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Don't forget to re-torque

Torque

Many wood prop torque issues are a result of moving from wet to extremely dry climates.
In AZ my prop torque has not changed in two years. I do check it regularly.
An under torqued prop that is about to depart is sometimes preceded by a smell of burning wood.
 
I don't get it ?? What's the failure mode?

Great job of SA and getting the airplane down to a convenient airport. Very calm for a truly "Oh Sh##" experience.

But now I'm curious about the how and why! I can see some wobble, wear, out of balance and track kind of bad stuff from not torqueing a wood prop annually but how does that cause a prop to "depart" the aircraft. Prop bolts that are safety wired should not back out ... ? The prop isn't going to shrink enough to actually have gaps between the prop plate and the prop .... or is it?

The assumption is that it didn't just shed the blades and the hub remained. Assuming the prop didn't just shred itself, how would it just "come off" ?? All of the bolts would have to back out which implies they had enough "counter force" (is that a technical term) to shear the safety wire(s) at the same time and then back out all at the same time so that the prop doesn't wobble enough to give you some indication that something is wrong. If only one bolt and the prop plate were left, it certainly would not have just idled very well with a swinging weight on the drive shaft. Wish I could see the tach on the dash... and the engine/airframe after they stopped moving.

I just don't know .... ? Anybody else ever actually had all the prop bolts back out and the prop just take a hike ?? What's the failure mode?




Bill
 
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Great job of SA and getting the airplane down to a convenient airport. Very calm for a truly "Oh Sh##" experience.
But now I'm curious about the how and why! I can see some wobble, wear, out of balance and track kind of bad stuff from not torqueing a wood prop annually but how does that cause a prop to "depart" the aircraft. Prop bolts that are safety wired should not back out ... ? The prop isn't going to shrink enough to actually have gaps between the prop plate and the prop .... or is it?
The assumption is that it didn't just shed the blades and the hub remained. Assuming the prop didn't just shred itself, how would it just "come off" ?? All of the bolts would have to back out which implies they had enough "counter force" (is that a technical term) to shear the safety wire(s) at the same time and then back out all at the same time so that the prop doesn't wobble enough to give you some indication that something is wrong. If only one bolt and the prop plate were left, it certainly would not have just idled very well with a swinging weight on the drive shaft. Wish I could see the tach on the dash... and the engine/airframe after they stopped moving.
I just don't know .... ? Anybody else ever actually had all the prop bolts back out and the prop just take a hike ?? What's the failure mode?
Bill

This can be cause by the prop bolts being too tight. If they were torqued in a very dry climate/season. Going to a very humid climate/season can cause the prop to swell to the point of breaking the bolts. I've seen prop bolts stretched to the point of breaking. I did a program on this very subject several years ago at our EAA Chapter. Less than a week later a Chapter member showed me his "stretched" bolts and thanked me for the program.
 
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I was at an airshow as a kid (many many years ago) when a friend of the family was doing an aerobatic performance (I think it was a Citabria, but that may not be correct). Down the runway inverted, pull up (or, I guess, push up :)) and roll wings upright and...whooosh!...the entire prop departed company with the plane. The pilot, an ex-fighter jock, managed his energy well and cranked her over and back around to the runway for a safe landing. An outstanding bit of airmanship.

Prop flew what to my young eyes looked like several hundred feet up, and landed in the middle of the field (fortunately, away from the crowd).

I was way too young to have the presence of mind later to ask him why it came off. But if he wasn't a hero in my youthful days of loving flight before, he was after that! :)

(ETA: I seem to recall quite some time after that that someone said the flange sheared off...but it was so long ago, I wouldn't swear to that).
 
Oh the adventures of ultra light flying.

2 strokes do the darnest things

looks like to me the prop and hub departed.
 
I know of an RV where the safety wire on the (constant speed) prop bolts broke and were slowly backing off... fortunately this was discovered before the prop departed the airframe, but IIRC the prop was scrap...
 
Great job of SA and getting the airplane down to a convenient airport. Very calm for a truly "Oh Sh##" experience.

But now I'm curious about the how and why! I can see some wobble, wear, out of balance and track kind of bad stuff from not torqueing a wood prop annually but how does that cause a prop to "depart" the aircraft. Prop bolts that are safety wired should not back out ... ? The prop isn't going to shrink enough to actually have gaps between the prop plate and the prop .... or is it?

The assumption is that it didn't just shed the blades and the hub remained. Assuming the prop didn't just shred itself, how would it just "come off" ?? All of the bolts would have to back out which implies they had enough "counter force" (is that a technical term) to shear the safety wire(s) at the same time and then back out all at the same time so that the prop doesn't wobble enough to give you some indication that something is wrong. If only one bolt and the prop plate were left, it certainly would not have just idled very well with a swinging weight on the drive shaft. Wish I could see the tach on the dash... and the engine/airframe after they stopped moving.

I just don't know .... ? Anybody else ever actually had all the prop bolts back out and the prop just take a hike ?? What's the failure mode?




Bill

The prop to hub assembly is dependent on the clamping force entirely to prevent failure. The bolts and lugs will not survive the shear forces without it.
A lot of us in the EZ community are moving to a system of Belleville washers, reinvented from long ago by Marc Zeitlin after he lost a prop near Twentynine Palms...after moving there from the East coast.
I understand Craig Catto now recommends these be used on his props.
Interesting reading here:
http://www.cozybuilders.org/Prop_Bolt_Bellville_Washer/2015_06_21-Belleville_Installation_Rev-N.pdf
And here:
http://www.cozybuilders.org/Oshkosh_Presentations/2007_Prop_Loss_Desert_Center.pdf
Tim Andres
 
I was at an airshow as a kid (many many years ago) when a friend of the family was doing an aerobatic performance (I think it was a Citabria, but that may not be correct). Down the runway inverted, pull up (or, I guess, push up :)) and roll wings upright and...whooosh!...the entire prop departed company with the plane. The pilot, an ex-fighter jock, managed his energy well and cranked her over and back around to the runway for a safe landing. An outstanding bit of airmanship.

Prop flew what to my young eyes looked like several hundred feet up, and landed in the middle of the field (fortunately, away from the crowd).

I was way too young to have the presence of mind later to ask him why it came off. But if he wasn't a hero in my youthful days of loving flight before, he was after that! :)

(ETA: I seem to recall quite some time after that that someone said the flange sheared off...but it was so long ago, I wouldn't swear to that).
reminded me of this prop departure vid...
https://www.youtube.com/watch?v=zl5TIR1_ciQ
 
Re; How do the bolts come off

The bolts fail due to fatigue, the fatigue life characteristics change between full load and partial load with cyclic loading. You see this on engine heads and wheel lugs. Ask me how I know about wheel lugs. :eek:
 
Post #18

That was Kirk Wicker, Cap 21, College Park Airport, sometime prior to 9/11. Kirk's ex wife was the Stearman wingwalker killed at Dayton Airshow a few years ago.
 
Holy cow! Bet they had to use a crow bar to get the seat cushion off his backside! Well done in a pretty bad situation. Wonder where the prop ended up, in some trees/forest or someone's house/yard/car?
 
I just don't know .... ? Anybody else ever actually had all the prop bolts back out and the prop just take a hike ?? What's the failure mode?

Bill

Bill/Others, I did read (or got a PM) about a saber extension that came loose and the prop departed on a 7. So it can happen the same way.

On this one, the comments said he missed a couple of retorques, and said he wished he had used the larger 8mm bolts. So the bolts were pretty small.

Failure progression - weak bolt tension, loosening (sliding) prop - the engine torque is transmitted by the shear force on the bolts alone, but since that load alternates (torsionally) it just fatigues and breaks the bolts - - -until it rips off the last one. Zing - it is gone.

At least the glide ratio was improved. Good to see a safe outcome, but the stare at the end is priceless.
 
Where is that thread the showed using some type of waved washer to accommodate TQ with the expanding contracting of the wood and composite props?
 
washers

I have heard that Catto is now using the special washers. I think the late Paul Lipps was one of the first to use them. Belleville washers???
 
Belleville washers are a good solution of holding nearly constant force in a variable distance (thickness). Much better than a bolt with low torque/tension/strain. A stiff plate is needed to lower contact stress (distribute the load) on the composite or wood prop.

I am not sure if a design exists to eliminate the retorque requirement, but it would extend the interval.
 
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