knot in a wood prop?

[CATPart]

Looking at an older sterba prop on an o320. It has a knot in it. Run away?

[Geico266]

How many hours are on it?

[CATPart]

maybe 900 ish

[bruceh]

If you've ever worked with wood, you'd know that's probably the strongest part of the prop!

[rockwoodrv9]

Disclaimer - I know nothing about props. I do know a bunch about wood. I would not worry about the strength of the portion with the knot. Like Bruce said, knots can be very hard. What I would make sure is it was sealed well with epoxy so it wasn't a place water could get into and freeze. Other than that, it looks pretty cool!

[CATPart]

I agree that a knot is a hard piece of wood. It is obviously cracked in the center and appears somewhat pulled away on the right edge. Lets imagine it falls out like you often see a board with a knot that has fallen out, are we still good? It doesn't give me that happy feeling.

Interestingly I see a logbook entry at 664hours that a defective sterba prop was removed and replaced with a catto. the airframe is now at 940 hours. I see no subsequent entries about props, but obviously the catto was removed and this sterba put on....hmm

[izzybear]

Consider yourself lucky...I'd say your prop is good for an "extra knot"

[CATPart]

I have collected some opinions from people who make propellers. To paraphrase:

1. "I would remove the knot and fill it with epoxy"

2. "I have not yet seen a knot in a prop and would replace this prop"

Bottom line, it is beyond my comfort limit to fly with it, so I am looking for a replacement. Aside from the knot and the age, it has a couple other small issues that need attention. I don't think it is worth servicing this old prop. It shall become a wall hanger.

[rockwoodrv9]

What a cool piece of wall art! You have to go with your comfort level - you are the guy flying behind it!

[Vansconvert]

Prop Maker Ed Sterba and I, unkown to either of us until years later, learned prop carving from a master - Harold Rhem from Dousman, WI. Harold had a position as a quality inspector at a very large well known corporation in the Milwuakee, WI area and had very high standards. So high, that he once told me because he refused to pass a set of parts that were in demand at his place of employment, that he was in so many words....encouraged to retire prematurely.

There was a very nice write up in Sport Aviation about Harold many years ago if you would care to research it. I am sure I saved the article but it is buried at the moment along with the formulas for calculating pitch and laying out a prop. Otherwise I could give you the month and year that it was written.

Anyway, Harold laid out the pieces of quartersawn hardwood boards carefully for subsequent gluing. He would lay them out in order that any loose knots would be carved/cut away in the rough cutting stages of the prop. He told me, if it was a smaller size, tight knot, don't worry about it at all as it would be the strongest part of the board, and he would lay that board out with the rest of them, like it didn't even exist, for gluing in the press.

I distinctly remember him telling me that he would not manufacture a prop where a loose knot appeared within 12 inches of the tip, but if it were closer to the hub, a small loose knot would not be a problem because there was a lot more strength in blade where it was thicker near the hub, as opposed to out in the thinner end section. And he went on to say, some guys would not like the cosmetics of it, but it would be plenty strong.

Because the props are laminated from X number of boards, there are unseen knots in many, many props. The knot pictured is relatively small and while I am by no means a prop expert, I do know that Harold Rhem would not have been concerned with it. I can't recall off hand the number of props he built (there was a number quoted in the Sport Aviation article) but it was a lot, and to my knowledge, he never had a customer complaint - of any nature actually.

As an aside, Harold built a lot of one-off props for unique aircraft. He had an extensive machinist background and was one heck of a nice guy!

Ed Sterba has built quality props for years. He is well known as a prop builder for the Sonerai series of aircraft. If it were me, I would fly behind that prop and not give it a second thought.