Pitch gage (was Re: YS Engines)

[David Lockhart]

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


----- Original Message -----
From: "David Harmon" <K6XYZ at comcast.net>
To: <discussion at nsrca.org>
Sent: Tuesday, March 30, 2004 2:46 PM
Subject: RE: YS Engines


> Dean...I just had a nice conversation with Terry Prather and he said the
> next run of pitch gauges will be next month.
> I also mentioned the current need for a larger size gauge for today's
> props and he said that he had been thinking about it and would develop a
> new model this summer capable of handling props up to 24".
>
> Personally, I have had no problem using the original gauge to repitch
> APC props up to 16".
> I use it quite a bit to determine if the prop in question is what the
> mfg says it really is.
> Usually, the blades are not equal in pitch and neither blade is the
> pitch as marked.
>
> Regards
>
> Dave Harmon
> NSRCA 586
> K6XYZ[at]comcast[dot]net
> Torrance, Ca.
>
> -----Original Message-----
> From: discussion-request at nsrca.org [mailto:discussion-request at nsrca.org]
> On Behalf Of Dean Pappas
> Sent: Tuesday, March 30, 2004 10:34 AM
> To: discussion at nsrca.org
> Subject: RE: YS Engines
>
> Prather make them up in batches fairly rarely, they are too small for
> 15, 16, and 17 inch props let alone the IMAC stuff, and it is otherwise
> a fine high-quality product. I've had mine since the great quiet
> struggle of '82/'83.
> Dean
>
> -----Original Message-----
> From: David Harmon [mailto:K6XYZ at comcast.net]
> Sent: Tuesday, March 30, 2004 1:33 PM
> To: discussion at nsrca.org
> Subject: RE: YS Engines
>
>
> Hey Dean....does Prather still make that pitch gauge?
> I have one and it works well.
>
> Regards
>
> Dave Harmon
> NSRCA 586
> K6XYZ[at]comcast[dot]net
> Torrance, Ca.
>
> -----Original Message-----
> From: discussion-request at nsrca.org [mailto:discussion-request at nsrca.org]
> On Behalf Of Dean Pappas
> Sent: Tuesday, March 30, 2004 10:11 AM
> To: discussion at nsrca.org
> Subject: RE: YS Engines
>
> Hi Anthony,
> The economics are not there, we have spoken to Fred at APC about this
> already. The propellor pitch gauge construction article will appear in
> FM. Now, all I have to do is write it ...
> Dean P.
>
>
>
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