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United States, MN, Minneapolis
Joined Feb 2011
48 Posts
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Question
Cause of sheared prop bolts
Yesterday while towing a glider with DA-150 powered PG Cmelak, my prob let go at 100 feet just as I cleared some trees...I could see the propeller look like a boomerang, the tow plane nosed up with the glider still attached, I pointed down and to the right to avoid the trees while the glider released but I stalled and spun and all 44 pounds came down hard as well as shredded itself through a barb wire fence. prop was a new menz 30x12 put on one month ago, bolts retightend a week later. I use a washer and split ring lock washer under all 6 bolts. All six bolts sheared right at the hub. The prop was found in the field undamaged and still had 3 of the 6 bolts and prop washer on it.
I have read to not over tighten prop bolts on wooden props for fear of crushing the wood. Did I fail to do "one more tightening" ? or was this a hic up with the engine ?? the Engine was overhauled at DA two years ago and has been running flawlessly. The plane is totally destroyed but I want to learn what went wrong. thanks. kevin in Minneapolis |
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United States, MN, Minneapolis
Joined Feb 2011
48 Posts
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forgot photo..
RIP Cmelak.
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Temperature, humidity, and time all can cause your wooden prop to shrink and swell.. technically it is recommended to check before each flying session. I have an inch pound torque driver I use to check them with so I am not cranking them down tighter and tighter each time. Usually I see them loosen up for 3-4 months then stop. I also use silicon sealer on the threads. Gums them up to not back out but you can get them apart..
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On my props I use no washers of any type other than the prop washer. I check torque the prop bolts every 5-7 flights and or the night before before a weekends worth of flying or,,, post/pre flight inspect the plane during the week before the weekend is what I do.
Prop bolts will always need attention. |
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If it sheared the bolts, the bolts were loose. As mentioned above the wood props tend to need the most attention. And it needs to be torqued from the very beginning. DA has a torque spec for wood props, I suggest to get a torque wrench (harbor freight works fine) and torque that prop.
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Man that sucks, that's a nice plane too
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Loose bolts. Seen it 100 times. On the ground, in the air.
Wood props only get torqued to about 40-45 pound inches, carbon gets torqued to 90-95 pound inches. Over torquing a wood prop will cause the prop disc to embed into the prop. Then you'll never get a good torque. |
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United States, AZ, Tucson
Joined Sep 2009
1,205 Posts
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I have lost several wood props over the years, but when I switched to carbon fiber...never again
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rhome , texas
Joined May 2008
683 Posts
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Bolts stretch everytime they are tightened and will eventually fail also
If you don’t think so get a 5/16 bolt place it in something place a nut on it and torque it to spec and remove it about ten times and look at the threads and the distortion We change props and it is easy to lose count how many times those bolts are cycled |
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Ouch....Sorry for your loss !!
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United States, MD, Parkton
Joined Jun 2008
1,508 Posts
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Let's add carbon fiber spinner back plates to the mix. They can soften up from heat.
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Quote:
CB |
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