Pitch gage (was Re: YS Engines)

[David Lockhart]

Tim,

Aside from patience, practice, the pitch gauge and some patience and
practice -

I use a good heat gun, a thick cotton towel or sock, and a large set of
channel lock pliers.  Basically, you slowly heat the prop blade near the hub
with the heat gun (heat evenly, and concentrate on the thicker areas of the
blade), and when the blade is hot enough, it will twist without "too much"
pressure.  For the 3 blade, I heat at Station 5 (of the Pitch gauge, which
is 2.5" from prop center) and then wrap the sock around the blade and hold
the prop at station 7 to apply the twist - I put the other 2 blades on my
leg (a block of foam on the bench would also work) and do the twisting bit
in my lap.  (For 2 bladers, I use 2 pair of pliers).  The sock is too keep
the teeth of the pliers from scarring the prop.  When the desired pitch is
achieved, I cool the prop in cold running water and move onto the next
blade.

The basics are pretty simple - but it does take some practice to get good at
using the gauge and a feel for how much to twist the prop.  Heating the prop
slowly (4-5 minutes before twisting) is better than heating it fast - you
want the prop warm on the surface and at the core (my IR gun says about 200
F) - then it will twist easier and have less memory.  Some of the props will
show some white coloration during heating, and some show the whitish
coloration after twisting.  If the prop is not evenly heated, it will tend
to wander - not good.  And when the prop is cooled, cool both sides evenly
or again, the pitch will wander.  It usually takes me 1 heating cycle, about
1 major twist, and a couple minor "tweaks" to get the pitch change I want,
then I cool the prop.  After I get all blades done, I re-check all blades.
On occasion, I do need to reheat a blade and tweak the pitch a bit.  I then
check the prop again the next day - and maybe 1 in 10 blades needs a small
adjustment - rarely more than .25" - which probably means the blade was not
thoroughly heated when the twist was initially made.

This same procedure works on the Bolly CF and FG props (and I believe Wayne
Powell verified that once in a prior thread).  And I know some who have
repitched the APCs by heating and then clamping in a jig with wedges equal
to the desired pitch.

For newbies to prop twisting, you might want to practice on little props
first - they are much easier to work with, and much cheaper too!!!

Dave

----- Original Message -----
From: "Pascoe,Tim [Burlington]" <Tim.Pascoe at ec.gc.ca>
To: <discussion at nsrca.org>
Sent: Tuesday, March 30, 2004 3:59 PM
Subject: RE: Pitch gage (was Re: YS Engines)


> 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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