Pitch gage (was Re: YS Engines)

[Pascoe,Tim [Burlington]]

For those of us who haven't entered the wonderful world of re-pitching an
APC prop, can you give us an idea of what's involved?


Tim

-----Original Message-----
From: David Lockhart [mailto:DaveL322 at comcast.net] 
Sent: Tuesday, March 30, 2004 3:18 PM
To: discussion at nsrca.org
Subject: Pitch gage (was Re: YS Engines)

A little more detail - the Prather Pitch gage has been around as long as I
can remember, and the guys I originally knew to use it were RC pylon (and a
couple pattern guys) and CL speed and stunt guys that were modifying
existing props or carving their own from scratch - the biggest props
commonly used at the time were 12" diameter.  The Prather gage would (and
could) accommodate up to 14" diameter (more than enough at the time).

You can measure pitch on a given prop blade as far as 7" from the hub with
the Prather.  My experience (I've measured hundreds) with stock APCs tells
me that if the pitch of the blades are equal to a distance of 4 or 5" from
the hub, they will also be equal at the tips.  With the larger props (over
14"), you do have to either assume the pitch at the tips is equal, or, go
through a fair bit of fuss to check symmetry by other means (surface gage
works well enough, but doesn't tell you the pitch).

As most of the thrust generated by the prop is centered somewhere around a
point about 75% of the way from the hub to the tip, and the Prather will
allow readings out to about 83% on a 17" prop, you can effectively measure
17" props.

For the props I've repitched, the majority of the repitching occurs within
3" of the hub, and outside of about 5", the blades are not subjected to any
stress.  I have come across props where the blades did not track in the same
plane - and it was due to a difference in pitch of the blades.  I have not
had this problem with any stock APCs I've run or any APCs I have repitched.

The gage itself is a pretty simple device - a jig that holds the prop, and
the jig is placed in various positions on a machined aluminum tray, and then
a hinged arm is matched to the backside of the prop blade.  What the gage
actually does is measure the angle of the bottom of the prop blade.  The arm
lines up on a graph where the pitch is labeled.  Note that this pitch
reading is really only a reference.  IF the angle of the bottom of the prop
blade represented zero angle of attack for the prop blade, the pitch reading
would be accurate.  However, most props do not use a flat airfoil, so the
true pitch reading could only be determined if the zero angle of attack of
the prop blade were known.

Getting consistent readings with the gage does take a bit of practice - and
I would say the gage is really only accurate to within an 1/8" of pitch.
Having gauged and fixed many a wood prop, I can tell you that the APCs
were/are the first truly mass produced prop that are consistent - blade to
blade, and prop to prop.  The largest pitch imbalance I ever found on an APC
was 1/2" - and that prop ran without blade tracking problems or vibrations.
I have found some props (not APCs) where the blades were rarely within 1/2"
of each other and the average pitch on a given sized varied by as much as
1.5" of pitch.

I  know a number of guys have been bugging Terry for at least a year now -
and more gages have "been in the works".  Hopefully they will arrive this
spring/summer, and a larger size would be a definite bonus!!!!!!!!!!!!!

Regards,

Dave Lockhart
DaveL322 at comcast.net
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