Pitch gage (was Re: YS Engines)

[Brian Young]

Where on the prop do you heat. 
I used to see Bill Thomas's props he heated within a
couple of inches of the hub.



--- Dean Pappas <d.pappas at kodeos.com> wrote:
> Use a monokote heat gun or two, heavy mittens, and
> some cold damp rags.
> Play with a couple of smaller throw-away props
> first!
> Oh yeah, add patience! The same technique (with a
> bit less heating) works on fiberglass and CF props
> too.
> 
> Dean
> 
> -----Original Message-----
> From: Pascoe,Tim [Burlington]
> [mailto:Tim.Pascoe at ec.gc.ca]
> Sent: Tuesday, March 30, 2004 4:00 PM
> To: 'discussion at nsrca.org'
> 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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