REFLECTOR: more MT prop data points...

[Sid Knox]

I bought my Velocity RG in early 1991.  It was built in '93 and had only a couple hundred hours on it. The previous owner/builder (in Florida) had installed an MT three-blade less than 100 hours before I bought the plane.  I had not even gotten the plane out of Florida before my MT problems began.  

1) the cheezy mount for the brush holder (surely they must be better nowdays??) would not keep the brushes in contact with the slip-rings.   I added a brace.  Ok since then.

2) About 50 hours later, the prop-pitch change mechanism quit.  The motor would not turn because the bronze bushing in the yoke stripped its threads and the debris jammed the threaded drive shaft (which stalled the motor.   Upon dissambly, I observed that what grease remained was dried-out and flaked away at the touch. (I am guessing that it was WWII surplus German Panzer tank grease).  I was not able to remove the bushing from the yoke (I tried pressing it out with no success) and  in speaking with a MT technician at the Florida facility, learned that it was screwed into the yoke and was not an easy thing to remove/replace.  So, I sent the yoke to him and he installed a new bushing.  The new bushing had an internal groove to retain the grease (there was nothing in the original design to prevent the grease from being spun-off due to centrigugal (?) force... which is probably what happened).  I was somewhat displeased with being a part of their product-improvement program but the charge was reasonable ( 50 or 60 dollars as best I remember).  And as a courtesy, he sent a small vial of "improved" (new) grease.

3) While I had the prop hub apart, I observed that the blade bearings appeared to have no grease. It apparently had long since been flung out thru the seals. Trying to rotate the blades by hand showed that the bearings would barely move and had that "gritty" feeling one feels when turning an old, dry bearing with crud jamming the balls.  I fabricated a simple wand from 1/8 copper tube with which I could reach way back to the back of the bearings (access is very limited inside the hub with the blades installed) and blow a mix of compressed-air and solvent into the races and flood-out all the old hardened grease remnants.  After a good cleaning and drying, I injected fresh CV-joint grease into the bearings.  They then moved smoothly as one would expect.  This problem was undoubtedly a contributer to the stripped bushing.
 I reassembled the thing and it has been ok since.

As an aside... after reassembly, I measured and recorded the current required to drive the pitch motor.  Now, periodically, I can again measure the current to get a feel for how hard the motor is working to move the yoke.  So far, the current has stayed as it was following my "Okie Overhaul".

4)  Shortly after that, I made a really bad, worse than usual, landing and got the prop tips into the asphalt!!  The factory wanted the prop and about $2400 to replace the blades.  My plan was simpler... a Makita grinder and JB Weld.  The blades are now about 1/2 inch shorter and I have noticed no change in performance.

I have photos of 2) and 4) for anyone interested.

Sid Knox
Oklahoma

Velocity    N199RS
Starduster  N666SK
KR2         N24TC
W7QJQ


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