In the previous explanation, we discussed the mechanism of the Comet crash and the impact of differences in testing between the RAE (Royal Aeronautical Institute) and the development company (de Havilland) on the results.
This time, we'll continue to discuss how the differences between RAE's testing and the development company's testing affected the results.
To briefly summarize, the differences between RAE and testing at development companies are as follows:
1. There was no test facility large enough to accommodate the entire aircraft.The pressurized chamber was divided and subjected to durability testing.(One of the main reasons was that the pressurized chamber was constructed by dividing the engine into parts, creating them, and then gluing them together.)
2. DividedTo determine how much load it would take for the pressurized chamber to fail (one-shot failure), a test was conducted applying a load twice the load experienced during flight.
3,A fatigue test was conducted by repeatedly applying loads to the divided pressurized chamber used in the second test.
4. The fatigue test involves repeatedly applying a load due to a pressure difference, a pressure of 0.6 atm (60 kPa).As a precaution, we applied double the pressure, 1.2 atm (120 kPa), every 1000 cycles.
5. To apply repeated loads to a pressurized chamberUsing compressed airThe pressurized chamber was repeatedly subjected to loads.
The finite element method, which has the greatest impact on the results due to these differences, was explained in the previous article.
So this time, we'll focus on and introduce the remaining numbers 2, 3, and 4.
The first major difference in this case was the difference in the content of tests 2 and 3: "double the load was applied for the one-shot destruction test" and "a fatigue test was then conducted using that part."
Reusing the same part in multiple tests (material properties: differences in failure modes)
Before discussing Comet, let me first explain the basic properties of the material.
First, imagine a spring (coil spring), something everyone has seen or touched before.
Obviously, if you stretch a spring within a certain range (load P), it will return to its original length.
Next, if you force it (load P' to stretch), the spring will remain stretched.
If stretched further (load P''), the spring will break.
In fact, the material basically has exactly the same properties as a spring.
Just like a springIf you pull it within a certain range, it will return to its original shape; if you pull it too hard, it will remain stretched; and if you pull it any further, it will tear.
These characteristics are called the elastic region, the plastic region, and the fracture point, respectively.'S.
Furthermore, the amount of elongation is called strain, and the force generated by this elongation is called stress.
このThe relationship between elongation (strain) and force (stress) is determined for each material, and there is a graph called a stress-strain diagram that represents this relationship.
fundamentallyMachines are used within the range of force (stress) that allows them to return to their original shape, just like a spring, that is, within the elastic range (although they are sometimes intentionally used in the plastic range).
vice versaJust as a stretched-out spring is unusable, materials that have entered the plastic deformation zone (stretched) are no longer usable.
For detailed information on material properties, click here. What is the elastic region?
Now, returning to the Comet, please recall that in Test Procedure 2, twice the load that would be applied to the pressurized chamber during flight was used.
Yes, you can enjoy Okesa Persimmons in the form ofIt is highly probable that the material (spring) entered the plastic region (the region where it does not return to its original shape) because it was subjected to twice the load (exceeding the range of return to its original shape).
Furthermore, in section 3 of the exam content...A fatigue test was then conducted on the part that had been tested (the stretched spring).
In other words, because twice the load was applied, the material in the pressurized chamber underwent fatigue testing in an area outside the design's intended range (plastic region: the region where it does not return to its original shape), rendering the test completely meaningless.
fundamentallySince one-shot failure and fatigue failure have completely different modes of failure, the same part should not be used for both tests.
It's completely meaningless unless both tests are conducted using brand new parts.
I have no idea why I made such a mistake.
In my humble opinion, the only explanations I can think of are that either the Comet was so huge and expensive to test that they skimped on the testing costs, or someone was pressuring them and they were pressed for time, so they reluctantly used the same parts for the test.
The next major difference relates to sections 2, 3, and 4 of the exam content.
Unnecessary overload (material property: compressive residual stress)
Furthermore, the basic properties of the material will be explained.
Now, instead of springs, I'd like you to think of things that every Japanese person knows, like Japanese swords or kitchen knives.
Japanese swords, for example, are known for being incredibly durable tools.
I think it's well known that one of the main reasons for their sturdiness is that Japanese swords are made by hammering them with a hammer (genno).
Now, let me explain what phenomena are occurring in the material at that time.
Actually,When a material is hammered (pressed, compressed), a portion of the force applied remains in the material.
That remaining strength allows it to withstand and become sturdy when a force is applied in the opposite direction to the striking direction. That's why Japanese swords don't easily bend or warp when cutting things.
That's why Japanese swords are so sturdy.
The effect is particularly strong when you strike a heated material, hence the proverb, "Strike while the iron is hot."
そのThe remaining force is called compressive residual stress.
For details on compressive residual stress, click here.
If used effectively, this characteristic can be extremely useful.
Unfortunately, Comet's actions backfired.
Returning to Comet, I'd like you to recall sections 2, 3, and 4 of the exam content.
2. To confirm instant failure, twice the expected load was applied.
3. Fatigue tests were conducted on the parts that were confirmed to have failed on the first attempt.
4. As a precaution, under fatigue test conditions, twice the expected pressure of 1.2 atm (120 kPa) was applied at a rate of once every 1000 cycles.
This tasted really bad.
First, because twice the load was applied as described in point 2, compressive residual stress (force remaining in the material) was generated, similar to how it would be if it were struck with a hammer, making it stronger.
Next, a fatigue test was conducted in the reinforced pressurized chamber, as described in item number 3.
Finally, in step 4, we applied double the load once every 1000 times, making it even more robust.
Furthermore, the corners of the windows and the antenna holes, which were the starting points of the actual accident, are prone to stress concentration, resulting in greater loads being placed on them, leaving compressive residual stress and making them more robust.
In other words, The further we progressed with the tests, the more robust the pressurized chamber became, rendering the tests meaningless.
Here again, just like before, the fact that the same part was used in both the single-shot destruction test and the fatigue test has a double impact on the difference in results.
Naturally, mass-produced aircraft do not undergo these kinds of tests (though they do perform some minor inspections), so their pressurized chambers are considerably weaker compared to those of the prototype..
The idea of doubling the load as a precaution turned out to be completely ineffective, which is a laughable situation.
This is a common occurrence in modern development: tests designed to be overly rigorous often turn out to be ineffective.
Generally, defects are discovered during field tests or final product testing, but it seems that the Comet's defects weren't discovered because, presumably due to the high cost of the prototype and the rushed production process, the field flight testing wasn't sufficient.
I, too, have had many similar bitter experiences, and the lessons I learned from them are"Testing with unnecessary loads without a reason, solely for the purpose of gaining a sense of security, is meaningless" (the same applies to simulations).
Applying an unrealistically excessive or insufficient load is pointless because the resulting phenomena will differ depending on the load applied.
IfIf you're going to concentrate the test, you need to use reliability engineering, as explained last time, to carefully determine the test conditions.
If you don't face reality squarely and bravely decide on the maximum load to place on the product, you're bound to regret it.
The final major difference was the use of compressed air to pressurize chamber 5.
The difficulty of handling air (the brilliance of RAE)
As you all know, air is invisible and light, making it difficult to detect leaks or measure whether the pressure is applied correctly.
Also, basically,According to Pascal's principle, pressure is the same at all points regardless of the fluid (air, water, etc.) used to apply pressure. However, in reality, the pressure at each point changes depending on how the fluid flows (whether vortices are generated or the fluid separates).
Given the difficulty of the measurement and the pressure distribution, if air was used during the test, it is highly questionable whether the appropriate pressure was applied to the pressurized chamber.
While compressed air is still sometimes used for testing, its purpose is limited to leak detection (leakage measurement).
If you're conducting tests using air, as you may have seen before, the basic method is to mix kerosene or white gasoline with the air to make it visible (wind tunnel experiments).
Even today, tests that use air are that difficult.
This was especially true back then.
However, since airplanes fly in the air, I think it's understandable that they would conduct tests using compressed air.
I'm impressed by the RAE that uses water. I don't think it's easy to come up with such a paradigm shift even in modern times.
このThe RAE's bold ideas and execution, along with Prime Minister Churchill's decisiveness, truly demonstrate the power of the British Empire.
in this wayDue to several critical differences between development tests, actual equipment testing, and RAE (Real-Aided Engineering) reproduction tests, the projected lifespan during development and the lifespan calculated from test results ended up being far stronger than the actual strength.
In other words, the pressurized chamber's strength was merely superficial.
This is the critical difference between the two exams.
The Comet, which underwent safety measures and fundamental improvements, and what followed
With this much information, designing countermeasures is easy (although things like how to distribute parts to mass-produced machines and the response schedule are difficult).
The pressurized chamber simply needs to be strengthened because its structural integrity is weak.
Specifically, this involves increasing the thickness of the walls of the pressurized chamber itself.
All that's left is to reduce the stress by making the corners of the windows, where stress concentrates, more rounded and by making the antenna holes larger.
While implementing these safety measures, we also addressed the operational challenges of the Comet, specifically the difficulties in takeoff and landing.
Specifically, they developed and installed a more powerful engine.
Up until now, in order to quickly complete the world's first jet passenger aircraft, the engine was also developed in a short period of time, and the British-made centrifugal jet engine Ghost (manufactured by de Havilland, thrust of over 2000 kgf) was adopted.
However, immediately after the war, the thrust limit of centrifugal jet engines was 2200 kgf, and the potential for developing more powerful axial-flow jet engines was attracting attention, but it was thought that it would take until after 1950 for them to be put into practical use on the Iris.
While the Comet was experiencing troubles and crashes, Rolls-Royce in the same country completed and made available the Avon axial-flow jet engine with a thrust of 3250 kgf, so it was decided to use it in the Comet as well.
With a single engine producing approximately 50% more power, and equipped with four cylinders, the total output was increased from 8800 kgf to 13000 kgf, a level almost equivalent to a completely different vehicle.
Fortunately, because the Comet used a centrifugal jet engine, it had a very large engine room, making engine replacement relatively easy (normally this would require major construction and modifications).
This not only eliminates the difficulties of takeoff and landing, but also increases the cruising speed from the original 720 km/h to 770 km/h.
To further enhance the appeal of the Comet, the capacity was initially increased from a 50-person class to a 80-person class.
The engine replacement and larger airframe allowed for increased fuel capacity and improved engine efficiency, resulting in an increased range from the original 2850 km to 4065 km.
To put it bluntly, the modifications were so extensive that it became a completely different machine, transforming it into the Super Comet (Comet MK3).
It was supposed to be a safer and larger jet passenger aircraft that would once again play an active role, but things aren't that simple in this world.
While they were dealing with this series of issues, rival American companies' jet passenger aircraft (such as the Boeing 707, Douglas DC-8, and Convair 880) debuted and were already in active service.
Furthermore, no matter how many safety measures are taken, public trust has been lost due to a total of six accidents, including three fatal crashes, and no one wants to fly with them.
It's no easy feat to regain lost trust. Moreover, because it's a world first, it comes with immense expectations, but also with the potential loss of trust.'S.
Ultimately, as a civilian passenger aircraft, orders did not grow, and the development and manufacturing company (de Havilland) was acquired for a pittance in 1959 and disappeared.
Just seven years after the Comet began commercial flights, the company disappeared.
Moreover, at the time, it was a top-tier company that everyone admired and expected to last forever.
Incidentally, this company was acquired by Hawker Siddeley (the company that developed the Harrier, the world's first vertical takeoff and landing jet fighter), and is now BAE Systems (the UK's No. 1 and one of the world's leading military companies, which handles everything from ships to airplanes).
Production of the Comet itself continued until 1964, and commercial flights continued sporadically until 1980, but it seems it wasn't a business success.
On the other hand, in military use, it appears to have been active until recently in the British Commonwealth (Canada, Australia) as a transport aircraft and anti-submarine patrol aircraft.
I believe that the pride of being the world's first four-engine jet passenger aircraft was, by the skin of its teeth, preserved.
However, Britain's attempt to seize air dominance in civil aviation ultimately resulted in it being completely dominated by the United States.
It is by overcoming such tragedies that modern jet passenger planes are able to fly safely and at reasonable prices.
Thus, the company and Comet disappeared due to the difficulties of venturing into uncharted territory, the way accidents were handled, and inappropriate strength testing.
Even in modern Japan, if a company loses a lot of trust, regardless of the cause, it will collapse (like O-jirushi Dairy or Toyota, the manufacturer of automotive airbags).
No matter how uncharted territory or difficult something may be, it's irrelevant to the customer. Poor product reliability and inadequate handling of accidents will drive the company crazy.
While the lessons learned from the Comet jet crash, as a general study of failures, focus solely on the findings related to metal fatigue and stress concentration, I think it's clear from the explanation so far that the cause was not limited to just those factors.
Because if it were simply a matter of insufficient knowledge, then after three consecutive crashes (which killed nearly 100 people), humanity would no longer be able to take on challenges into the unknown (would we incur casualties every time?).
While the physical causes of the accident were indeed metal fatigue and stress concentration, the real causes were poor accident response (delays in the initial response), inadequate efforts to address defects prior to the crash, and organizational problems..
Next time, I will summarize the Comet jet crash and introduce the generally accepted lessons learned. I will also explain the introductory part of my own analysis.
To those who found this article helpful in understanding design:
Since we're on the subject, I'd like to recommend a book that's essential for mechanical design.
To be honest, the content is extremely unhelpful, but it can be used like a dictionary when you forget the details. If you read this article, you should be able to understand the content and use it effectively. It also includes commonly used standards, making it quite useful.
If you don't already own one, I highly recommend getting one, even though it's a bit pricey. However, new ones are expensive, so if you're considering buying a used one, I strongly recommend checking that the surface roughness conforms to the new JIS standard.
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