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Author Topic: FAQ: Electra bouyancy, Ditching at sea  (Read 110158 times)

Gary LaPook

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Re: FAQ: Electra bouyancy, Ditching at sea
« Reply #45 on: October 26, 2011, 09:01:19 PM »

I wonder which of these photos was taken earlier and/or if the fabric was removed as a lightening measure after the Luke Field accident.  Vent lines aside, perhaps this might provide insight into modifications made before the 2nd World flight attempt.

It's the other way around.  The photo without the fabric is quite early - probably August or September 1936.  Initially they tried to use a manifold system for filling the fuselage tanks.  Note that there is no filler neck over the starboard 118 gallon tank forward and only two fueling ports on the port side of the cabin. Later they gave each tank its own fueling port.

Dating the various photos of the cabin interior is important - and tricky.  The aircraft went though many changes in the year between its delivery in July '36 and its disappearance in July '37.  The fuel system was jiggered around, radios and antennas came and went, interior furnishing were added and removed.  Many photos were taken of preparations for the first world flight attempt because Earhart was courting publicity.  Unfortunately, there are no known photos of the cockpit or cabin interior after the April/May repairs.
---------------------
The tanks were just being installed and the filler neck for the starboard 118 gallon tank had not yet been installed, it is visible in  the other photos.
I am attaching two diagrams of the tanks filling setup.

gl
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Ric Gillespie

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Re: FAQ: Electra bouyancy, Ditching at sea
« Reply #46 on: October 26, 2011, 09:13:56 PM »

The tanks were just being installed and the filler neck for the starboard 118 gallon tank had not yet been installed, it is visible in  the other photos.
I am attaching two diagrams of the tanks filling setup.

As I said, initially they tried to use a manifold system for filling the fuselage tanks.  The airplane was delivered with only one fueling port on the top of the fuselage and two on the left-hand side of the cabin.  Apparently if didn't work well and they gave each tank its own port.
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Ric Gillespie

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Re: FAQ: Electra bouyancy, Ditching at sea
« Reply #47 on: October 26, 2011, 09:15:44 PM »

So, water still ain't going to flow uphill.

It doesn't need to if waves are breaking over the airplane.
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John Ousterhout

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Re: FAQ: Electra bouyancy, Ditching at sea
« Reply #48 on: October 26, 2011, 09:31:09 PM »

Most vehicle fuel tanks with sealed filler caps and flat-top construction have two vent locations in my limited experience, connecting to opposite sides or ends, and commonly opposite corners. Alternatives are to have a vapor dome, or curve the top of the tank with the vent at the highest point, or route a single vent line to a small header tank with it's own vent arrangement.  Here's why flat top tanks benefit from two vents:
Imagine a tank almost completely filled with cool gasoline.  If the port wing is a little bit low, the tank will also have a bit of a tilt.  The port vent connection on the tank will be submerged in gas, while the starboard one is above the liquid level. Now warm the fuel slightly, so it expands a bit.  If there were only one vent on the port side, then the warming and expanding liquid gasoline would be pushed out the vent, even though there is some room left in the tank.  With two vents, there is always some room to expand at one of the vent locations, unless the tank is 100% full. In that case, with luck, the manifold will route any displaced gasoline into another tank, rather than on the ground.

Wasn't "gas on the ground" one of the 3 useless things, along with altitude above you, and runway behind you?
The pictures are a big help.  I've got to get the CD's as soon as my membership arrives.

Regarding the tank internal bracing - lots of rivets do indicate lots of internal structure.  If the apparant thinness of the outer surface is an indication, the internal bracing may also be equally thin.  That works fine for holding fuel in, but not resisting external pressure.  Any internal structure is certain to be heavier than foil, but not much more.  I hope to find some contemporary tank construction for simple analysis.  Until we know for sure how the tanks were made, it's only conjecture how they would respond to submersion.  With vent lines out the bottom, trapping the air inside them, intact tanks would offer a LOT of excess buoyancy, even if they partially crushed until pressure was equalized.  In that case they would be metal balloons.  However, if the plane tried to float nose-down, as Ric suggested, then the vent lines exiting from the belly behind them would be "above" the tanks.  If the tanks crushed, then they would "deflate" and quickly lose all buoyancy as the air escaped through the vent line.  If they stayed "inflated" due to strong internal bracing as Gary suggests, then they would continued to provide a lot of excess buoyancy even if vented to the open air.
If the plane was banged around on the reef, then anything might have happened, but damage would be certain.
Cheers,
JohnO
 
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Gary LaPook

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Re: FAQ: Electra bouyancy, Ditching at sea
« Reply #49 on: October 26, 2011, 09:36:49 PM »

So, water still ain't going to flow uphill.

It doesn't need to if waves are breaking over the airplane.
-----------------------------
Get one of those straws that bend in the middle, bend it into an "L" shape, hold it with the bend uppermost, immerse the whole straw to simulate waves splashing higher than the vent lines, observe if any water flows up through the straw and over the bend in the middle. You can also slosh the water in the sink at the same time.

gl
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Gary LaPook

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Re: FAQ: Electra bouyancy, Ditching at sea
« Reply #50 on: October 26, 2011, 09:50:40 PM »

Most vehicle fuel tanks with sealed filler caps and flat-top construction have two vent locations in my limited experience, connecting to opposite sides or ends, and commonly opposite corners. Alternatives are to have a vapor dome, or curve the top of the tank with the vent at the highest point, or route a single vent line to a small header tank with it's own vent arrangement.  Here's why flat top tanks benefit from two vents:
Imagine a tank almost completely filled with cool gasoline.  If the port wing is a little bit low, the tank will also have a bit of a tilt.  The port vent connection on the tank will be submerged in gas, while the starboard one is above the liquid level. Now warm the fuel slightly, so it expands a bit.  If there were only one vent on the port side, then the warming and expanding liquid gasoline would be pushed out the vent, even though there is some room left in the tank.  With two vents, there is always some room to expand at one of the vent locations, unless the tank is 100% full. In that case, with luck, the manifold will route any displaced gasoline into another tank, rather than on the ground.

Wasn't "gas on the ground" one of the 3 useless things, along with altitude above you, and runway behind you?
The pictures are a big help.  I've got to get the CD's as soon as my membership arrives.

Regarding the tank internal bracing - lots of rivets do indicate lots of internal structure.  If the apparant thinness of the outer surface is an indication, the internal bracing may also be equally thin.  That works fine for holding fuel in, but not resisting external pressure.  Any internal structure is certain to be heavier than foil, but not much more.  I hope to find some contemporary tank construction for simple analysis.  Until we know for sure how the tanks were made, it's only conjecture how they would respond to submersion.  With vent lines out the bottom, trapping the air inside them, intact tanks would offer a LOT of excess buoyancy, even if they partially crushed until pressure was equalized.  In that case they would be metal balloons.  However, if the plane tried to float nose-down, as Ric suggested, then the vent lines exiting from the belly behind them would be "above" the tanks.  If the tanks crushed, then they would "deflate" and quickly lose all buoyancy as the air escaped through the vent line.  If they stayed "inflated" due to strong internal bracing as Gary suggests, then they would continued to provide a lot of excess buoyancy even if vented to the open air.
If the plane was banged around on the reef, then anything might have happened, but damage would be certain.
----------------------------------------
I don't know what it looks like inside those tanks but I am going with the theory that the vertical rows of rivets indicates a baffle to stop right to left sloshing that would resist some level of compressive load from the front and back of the tank.

gl
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John Ousterhout

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Re: FAQ: Electra bouyancy, Ditching at sea
« Reply #51 on: October 27, 2011, 06:58:12 AM »

Gary sez: "... the vertical rows of rivets indicates a baffle to stop right to left sloshing..."

I agree.  Also the pictures clearly show the several round plugs where access was had to the tank interiors.  Those round holes were necessary to contruct the tanks, especially when bucking rivets - you need access to both ends of a rivet (except with modern Cherry "pop" rivets or explosive rivets).  The holes were then sealed with a circular plug that fit from the inside of the tank.  Those plugs had a "lip" that was a bit bigger than the hole, so internal pressure helped hold them in place.  The ones I've seen were soldered or welded in place, and not removable.  They also would not pop into the tank from external pressure, although they weren't quite as strong as a tank wall that had no hole in it. 
I'm curious what the tanks material was - aluminum, or tern, or stainless steel?  I haven't stumbled across any references.  A SS tank would survive in salt water just fine. Tern might last nearly as long.  The vent lines might be SS, in which case they would remain after the aluminum fuselage dissolved.
Cheers,
JohnO
 
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Ric Gillespie

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Re: FAQ: Electra bouyancy, Ditching at sea
« Reply #52 on: October 27, 2011, 01:27:29 PM »

Tern might last nearly as long. 

What is tern?

The vent lines might be SS, in which case they would remain after the aluminum fuselage dissolved.

Aluminum aircraft immersed in salt water don't "dissolve."  They corrode (quickly and badly) when recovered if not properly conserved but aircraft underwater hold up quite well.  The deeper the better.
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Ric Gillespie

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Re: FAQ: Electra bouyancy, Ditching at sea
« Reply #53 on: October 27, 2011, 02:31:53 PM »

Stainless is a posssibility - but I believe NR16020's tanks would have not been a strong candidate for that in the 1930's.  Welded / spot-welded construction would be more the norm for stainless, not the riveted / welded construction type we see in NR16020. 
I am not so familiar with the use of terne metal except that I think it commonly refers to terne-cladded copper and stainless sheet.

Wouldn't any those materials be a LOT heavier than 1100 aluminum?

Vents do not themselves allow a dramatic exchange of liquid - it takes time since they are relatively small and meant to allow for the evacuation of fuel at normal burn rates plus some margin by permitting the flow of fluid (air) into the space as liguid leaves.  But, they DO allow fluid to move readily once there is any substantial breach by allowing the relatively rapid expulsion of air - another possibility to consider; if a tank (or tanks) shifted during a hard landing, crash or during abuse in the surf, enough breach(es) could occur to allow a great deal of water into the tanks fairly quickly.

Here's another thought.  There were multiple fuselage tanks, rather than one big one, because they had to fit through the cabin door.  The tanks were anchored to the floor - or rather to the structure under the plywood floor - by means of what appear to be thin metal rods with dark felt?, leather?, or rubber? padding between the tanks and the rods.  The system for securing the tanks in the cabin was, logically, designed to keep the tanks from shifting in flight.  Just as logically, the system was not designed for the tanks to be flotation devices with the weight of the aircraft suspended from the rods.

If the aircraft floated nose down with the tanks providing the buoyancy, it seems like there would be tremendous tension upward and rearward on the rods. If the tanks buckled under the rods or the rods cut through the padding into the tank it would be Katie-Bar-The-Door.  If the rod anchors failed, the tanks would probably tear loose from the vents and filler necks and pile up in the rear of the cabin with equally entertaining results.
 [/quote]

It would be good to have a breakthrough in finding the technical details on NR16020's tanks somehow - it could tell us a great deal about how the ship might have behaved with a low-fuel state in water, heavy surf, etc.

The only paperwork on the Earhart Electra that has surfaced is what the FAA has - and it's not much.
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John Ousterhout

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Re: FAQ: Electra bouyancy, Ditching at sea
« Reply #54 on: October 27, 2011, 03:44:03 PM »

Terne plate was a common gas tank material at one time, I'd guess roughly before 1960.  In my experience, it refers to light gage sheet steel with a lead or lead/tin coating, although I've also seen it used in reference to a copper tubing coating.  The coating made it non-sparking, and easy to fabricate by soldering or welding.  The steel used underneath was dead-soft/low-alloy, making it very easy to form, especially deep-drawing.  My experience is with old farm equipment with Terne-plate tanks.  As long as the coating held up, they didn’t rust or corrode.  Unlike galvanizing though, scratches weren’t self-healing, and the coating wasn’t very abrasion resistant – they’d rust through in a year once the steel was exposed.  My strongest suspicion is that the fuselage tanks are soft aluminum, and pretty thin at that, to save weight.  Besides, Northrop is good at fabricating things out of aluminum.
Cheers,
JohnO
 
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Gary LaPook

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Re: FAQ: Electra bouyancy, Ditching at sea
« Reply #55 on: October 27, 2011, 06:37:49 PM »

Most vehicle fuel tanks with sealed filler caps and flat-top construction have two vent locations in my limited experience, connecting to opposite sides or ends, and commonly opposite corners. Alternatives are to have a vapor dome, or curve the top of the tank with the vent at the highest point, or route a single vent line to a small header tank with it's own vent arrangement.  Here's why flat top tanks benefit from two vents:
Imagine a tank almost completely filled with cool gasoline.  If the port wing is a little bit low, the tank will also have a bit of a tilt.  The port vent connection on the tank will be submerged in gas, while the starboard one is above the liquid level. Now warm the fuel slightly, so it expands a bit.  If there were only one vent on the port side, then the warming and expanding liquid gasoline would be pushed out the vent, even though there is some room left in the tank.  With two vents, there is always some room to expand at one of the vent locations, unless the tank is 100% full. In that case, with luck, the manifold will route any displaced gasoline into another tank, rather than on the ground.

Wasn't "gas on the ground" one of the 3 useless things, along with altitude above you, and runway behind you?
The pictures are a big help.  I've got to get the CD's as soon as my membership arrives.

Regarding the tank internal bracing - lots of rivets do indicate lots of internal structure.  If the apparant thinness of the outer surface is an indication, the internal bracing may also be equally thin.  That works fine for holding fuel in, but not resisting external pressure.  Any internal structure is certain to be heavier than foil, but not much more.  I hope to find some contemporary tank construction for simple analysis.  Until we know for sure how the tanks were made, it's only conjecture how they would respond to submersion.  With vent lines out the bottom, trapping the air inside them, intact tanks would offer a LOT of excess buoyancy, even if they partially crushed until pressure was equalized.  In that case they would be metal balloons.  However, if the plane tried to float nose-down, as Ric suggested, then the vent lines exiting from the belly behind them would be "above" the tanks.  If the tanks crushed, then they would "deflate" and quickly lose all buoyancy as the air escaped through the vent line.  If they stayed "inflated" due to strong internal bracing as Gary suggests, then they would continued to provide a lot of excess buoyancy even if vented to the open air.
If the plane was banged around on the reef, then anything might have happened, but damage would be certain.
-------------------
The regulations in 1937 required that the tank withstand an internal pressure of 3.5 pounds per square inch. Although this is from the inside pushing out it is also possible that material that can deal with this much internal pressure might be strong enough to withstand pressure from the outside too that is only about half as much, and not collapse like a balloon.
gl
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Gary LaPook

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Re: FAQ: Electra bouyancy, Ditching at sea
« Reply #56 on: October 27, 2011, 06:46:24 PM »

Gary sez: "... the vertical rows of rivets indicates a baffle to stop right to left sloshing..."

I agree.  Also the pictures clearly show the several round plugs where access was had to the tank interiors.  Those round holes were necessary to contruct the tanks, especially when bucking rivets - you need access to both ends of a rivet (except with modern Cherry "pop" rivets or explosive rivets).  The holes were then sealed with a circular plug that fit from the inside of the tank.  Those plugs had a "lip" that was a bit bigger than the hole, so internal pressure helped hold them in place.  The ones I've seen were soldered or welded in place, and not removable.  They also would not pop into the tank from external pressure, although they weren't quite as strong as a tank wall that had no hole in it. 
I'm curious what the tanks material was - aluminum, or tern, or stainless steel?  I haven't stumbled across any references.  A SS tank would survive in salt water just fine. Tern might last nearly as long.  The vent lines might be SS, in which case they would remain after the aluminum fuselage dissolved.

It would be very useful to know what the tanks on NR16020 were made of and how they were constructed. 

In mulling over these posts, I'd like to share a bit of thought from experience -

It is not unheard of to find older airplane tanks made of 1100 aluminum alloy in "0" (annealed) state.  Large "A" (1100 - "soft") rivets were also commonly used in such tanks - they are more compatible with the soft base material in terms of being worked (bucked) and relative strength.  The large size of the rivets also is compatible in terms of the needed bearing area in soft material.  The 1100 "0" state aluminum also allows easy welding of apertures for fittings, etc. during the build process very readily.  1100 can also be very corrosion resistent since it has little alloy inclusion in it. 

These tanks have the appearance of such construction in the photos but we can't know for certain without more information. 

Stainless is a posssibility - but I believe NR16020's tanks would have not been a strong candidate for that in the 1930's.  Welded / spot-welded construction would be more the norm for stainless, not the riveted / welded construction type we see in NR16020. 

I am not so familiar with the use of terne metal except that I think it commonly refers to terne-cladded copper and stainless sheet.  My understanding also is that terne metal has to be painted to effectively weather the elements, but that it lasts well with an organic coating.  I am familiar with 'metalizing', i.e. the application of a soft 1100-type aluminum to steel parts such as naval aircraft cylinders to protect them from the elements, but do not know of terne metal being used in aviation per se.

It cannot be said for certain what the tanks were made of without real records, but I know from experience you cannot easily buck large, hard rivets (like big brazier head 2117s or similar of the day) that I see in these pictures without severly deforming and tearing the base material.  It is also possible that soft-state tanks could be heat-treated after construction (if other alloy than 1100 which doesn't heat treat so well), but the size of these tanks could have made that problematic.  Heat treatment proably would not be necessary for the use such tanks would see anyway.

It is entirely possible that the tanks were of soft material and considered adequate for their specialized, limited use.  If they were, they could have been quite crushable under pressure - even with baffling within.  The large surface area means you could generate substantial crushing forces at relatively meager pressures per square inch. 

My experience includes outfitting a certain large-cabin business jet with large, temporary in-cabin tanks for an around-the-world record flight - the construction method was quite similar to what I have described and one good reason was ease of manufacture and installation.  Malleable material is easier to work with.  The tanks did not need a long-life - just enough to get them reliably through the intended mission.  Such may well have been the case for NR16020.

Of course we can't know how these things might apply to NR16020 without direct evidence of how they were constructed - that would be good to know, for sure.  It may well have been a factor in how long the ship could have floated after going into the water.

As to the venting, everything I can discern from the pictures and from what I know of tank venting suggests that the vent outlets would have been placed as high as possible on the airframe, not low.  Venting would have also been by the most direct route.  We see vents for wing tanks on top and on the bottom of wings - often both as a redundant means of venting.  Again, it would help to know even gas-cap details - many are vented, and it would not be a surprise to find that NR16020 had vented caps of some sort. 

Vents do not themselves allow a dramatic exchange of liquid - it takes time since they are relatively small and meant to allow for the evacuation of fuel at normal burn rates plus some margin by permitting the flow of fluid (air) into the space as liguid leaves.  But, they DO allow fluid to move readily once there is any substantial breach by allowing the relatively rapid expulsion of air - another possibility to consider; if a tank (or tanks) shifted during a hard landing, crash or during abuse in the surf, enough breach(es) could occur to allow a great deal of water into the tanks fairly quickly.

It would be good to have a breakthrough in finding the technical details on NR16020's tanks somehow - it could tell us a great deal about how the ship might have behaved with a low-fuel state in water, heavy surf, etc.

LTM -

-------------------------
But the hypothesis is that the plane made a normal soft landing on the smooth reef surface, soft enough that the landing gear was not damaged, allowing the engines to be run. At the time of the landing the tanks held very little fuel (and most likely all the cabin tanks were completely empty with the remaining fuel being in the wing tanks) so there should have only been light, normal stresses on the cabin tanks so the tanks should not have suffered any damage during the landing. As for damage by the surf, the cabin tanks were inside the cabin so the cabin itself protected the tanks from dynamic pressure from the waves so, again, unlikely to have suffered any damage.

gl
« Last Edit: October 27, 2011, 06:55:36 PM by Gary LaPook »
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Ric Gillespie

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Re: FAQ: Electra bouyancy, Ditching at sea
« Reply #57 on: October 27, 2011, 07:35:35 PM »

But the hypothesis is that the plane made a normal soft landing on the smooth reef surface, soft enough that the landing gear was not damaged, allowing the engines to be run. At the time of the landing the tanks held very little fuel (and most likely all the cabin tanks were completely empty with the remaining fuel being in the wing tanks) so there should have only been light, normal stresses on the cabin tanks so the tanks should not have suffered any damage during the landing. As for damage by the surf, the cabin tanks were inside the cabin so the cabin itself protected the tanks from dynamic pressure from the waves so, again, unlikely to have suffered any damage.

If tank connections could be dislodged in a hard landing, they could certainly be dislodged by the shock of the aircraft being repeatedly hammered by waves with enough force sufficient to knock it off its gear and drive it over the edge of the reef.
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Gary LaPook

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Re: FAQ: Electra bouyancy, Ditching at sea
« Reply #58 on: October 27, 2011, 11:01:22 PM »

But the hypothesis is that the plane made a normal soft landing on the smooth reef surface, soft enough that the landing gear was not damaged, allowing the engines to be run. At the time of the landing the tanks held very little fuel (and most likely all the cabin tanks were completely empty with the remaining fuel being in the wing tanks) so there should have only been light, normal stresses on the cabin tanks so the tanks should not have suffered any damage during the landing. As for damage by the surf, the cabin tanks were inside the cabin so the cabin itself protected the tanks from dynamic pressure from the waves so, again, unlikely to have suffered any damage.

If tank connections could be dislodged in a hard landing, they could certainly be dislodged by the shock of the aircraft being repeatedly hammered by waves with enough force sufficient to knock it off its gear and drive it over the edge of the reef.
----------------------------
I didn't say they could be damaged by a "hard landing," maybe by a crash or a bad ditching. Since they were empty, the fuselage being batted around by waves shouldn't damage the tanks because they would not resist the movement since they were very light so no great inertial forces so again, no damage.

gl
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Gary LaPook

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Re: FAQ: Electra bouyancy, Ditching at sea
« Reply #59 on: October 28, 2011, 01:38:10 AM »

The only paperwork on the Earhart Electra that has surfaced is what the FAA has - and it's not much.

------------------------------------------------------


Let's do the math. The airplane was produced under Air Bulletin 7a and 26 effective 1934. Using the method provided in those bulletins to calculate the allowable design load factors is more complicated than in the subsequent CAR 3 and in the current FAR part 23 but you end up with the same result plus 3.8 and minus 1.52 Gs. These bulletins also require the same 50% safety margin.
Assume the 149 gallon tanks weighed 100 pounds and were made out of aluminum. Fill them with 149 gallons of aviation gas weighing 894 pounds so the total is 994 pounds. The hold downs will have to be strong enough to keep this tank from displacing upward either due to maneuvering loads or from buoyancy from the immersed empty tank. This must be 1.52 times that total weight or 1511 pounds. Add to this the 50% safety factor and the strength must be, at a minimum, 2266 pounds. And at this load the straps could suffer permanent deformation but would not fail completely even at that load.

 Now let's calculate the maximum buoyancy force that could be created by this empty tank. The buoyancy of the air in the tank is equal to the weight of the water that would have filled the tank. Seawater weighs 8.5 pounds per gallon so the buoyancy of the air in the tank is 1266.5 pounds. To this we must add the buoyancy of the aluminum in the tank itself. The specific gravity of aluminum is 2.7 meaning that it has a density of 2.7 times that of fresh water. So just divide the 100 pounds of aluminum by the specific gravity and you find the buoyancy of the aluminum tank itself. One hundred pounds divided by 2.7 equals 37 pounds of buoyancy in fresh water. Seawater is denser than fresh water by 3% so buoyancy in seawater is 3% greater so the tank makes an upward buoyancy of 38.1 pounds. Add this to the buoyancy of the 149 gallons of air and the total upward force on the tie downs is 1304.6 pounds which is well below the 1511 pounds designed strength of the hold downs and 961 pounds below the strength of the tie downs required by the 50% safety factor. This is a safety margin actually 74% greater than the buoyancy of the fuel tank.

So, the tank tie downs will not fail due to the maximum buoyancy force that could be created by the empty fuel tank. You can work your own examples using different assumptions about the material and the weight of the tank and you will see that no combination comes anywhere close to exceeding the strength of the tie down straps. For instance if the tanks were made out of steel then there would be even less buoyancy and, since the tank would be heavier, then the strength of the tie downs would have to be even stronger so an even greater safety margin would exist.

This computation also shows why the tanks would not have been displaced in a normal or hard landing. The strength of the the tie down in the upward direction 2266 pounds and the assumed weight of the tank is only 100 pounds so it would take more than 22 Gs for an empty tank to displace upward in a crash or hard landing and the strength requirements in other directions are even greater. If the plane had been subjected to 22 Gs when landing on the reef it would not have ended up standing on its own legs so no running engines and no radio messages. It would also take more than 22 Gs from wave impacts to tear the tanks loose so that is not too likely either.

gl
« Last Edit: October 28, 2011, 04:30:40 AM by Martin X. Moleski, SJ »
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