Wanted - small propellors

Alta Vista found :

Also, inside of every 6x3 wooden prop there's a 5.25x3 wooden prop just waiting to be freed. You _do_ own a razor saw, yes?

Ook,

Tower Hobbies has an APC 5x3 $3.79 'CLOSE' at:

I had a couple of those. I don't have the time or ambition to make a bushing and then try to balance the thing. I have a top flite 5.25x3 prop in my collections, but it's the only one I have. I saw some on eBay a while ago, but they went for $4-5 each. I wish now I would have got them! I actually have several regular 5x3 with the right size hole, but I really want the

By the time I got done whittleing it down, it would be more like a 5.25x2.75 or so. I'd have to start with a 5x4 wood prop. Not sure if it would be worth the time or trouble or not. I'm running them on a teedee .051, and it doesn't take much of an imbalance to cause significant vibration. I'm having a challenge getting a good smooth run on this thing - one of my prop screws was bent slightly, and that was all it took to set the thing off.

How can you reduce the pitch?

Surely it's just a matter of cutting 3/8" off each tip - little bit of shaping - and balance.

Cheers,

Nigel

Nigel Heather > By the time I got done whittleing it down, it would be more like a

Not by reducing the diameter. That's for sure.

For diameter, yes.

Pitch requires a different blade angle all round.

That's the point I'm making. The OP wants a 5 1/4 x 3 but can only find a 6 x 3

When suggested that he just cut the 6 x 3 down the OP says that by the time he had finished he would probably end up with 5 1/4 x 2 3/4.

I was just saying that he couldn't reduce the pitch (maybe I just missed the joke though).

Cheers,

Nigel

"Nigel Heather the-heathers.co.uk>"

You might be right. Isn't pitch how far the propellor propels the plane in one revolution? If you cut off 3/4 of the tip, would not that reduce the pitch? Maybe someone who knows more then I do can explain pitch and how shortening the propellor doesn't change the pitch. I have a set of brand new master airscrew 6x3 props right here in front of me, and if I can safely cut off 1/2 - 3/4 of the tip and retain the pitch, I'm ready to start chopping :-). I just need some info - like, yes, you can do it, it's safe, it won't make the props fly apart if they are balanced, etc.

"Ook" > wrote

Pitch is figured in an ideal, mathmatical world, with no slip. Think of it like cutting through jello.

You can cut them safely, if you balance them right, but you will lose some performance, vs. the real manufactured smaller prop. It has to do with the airfoil shape, thickness, and whatnot that you cut off.

I wonder how much pull comes from the outer half inch of a 6x3? The prop is thinnest out there, with a minimal airfoil. However, that last half inch moves the fastest, so maybe it doesn't need much of an airfoil? I'd be curious to see some charts that show which part of the prop does the most pulling. Would it peak at the tip? Or be more of a bell curve?

The reason I want a smaller diameter 3" pitch prop is for higher rpms. I remember clearly the day I went from a 6x3 to a 5 1/4x3 prop on a small airplane with a Black Widow engine. Dang thing went so fast, the fuselage broke apart from the wing. It was an old Scientific model, formed fuselage and wing, Lil Devil, I think. It most definitely was not designed to go that fast! Anyhow - keep the same pitch, reduce diameter, airplane went a lot faster. That is why I want a smaller prop. I have some 5x3 props, but I want to play with some props inbetween 5 and 6 to see which do best on which airplane.

I'm pretty sure that most of your thrust comes from the outer part of the propeller.

I have heard it said to never cut down a prop because of saftey but I do it. mk

On Fri, 18 May 2007 13:06:31 GMT, "MJKolodziej" wrote in :

Yes.

Not really.

The pitch may actually vary at different stations on a prop. It's not a simple measurement. There are prop pitch tools that can be purchased for those who like to fool around with such things.

Chop away! It probably won't resemeble a 5.x prop designed from scratch, but it will probably improve the performance on your aircraft. If you're patient, you could make a series of tests with one prop or cut three 6" props down to three different sizes and see which one performs best on your airplane.

"One test is worth 10,000 expert opinions."

Yes, yes, yes. If the material is strong enough to hold the tip on in the first place, it is strong enough to become the tip.

I believe in safety. I hate hurting myself and others. I prefer to fly and land safely because I enjoy flying more than building or rebuilding. But in this case, the principles of safety are on our side. A 6" prop that is safe enough to run on the engine can be cut down to 5" quite safely.

Instructions on how to break in an OS .91 ducted fan recommended using a cut-down prop to get more RPMs out of it. If I remember correctly, they suggested cutting it down in stages.

Up until APC came along and allowed contest directors to provide very nearly uniform props to all competitors, folks used to carve their own props from scratch or modify commercial wooden props.

Some pattern guys repitch APC props by heating them, twisting, then allowing them to cool in the new pitch. I don't think I'm going to try that myself, but I haven't heard any reports of the repitched props breaking in flight.

I think someone else speculated that most of the thrust comes from the tip. I think that depends on RPM and angle of attack. I would expect most thrust to come from the mid-section because airfoils tend to cause vortexes at the tip, which raises drag and reduces efficiency. That's why some wings are capped with winglets--it makes the tip of the wing more efficient. On some electric composite props, people are experimenting with really wild, swept-back tips. I imagine they're trying to improve tip efficiency.

For most of our sport-flying, I doubt we have to worry much about those kinds of refinements.

Marty

Having examined 4 or 5 different makes of props, it appears that the outer

On Fri, 18 May 2007 09:19:19 -0700, "Ook" wrote in :

Yes.

APC clearly has done a lot of homework.

IF I was chopping down one of their props, I'd probably try to fashion the tip to look something like the original.

But after two or three engine runs, you'll know more about this than I do. Let us know what you learn!

Marty

That's crap. Forces on the blades reduce as they get smaller. Try running a full size prop at 20k RPM.

Pitch does not change in a true helically cut prop when you change the diameter.

Are props helically true? I'm guessing no, the blades for high speed air props have airfoils for increased efficiency. Also, wouldn't decreasing diameter by cutting off the tips increase prop slip, which effectively decrease actual, or measured, pitch? Theoritical pitch means little to me - it's what happens when you mount it to the engine and fly it that counts. I would guess that cutting off the tip would allow the prop to spin faster, but if the prop isn't designed to spin that fast it might not be as efficient as one that was made that size to start with.

Hmm...are small propellors designed for different rpms? Is a 5x3 prop designed to spin faster then a 6x3? I would think yes, because there aren't many engines that you can mount a 5x3 on to start with. Can you run a 5x3 on a .02? (I don't know the answer). Can you mount a 5x3 on a standard .049? Yeah, yeah, I know...try it and let us know what happens :-)

Pitch, theoretical or otherwise, is calculated based on a single turn of the propeller. The rating has nothing to do with RPM. When he said "true helically cut prop" what he meant was that if a prop has a twist that will take it forward 3 inches in one rotation (assuming no slip), the pitch will be true through the entire span of the prop. Imagine a spiral staircase. It is steeper in the middle than at the outside, but the entire stairway has the same pitch per rotation.

To answer your question about small props being designed for different RPMs, I don't know if that's exactly the right way to look at it. Diameter is a load on the engine, and pitch is a load on the engine. Different engines will turn these loads into thrust based on their capabilities. Lots of factors affect this equation, including the speed at which a particular engine develops top horsepower, how much thrust a propeller makes at a particular RPM, how much thrust a propeller makes at a particular airspeed, and how draggy the airplane is. Your best bet is to get as many different props as you can find to try on your engines and your intended airplanes, and look for trends. If you find that high pitch and small diameter is good, then you can see if trimming another half inch off would make it even better. Try as many different factory props as you can at first.

Here's a rule of thumb that I figured out myself. I like low pitch and large diameter on a draggy biplane. Higher pitch and smaller diameter works better on a faster airplane. Most planes fall somewhere in the middle, which is why I'm glad I have a huge drawer full of different propellers to try every time I build a new plane.

Some are. Depends really. For best top sped that's what you want, for best hover you want constant angle.

Tat not what we call 'pitch'

Pitch is either the geometrical pitch or the 'no thrust' pitch. netther are affected by shortening a heliceal blade.

That is true.

Props are designed to suit engines.

In general the smaller the engine the faster it revs