3-bladed props for OS160FX

Mon Nov 15 15:57:06 AKST 2004

Claudio,

Below I've posted some info I've written before - I think most of it has
been on this discussion list before.  In short, progressive twisting is not
needed - a single twist near the root is sufficient.

--------

I've been repitching APCs for years - and in general not had a problem with
springback/memory - but I have found that the technique is critical.  Using
the Prather pitch gauge, I feel comfortable saying I can repeatedly measure
prop pitches within 1/8" pitch - which I feel is pretty much the pushing the
precision of the gauge.  I would say that 10-20% of the blades I twist do
have some amount of springback (1/8 to 1/2") in the first 24 hrs - but I've
not seen them springback anymore after that, and it is rare that I have one
springback as much as 1/2".

I mark every prop front/back of the blade where I am going to heat with the
heatgun and where I am going to apply the twist.  I use a good heat gun, a
thick cotton towel or sock, and a large set of channel lock pliers.
Basically, I slowly heat the prop blade near the hub with the heat gun
(alternating front/back, concentrating on the thicker areas of the blade),
and when the blade is hot enough, it will twist without too much pressure.
For the APC 15.75-13 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 (3.5" from prop center) to apply the twist - I put the
other 2 blades on my leg 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, you can either cool
the prop in cold running water (and move onto the next blade) or let it cool
overnight (don't mess with the prop while it is cooling).

The basics are pretty simple - but it does take some practice to get good at
using the Pitch Gauge and being able to get consistent readings from the
Pitch Gauge.  Heating the prop slowly (I spend 4-5 minutes before twisting
on the 15.75-11) is better than heating it fast - think slow cooked prime
rib, not "black and blue".  You want the prop warm on the surface and at the
core - then it will twist easier and have less memory.  If the prop is not
evenly heated, the pitch will tend to wander as it cools - not good.  It
usually takes me 1 heating cycle, about 1 major twist, and a couple minor
"tweaks" during the first couple minutes of cooling to get the pitch change
I want.  On occasion, I do need to reheat a blade and tweak the pitch a bit.
Some of the props I twist get a little bit of whitish discoloration when
heated, some develop the whitish coloration after I twist them, and some
don't show any change at all.  Near as I can tell, the prop is at about 200
degrees when I twist (using an InfraRed temperature gun).

I used to quench the prop in cold running water (very important to cool both
sides equally) and repitch each blade consecutively in one session.  At this
time, I'm having better luck letting the prop air cool - the pitch seems to
wander less.  I then let the prop sit overnight and check again the next day
and make minor pitch adjustments if needed.  I've got a couple 13.5-11.5N
prototypes I made back in 1992 that were the result of twisting/reshaping
and they are still at 11.5" pitch, after 12 years, and hundreds of flights.
And I've got some prototype 17-13Ns that are 5 years old with hundred of
flights and still ok.

Regards,

Dave Lockhart
DaveL322 at comcast.net

----- Original Message -----
From: Claudio Garcia Rosa
To: discussion at nsrca.org
Sent: Monday, November 15, 2004 7:06 PM
Subject: RE: 3-bladed props for OS160FX

Gracias Dean,

Just to see if I´ve understood,
I must twist the blade about 1-1/2" from the root untill it reaches the 12"
pitch?.
Or must I twist it progressively from the root to the tip ?

Saludos.
Claudio
----- Original Message -----
From: Dean Pappas
To: discussion at nsrca.org
Sent: Monday, November 15, 2004 7:05 PM
Subject: RE: 3-bladed props for OS160FX

Saludos Claudio,
First you need a pitch gauge, and then a monokote hot air gun. For this size
prop, I actually use 2 heat guns, to warm the thick blade root. You will
probably have to apply heat for 30 ~ 60 seconds with two heat guns. Warm the
blade about 1-1/2" from the root, and twist the blade until it cools. You
will need something to prevent burning your hands, like a block of wood that
has a narrow hole in it. The blade can be restored to its original shape by
warming, if and when you need several tries to achieve the dewsired pitch.
It will take practice to keep from bending the blade along its length, while
just twisting it.

Try practicing on a small, disposable, two-bladed prop!

Have Fun,

Dean Pappas
Sr. Design Engineer
Kodeos Communications
111 Corporate Blvd.
South Plainfield, N.J. 07080
(908) 222-7817 phone
(908) 222-2392 fax
d.pappas at kodeos.com
-----Original Message-----
From: discussion-request at nsrca.org [mailto:discussion-request at nsrca.org]On
Behalf Of Claudio Garcia Rosa
Sent: Monday, November 15, 2004 4:59 PM
To: discussion at nsrca.org
Subject: RE: 3-bladed props for OS160FX

Hello Dean,

I have the same prop.
Can you please tel me how to de-pitch it to 12 ?

Thanks in advance.
Claudio
----- Original Message -----
From: Dean Pappas
To: discussion at nsrca.org
Sent: Monday, November 15, 2004 6:42 PM
Subject: RE: 3-bladed props for OS160FX

Try the stock 15-3/4 X 13 APC.
You migh de-pitch it to a 12.

Dean Pappas
Sr. Design Engineer
Kodeos Communications
111 Corporate Blvd.
South Plainfield, N.J. 07080
(908) 222-7817 phone
(908) 222-2392 fax
d.pappas at kodeos.com
-----Original Message-----
From: discussion-request at nsrca.org [mailto:discussion-request at nsrca.org]On
Behalf Of randy10926 at comcast.net
Sent: Monday, November 15, 2004 4:41 PM
To: discussion at nsrca.org @nsrca.
Subject: 3-bladed props for OS160FX

So what 3 bladed prop would you use on an OS160FX?

Any reason to think a bolly carbon prop is better than an APC on this
engine?

Randy

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