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Author Topic: Grand Rapids trip (2-2-V-1)  (Read 147845 times)

Walter Runck

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Re: Grand Rapids trip
« Reply #60 on: January 28, 2014, 07:31:49 PM »

- for an aircraft "skin" to have flush mounted rivets, that skin would need to be dimpled to begin with wouldn't it? See the above link re: rivets.
   

Kind of.  To have the head of the rivet end up flush with the skin, you have to use a countersunk style of rivet and either drill or dimple the skin before you set the rivet.  There are some drawings here, but not a lot of detail on installation technique.  I'm not aware of any use of this style on the Electra.  It's more expensive and only makes sense if you're more interested in performance than cost.

.
For Walter - the underlying structural shape might have been in the shape of an "L" or "Z", with 90-degree bends, perhaps only 1/2 inch tall, although I've seen them more commonly about an inch tall. Using material twice as thick as the skin is about right for most stiffeners - the skin provides stiffness at a right angle to the stiffness of the structure, but the combination is still flexible across the structure (the 3rd angle),which would be consistent with a non-load-bearing piece that spanned a relatively narrow space.  The small diameter (3/32") also indicates non-load-bearing use.
The Van's Aircraft Site has lots of very nice photos and descriptions of modern riveting.  Note that they use lots of flush rivets and "pop" rivets, not the domed rivets used on the artifact.
Attaching the stiffeners to surrounding structure could have been done in a variety of ways.  I think Ric or others might be able to offer insight into the methods used in the 1930's.  Otherwise there are a variety of examples to be seen in the Van's aircraft photos.
Also for Tim - the "dimpling" I mentioned was described in the NTSB report:"The skin around these holes was, in general, dimpled inward toward the concave side of the sheet suggesting that the sheet had been area loaded from the concave side while the rivets and underlying structure were intact."
More later...

Thanks John, I get the sense that the FAA is implying that the dimpling came during whatever event caused them to separate from the sheet, rather than during installation.  I'll do a sketch to show what I mean when I get back to the office.


I just can't see how the rivets could all pop off without leaving signs of force to the holes, beyond mere dimpling, than observable (at least as I can see) on the item. Unless of course the rivets were made of something ridiculously soft.  Has anyone done any experiments on similar materials?   


It's the worst explanation for what we're looking at, with the exception of everything else.  There are rivets made of 1100 series aluminum that are much softer, but the FAA claims there is a dimple on the surviving rivet which should indicate a much stronger 2117 T4 material.  This could be settled by chemical or metallographic analysis if desired.
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Jon Romig

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Re: Grand Rapids trip
« Reply #61 on: January 28, 2014, 07:59:18 PM »

A link to aircraft rivets/fasteners and their use in repairs to aircraft structures and skin to maintain integrity and strength.

http://aviation.spenner.org/AircraftRivetsandSpecialFastners.pdf

I am interested in understanding what level of force would accomplish this.

The linked document suggests that the reference 3/32 rivets will fail in "sheer" at a force of 200-300 pounds. It is unlikely that these alloy rivets are the same strength as the "soft aluminum" rivets that may have been used in the 1930s, in fact they are very likely to be stronger.

But assuming 200 pounds to failure for each, simultaneous failure of all the rivets would have required 83 (my rough count of rivet holes) x 200 pounds = a force of 16,600 pounds, spread over about 437 square inches, or 38 PSI.

Bernoulli's equation: P = 0.5 * rho * v^2 where rho is density of water at 1000 kg/m^3 (metric units)

38 PSI = 261 kPa

Solving: V = 47 MPH

That is a pretty fast-moving wave, even assuming some hydraulic focussing, which would likely be minor considering the geometry of the fuselage.

It makes me wonder if the artifact popped loose in a crash into the sea, not in wave action. Or in the fuel explosion discussed in the report.

Could somebody please check my work?

Thanks!

Jon
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« Last Edit: January 28, 2014, 08:24:18 PM by Jon Romig »
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John Ousterhout

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Re: Grand Rapids trip
« Reply #62 on: January 28, 2014, 10:28:19 PM »

Jon - Bernoulli would apply if the pressure were a difference being created by high velocity flow on one side of the panel and none on the other.  I think the more likely event was impact(s) by waves, which would abruptly pressurize one side of the panel due to a change in momentum.

Start with a quantity of water moving at normal wave velocity encountering the panel - tons of mass x some velocity, that is forced to a halt or change direction in a short distance.  If there is no room for the mass of water to splash sideways then it acts like a hammer, and more of the mass is brought to a halt which increases the hydraulic pressure on the panel momentarily.

A quick google search didn't provide me with more useful calculations, and I'm 600 miles from my reference books at present, so I hope this arm-waving explanation helps.
Cheers,
JohnO
 
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Walter Runck

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Re: Grand Rapids trip
« Reply #63 on: January 28, 2014, 11:30:53 PM »

With apologies to Jon and Cool Hand Luke, what we have here is failure to orientate.  We're dealing with tensile loads, not shear.

Imagine a rivet looking kind of like a muffin with a top and a stump

The published numbers for shear strength are based on loading a rivet in the manner they were intended to be loaded - sideways.  It's the amount of force necessary to knock the muffin top off of the stump by pulling sideways on the top while holding the stump in your hand.

The type of failure we're suspecting here is tensile rather than shear.  Pulling the muffin top off of the stump by prying it up out of the baking pan after we forgot to put the little paper thing in to keep the batter from baking itself into the pan.  Rivets as used in stressed skin construction aren't intended to carry a lot of load in this direction.  If you know something is going to be carrying a longitudinal load like a pulley in a control cable, you don't attach the pulley to the bulkhead with rivets, you use machine screws (bolts).

Other than that, Jon's exercise is similar to what I'm pursuing.  Figure out the tensile load necessary to pop the head off of one rivet.  Figure out how much skin each rivet would be holding in place (by dividing total area by number of rivets).  Don't forget to account for the area shielded by the stiffeners (this is problematic if you don't know what the stiffeners were).  Figure out what kind of pressure loads your presumed cause of failure (hydraulic, explosive, coconut crabs, etc.) placed on the inside and outside of the structure.  Pressure times area equals force.  Once you have the amount of force per rivet, compare it to the tensile yield load and see whether your structure is still in one piece or not.

I've attached a stress distribution diagram of how a tensile load resulting from pressure on the skin would resolve itself within the rivet body.  The red areas indicate the highest stresses and location of probable failure.  If you glue the muffin stump to the counter and lift up on the bottom lip of the top, it will eventually separate right at the junction of stump and top.  This is simulated by fixing the end of the shaft and applying a uniform pressure to the underside of the head. 
« Last Edit: January 29, 2014, 03:49:12 PM by Walter Runck »
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Tim Gard

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Re: Grand Rapids trip
« Reply #64 on: January 29, 2014, 01:41:26 AM »

There is evidence of intense wave action at Nikumaroro.

As Ric explains in "Aerial Tour of Nikumaroro", sections of reef the size of bulldozers were broken off and flung around like so many basket balls. Sections of steel hull have been frisbeed.

http://www.youtube.com/watch?v=DL9FGsvB3E8
/ Member #4122 /
/Hold the Heading/
 
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Jon Romig

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Re: Grand Rapids trip
« Reply #65 on: January 29, 2014, 07:01:01 AM »

With apologies to Jon and Cool Hand Luke, what we have here is failure to orientate.  We're dealing with tensile loads, not shear.
Walter, thanks. I agree we are looking at tensile failure, not sheer. I could not find any data on tensile load capacity, so I used sheer as a surrogate. I did NOT know that rivets are much stronger in sheer, but it makes sense once you think about it. I assume we can do a failure calc based upon known characteristics of the material (soft aluminum or an alloy).

John: I was motivated to use Bernoulli based upon some sources on the web that were calculating the pressure (in PSI) of a "jet" of water based on a known velocity. The weight or amount of the water (in our problem the size of the wave) was considered irrelevant in this calculation as the force is considered instantaneous. The jet must be absolutely normal to the surface being measured, or the pressure will be less (or the velocity must be higher). Believe me, I am no hydraulic engineer so that may be all wrong ;-]

Tim: I agree that there is intense wave action on Niku, and that water in one of these waves could very well have been moving at 47 MPH. I have tried to research water velocities within waves of various sizes, but cannot find a source.

It is worth noting that a wave pressure of 38 PSI, or even less, adds up quickly (for example 16,000 pounds on our <3 SF artifact) so I am not at all surprised that large heavy objects get tossed around.

Jon
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« Last Edit: January 29, 2014, 07:08:01 AM by Jon Romig »
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John Ousterhout

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Re: Grand Rapids trip
« Reply #66 on: January 29, 2014, 07:10:35 AM »

A 1944 NACA report gives tensile failure numbers for AN456 3/32 inch Brazier head rivets in a variety of sheet thicknesses, and describes the heads failing at around 200 pounds force.
Note that that works out to Jon's 37psi to fail the rivets in tension, given that each rivet supported about 5.3 square inches.
If you applied a bit more than 37 psi to the whole sheet at once (as if a wave smacked into it), then there will be one rivet somewhere to fail first, which increases the load on the neighboring rivets, so they fail, and away we go...
Cheers,
JohnO
 
« Last Edit: January 29, 2014, 07:31:58 AM by John Ousterhout »
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John Ousterhout

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Re: Grand Rapids trip
« Reply #67 on: January 29, 2014, 07:19:32 AM »

The 2-2-V page has some nice photos showing the C-shapes (sometimes called "channel" or "U" shaped, but I don't know what they were called back then) of structural stringers in the fuselage, clearly showing the rivets from the interior and exterior. Some of the photos show how the stringers were connected to other structural elements.  It's hard to imagine the connections between the stringers and the main structure being capable of resisting an impact load without coming loose.  The "bellying" of the aluminum and dimpling around the rivet heads may well have been all that happened while the assembly tore loose from the aircraft, and only later were the stringers removed from the skin by removing the rivets (as proposed above by Walter, Jon and probably some others I've overlooked).
Cheers,
JohnO
 
« Last Edit: January 29, 2014, 09:09:00 AM by John Ousterhout »
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Walter Runck

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Re: Grand Rapids trip
« Reply #68 on: January 29, 2014, 09:32:27 AM »

A 1944 NACA report gives tensile failure numbers for AN456 3/32 inch Brazier head rivets in a variety of sheet thicknesses, and describes the heads failing at around 200 pounds force.
Note that that works out to Jon's 37psi to fail the rivets in tension, given that each rivet supported about 5.3 square inches.
If you applied a bit more than 37 psi to the whole sheet at once (as if a wave smacked into it), then there will be one rivet somewhere to fail first, which increases the load on the neighboring rivets, so they fail, and away we go...

Nice find John.  Kind of funny to see that a report about popping the heads off rivets was classified during WWII.

We still need to account for interior surface area covered by the stringers.  Commercial sizes for 1/16 thick aluminum channel seem to range from 3/8 to 3/4" wide , so you have to debit the overall surface area by something around 12% (assuming 1/2 wide channel on 4 inch centers) and rerun the pressure calculations.

Personally I think modeling the dynamic effects of crashing waves or exploding batteries is a step or two beyond the current state of this investigation.  There are too many unknowns to develop any knowledge with a reasonable level of confidence.  Without knowing the structure behind the skin or even where the skin was located, we can't even do a decent static analysis (if you filled the Electra up with water, would the window blow out?), let alone a dynamic one.  It's a good mental exercise to work through the process, but don't expect any conclusions.
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John Ousterhout

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Re: Grand Rapids trip
« Reply #69 on: January 29, 2014, 10:50:30 AM »

Walter sez: "...It's a good mental exercise to work through the process, but don't expect any conclusions."
Well put.  The same general statement applies to a lot of TIGHAR's work.
Not only don't we know much about what happened, we also don't know what we don't know, so to speak.
Cheers,
JohnO
 
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Ric Gillespie

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Re: Grand Rapids trip
« Reply #70 on: January 29, 2014, 12:30:04 PM »

It's the worst explanation for what we're looking at, with the exception of everything else.  There are rivets made of 1100 series aluminum that are much softer, but the FAA claims there is a dimple on the surviving rivet which should indicate a much stronger 2117 T4 material.  This could be settled by chemical or metallographic analysis if desired.

The surviving rivet is an AN455 or AN456 3/32 inch Brazier head made of 2117 T4 aluminum.  That was confirmed long ago by the NTSB lab.

Also, please note, Aris Scarla works for the FAA but his observations and opinions regarding this artifact are offered as an individual, not as a representative of the FAA.  So let's not say "the FAA says."
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Greg Daspit

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Re: Grand Rapids trip
« Reply #71 on: January 29, 2014, 12:35:11 PM »

Assume a plane part is stuck in a groove at the steep edge of the reef like has been speculated. There is a storm.  Between the waves the plane may be exposed. (The water level of the wave below sea level about the same as the wave height above sea level) Then a fresh wave comes in. What happens to the water pressure of the next incomming wave in the groove?
And what happens to the velocity?
3971R
 
« Last Edit: January 29, 2014, 12:48:47 PM by Greg Daspit »
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Ric Gillespie

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Re: Grand Rapids trip
« Reply #72 on: January 29, 2014, 12:58:26 PM »

Aris Scarla, Bob Brandenburg and I are designing an experiment to confirm how much and what kind of force is needed to cause the damage we see on the artifact.  Aris can build a reproduction of the belly of the Electra in the area where we think the failure occurred. The same aluminum sheet and stringers are commonly available today but finding the old-style brazier head rivets might be tricky.  Bob can calculate the required force.  We'll need to partner with a university or research facility that has the capability to generate the kind of fluid force we need.

Knowing what it takes to cause this kind of damage will tell us a great deal about what circumstances the aircraft the artifact came from must have experienced. That, in turn, should enable us to refine our hypothesis of what happened to NR16020 and how much of it is likely to be left to find.

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Walter Runck

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Re: Grand Rapids trip
« Reply #73 on: January 29, 2014, 01:04:41 PM »

It's the worst explanation for what we're looking at, with the exception of everything else.  There are rivets made of 1100 series aluminum that are much softer, but the FAA claims there is a dimple on the surviving rivet which should indicate a much stronger 2117 T4 material.  This could be settled by chemical or metallographic analysis if desired.

The surviving rivet is an AN455 or AN456 3/32 inch Brazier head made of 2117 T4 aluminum.  That was confirmed long ago by the NTSB lab.

Also, please note, Aris Scarla works for the FAA but his observations and opinions regarding this artifact are offered as an individual, not as a representative of the FAA.  So let's not say "the FAA says."

My mistake.  I conflated the NTSB and the FAA.  The NTSB report does contain the rivet ID I was thinking of, so that would seem to be pretty official.  As far as the FAA goes, I'm grateful that people with expertise in this area are willing to donate their time and energy to the project. 

I am curious if the FAA has released any engineering analysis regarding the effects of internal explosions on stressed skin.  I'm sure they've done the work, but could understand it not being publicly available.

Once you have the solid models, it's not a huge amount of extra work to do the stress analysis if you have the software.  Plus it's a good clean fun way to stay in practice.
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Ted G Campbell

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Re: Grand Rapids trip
« Reply #74 on: January 29, 2014, 01:24:35 PM »

The FAA and the NTSB did a lot of work on explosions re the TWA 800 incident and I would think they may have some data they could share concerning effects on aircraft skins.
Ted Campbell
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