Where is the prop?

[piperboy84]

So I am waiting for my DE to show up for my commercial checkride on Sunday and he arrives rather late, his excuse? He had a student taxi into position and hold and go to full throttle, upon reaching full RPM there is loud banging and the cowling gets all twisted, he gets out and notices there is only a six inch stub left of the prop on one side with no ground strike visible. A search of the area revealed nothing.

So here's a question for you engineering types, one side of the prop turning at 2500rpm flies off not touching the ground, what would be the distance it could travel?

[9 lives]

So that's what went over!

I'm not good enough with math to work out the numbers, but that is a highly alarming situation. Did the examiner stay what kind of prop/engine it had been? It's probably a wonder that the engine is still in the mounts!

[piperboy84]

The way he described it it sounds like the whole front end/firewall is twisted too hell, the cowling got totally bent, he had a photo of the stub, it was interesting to see there were different surfaces across the breaking point, one part smooth the other pitted.

[mad_jock]

Depends how heavy it is and what angle its come off at. The force required to hold it in the hub is mass x radius x 4 x PI(SQ) that lot divided by the time for one revolution squared. I wouldn't be surprised if it was over 500m away but would be over Km. You have to use a bit of calculus to work out the equivalent radius and mass of a point mass spinning round the centre of rotation.

Once had a screw up doing what I thought was an inappropriately large volume of pressure test on a live test pressure pipe system to be rated and proofed at 50bar for the oil industry.

Normally we would fill them full of water so keep the volume of air down so if it did go bang things wouldn't move much.

But oh no this one had to be full of gas at extremely high pressure.

Did it at the weekend so nobody was in the building apart from us and some radiographers.

I had arranged everything so valves plus attachments were pointed towards the outside of the building pointing slightly up.

Nested behind 4 thick of rail sleepers watching the pressure increase we were just getting to the soak pressure ready to shut the supply off to see if there was any leaks, strain gauges were looking good and about to give a sigh of relief when the was an almighty bang.

A 175kg valve had blown off a 20" dia pipe with a 24" OD flange ripped the bolts right through the flange.

Never saw that again either and when I went outside across the bay all I could see was the backside of a supply boat going into Aberdeen harbour in the general direction of where it had been pointing. It was a worrying couple of hours listening to Northsound for reports of a supply boat sunk in front of Torry battery.

[BackPacker]

Back of the envelope, worst case scenario math here.

I'm assuming that it's just the very tip of the prop that separated (as that's rotating the fastest), and I'm not incorporating any aerodynamic drag or other effect here (if that prop bit becomes a glider-like object, then it can travel for tens or hundreds of miles).

I'm also assuming that the prop tip at redline RPM is near-supersonic speed (300 m/s), but in reality it will be less, perhaps just 90% of that number. And if a significant bit of the prop is broken off, like you said, the initial speed of that separated bit will be some sort of weighted average of the whole separated bit, not the speed of the tip.

Having said that, here's the math:

If the prop tip is separated and goes straight up, the altitude reached is v^2/2g = 90000/20 = 4.5 km. But obviously you'd eventually find the prop tip back, right next to the plane. The time it takes to fly up and fall back down is 2 * ( v/g ) = 2 * 300 / 10 = 60 seconds.

If the prop tip separates on a 45 degree angle (more or less the optimal angle to get maximum distance) the initial vertical speed is 300 / sqrt(2) = 212 m/s. The altitude reached is 2.25 km and the time it takes to fall back to the earth is 42 seconds. In that 42 seconds the tip can travel a horizontal distance of 8.9 km. (Again, I'm not considering a deceleration due to aerodynamic drag here.)

So that 8.9km is your absolute upper limit. But in reality it will be far, far less than that. A wooden prop tip will incur significant drag, especially when traveling at near-supersonic speed and while tumbling. How to calculate that is beyond my ability, unfortunately. Still, I would not be surprised if you eventually find the prop tip maybe half a kilometer away.

[piperboy84]

It was a C172 , metal prop

[Jetblu]

197.54 metres (dependable upon wind and other factors)





......and it wouldn't float. :-)

[worrab]

Probably not too far. Early guns fired projectiles at near supersonic speeds but with no rifling in the barrel, the projectile just tumbled and air resistance brought the thing down to earth in short measure. It wasn't until rifling was introduced that distances increased. Having said that, even if it only flew 500m I'm sure you'll struggle to find it.

I'm slightly surprised that the engine stayed mounted. The out-of-balance forces will have been high and the (few) designs I've seen don't look to have a superabundance of lateral strength.

[abgd]

I've had model helicopter blades come off at slower tip speeds of around 200mph. Thankfully they don't go more than a few tends of yards. They presumably tumble and slow down quite quickly.

Wind turbines that overspeed can fling their blades 2-500 metres.

It's not going to be an easy problem to solve analytically. When you find the debris, let us know the answer.

[9 lives]

A 172?!? May we please have a link to a formal report of this when it's available? This sounds critically serious, I'd like to know the details. There's got to be something to learn from this!

[piperboy84]

There is a bit of a political dimension to the story, I am not sure how the broken aircraft has been handled, as i understand it, it has just been towed to an on-field repair station, however I assume when the insurance gets involved there will also be some type of governmental agency report. In the meantime there is a lot of controversy surrounding the continued use of the facility as an airfield and a ballot measure scheduled for vote soon to determine the fields future that is generating huge amounts of press locally.

It probably would not help matters if some neighbor staggers out their garden with a Sensenich sticking out of his head. The best they can hope for is it is never found, or at least not till the ballot is over and perhaps during the forthcoming rainy season a roof inspection on one of the neighboring houses will reveal a prop impaled on the roof as the leak source.

As far are maintenance goes, the current owner is a friend of mine of 20 years and used to leaseback my aircraft, he is a totally no expense spared guy when it comes to maintenance, and the shop who does the maintenance is very highly regarded, the previous owner of the plane (a big shot movie star with plenty money to spend on maintenance) had the plane for quite some time and used the same repair shop as the current owner so I wouldn't think that is the issue. The owner thinks the cause is probably a manufacturing defect.

Will keep you updated.

[27/09]

The owner thinks the cause is probably a manufacturing defect.

I'd say very unlikely.

How old was the prop? When did it last have an overhaul/calendar inspection?

I'd say more likely caused by an un reported prop strike or corrosion.

I really hope they find the true cause.

Some on here will remember the Lycoming crankshaft issue. There was a crankshaft which broke near the prop flange in the UK. The cause was determined to be due to internal corrosion in the hollow part of the crankshaft. Some time later it was discovered that that aircraft had had a prop strike which was the real reason for the crankshaft breaking. However not before many crankshafts were unnecessarily replaced at great expense.

[nonsense]

...he had a photo of the stub, it was interesting to see there were different surfaces across the breaking point, one part smooth the other pitted.

It was a C172 , metal prop.

Fatigue. The smooth part is where the fatigue crack progressed quietly on its way, the pitted part is where what was left was no longer strong enough and was torn apart.

If you search for images of "fatigue failure" on google (https://www.google.com.au/search?q=fatigue+failure&tbm=isch) you'll see many examples of similar failures:

http://practicalmaintenance.net/wp-content/uploads/Surface-of-a-Fatigue-Fracture.jpg (http://practicalmaintenance.net/?p=989)

http://www.corrosionlab.com/Failure-Analysis-Studies/Failure-Analysis-Images/29097.mechanical-fatigue.stainless-steel-tube/fracture%20surface.jpg (http://www.corrosionlab.com/Failure-Analysis-Studies/29097.mechanical-fatigue.ss-shaft.htm)

[mad_jock]

A material issue is a possibility. The occlusion then leads to a stress raiser which then leads to fatigue.

It will be relatively simple to spot as you have the remaining fracture surface.

As a matter of interest any chance you could get a picture of the surface?

[N707ZS]

Good job it was on the ground then, this must be a serious incident to be investigated as there must be plenty more of these propellers flying around the world.

[FullWings]

there must be plenty more of these propellers flying around the world.
I thought we’d already worked out that it couldn’t make orbit? ;)

[172driver]

Piperboy, it sounds we are flying out of the same airfield - mind sending me a PM with details of the a/c involved?

[9 lives]

there must be plenty more of these propellers flying around the world.

I thought we’d already worked out that it couldn’t make orbit? ;)

I spit OJ reading that!

[dsc810]

Once heard of an industrial generation type gas turbine which lost a turbine disc...somewhere in the world - this is going back decades now. (I was in the business)

So I was told it exited out horizontally from the engine/casing, out though the canopy/noise protection. continued through the site and through the site fence, crossed a road, in though the front brick wall of a house, 'climbed' the back wall internally, through the ceiling and first floor, up through that ceiling into the loft continued out though the rafters/tiles back into the air - now going mostly vertically and finally ran out of energy a bit later.

[mad_jock]

And then there was the incident in the IOM where someone put jet A into the water meth booster on a turboprop.

There was bits hitting the terminal 2km away when it kicked in and both engines exploded.

[BackPacker]

Oh, we're into strong stories now, eh?

In Northern Iceland, in the Krafla region, a magma chamber sits relatively close to the surface. This boils the ground water, and since that water has no way to escape, the pressure builds up. The Icelandic people drill holes in the ground and tap into these high pressure steam chambers to run turbines to produce electricity.

One such drill action went spectacularly wrong, and led to the steam chamber exploding. Bits of the drill were apparently found three kilometers away. Fortunately nobody was hurt.

Krafla - Lonely Planet (http://www.lonelyplanet.com/iceland/the-north/myvatn/sights/natural-landmarks/krafla)

[mad_jock]

I had a mate on a drilling rig that the driller made a slight cockup with his mud weights.

This led to a 800m string of drill pipe getting blown out the drill hole and the BOP didn't fire. It went straight up through the Derrek.

Then another one that was stall testing a tunnelling unit gear box when the wrong end of it became untethered and the whole bloody engine started rotating. He only managed to stop it by taking an axe to the fuel line which was of fire hose diameter.

To be honest I think all of us survived our 20's more by luck than skill or common sense.

[Pirke]

A diving buddy witnessed what happens when you drop a filled air tank (230 bar) and the fall breaks off the valve on the end. The whole air tank went straight thru a concrete wall 1 meter thick and ended up sticking in the next wall on the other side of the room.

And we strap these things to our back willingly :}

[172driver]

To be honest I think all of us survived our 20's more by luck than skill or common sense.

Truer words have rarely been spoken!

[Crash one]

Oxygen tanks on a trolley , greasy fingers, hangar deck Ark Royal all aircraft fully loaded ready for flight next morning. Total mayhem, then some pillock lowered the fire curtain between us fire crew & the fire.
I'd like to have seen 800m of drill going through the Derrick though!

[piperboy84]

I have a pic of the prop stump, anyone know how i can post it on here?

[mad_jock]

you have to find somewhere that will host it and then get the link out of it.

Either that or upload it into dropbox and post the link of that.

It won't be viewable with dropbox but we will get it full sized without it screwing with the forum.

[piperboy84]

https://onedrive.live.com/redir?resid=1EDF911B882D2AC5!531&authkey=!AMTjzx08F25yq7A&v=3&ithint=photo%2cJPG

https://onedrive.live.com/redir?resid=1EDF911B882D2AC5!531&authkey=!AMTjzx08F25yq7A&v=3&ithint=photo%2cJPG

Let me know if you can see this

or maybe this link below
https://onedrive.live.com/redir?resid=1EDF911B882D2AC5!531&authkey=!AMTjzx08F25yq7A&v=3&ithint=photo%2cJPG

https://onedrive.live.com/redir?resid=1EDF911B882D2AC5!531&authkey=!AMTjzx08F25yq7A&v=3&ithint=photo%2cJPG

[mad_jock]

that site doesn't support sharing of photo's.

BUt I got to see it from the bad link properties.

For the engineers among us can you try and get a photo of the flat fracture face please like the picture by Nonsense post as an example.

Does look like a fatigue fracture followed by fast fracture.

[piperboy84]

It still has not been found so it must have traveled outside the field boundary as the area directly to the left and right of the planes position when it came of his heavily trafficked.

[mad_jock]

I would go and have a look in Venice reservoir site.

Some one would have found it by now if it had gone into housing.

But if it speared in it may well be under ground.

[funfly]

What are the chances of oscillation on a prop blade?

[mad_jock]

loads which is why commercial operators get them dynamically balanced every so often.

And when something changes you know about it as a pilot you can feel it through your backside and your feet on the rudder pedals.

[nonsense]

https://oijb2g.by3302.livefilestore.com/y2mu6Y8BrcDvET6W1p_LDav34YSGY40kSVjiVN38YnKT1D7CleUB_6BeyOxo 6lHSOcHzo0xsld69DbGZDzxBvmN0OJ7h9H9azklJzvOhCDlHWq0vP0-zorHR0ZzHwqWVmB6/IMG_1201.JPG

Hopefully you can all see the image above.

[mad_jock]

yes but its utterly useless.

We need to see the fracture face perpendicular.

it looks very spectacular which it is but we can see sod all apart from the fast fracture failure and nothing about the fatigue surface.

[Mach Jump]

:eek: How far out from the spinner was the fracture?


MJ:ok:

[N707ZS]

A shot of the whole blade section would be more interesting, those black marks on the near top left hand side might be the start of something.

[Mach Jump]

As Jock says, it's hard to determine the cause for sure from the pic, but it looks like it may, unusually, have started from the trailing edge of the blade, and have been started off by some damage to the rear face of the blade, near the trailing edge.

We all check the leading edge of blades for chips, but perhaps we don't pay as much attention to the rear face of the blade as we should?


MJ:ok:

[phiggsbroadband]

.
Sherlock Holmes here.... Look at the shadow on the cowling. It looks as if the break was 6-8 inches from the hub root.


I did a Parabolic calculation for a perfect projectile, and the range was about 6.3 km in 36 seconds (starting at 250m/s at 45deg.). However this blade would have been spinning and tumbling, so the variable aerodynamic forces would severely reduce the range. If during one of its tumbles, it went leading edge first, then the lift created by its 'wing section' would dramatically change the direction of the trajectory.


Use the Peak Height, Range and Time of Flight Calculator at....
Trajectories (http://hyperphysics.phy-astr.gsu.edu/hbase/traj.html)


.