REFLECTOR: Fixed pitch props, freeze plugs, and crankshaft plugs

[davedent at comcast.net]

There is one thing you forgot related to removing the CS prop and going to 
fix pitch.  You have to either remove the inner plug in the crank or if no 
small plug in the inner large plug you must drill and tap it if going back 
to the CS a tap size hole in the large plug behind the governor transfer 
tube. This way no pressure can build up in there and the oil that gets in 
there drains back into the engine.  If you don't do this you can cause slug 
to build up in there and even plug up your transfer tube.  Let alone build 
up pressure to blow out the plug in the end of the crank.  In the area that 
the oil is pumped into will create a greater pressure then what you think 
it's receiving.  So don't try and run the engine without putting a hole in 
the inner plug.

The plug that is used is not a standard freeze plug.  It is a special part 
number and it is concave to be flattened when installed properly.  You can 
buy then for very little from any dealer that deals with aircraft engines. 
I think you can even buy them from A/C Spruce.

Trying to remove the inner plug is next to naught other then when the crank 
is removed.  That is why we drill and tap the plug with a 1/4-20 pitch and 
leave open till we install the CS prop again. Then put an Allen type screw 
in it with Loc-tite when you re-install it.  If drilling the hole and 
taping, use some bee's wax on the drill and tap to keep chips from getting 
into the engine.  Toilet bowl rings are made of it.

Do not run the engine with the governor installed and the plug in the end of 
the crank.  If going to fix pitch the governor must be removed and blanked 
off.

Oil pressure is used to get the prop into greater pitch or more bite.  The 
MT has a 4" hole in the back of  prop hub that sees the pressure from you 
governor. Inside the forward end of the crank. The governor can put out 700 
or 900 psi depending on the type and that against a 4" hole could be 2000psi 
or greater, going to move you blades.   So you can see what it takes to move 
the blades from a flat or take off position to a climb or cruse position and 
hold it there.  If the pressure drops off then the prop blades return to the 
flatter position or  high RPM. This is done with the large spring in the 
front of the hub.  The governor holds this pressure by the position of the 
control arm that allows the fly weights to maintain the selected RPM by 
porting this pressure required to the back of the prop.

I hope this help you understand how this all works.
Dave
----- Original Message ----- 
From: "Dave Philipsen" <velocity at davebiz.com>
To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Sent: Monday, October 29, 2007 6:13 PM
Subject: REFLECTOR: Fixed pitch props, freeze plugs, and crankshaft plugs


> Here's a technical question for the group:  I just replaced my
> constant-speed prop with a fixed-pitch loaner prop (the MT prop is off
> for repair & overhaul).  I was told to remove the crankshaft plug (welch
> plug) and drive a freeze plug into the end of the crankshaft before
> putting the prop on.  Well, I could not get the crankshaft plug out but
> I went ahead and put the freeze plug in the end of the crankshaft.  The
> fit was perfect and very tight.  Is there any reason for me to worry
> about the crankshaft plug remaining?  I realize that there is a
> possibility oil pressure could build up against the freeze plug but I
> seriously doubt that it could be pushed out even at 100-120 psi.
>
> Does the MT prop use oil pressure to keep it in flat pitch or does the
> oil pressure keep it in coarse pitch?  If I can fly it with the
> crankshaft plug still in, would it be advisable to pull the prop lever
> to low rpm or leave it at high rpm?
>
> If you have an intelligent answer for this could you explain to me how
> you came to the conclusion?  I'm getting conflicting or no opinions on
> what to do.
>
>
> P.S. Travis Holland is investigating this for me as well since he is the
> one who loaned me the prop.
>
> -- 
> Dave Philipsen
> Velocity STD-FG
> N83DP
>
>
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