loose wood prop

i live in michigan but spent last three weeks in the hot dry west when i got home my wood prop was loose. I turned the bolts aproximatly 1/2 a turn to get a tourqe of 22 foot pounds. i have heard storys of the wood getting hot if too loose do i need to do more then recheck bolts and keep flying? I checked the bolts before trip so they got loose in the last 20 hours thanks

bill v said:

i live in michigan but spent last three weeks in the hot dry west when i got home my wood prop was loose. I turned the bolts aproximatly 1/2 a turn to get a tourqe of 22 foot pounds. i have heard storys of the wood getting hot if too loose do i need to do more then recheck bolts and keep flying? I checked the bolts before trip so they got loose in the last 20 hours thanks

Click to expand...

Wood props are a living tree. The prop is sensitive to changes in temperature and humidity and bolt torque requires frequent checking. A good technique in re-checking bolt torque is to back the bolt a turn or two off, then reset the final torque value. This will prevent the inadvertant over torquing of the bolts.

If the prop was loose, you might have damaged the bolts or ferules. It's the clamping force that keeps the prop from moving, not the bolts. When the clamping force goes away, it will rapidly destroy the bolts. For example:
http://cozybuilders.org/Desert_Center/Emergency_Landing.html

Also be aware that if the bolt threads bottom in the flange, you will get a false torque reading. I'd check with the prop manufacturer for the recommended torque, there's a lot of variation depending on materials and number of laminates.

Kent is dead right. We don't know how loose your prop might have been, but you should remove it for inspection. A burned friction face or any freeplay in the bushing counterbores means a new prop or skilled repairs. If the counterbores have freeplay, you also need new bolts.

Henry Rose, at the time Chief Engineer at Sensenich, published the calculations for wood prop hub torque capacity as a SA article many years ago. Here's a clip from that article:



As you can see, the drive bushings are only minimal insurance in the event friction fails due to loss of clamp pressure. When they fail due to wood crushing, the bolts go very quickly due to bending.

I'm looking to learn...

I have heard of folks with wood props using two Belleville washers per bolt with longer bolts to get the right grip length. I think this is an attempt to maintain torque or smooth the variations due to temp or humidity. Anybody heard of this? Does it work?

I don't have a horse in this race...I have a WW 200RV

belleville washers

The late Paul Lipps wrote about belleville washers in a Contact Magazine article. Don't know if any of these back issues are still available.

Marc Zeitlin, Cozy builder & flyer and employee at Scaled Composites, wrote extensively about the Belleville washer method for securing wood props after his near-accident a few years ago. I chatted briefly with Craig Catto about this some months ago when I was torquing my 3-blade. ( It seemed like Craig and Marc Zeitlin had discussed the securing of Catto props and the Belleville "thing".) Craig indicated to me that the only incidents of a Catto prop with a tightness problem occurred on composite aircraft that had composite spinner plates rather than the metal ones the rest of us use. He said he knew of no instances where a Catto prop with a conventional spinner and backplate had come loose if installed, torqued, and safety wired properly. At least that's how I recall our discussion. I have huge respect for both Craig and Zeitlin, and if I represented this incorrectly I would hope either would straighten me out.

Catto

I have heard that Catto uses a metal plate bonded to the prop hub on at least some props. My knowledge of this is limited to one person who tested a couple of props for Catto several years ago.

Prop me up!

bill v said:

i live in michigan but spent last three weeks in the hot dry west when i got home my wood prop was loose. I turned the bolts aproximatly 1/2 a turn to get a tourqe of 22 foot pounds. i have heard storys of the wood getting hot if too loose do i need to do more then recheck bolts and keep flying? I checked the bolts before trip so they got loose in the last 20 hours thanks

Click to expand...

Bill,
Having put 1000 hours on 3 wood props I can relate. First thing you should do is remove your prop and inspect the bolts, prop face and crush plate, as mentioned above. All manufacturers have their own torque specified. Use it.

The biggest factor I found in bolts loosening is bolt size. You didn't specify which engine or bolt size you have. If it's 3/8" I recommend switching out the lugs and installing 7/16" bolts for the 0-320, far superior to holding torque between oil changes. You can order the bolts from Saber MFG.

V/R
Smokey

PS: If your prop ends up being unserviceable and headed for your mantle, consider switching to a Catto Composite, I tested Craig's first RV prop on my RV4 and yes it does have a metal plate on the back. Far superior to handling weather changes and torque.

Catto

I know Catto has in the past modified other manufacturers props for the metal plate on the aft side of the hub. With his backlog he may not do that anymore.