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prop measurement

Old 12-10-2009, 12:25 PM
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sautterjw
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Default prop measurement

May be a dumb question, but how do you measure prop pitch?

Thanks
John
Old 12-10-2009, 12:53 PM
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mclina
 
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Default RE: prop measurement

I believe there are pitch guages that can tell you prop pitch. I don't think it's something you can easily measure otherwise.

I just go by what's stamped on the prop.

Definitely not a dumb question.
Old 12-10-2009, 01:52 PM
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Default RE: prop measurement

Considering that the pitch is more coarse at the root of most props than it is at the tip, accurately measuring the pitch would be very difficult. But, the question has peaked my curiosity. Anyone out there know of a good method?
Old 12-10-2009, 03:00 PM
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Default RE: prop measurement

I'm not sure but I think the OP is asking about how to apply the pitch to a plane/engine? There used to be a pitch gauge sold at one point. I may have some info on them in a book? I think Brian Winch wrote about prop pitch in RC Report magazine not too long ago, RCR is now online so the OP can write Brian and ask him. It was found that the pitch on a lot of props sold today were off, some quite a bit. Nothing you can do about it, sort of an is what it is thing.
Old 12-10-2009, 03:15 PM
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Default RE: prop measurement

Prather pitch gauges used to be available. You can find them used on auction sites.

Since I worked for a major prop maker part time while in college and know another from my pylon racing days, I have some insight that the average modeler might not have on the subject.

First off, most props do not have a simple helical pitch. That means that the pitch is not constant down the length of the blade. Depending upon the application of the prop, the pitch distribution is different on the blade.

For instance, on many sport props, the hub and the tips are washed out from the maximum pitch. On this type of blade, the pitch is measured as about the 75% point of the blade.

On racing props, the hub has lower pitch, and the pitch at the 75% point carries out to the tips.

Making it more difficult to measure, many props today have a curved surface on the under side of the prop.

Some manufacturers use the zero lift angle of the airfoil when figuring out the pitch curve, and the airfoil changes down the length of the prop.

Wood props are easy to modify if you are so inclined. Most of them have flat backsideds.

The most important thing is to have the blades track, and match in pitch.
Old 12-10-2009, 04:16 PM
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Default RE: prop measurement

It's been 15 years, but I had someone tell me you could use a tach and a wind speed indicator. A prop can be mounted to a balancer and set out in the wind and pitch can be calculated knowing the wind speed and RPM. This kind of math is over my head and the guy could have been full of it. It makes sense but seems like a lot of variables could change the out come.

David
Old 12-10-2009, 04:38 PM
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Villa
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Default RE: prop measurement

The pitch of a prop is defined as the distance the plane will travel forward in one prop revolution if the prop was in something like jello. On a 12X4 prop that is 4 inches. On a 12X4 prop let us put a dot at 3 inches and at 5 inches from the prop center. The angle of the prop at 3" is greater that the angle of the prop at 5", but both points move forward 4 inches when that dot makes a full circle. Determining the angle at that point is either a sine, cosine or a tangent function. At age 73 I'm too old to go any further.
Old 12-10-2009, 06:00 PM
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sautterjw
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Default RE: prop measurement

Hey yall,

Thank you for the wonderful insight. The reason i asked is that i have an old wooden prop with no info on it!  It seems as though it wasn't a dumb question after all!

john
Old 12-11-2009, 01:06 AM
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Default RE: prop measurement

John, I know there are modelers in western NC who have pitch gages! You ought to be able to find someone who could measure your prop, so you don't have to build or buy a pitch gage. Ask around at some of the better model airplane stores. Most everyone who has raced for any length of time will have one. It's a quick and simple process to check the pitch. Over the years there have been many articles about makng pitch gages, and checking props. The Prather pitch gage is one of the best. It's probably been discontinued, but it can't hurt to contact Prather Products? As already stated the pitch is the distance the prop will travel forward in one revolution if there is no slipage, such as in a solid. I like the comparison of one revolution in butter. A 9"/6" prop will travel forward six inches in one revolution through a solid substance. If a prop has perfect pitch distribution the back of the blade will have a constant change of the angle from root to tip. There are several reasons (both design, and manufacturing reasons, and wood charteristics) why props do not have exactely the same pitch from root to tip. As a result most people are interested in the pitch at the 60% to 75% range measuring from the root. If you can't find someone to help you locally, send me your prop, and I'll check it for you. Greg
Old 12-11-2009, 10:00 AM
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Default RE: prop measurement

sautterjw,

If you do some measurements on your wood propeller, I can do the math for you and explain the process.

Place the prop on a flat board, and just measure the angle of the flat side at 75% of the diameter respect to the flat board.

If you have nothing to measure angles with, just put any straight piece against that flat side of the prop and tell me the height of both ends of the piece above the flat board and the length of the piece.
Old 12-13-2009, 12:08 AM
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frets24
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Default RE: prop measurement

Or you could just take the prop to your local hobbyshop and eyeball it on the counter next to a few props of the same size and different pitches. It then is not too difficult to see what pitch it "looks" closest to.
Old 12-13-2009, 03:40 PM
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vicman
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Default RE: prop measurement

Here is the Gage Greg is talking about. I find it mysterious how he knew I was so close to Boone[X(]

On the gage is an APC 8x6 prop. The block is in slot 5 and as you look down the scale for slot 5 you will see the pitch is 6. In other slots it varies a little but never out of the 6 range. So this prop is pretty close to accurate. The big thing I look for is that the prop is the same on both sides for a given pitch.
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Old 12-13-2009, 04:14 PM
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da Rock
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Default RE: prop measurement

ORIGINAL: vicman

Here is the Gage Greg is talking about. I find it mysterious how he knew I was so close to Boone[X(]

On the gage is an APC 8x6 prop. The block is in slot 5 and as you look down the scale for slot 5 you will see the pitch is 6. In other slots it varies a little but never out of the 6 range. So this prop is pretty close to accurate. The big thing I look for is that the prop is the same on both sides for a given pitch.

Nice pictures.

could it be because you're both NMPRA and have been since back when Charlotte was a hotbed?
Old 12-13-2009, 04:23 PM
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vicman
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Default RE: prop measurement

Greg is a legend around here!
I'm a newcomer trying to get it all started again. He most likely would'nt recognize me from a lineup but I did meet him at OJA last summer and look forward to racing with him the next time he makes the trip over here.
Old 12-13-2009, 10:40 PM
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Default RE: prop measurement

Thankyou for the flattering comments! Back when QM .15 rules did not allow reworked props we used to search for the worst out of ballance props we could find, so that we could trim one blade shorter than the other. Later when we got into composit props, I experimented with props that had one blade made from carbon fiber, and one blade made from glass fiber. This produced a prop that was so out of ballance that the glass blade was a 1/2" shorter than the carbon blade. Finished second at the Nats. The most efficient prop
is a single blade! Later, Greg
Old 12-14-2009, 12:55 AM
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Default RE: prop measurement

The Prather prop pitch gage I have measures the bottom of the prop airfoil which is not the true center line of the airfoil

But it seems to get the job done.

I have had it a number of yrs and have never measured a prop that was not what it said it was.

Comes in handy for props with no markings


Old 12-14-2009, 07:30 AM
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da Rock
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Default RE: prop measurement

Keep in mind that the number the manufacturer stamps on the prop is their choice, their opinion really.

I've not been a lucky as others. I quit buying one brand years ago because the readings from one side to the other varied too much, and differed from one prop to the next more than seemed acceptable for something mass produced.

Check a handful of different brands using one dia/pitch, and you'll find no uniformity in the readings from hub to tip by brand. APC for example washes out the tips so much on some props you wonder about the value of the diameter reading. Some actually have a bit more tip pitch than pitch at 75S. Keep in mind that the airfoil differences matter as much as anything and it's really easy to see that there is a wide choice of airfoils by mfg. Pretty much proves the advertised pitch doesn't mean the same thing to all the mfgs. It also suggests that the measurement of the airfoil's bottom surface isn't anything more than a hint. Like everything in modeling, using the gauges takes some skill in technique and some understanding for intelligent interpretation.
Old 12-14-2009, 10:21 AM
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da Rock
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Default RE: prop measurement


ORIGINAL: vicman
I'm a newcomer trying to get it all started again.
A newcomer???? with a two digit registration number.... Commmon...... fess up....... you're like a lot of us, old guys with the time to do it right this time around.

Old 12-14-2009, 10:42 AM
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da Rock
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Default RE: prop measurement

Guys, keep in mind also that the pitch gauges are usually based on theory but can't apply any part of it correctly. All they do is measure the angle of the bottom line of the airfoil. Think about how vague a relationship that is to an unknown airfoil that could be almost any camber, max thickness, thickness distribution, etc etc etc etc......... Would AEs trust anything they figured out if all they knew was the angle of a line that runs along the bottom of the wing profile?

The idea that those readings would match up to the actual results the airfoils are going to produce would actually bother you more than reassure you. If you were measuring machine screw thread pitch, it'd be spot on. You're not. You've really only got a suggestion of what you might have above that line.

Someone years ago made the guess that the pitch gauge was made to work for a prop with a Clark-Y profile. I wonder what it tells us about the flat plate outboard part of an APC......

Test a number of props of the same dia/pitch and you'll see that the pitch printed on the suckers is just advertising and there is a world of difference in what the same numbers can produce. Pitch gauges are good things to use, but the numbers are just advertising.

Greg showed you an excellent example of how far from theory practice can be, how far from theory a clever craftsman can alter the actual results. His prop was accepted by tech inspection I'm assuming. And they checked what? with what?

(In theory, practice and theory are the same. In practice they are not.)
Old 12-14-2009, 02:03 PM
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Default RE: prop measurement

I still like my Prather Pitch gage, and use it when I think I can get some useful information. I used my first one so much the numbers started to ware off. I put clear sticky back tape over the face to protect the printing. The stuff I used is what the Controline Combat guys called Fast Cal. If it is getting harder and harder to read the numbers, don't blame your eyes, maybe the printing is wearing off! I can't take credit
for this idea, someone else suggested it, but I can't remember who? If you have a Prather pitch gage, think about doing this sooner rather than later, before your numbers go away. Greg
Old 12-14-2009, 02:22 PM
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Default RE: prop measurement

It's no wonder I can't remember who made the suggestion about covering the face of the Prather pitch gage. I can't even remember for the 10 minutes the response I wanted to make to the previous post! My wife says if we get Alshemers ( spelling?) that memory desease, how will we know?
When we had a stock prop rule the manufacturers logo had to be visible on at least one blade. You could remove material from one blade only, for ballancing purposes. When composit props were approved the rule
was two bladed, fixed pitch, continuous filament, compression molded props. No rule said the blades had to be equal in length. The theory is that one blade is working better because it is not following in the wake of the other blade. Can I prove it helped? No. I was just experemented with an idea. Greg
Of all the things I've lost, I miss my mind the most!
Old 12-14-2009, 04:59 PM
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Default RE: prop measurement

ORIGINAL: lnewqban

sautterjw,

If you do some measurements on your wood propeller, I can do the math for you and explain the process.

Place the prop on a flat board, and just measure the angle of the flat side at 75% of the diameter respect to the flat board.

If you have nothing to measure angles with, just put any straight piece against that flat side of the prop and tell me the height of both ends of the piece above the flat board and the length of the piece.
For those who don’t have a pitch gauge and own propellers that show no markings, there is a graphic method to determine the approximate nominal pitch of a propeller (as stamped by the manufacturer on it).

As explained above, this nominal pitch seldom reflects how much the model actually moves forward per each revolution of the engine, because there are always slip, effective AOA, stall of the blade, airfoil efficiency, etc.

The zones of the rotating disk that are near the center and the tips cannot do much more that creating drag and noise.
I believe that because of that fact, the nominal pitch, by convention, is measured for the flat side of the blade and at 75% of the diameter of the propeller (12.5% from the tip).

If we can measure that angle or slope, we can take it to a piece of paper and graphically determine the nominal pitch of most of the propellers used for sport flying.

Please see attached schematics.

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