[alpinelakespilot2000]

I've always known that wood or wood-core props, like my Catto, need to be retorqued periodically to compensate for seasonal humidity changes. For parts of the country that are pretty humid year round, when (or if) this happens probably doesn't matter very much. However, for those of us in the dry part of the west, where the humidity fluctuations throughout the year are pretty significant, how often should this be done? Just twice yearly (once during the 'dry' season and once during the 'wet' season) or more often that that?

Based on the following monthly average humidity chart for where my plane is based, when would you recommend I do my periodic retorques?

FYI, the chart came from:
https://weatherspark.com/averages/30171/Ellensburg-Washington-United-States

 

[Greg Arehart]

Steve,

I believe that there was a thread here that Craig responded to a couple years back that indicated the props were pretty much sealed up with epoxy (even the center, that looks more like wood) and thus not responsive to humidity swings. But you might give him a call and ask. My prop (2 years old now) has never needed retorquing (I've tested it a couple times in addition to at 2 annuals).

Greg

 

[jrs14855]

Catto

Steve,

I believe that there was a thread here that Craig responded to a couple years back that indicated the props were pretty much sealed up with epoxy (even the center, that looks more like wood) and thus not responsive to humidity swings. But you might give him a call and ask. My prop (2 years old now) has never needed retorquing (I've tested it a couple times in addition to at 2 annuals).

Greg

That is my understanding also but in addition Catto is now bonding and aluminum plate to the aft side of the "hub", I believe he has been doing this for some time. But check with Catto.

 

[Ron RV8]

I checked mine at inspection time in December and all the bolts took 1/2 - 3/4 of a turn... We're in a semi desert...

 

[alpinelakespilot2000]

Steve,

I believe that there was a thread here that Craig responded to a couple years back that indicated the props were pretty much sealed up with epoxy (even the center, that looks more like wood) and thus not responsive to humidity swings. But you might give him a call and ask. My prop (2 years old now) has never needed retorquing (I've tested it a couple times in addition to at 2 annuals).

Greg

That is my understanding also but in addition Catto is now bonding and aluminum plate to the aft side of the "hub", I believe he has been doing this for some time. But check with Catto.

I've heard both yes it's needed and no it's not needed secondhand, so I may need to hear it from Craig himself.

The pic below shows two of my washers from a retorque today, right at the high humidity point of the year. The last time I retorqued was at the height of the dry season in July. The washer on the left shows the forward face, the washer on the right, the aft face. My Saber crush plate has a slight chamfer on the bolt holes and on the washers you can see where the washers took on that chamfered form. (The washers are standard cad-plated 7/16"; upon inspection, the bolts look just fine.)

When I last retorqued in the summer my washers looked just fine, but that's probably because the prop had dried out and shrunk since the previous January's retorque.

Based on today's discoveries, and to get a start on answering my own question, it appears that I definitely need to shift my retorques to around October and March and/or shift to retorquing 4 times per year.

 

[alpinelakespilot2000]

I checked mine at inspection time in December and all the bolts took 1/2 - 3/4 of a turn... We're in a semi desert...

That's interesting Ron. Summerland has pretty much the exact same climate as Ellensburg. Maybe you have a different generation/better sealed prop? Mine's 2009, and I can definitely see wood in the interior crankshaft hole, so I don't think it is sealed.

 

[roadrunner20]

I've been flying my 3 blade vintage 2005 Catto prop since 2006. I usually check it a couple times a year. It's always maintained it's proper torque tolerance.
I live in Florida.

 

[jjconstant]

We had Marc Zeitlin do a talk at Livermore Chapter 663 and he is now a big believer in the belleville washer approach to wood prop installations.

http://www.vansairforce.com/community/showthread.php?t=13712&highlight=belleville+washers+props

 

[SHIPCHIEF]

I have a 1980'ish 5 lamination Ted Hendrickson 68x68 on my T-18. It would be a worst case scenario for a prop (only 5 wood layers, urethane varnish, fiberglass over tips only) . I live in Western Washington, which is about the best climate for a wood prop if you don't abuse it in the rain.
I re-torque it during condition inspection, it takes little to nothing. I've had the prop off a few times, and no significant crushing was observed. The fit over the drive lugs and the bolts thru the wood are still very tight. It has a little over 400 hours in service. I think it has always been hangared.
I'm on my second CATTO, both seem to hold torque well, but I have not had them long enough to add anything to the conversation.

 

[wjb]

We had Marc Zeitlin do a talk at Livermore Chapter 663 and he is now a big believer in the belleville washer approach to wood prop installations.

http://www.vansairforce.com/community/showthread.php?t=13712&highlight=belleville+washers+props

+1 on this .. very interesting presentation. His prop departed the aircraft at 13,000 ft (with minimal damage to the plane). Safe, though very quiet landing. The plane was originally based on the east coast, then came to CA where the prop dried and shrunk just a bit. check it out.

http://www.cozybuilders.org/Prop_Bolt_Bellville_Washer/index.html

 

[DanH]

His prop departed the aircraft at 13,000 ft

A sanded carbon fiber spinner backplate clamped between the prop and an aluminum hub may have been a contributing factor.

In the case of a conventional wood prop, torque transmission is via friction. The drive lugs do nothing, unless the friction joint fails, and then they only extend the time before bolt breakage.

Torque capacity of the friction joint between a steel crankshaft flange and the varnished hub of a conventional wood prop is usually based on a static coefficient of friction around 0.6. That can be increased with a different surface coating, or in some cases, by increased temperature. Epoxy and urethane coatings, for example, usually show an increase in static CoF as temperature moves toward Tg.

In general, carbon and carbon composites exhibit significantly lower static CoF when clamped to metals (ballpark 0.1~0.25), plus variability is high due to several factors. In addition, there is a loss of friction capacity when the metal surface forms an oxide film. In that context, consider the galvanic behavior of carbon and aluminum in contact.

Moral of the story? Clamp force is very important, but it's only one factor. Make sure your flange and hub are oil and oxide free, and be aware that inserting random materials into the friction joint has the potential for adverse consequences.

Postscript; readers who run an aluminum sheet spinner backplate clamped between the engine flange and a wood prop hub (i.e. standard old-school Vans) will be comforted to know that the static Cof for aluminum on steel is about 0.6, and aluminum on an aluminum shaft extension is in excess of 1.0

 

[smokyray]

Torqued up!

I've always known that wood or wood-core props, like my Catto, need to be retorqued periodically to compensate for seasonal humidity changes. For parts of the country that are pretty humid year round, when (or if) this happens probably doesn't matter very much. However, for those of us in the dry part of the west, where the humidity fluctuations throughout the year are pretty significant, how often should this be done? Just twice yearly (once during the 'dry' season and once during the 'wet' season) or more often that that?

Based on the following monthly average humidity chart for where my plane is based, when would you recommend I do my periodic retorques?

FYI, the chart came from:
https://weatherspark.com/averages/30171/Ellensburg-Washington-United-States

I've personally witnessed two prop "loosening" failures, Van even experienced one back in the 90's on the prototype RV6. Having owned and flown 8 different props on my RV4 including a Sterba, Warnke, Sensy wood and 3 Cattos I found two things that were crucial. Bolt size and humidity. Living in FL humidity was prevalent to say the least. Using 7/16 prop bolts on my 0-320 and 1/2 on my 0-360 (RVX) helped immensely with holding Torque. I used 25 hours as a standard on my wood props, the Catto, 50 hours.

Hope that helps...
V/R
Smokey

 

[Rupester]

I've been flying my 3 blade vintage 2005 Catto prop since 2006. I usually check it a couple times a year. It's always maintained it's proper torque tolerance.

Mine was installed in summer of 2012, and my experience is the same as Dan's. Torque is fine whenever I check it.