allow the engine to achieve the same peak power output. The same applies to electric motors. If the props draw the same current, and therefore the system is using the same number of watts, then the 2 bladed prop will be more efficient than a multiblade prop. The more blades the worse the thrust.

jetmech05

that is a good link. look at a full scale c-130 the latest version is a 6 blade design. remember its a variable pitch prop, and they have done away with the square prop tips.
also look at the p-3 prop compaired to a c 130 prop, same engine (allision t-56) but the p-3 prop is a little wider with rounded prop tips the 130 is thinner with square tips. I was told its a speed prop on a p-3 and a truck prop on 130.
tip noise is reduced by "bending the tip over a bit" i guess this is what is meant by rake. an example of this is the newer huey blade tips.
all things considered i'd use a 3 or 4 blade prop on a war bird, but i've always had an eye for scale looks.
ps i dont buy the disturbed air theory for the trailing blade. in flight you have forward motition so all blades pass through "undistrubed air" at idle on the ground all blades pass through semi distrubed air due to the airflow coming to the prop due to the low pressure created created to produce lift in this case thrust.
just my 2 cents. look i just fix 'em i dont design 'em.
bottom line flight is life have fun

Ericr7

Let me know when you figure out the exploding prop design. I'll be your first customer! Very clever!

da Rock

Do you have any figures that would help everyone understand what the difference in efficiency is? The efficiency argument has been around forever and probably is one of the most stifling old wives tales our hobby has been cursed with. The unfortunate part of the "expert advice" is the assumption the efficiency is significant and ruinous. It's not even close, which is proven by anyone who is lucky to find a suitable 3 blade to try. And proven further by events such as Pattern. Pattern experienced rule changes and design evolutions that favored 3 blades for the usual reason: ground clearance. The development of available 3 blades produced planes with thrust that was outstanding, along with proof that efficiency was unimportant. Good numbers of pattern guys moved on to IMAC and had no hesitation to go with 3 blades. Ground clearance? Look at how much they have now and ask yourself if they wouldn't get more 'efficiency' quite easily with cheaper 2 bladers.

The real reason nobody much goes with 3 bladers today is cost. The next reason is availability. Try and find one if you're not flying IMAC. Probably the predominant reason is the sage advice from experts about efficiency. How many non-IMAC modelers at your field have ever bought a 3 blade? Why would they have, given that everybody knows they won't get efficiency or thrust. Is it really true about thrust? Do 3 blades really give less than 2...... That is oversimplified to make a point. The point? What amount of efficiency is there?

Take a look around the pits at an IMAC meet and check the ground clearances. Then ask yourself why all those guys aren't using 2 bladers to get better efficiency.

speedracerntrixie

My Feedback: (29)

Rock, you are correct that outside of IMAC sized props there is a dismal selection of sizes. You may be a bit off on your reasons why some IMAC competitors use 3 blades rather then 2. In a nutshell, its about noise control. A few years ago IMAC adopted a noise score. It is a small percentage of a competitors total score for the event. As a result a good number of competitors these days use canister exhaust systems and props that keep the engine below 6,200 rpm. For a 150cc engine a 32X10 was the standard for many years but an engines got more powerful and displacement went up we started spinning these props too fast and the resulting prop noise became an issue. At the time the best solution was to bolt on a 3 blade prop to load the engine more. Now that prop manufacturers have caught up with more appropriate sizes for the 170cc, 200cc and 222cc engines available some guys are going back to 2 blade props. Currently for a 150cc a 30X13 is a popular choice and 31x13 for a 170cc. I can tell you first hand that when the sound score came about my 150cc powered airplane with mufflers was not getting a decent sound score. Simply bolting on a 28.5X12 3 blade prop made it much quieter and I was getting a sound score of 10. I did however loose a noticeable amount of vertical with the 3 blade prop and had to adjust my flying style some to compensate.

DagTheElder

Hello!
Here you will find all you need to know re multiblade-propellers �� lhttp://www.mh-aerotools.de/airfoils/index.htm

da Rock

Whenever I've observed what you did, that you're getting less vertical, I've done what I always do, test some more. It's most unfortunate that we don't have a pitch control on our transmitters that'd give us the same control full scale pilots often have in their cockpits. We're saddled with a rather gross availability of pitches, that often are simply numbers pulled out of the mfg's uh.. numbers the mfg's hope describe the performance us consumers might get from whatever engines we've chosen on whatever models those engines find themselves.

I love to test props. It's provided a lot of eye opening discoveries. For example, the best prop for my OS91FX on a H9 Corsair is a Master Airscrew 3 blade. The MA prop that easily busts 100mph and pulls straight up is one that MA recommends be run on 1.5-1.8 glow engines. They suggest 11x7(3) to 12x8(3) for .90 displacement engines. When I ran those, I noticed a significant sound reduction and noticeable loss of vertical performance. So with the "wrong" MA (according to MA) prop that proved itself in testing, I get significant sound reduction and got back vertical, speed, acceleration performance.

It's quite sad how few modelers go beyond asking for prop advice from field experts whose expertise stops around what they heard about the mystical "efficiency" and went no further.

da Rock

???? Actually, I really didn't mention IMAC flyer's reasons in that post? They (IMAC and pattern) were mentioned simply as proof that 3 blades are in use quite successfully. It took an external force (their rule books) to push both groups to seriously test something they really couldn't find to begin with. Both groups basically forced their members to search for something that wasn't there. A couple of unknown companies showed up. etc etc