Out of the Big Three WW2 bombers (B-17, B-24, Lancaster), was the Flying Fortress the most redundant?

[Reluctant Poster]

Good catch. I knew that the deployment was planned, did not know the final reason they were stood down. Do you have the date for this?

The paper I posted on the change in deployment is dated August 9, 1945, which is ironic as the second atomic bomb was dropped on Nagasaki that day rendering the decision moot. The comparison paper is dated August 11, 1945, so I presume it was the basis for the decision.

 

[Greg Boeser]

The accident rate for the B-26 got progressively lower as the war progressed. Improvements in training and the adoption of the longer wing contributing greatly. Both A-26 and A-20 had higher accident rates.

 

[Shortround6]

On the B-17 vs B-24 thing the B-17 may have been overbuilt as they weren't sure how large aircraft structures behaved. One of the YB-17s had been slated for destructive testing to see how well the structure would standup but when one of the early ones survived being flipped upside down in a thunderstorm they canceled the tests and completed the airframe as a regular flying aircraft.
I believe the B-17 main spar was also steel. This was supposed to give a bit more time before the spar failed in wing fuel tank fires but that would be rather hard to prove. The B-24 did have the reputation of wing failure due to fire/combat damage. Basically any advantage was going to mean a few more crewmen might have the opportunity to bail out. A very hard thing to prove as with hundreds if not thousands of planes shot down you are going to find examples of both types surviving extreme damages and examples of both types failing in a matter of seconds from unknown damage. An awful lot of variables.

You would be surprised (or maybe not) what some people will argue over. I was on a committee to select a new ladder truck for the Fire Dept and some members insisted we should get a steel ladder instead of aluminum ladder because it would last longer if exposed to heat/fire. We were looking at a 100ft bucket truck that could hold 3-4 men in the bucket. My view was that if the ladder structure was exposed to enough heat/fire for it to fail with either material I wasn't going to worry about it because I was going to be 10-30ft above the heat/flames and even with Nomex and an air tank I was going to be dead before the ladder collapsed.

 

[Reluctant Poster]

On the B-17 vs B-24 thing the B-17 may have been overbuilt as they weren't sure how large aircraft structures behaved. One of the YB-17s had been slated for destructive testing to see how well the structure would standup but when one of the early ones survived being flipped upside down in a thunderstorm they canceled the tests and completed the airframe as a regular flying aircraft.
I believe the B-17 main spar was also steel. This was supposed to give a bit more time before the spar failed in wing fuel tank fires but that would be rather hard to prove. The B-24 did have the reputation of wing failure due to fire/combat damage. Basically any advantage was going to mean a few more crewmen might have the opportunity to bail out. A very hard thing to prove as with hundreds if not thousands of planes shot down you are going to find examples of both types surviving extreme damages and examples of both types failing in a matter of seconds from unknown damage. An awful lot of variables.

You would be surprised (or maybe not) what some people will argue over. I was on a committee to select a new ladder truck for the Fire Dept and some members insisted we should get a steel ladder instead of aluminum ladder because it would last longer if exposed to heat/fire. We were looking at a 100ft bucket truck that could hold 3-4 men in the bucket. My view was that if the ladder structure was exposed to enough heat/fire for it to fail with either material I wasn't going to worry about it because I was going to be 10-30ft above the heat/flames and even with Nomex and an air tank I was going to be dead before the ladder collapsed.

 

[pbehn]

The paper I posted on the change in deployment is dated August 9, 1945, which is ironic as the second atomic bomb was dropped on Nagasaki that day rendering the decision moot. The comparison paper is dated August 11, 1945, so I presume it was the basis for the decision.

Maybe not moot, maybe already preparing for the post war situation?

 

[Reluctant Poster]

I would not be surprised at all. I design subway stations and fire protection is obviously important. I hear statements like we have to design for extreme temperatures so the station can get back to service asap. I point out that if the fire is big enough to develop those temperatures you have more worries than getting the station up and running

 

[Deleted member 68059]

On the B-17 vs B-24 thing the B-17 may have been overbuilt as they weren't sure how large aircraft structures behaved. One of the YB-17s had been slated for destructive testing to see how well the structure would standup but when one of the early ones survived being flipped upside down in a thunderstorm they canceled the tests and completed the airframe as a regular flying aircraft.
I believe the B-17 main spar was also steel. This was supposed to give a bit more time before the spar failed in wing fuel tank fires but that would be rather hard to prove. The B-24 did have the reputation of wing failure due to fire/combat damage. Basically any advantage was going to mean a few more crewmen might have the opportunity to bail out. A very hard thing to prove as with hundreds if not thousands of planes shot down you are going to find examples of both types surviving extreme damages and examples of both types failing in a matter of seconds from unknown damage. An awful lot of variables.

You would be surprised (or maybe not) what some people will argue over. I was on a committee to select a new ladder truck for the Fire Dept and some members insisted we should get a steel ladder instead of aluminum ladder because it would last longer if exposed to heat/fire. We were looking at a 100ft bucket truck that could hold 3-4 men in the bucket. My view was that if the ladder structure was exposed to enough heat/fire for it to fail with either material I wasn't going to worry about it because I was going to be 10-30ft above the heat/flames and even with Nomex and an air tank I was going to be dead before the ladder collapsed.

The properties of nearly any steel are so dramatically superior to Aluminium at high temperatures, that I dont think any "proof" would be needed of the contention that a steel spar would last dramatically longer in exposure to fire than an aluminium one. There is of course a "slight" element in favour of the Aluminium in that the conductivity is very high, but I do not think that anywhere near balances out the difference.

 

[Howard Gibson]

The properties of nearly any steel are so dramatically superior to Aluminium at high temperatures, that I dont think any "proof" would be needed of the contention that a steel spar would last dramatically longer in exposure to fire than an aluminium one. There is of course a "slight" element in favour of the Aluminium in that the conductivity is very high, but I do not think that anywhere near balances out the difference.

As noted above, if the aluminium is in an environment hot enough to make it fail, the people on it will be in no shape to care. Steel does have better fatigue properties, but this may not count with a lightweight structure.

The B-17 wing looks a lot sturdier than the B-24's wing.

 

[Shortround6]

There is no doubt that the steel lasts longer and will stand up "better".

However when it comes down to crew survival we have to figure out what it means.

Hypothetical numbers, steel lasts 3 times longer than aluminum before failing.
Aluminum spar lasts 12 seconds under XXX Flame impingement.
Steel lasts 36 seconds.
You are a turret gunner getting out of a turret, grabbing parachute from rack on fuselage, attaching parachute to harness and getting out of escape hatch.

How long?

I have seen aluminum components reduced to puddles on the floor/ground. I have also seen iron water pipes (Sprinkler pipes 6in in diameter) and structural components that looked like cooked spaghetti draped over parts/rubble that were lower. Steel, if I remember correctly, becomes "plastic" at around 800 degrees. The steel pipe or beam will maintain it's "shape" like tube or I beam but can no longer support itself, and droops/sags down onto whatever will support it. It hasn't "melted" like aluminum but it isn't holding up a lot either.
Steel buildings are known as "non-combustible" buildings which basically means that the building structure (beams, siding, etc) will not contribute to the fire load over and above what the contents are doing. It does NOT mean the steel building is fire proof. Steel structure can be fire proofed with the application of layer/s of insulation or coatings to give a certain rating for flame impeachment, some times given as how many minutes before failure.
Now with a damaged aircraft how big is the fuel fire, how big is the airflow through the compartment/s where the fire is, how fast/volume is the airflow and what is the altitude (air density) of the air feeding the fire?
That is just stuff I can think of off the top of my head. I would guess that steel stands up better there is a lot of stuff that affects the result in each case.

 

[FLYBOYJ]

IMO the B-17's wing was built better and stronger.

The B-24 had a wing that was built up with ribs, stringers and longerons, all riveted together and then the outer skin riveted to the structure. Between the structure sat fuel cells.

The B-17 had a similar construction except the outer skin was riveted to a corrugated skin and then riveted to the ribs

The P-38's wing was also built very similar

 

[Deleted member 68059]

As noted above, if the aluminium is in an environment hot enough to make it fail, the people on it will be in no shape to care. Steel does have better fatigue properties, but this may not count with a lightweight structure.

The B-17 wing looks a lot sturdier than the B-24's wing.

A fire in the wing can make the wings fail in a few minuites, there is no fuel in the fuselage of a B-17 so the fire can easily make the wing spar fail
with zero crew exposure to the flames.

What do fatigue properties have to do with a fire ? Or if a structure is lightweight ?

The only properties of merit in that are what is the yield strength at a certain temperature. Most aluminiums will be down to 100megagpascals stress for example,
(not even good enough for a biscuit tin) at not much over 220 deg C, to get fairly average steel down to that by heat will need over 500 Deg C.

 

[Shortround6]

For me there is just too much unknown.
I am not saying the aluminum is better, not even close, but even in well know photos we don't know the duration of time, even in some photo sequences.
We don't know the actual flame (or hot gas ) temperature at the impingement area.

We can see the size of the fire blowing over the outside of the wing but that doesn't tell us the size of the fire inside the wing structure or what the airflow (hot gas/flame travel) inside the wing compartment/s was like.

And we haven't even gotten to at what point the wing spar/structure can not handle the aerodynamic load and starts to fold up. How fast, what altitude, what angle of attack is the wing, what side load? all of which may take a few seconds or less than a second?

And if the majority of the crew cannot successfully exit the aircraft in the time available does it matter what the wing spar was made out of?

 

[FLYBOYJ]

Some things to consider -

Calum hits the nail on the head with regards to alloy strengths and heat resistance. With regards to construction, those steel components, while taking heavy structural loads and basically holding the machine together, are along for the ride as it's the aluminum structure that is actually lifting the whole aircraft. IIRC most of the structural steel components were made from 4130 steel (stainless steel in high heat areas) and I believe the lower eutectic temperature is about 1600F. Structural aluminum 7075 (or 75T back in the day) is about 1000F, so I think it's pretty obvious what's going to fail first under excessive heat.

The only time you have too much fuel on board if you're on fire.

 

[wlewisiii]

I was surprised by the numbers for the A-20. Perhaps because it was one of the first fairly high-speed attack profile aircraft, tricycle gear and single pilot with multiple engines to control? Did the train on low level stuff state side? If so, that would certainly increase the accident numbers.

 

[Shortround6]

I am not familiar enough with aluminum and it's fail modes.
I do now that steel can fail as a loading bearing component well below it's melting temperature.
But that is low grade steel. How well alloys do could be very different.

 

[Jugman]

The really important advantage of the B-24 over the B-17 was it's range while still carrying a useful load which only expanded as the war went on.

For example, RAF Coastal Command B-17 could fill the bomb bay with fuel but could then only carry 4 depth charges externally. The B-24 with extra fuel filling the forward bomb bay could still carry 8 depth charges or 4 plus a Mk24 homing torpedo internally and still fly further.

In 1945 the Liberator in SEAC was flying 24 hour missions out of Ceylon. Before the introduction of the B-29 it was the B-24 that were flying the longest ranged bombing missions of the war in 1942 and 1943. Check out the failed HALPRO mission to Ploesti in 1942 or the Shady Lady mission to Balikpapan in 1943.

There was a reason why the USAAF swapped B-17s for B-24s in the Pacific from early 1943. Range.

The reason was the 8th greatly preferred the B-17 and the Germany first doctrine meant they got their way.

Now early B-24s did have a practical range/payload advantage because they had 35% more internal fuel. But this only lasted until B-17s with Tokyo tanks entered service around the mid 1943. After that the B-24 only had an advantage when carrying depth bombs or GP bombs over 1000lbs.

 

[pbehn]

There is no doubt that the steel lasts longer and will stand up "better".

However when it comes down to crew survival we have to figure out what it means.

Hypothetical numbers, steel lasts 3 times longer than aluminum before failing.
Aluminum spar lasts 12 seconds under XXX Flame impingement.
Steel lasts 36 seconds.
You are a turret gunner getting out of a turret, grabbing parachute from rack on fuselage, attaching parachute to harness and getting out of escape hatch.

How long?

I have seen aluminum components reduced to puddles on the floor/ground. I have also seen iron water pipes (Sprinkler pipes 6in in diameter) and structural components that looked like cooked spaghetti draped over parts/rubble that were lower. Steel, if I remember correctly, becomes "plastic" at around 800 degrees. The steel pipe or beam will maintain it's "shape" like tube or I beam but can no longer support itself, and droops/sags down onto whatever will support it. It hasn't "melted" like aluminum but it isn't holding up a lot either.
Steel buildings are known as "non-combustible" buildings which basically means that the building structure (beams, siding, etc) will not contribute to the fire load over and above what the contents are doing. It does NOT mean the steel building is fire proof. Steel structure can be fire proofed with the application of layer/s of insulation or coatings to give a certain rating for flame impeachment, some times given as how many minutes before failure.
Now with a damaged aircraft how big is the fuel fire, how big is the airflow through the compartment/s where the fire is, how fast/volume is the airflow and what is the altitude (air density) of the air feeding the fire?
That is just stuff I can think of off the top of my head. I would guess that steel stands up better there is a lot of stuff that affects the result in each case.

I am not familiar enough with aluminum and it's fail modes.
I do now that steel can fail as a loading bearing component well below it's melting temperature.
But that is low grade steel. How well alloys do could be very different.

A lot of this is "engineering". Steel is a remarkable series of alloys, with a huge range of properties. A steel "can" with a large diameter and low wall thickness will deform under its own weight at room temperature. Above a certain ratio of diameter and wall thickness you cannot measure ovality, you are measuring gravity, and if you roll the "can" as we called them a quarter turn the ovality has moved with the rotation. Steel used for pipelines is certainly affected above 137C because I have worked on projects where the steel was tested at room temperature and at the exit temperature of the product which was 137C on 1 project and 145C on another. The difference is only noticed statistically, after performing hundreds of tests on the same material at both temperatures there is a lower average result at the higher temperature, however they are not exactly the same size of test piece, or the same type of extensometer and strain regime.

All of this is bye the bye, I once had to do a thermographic survey of pipes in a cracking furnace in Jubail Saudi Arabia. The dry air temperature was 150C, we were only allowed to work for 10 minutes then rest in an airconditioned room and drink for 50 minutes. We worked in a team of 4 although only two were needed for the job, the other two were for if someone collapsed from the heat. In practice the computer attached to the camera gave out before the humans did and the screen went blank from the heat after 7 or 8 minutes. At the end of the day, despite just "working" for about one hour I was knackered with light burns all over but the (creep resistant) steel pipes we had been surveying were happily still doing their stuff inside the cracker at around 1000C.

With regard to aircraft Snowygrouch speculated that a wing could burn through in minutes, I would say that you are more likely to read about it if it took minutes. If the plane was heavily loaded and the fire intense it would happen in seconds, or at best fractions of a minute.

 

[manta22]

Sorry if I may not be right. I'm no native English speaker.

How come?

No disrespect intended, it may be that you wanted to ask if the B-17 was more numerous.

 

[Thumpalumpacus]

The accident rate for the B-26 got progressively lower as the war progressed. Improvements in training and the adoption of the longer wing contributing greatly. Both A-26 and A-20 had higher accident rates.

IIRC the Marauder wound up with the lowest overall loss-rate per sortie of two- or four-engine bombers.

 

[Thumpalumpacus]

There is no doubt that the steel lasts longer and will stand up "better".

However when it comes down to crew survival we have to figure out what it means.

Hypothetical numbers, steel lasts 3 times longer than aluminum before failing.
Aluminum spar lasts 12 seconds under XXX Flame impingement.
Steel lasts 36 seconds.
You are a turret gunner getting out of a turret, grabbing parachute from rack on fuselage, attaching parachute to harness and getting out of escape hatch.

How long?

I have seen aluminum components reduced to puddles on the floor/ground. I have also seen iron water pipes (Sprinkler pipes 6in in diameter) and structural components that looked like cooked spaghetti draped over parts/rubble that were lower. Steel, if I remember correctly, becomes "plastic" at around 800 degrees. The steel pipe or beam will maintain it's "shape" like tube or I beam but can no longer support itself, and droops/sags down onto whatever will support it. It hasn't "melted" like aluminum but it isn't holding up a lot either.
Steel buildings are known as "non-combustible" buildings which basically means that the building structure (beams, siding, etc) will not contribute to the fire load over and above what the contents are doing. It does NOT mean the steel building is fire proof. Steel structure can be fire proofed with the application of layer/s of insulation or coatings to give a certain rating for flame impeachment, some times given as how many minutes before failure.
Now with a damaged aircraft how big is the fuel fire, how big is the airflow through the compartment/s where the fire is, how fast/volume is the airflow and what is the altitude (air density) of the air feeding the fire?
That is just stuff I can think of off the top of my head. I would guess that steel stands up better there is a lot of stuff that affects the result in each case.

IIRC from fire school, 30 minutes at 1300 degrees F will ruin structural steel. This is obviously well below the melting point of steel, but metal need not melt to be useless.

I'd still take an aluminum ladder to a fire any day of the week. If nothing else, if SHTF we can unass from danger quicker if we realize what's going down.