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kboatright1(at)comcast.ne
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Hopperdhh(at)aol.com
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 Posted: Sun Dec 17, 2006 6:53 am Post subject: Wood Prop Loss - Cozy |
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In a message dated 12/17/2006 8:40:46 AM Eastern Standard Time, kboatright1(at)comcast.net writes:
| Quote: | For those of us who fly behind wood props, this is a good read. It may give some indication of the symptoms of a wood prop beginning to fail. And then failing... 
http://www.cozybuilders.org/Desert_Center/
Kyle Boatright
2001 RV-6
Aymar Demuth Prop
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Great story and pictures, Kyle. Marc did a great job. Things could have been a lot worse.
do not archive
Dan Hopper
RV-7A
[quote][b]
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ceengland(at)bellsouth.ne
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| Posted: Sun Dec 17, 2006 5:32 pm Post subject: Wood Prop Loss - Cozy |
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Hopperdhh(at)aol.com wrote:
| Quote: | In a message dated 12/17/2006 8:40:46 AM Eastern Standard Time,
kboatright1(at)comcast.net writes:
For those of us who fly behind wood props, this is a good
read. It may give some indication of the symptoms of a wood prop
beginning to fail. And then failing...
http://www.cozybuilders.org/Desert_Center/
Kyle Boatright
2001 RV-6
Aymar Demuth Prop
Great story and pictures, Kyle. Marc did a great job. Things could
have been a lot worse.
do not archive
Dan Hopper
RV-7A
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Haven't read the whole thing yet, but there's a glaring error in his
analysis:
' My theory is that there was a crack in a blade which was growing,
bringing the resonant frequency of the prop in and out of the RPM
operating range as it got bigger and the vibration mode frequencies
changed. One blade leaves, then 10 seconds later the hub gives up the
ghost.'
If he had lost a blade, it wouldn't have been 10 seconds; more like 1
second. And it wouldn't have been just the prop that departed.
Less than an ounce off a tip will feel like the airframe is coming
apart. An entire blade will likely cause the engine to leave with the prop.
Charlie
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pitts_pilot(at)bellsouth.
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| Posted: Mon Dec 18, 2006 3:43 am Post subject: Wood Prop Loss - Cozy |
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Agreed! I'm no expert and wasn't going to comment, but here goes.
The black scorch marks on the engine prop hub is indicative of prop
bolts that weren't torqued to spec. This can occur when a wood prop
changes climates ...... a more moist climate will cause the wood to
swell, and a drier climate will cause the wood to shrink. On a new
prop, it my take quite a while for the changes to take place due to the
really good varnish job it gets. So, it's scenario time: Let's say the
prop was made in a moist climate expressly for an experimental aircraft.
Props made for certificated aircraft have strict moisture content specs
and may/may not be comparable to our scenario. The new prop is shipped
to the desert SW, is installed and torqued to spec. The airplane is
flown and the varnish is compromised through rock nicks and maybe some
rain. Now the prop can dry out a little, shrink, and the bolt torque is
now out of spec on the loose side. This will allow the prop hub to flex
and rub on the prop flange and the pressure plate. This creates heat,
which further dries out the hub, making it looser still. The
heating/cooling cycles now will accelerate the damage until the prop
bolts start to flex ..... and eventually they will fatigue and fail.
The end (and predictable) result is the glider ride. For a new prop,
it's prudent to check the bolt torque often.
Linn
do not archive
Charlie England wrote:
| Quote: |
Hopperdhh(at)aol.com wrote:
> In a message dated 12/17/2006 8:40:46 AM Eastern Standard Time,
> kboatright1(at)comcast.net writes:
>
> For those of us who fly behind wood props, this is a good
> read. It may give some indication of the symptoms of a wood prop
> beginning to fail. And then failing...
> http://www.cozybuilders.org/Desert_Center/
> Kyle Boatright
> 2001 RV-6
> Aymar Demuth Prop
>
> Great story and pictures, Kyle. Marc did a great job. Things could
> have been a lot worse.
>
> do not archive
>
> Dan Hopper
> RV-7A
Haven't read the whole thing yet, but there's a glaring error in his
analysis:
' My theory is that there was a crack in a blade which was growing,
bringing the resonant frequency of the prop in and out of the RPM
operating range as it got bigger and the vibration mode frequencies
changed. One blade leaves, then 10 seconds later the hub gives up the
ghost.'
If he had lost a blade, it wouldn't have been 10 seconds; more like 1
second. And it wouldn't have been just the prop that departed.
Less than an ounce off a tip will feel like the airframe is coming
apart. An entire blade will likely cause the engine to leave with the
prop.
Charlie
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aerobubba(at)earthlink.ne
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| Posted: Mon Dec 18, 2006 5:36 am Post subject: Wood Prop Loss - Cozy |
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| Quote: | Haven't read the whole thing yet, but there's a glaring error in his
analysis:
' My theory is that there was a crack in a blade which was growing,
bringing the resonant frequency of the prop in and out of the RPM
operating range as it got bigger and the vibration mode frequencies
changed. One blade leaves, then 10 seconds later the hub gives up the
ghost.'
If he had lost a blade, it wouldn't have been 10 seconds; more like 1
second. And it wouldn't have been just the prop that departed.
|
| Quote: | Less than an ounce off a tip will feel like the airframe is coming
apart. An entire blade will likely cause the engine to leave with the prop.
Charlie
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I've got to agree with you, Charlie (not that you asked ; - ) )
| Quote: | From my perspective there is a pretty clear chain of events here. I won't
bore the list with my ramblings, but the key cause and effect are that
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something (such as the victim's move from coastal MA to desert CA) caused
dimensional shrinkage of the prop, which loosened it, and led to torsional
vibration. This is borne out by the photos of the charring on the face of
the prop extension.
The reason I comment on the whole affair is that 'back in the day', when I
was flying Cubs and Champs, a fellow I think knew Wilbur and Orville
advised me to sniff the prop / crank interface when preflighting a wooden
prop. If the prop were loose, as can happen from season to season with
humidity change, you could smell the charring of the varnish.
Perhaps this preflight technique will help another avoid the same fate.
glen matejcek
aerobubba(at)earthlink.net
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cjensen(at)dts9000.com
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| Posted: Mon Dec 18, 2006 6:12 am Post subject: Wood Prop Loss - Cozy |
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Thanks,
Chuck Jensen
Diversified Technologies
2680 Westcott Blvd
Knoxville, TN 37931
Phn: 865-539-9000 x25
Cell: 865-406-9001
Fax: 865-539-9001
cjensen(at)dts9000.com
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hawk(at)digisys.net
Guest
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| Posted: Mon Dec 18, 2006 8:29 am Post subject: Wood Prop Loss - Cozy |
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If the prop bolts were "bottomed out" in the drive bushing threads and
actually torqued on the bolt shoulders, the resulting failure will look like
the instance described here. Examination of the remnants of the threaded
portions still retained in the extension should reveal if this might have
been the case.
Hawkeye Hughes
Skyote and RV-3
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