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Author Topic: Grand Rapids trip (2-2-V-1)  (Read 139326 times)

Tim Collins

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Re: Grand Rapids trip
« Reply #75 on: January 29, 2014, 05:24:42 PM »

For Tim - the rivets weren't flush, they had shallow domed heads, that wouldn't cause much drag in designs of the era. The heads of the installed rivets looked rather like the heads of modern-day thumbtacks, but smaller - a low rounded profile with a wide contact area. There was no dimple or countersink involved in the process.  The rivets were made of soft aluminum.  3/32" is the shank diameter, which is quite small.

I defer to your knowledge on the subject. I was clearly wrong about flush mounted rivets. However, looking for pictures of the Electra that have good views of the riveting I came across this: http://explorerworld.hu/wp-content/uploads/2012/08/earhart-cockpit-1024x817.jpg. Sure seems like there's a lot of dimpling going on with those rivets. Granted not all of them, but certainly more than just a few.
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richie conroy

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Re: Grand Rapids trip
« Reply #76 on: January 29, 2014, 08:02:03 PM »

Hi All

Just thinking aloud here, Is it possible this artifact is a result of the landing on the reef giving it's location seem's to fit with hypothesis of landing ?

If say the artifact is a part of a bigger piece that would explain why so little damage to rivet holes 2 things can happen either the rivet head thinness can buckle or due to rivets having the hole in middle they can sort of implode at certain pressure, Bare in mind temperature in cabin at touch down and temperature on outside

sort of makes sense to me like ? 
We are an echo of the past


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Jeff Victor Hayden

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Re: Grand Rapids trip
« Reply #77 on: January 29, 2014, 08:40:39 PM »

Something that might have contributed to the rivet heads shearing off? The amount of tensile force needed would be significantly reduced under these circumstances...

"Ferrous metals placed in contact with moist salt air will rust if not properly protected. Nonferrous metals, those without an iron base, do not rust, but a similar process known as corrosion takes place. The salt in moist air (found in the coastal areas) attacks the aluminum alloys. It is a common experience to inspect the rivets of an aircraft which has been operated near salt water and find them badly corroded.
If a copper rivet is inserted into an aluminum alloy structure, two dissimilar metals are brought in contact with each other. Remember, all metals possess a small electrical potential. Dissimilar metals in contact with each other in the presence of moisture cause an electrical current to flow between them and chemical byproducts to be formed. Principally, this results in the deterioration of one of the metals.
Certain aluminum alloys react to each other and, therefore, must be thought of as dissimilar metals. The commonly used aluminum alloys may be divided into the two groups shown in figure 6-32.

Aluminium groupings

Group A       Group B

1100            2117
3003            2017
5052            2124
6053            7075

figure 6-32

Members within either group A or group B can be considered as similar to each other and will not react to others within the same group. A corroding action will take place, however, if any metal of group A comes in contact with a metal in group B in the presence of moisture.
Avoid the use of dissimilar metals whenever possible. Their incompatibility is a factor which was considered when the AN Standards were adopted. To comply with AN Standards, the manufacturers must put a protective surface coating on the rivets. This may be zinc chromate, metal spray, or an anodized finish."

Now Ric's rivet is a AN455 or AN456 3/32 inch Brazier head made of 2117 T4 aluminium, Group B.
The skin is a sheet of 0.032 Alclad which is a 2024 alloy? Group A doesn't include 2024 aluminium but, on checking the aluminium alloy composition limits (% weight) table it is of remarkably similar composition to series 1100 and series 3003 aluminium alloys.

http://avstop.com/ac/apgeneral/rivets.html

The rivets may well have been weakened significantly by the environment the aircraft ended up in.
This must be the place
 
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John Ousterhout

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Re: Grand Rapids trip
« Reply #78 on: January 29, 2014, 10:57:06 PM »

Tim - yes, that photo does clearly show a little dimpling around many of the rivet heads.  I've seen similar in other photos and also in other aircraft, especially when the rivet was driven with an airgun from the factory-head side, and supported with a buck on the back side.  When such rivets are drilled out, it is common in my experience to see an imprint of the head on the surface of the aluminum sheet, but the dimple doesn't abruptly start at the edge of the head - it's more of a subtle buckling between rivets as the slight excess material gets pushed up.  The amount of deformation seen in the artifact may be a difference in degree.
Having thought about it for a day, I am now of the opinion that the rivets likely survived the initial expulsion of the sheet from the surrounding structure, with the stringer intact.  I find it hard to believe the stringer would have been strong enough to remain connected to the aircraft structure, considering the apparent light-duty the panel seemed designed for.
Cheers,
JohnO
 
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Walter Runck

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Re: Grand Rapids trip
« Reply #79 on: January 30, 2014, 08:55:36 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 looks like some of the channels on the NEAM aircraft were formed by folding thinner stock over onto itself.  Other than not having thicker material or preformed channel around, why would you do this?  It looks like a lot of work.  I'm guessing that the approximately .06 of phantom material behind the surviving rivet might have been two layers of .032?  Just can't figure out why.

I don't have an aircraft museum around here, so I'm stuck looking at pictures.  Any help or practical experience?

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John Ousterhout

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Re: Grand Rapids trip
« Reply #80 on: January 30, 2014, 09:06:26 AM »

Walter - you've got sharp eyes. I hadn't noticed that before, but it looks to me to be a double-image rather than two layers of metal.  Cheap camera optics, such as from a cell phone, tend to create that sort of problem at the edges of the field of view.  Looking further to the right the visual effect is more pronounced and looks less like two layers and more like an even more distorted image.

If someone found the bulged assembly on the beach, they might cut off the rivet shanks flush with the stringer then punch out the rivet remnant, leaving no mark on the sheetmetal.  The one intact rivet was left because it wasn't well attached to the stringer and came out as one piece.  Perhaps the  person doing such work thought the channel was more valuable than the sheet at the time.  It's a curious piece of a puzzle.

Sometimes I feel as though TIGHAR were compiling one of the most thorough histories of a randomly picked island.
Cheers,
JohnO
 
« Last Edit: January 30, 2014, 01:22:22 PM by John Ousterhout »
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Walter Runck

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Re: Grand Rapids trip
« Reply #81 on: January 30, 2014, 10:51:33 AM »

I am trying to work up a cross section for likely stiffener shapes and haven't had much luck.  Without artifacts or drawings, my preference would be to measure some similar, original structure on an Electra, but they're kind of thin on the ground around here.
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Jon Romig

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Re: Grand Rapids trip
« Reply #82 on: January 30, 2014, 09:19:16 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?

I have found some good sources for speed of waves, but not much for the speed of water in a breaking wave.

A wave caused by a 92 MPH wind with a fetch of 1,633 miles (!) will be 49 feet high and will be traveling at (coincidentally) 49 FPS. I think this should be considered an extreme boundary condition. With little or no shallows at the edge of the reef, I don't think we have to consider the slowing and heightening that would be experienced by a wave approaching a normal shore that has a shoaling bottom. So our wave hits the Niku reef at its deep-ocean speed and height.

A "plunging" wave would have to be about 72 feet high to dump water on our artifact at our calculated 47 MPH, so I think we can discount the cresting of a wave as the source for this speed.  It is the horizontal speed of the wave that we have to consider.

Our giant 49' wave traveling horizontally at 49 FPS is moving at 33.4 MPH. Compare this to the 47 MPH minimum (not considering the protection provided by the stringers) required to pop all the rivets and you are not too far off. If this wave were further focused by one of the reef grooves it could achieve the 47 MPH, although a wave that big will feel the groove only on its underside, where its speed is considerably reduced, even negative.

A much smaller wave arriving at low tide could experience considerable focusing in a reef groove, but smaller waves travel much slower. For example, a much smaller but still decent-sized 14' wave is only traveling at about 20 MPH. The geometry of the groove becomes increasingly important as you consider the even smaller waves that would be well-focused by the reef groove.

My sense is that reef grooves do not widen a lot towards deeper water, but I may be wrong.

Jon
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« Last Edit: January 30, 2014, 09:52:26 PM by Jon Romig »
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Jon Romig

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Re: Grand Rapids trip
« Reply #83 on: January 30, 2014, 09:35:45 PM »

....
"The salt in moist air (found in the coastal areas) attacks the aluminum alloys. It is a common experience to inspect the rivets of an aircraft which has been operated near salt water and find them badly corroded.
....
The rivets may well have been weakened significantly by the environment the aircraft ended up in.

Re dissimilar metals:
If the rivets were weakened by corrosion caused by the galvanic action between two dissimilar metals, then I would expect to also see corrosion on the skin that they were in contact with. Or does only one of the two dissimilar metals experience the corrosion?

Jon
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Jon Romig

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Re: Grand Rapids trip
« Reply #84 on: January 30, 2014, 10:01:22 PM »

....
I find it hard to believe the stringer would have been strong enough to remain connected to the aircraft structure, considering the apparent light-duty the panel seemed designed for.
If the artifact were subject to 16,000 pounds of force that I calculated is necessary to pop out all the rivets, I too find it difficult to believe that light stringers remained attached to the frame of the Electra. Do we know from the thickness of the stringer material that these were in fact secondary framing members?

Jon
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Tim Gard

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Re: Grand Rapids trip
« Reply #85 on: January 31, 2014, 02:52:33 AM »

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

Agreed, and it's spawned a great deal of concern about the potential involvement of missiles.

On another note, I can't help feeling that the forces that may have acted on the aircraft skin may have acted uniquely, as does directed explosive. See 43:00 at this video ...

http://www.youtube.com/watch?v=01pjt_K-94M

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Monty Fowler

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Re: Grand Rapids trip
« Reply #86 on: January 31, 2014, 08:26:47 AM »

I'm going to nose around with my contacts and see who might have a good fluids lab that is looking for a really cool historical research project.

LTM,
Monty Fowler, TIGHAR No. 2189 CER
Ex-TIGHAR member No. 2189 E C R SP, 1998-2016
 
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Todd Attebery

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Re: Grand Rapids trip
« Reply #87 on: January 31, 2014, 12:30:14 PM »

I mostly lurk in the forum during my lunch hour.  This topic does relate to my area of expertise as an aircraft structural engineer.

Rivets installed in thin sheet and loaded in tension will often fail by other mechanisms other than head failure... though that is the maximum possible value.    A modern universal head 2117 fastener will fail in tension in 0.032" sheet at approximately 175 lbs.  The same countersunk fastener will fail at about 90 lbs.  A brazier head would likely be somewhere in between those values and the fastener "pulling out" could account for the dimpled appearance.  Adjust your wave speed calculation accordingly.  I am NOT a fluids expert, but your calculations seem reasonable.

LTM 
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Randy Conrad

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Re: Grand Rapids trip
« Reply #88 on: January 31, 2014, 02:14:06 PM »

This is a question for Ric, Richie, and Jeff.....Let's say your leaving New Guinea...things are going great....and something breaks from being pulled from the tail of the plane...such as an antenna cable. My question is in the vintage WWII days, and in this case the Electra...Can an aircraft continue flying if part of the skin or plane comes loose due to some form of damage?
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Walter Runck

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Re: Grand Rapids trip
« Reply #89 on: January 31, 2014, 02:50:31 PM »

Can an aircraft continue flying if part of the skin or plane comes loose due to some form of damage?

 I'm gonna say yes.
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