Digital Prop Gauge

[wkevinm]

I have wondered for while if there was value in having a Prather propellor gauge, but was never ready to buy when they occasionally come on sale. That and having more time than money (retired) I thought I would make my own digital prop gauge.

I have attached a couple self explanatory pictures in case any one else is looking for something to measure props with. Of course this unit does not measure pitch but rather angle. So I have built a conversion table that will convert angle to pitch based on radius from the centre.

I am not fully confident yet that the formula used for the conversion is correct, I think it is close but I have yet to find a prop that is the same as the label. If anyone can check that would be appreciated. The formula I used to build the table (found on the internet) is as follows: '=TAN(RADIANS(angle))*((radius)*2*PI()).

The gauge I am showing is made from a digital angle protractor available at most building stores, attached to a piece of 3/8" aluminum plate, etched at a number of stations including a station at 75% of the diameter of a Q40 prop and a Quickee prop as well. I read propellor pitch can be measured at 75% of radius? Also added to the base is a dial indicator to measure how true the propellor is, I am not sure the value of this yet.

[fizzwater2]

the few spot checks I did in the table look right to me - but I'll admit I didn't do a spread sheet and check 'em all...
I like your idea, especially if you have a digital gauge that can measure in 0.1 degree increments or so. It'll also take a practiced eye to insure you get the angle up against the back of the prop consistently from measurement to measurement. I've found it helps to get a light just right so you can look for how well the gauge meets up with the blade.

I think part of the discrepancy between the propeller markings and a measurement such as the Prather (or yours) is the gauge rests on the flat back of the propeller (or at least the flattest part) and doesn't take into account any effective pitch impact of a leading edge radius and the chord line not being parallel to the flat part of the back of the blade - I think the prop makers most likely look at the pitch angle of the airfoil on their computer models, etc.

While there might be a discrepancy between markings and measurements, consistent measurements let you compare prop to prop, and sort out your props that way.

Now, if you only had a way to measure the radius real-time and run a small processor to do the math for you.. read out in inches of pitch no matter what radius you are measuring at..

[wkevinm]

Hi Fizzwater2

Thanks for checking the formula, I have rebuilt the table, and have checked a number of propellors, you are right a good light behind helps to see the closing angle between protractor and prop. I built a worksheet to log props and help see through all of the numbers to determine average pitch. I am surprised to find out most props have some degree of runout. After checking 8 propellors both new and used I find runout .003-.022" at about 3" radius. Not sure if that is an issue, but I am sure 0.000 is best. This suggests one needs to make a tool to remove runout....and another project.

Regards, Kevin

[fizzwater2]

are you using the drilled hole in the prop for center, or the molded in recess in the back of the hub?

The drilled holes aren't known for terrific accuracy - the molded recess is better used for center. I'm a Jett Q40 engine user, and the spinners have a spacer ring that fits the spinner backplate and the molded recess in the props to center them.

[wkevinm]

I am using the molded centre as well the front and back face, as would be used in clamping a prop to an engine, I am not using the drilled hole. I have been playing around a bit to understand what is out on a prop that has .022" of difference on the leading edge high point, one blade to the next from about 3" to the centre line. It appears on this particular prop the hub thickness varies a bit and .003" over 1/2" radius will equal.018" over a 3" radius. I have tried to correct by filing but find this is not accurate, so I am thinking of making a mandrel to fit a collet and run true off of the molded front and back centres.

Turns out my mini lathe will only swing 7" so I cannot see if this would work.

I don't think this is as much an issue as I am making it out to be but I am curious why some props have more runout than others.

I have also taken a look at the rules and it appears any work like this is not allowed... so I guess all one can do is select a prop based on its given characteristics and keep the rest for practice.


Regards, Kevin

[wkevinm]

I discovered the reason for so much propeller run-out on some propellers. Much of it seems to be from the drilled hole not being true to the molded centres.

I found that when I had drilled out Q500 props I drilled / reamed holes to 5/16 “ which is too tight and will lead to misalignment. To fix this I built an alignment jig to help center a hole to the molded centers and then purchased a 21/64” bit which gives about .014” clearance. This has worked well and I went back and checked some of the Q40 props and found in several instances where run-out was high (in excess of .01”) the holes were not centred, so much that the clearance was taken up on one side and this mis-aligned the prop. The prop alignment jig takes care of this and removes much of the run-out.

[fizzwater2]

I like your drilling fixture! That's great.