In the previous introduction, we discussed the fatigue testing of the pressurized chamber during the ground testing of the Comet rocket.
As explained last time, fatigue testing is generally called durability testing (combined durability testing), and it is an important final test performed in the development process.
This time, let's consider the test flight, which is the final stage in the development of the Comet.
Before that, as usual, let me explain the overview and basic contents that are confirmed during field testing in the development of products and machinery—in the case of automobiles, this means actual driving tests, and in the case of airplanes, this means test flights.
General field tests (actual running tests, test flights)
一般 的 なIn product and machine development, field testing (actual driving tests, test flights) is the very last step and represents the culmination of the development process.
Here is the development flow diagram that I introduced before.
We're now at a crucial stage, just before mass production.
Now, let's look at what and how we verify things during field tests (actual running tests, test flights).
In fact, the concept is exactly the same as the ground test I introduced two posts ago.
Similar to the ground tests described previously, we will check each component in the order of function, performance, and durability.
In other words, all the functions and performance anticipated in the concept and design are checked, and finally, durability is verified.
However, a major difference from ground testing is that it is a very dangerous test because people actually use, ride in, and operate the machine to verify its operation.
All previous tests have been conducted using ground-based test equipment and facilities, with some method of simulating human use, but this final field test isWe conduct tests in an environment that is exactly the same as the actual market conditions.
Therefore, if we don't identify and address all the problems and defects from previous tests, it will be extremely dangerous if new problems arise during this field test.
It's a very tense exam.
Therefore, these field tests (actual driving tests and flight tests) are special tests that can only be conducted by a limited number of trained individuals.
Comet isAs this is the world's first jet passenger aircraft, it's likely to be an extremely dangerous test, as it's unclear what might happen.
Therefore, it becomes a difficult test that can only be passed by a very limited number of highly professional pilots.
However, the test content itself is nothing special; it simply verifies all the functions, performance, and durability that were considered during the conceptualization and design phases.
Examples of field tests (actual running tests, test flights)
Specifically, when considering the Comet, we check every detail, from basic functions such as being able to fly, the pressurized cabin being pressurized, and the rudder working properly, to more detailed functions such as whether the instruments are functioning correctly, by actually flying the aircraft.
Similarly, we check all performance metrics, such as whether the aircraft actually reaches the expected maximum speed during flight, whether the takeoff distance is as expected, whether the landing approach speed is as expected, and whether the pressure inside the pressurized cabin is 0.6 atm.
The goal is to verify through test flights whether the mission, as envisioned for a military aircraft, can be successfully and accurately accomplished.
Similarly, durability testing involves performing limit flights (acrobatic flights) in stages to confirm that the aircraft does not break even when performing limit maneuvers as conceived and designed.
For example, the aircraft is subjected to repeated rapid ascents, descents, and sharp turns, relentlessly pushing its limits within predetermined performance boundaries.
As expectedSince the aircraft is actually piloted by a person, it would be dangerous to apply a load that would cause it to break, as in a ground test's single-shot destructive test (limit load test). Therefore, the limit of the aircraft's strength is determined by dividing the destructive load recorded in the single-shot destructive test by a safety factor (the safety factor is like a margin of safety).
For details on the safety factor, please click here.
The goal is to verify whether the aircraft is truly safe when actually flown within the determined structural strength limits.
As I've mentioned before, the exact same thing was done during the development of Japan's pre-war Zero fighter, with the military's elite test pilots gradually testing the limits of flight.
Unfortunately, during the steep dive, a phenomenon called flutter (vibration caused by shock waves generated when air exceeds the speed of sound) occurred in the aircraft's tail fin, which was not apparent on the ground. As a result, the aircraft disintegrated in mid-air, and precious lives were lost.
Naturally, all parties involved will take the situation seriously, conduct thorough investigations and countermeasures, and proceed only after confirming everything is in order.
Unfortunately, at the time, there was no expertise regarding flutter (the behavior when exceeding the speed of sound), so the countermeasure was to prohibit dive speeds above 620 km/h. This did not lead to a fundamental solution, but thanks to the tests, the limit of performance for safe flight could be determined. (Later, proper countermeasures were taken, and the dive speed was returned to 750 km/h in the Type 52).
I apologize for being persistent, but this book provides a great understanding of the social conditions, technological background, and the thoughts of engineers at the time, so if you're interested, please do pick it up.
Ultimately, no matter how advanced the development methods, testing techniques, and equipment become in product or machine development, and no matter how sophisticated the simulations become, there are still many things that can only be understood by actually using the product or machine.
SomedayThe day may come when all the mysteries are explained, but it will absolutely never happen during my lifetime.
In addition, No matter how much science advances, I think there's a 99.9% chance that this final field test (actual running test, test flight) will never disappear.
Fatigue testing during field tests (actual driving tests, test flights)
Once all of this is done, the final fatigue test (endurance test) will be conducted.
Presumably, with passenger airplanes, they have meetings with the operating company to decide how the aircraft will be used, and then they just keep flying it according to those plans.
In other words, the aircraft is launched from the airport under loads similar to those of a commercial flight (maximum passenger capacity, maximum cargo weight), and the flight and landing are carried out according to the same procedures as a commercial flight.
This process is repeated many times to confirm whether it is safe.
In reality, if we were to fly the aircraft for the same duration and number of flights as it would in commercial flight, the development time would be too long and it would be inefficient. Therefore, we consider conditions that allow for some degree of concentration when determining the content of the test flights (however, it is not possible to concentrate the tests to the level of ground testing due to safety concerns).
Click here for more details on the concept of concentration.
For example, with Comet, it's easy to think about what conditions result in a high load.
The most stressful aspects are likely the load from rapid ascent and descent, and the sudden pressure changes due to altitude differences. Therefore, by repeatedly performing rapid ascents and descents from around 0m altitude to a maximum altitude of 12000m, the number of times the load is applied is increased even in a short test period, concentrating the stress and creating a condition similar to that of commercial flight.
like thisThe content of field tests, like fatigue tests, is a treasure trove of know-how and becomes a critically important confidential matter for companies.
Since it's impossible to concentrate the testing as much as in ground tests, even in my area of expertise, automobiles, the tests are a long process, lasting at least two months, and sometimes as long as three months.
Ideally, I would like to personally inspect the ground tests (durability tests) every day, just like the fatigue tests, but since the actual driving tests are conducted in various locations (often overseas, or rather, almost entirely overseas), it becomes impossible (and I also have other work to do, so it's impossible for me to go along).
StillSince it's such an important test, it's normal to check in with the on-site test staff by phone or email at least once a day to see how the actual test is going and what the situation is like.
This know-how, though I'm being persistent, is essential because it reveals a company's true capabilities that can't be understood through economics alone.
Of course, thisAfter the fatigue test (durability test) is completed, the aircraft is disassembled completely, just like in the previous ground fatigue test, and each component is carefully checked for any abnormalities.
Furthermore, a special solution is used to check for cracks, and all parts are displayed for many people to see and confirm that there are no problems.
Only after completing these tests can we begin preparations for mass production.
As these test flights progress and a certain level of safety is confirmed, pilots from commercial airlines that use the aircraft, as well as military pilots for reference, will be invited to actually fly the aircraft to gather feedback and make fine adjustments and improvements to usability and other aspects.
Since this is the world's first jet passenger aircraft, the airlines naturally have no operational experience. Therefore, they will begin development only after a certain level of safety has been confirmed through test flights, and will proceed with creating flight manuals, air traffic control systems, and pilot training programs for commercial flights.
Once the test flights have been conducted in this manner and all the necessary checks are complete, preparations for mass production of the aircraft (such as the creation of jigs, molds, and other production equipment) will be made, and then mass production will begin.
Furthermore, once the test flights are complete, the prototype aircraft are loaned to commercial airlines and other organizations to allow pilots to familiarize themselves with them, while the development of flight manuals and air traffic control systems continues.
The concepts and content of field testing for mechanical and product development, electrical and electronic products, and intangible systems, organizations, and policies are almost identical.I think that's what will happen.
Next, let's consider the test flight of the Comet.
Comet test flight
We'll start by considering what the Comet's test flights were like, but there's not much information available, so it's difficult to imagine anything in detail, but we'll try our best.
Based on the limited information available, it appears that the Comet's test flights did confirm a full range of functions, performance, and durability.
As evidence of this, test flights were apparently conducted at several airports, and based on the results, considering the low stability during takeoff and landing and the importance of using runways with poor pavement, the landing gear, which initially consisted of a single large tire, was changed to a bogie-type trolley with four tires to reduce impact resistance and ground pressure.
Furthermore, it seems they also conducted acrobatic flights (extreme maneuvering flights) in various locations as a demonstration of its durability.
Analysis of the Comet's Limit Maneuverability in Test Flights
Looking back at the results, it seems that the Comet's test flights were largely meaningless.
As I mentioned in the previous-to-last post...Since the aircraft was not actually destroyed in the single-shot destruction test (limit load test), test flights were conducted without knowing the true limit of the aircraft's strength.
In other words, no matter how many acrobatic flights are actually performed, the limits of the aircraft's structural integrity are unknown, so it's impossible to know if the aircraft is truly reaching its limits in terms of flight stress.
Perhaps, by sheer luck, the aircraft was able to fly at the very limit of its structural integrity, or conversely, it may have been flying with a considerable margin of safety relative to its actual structural strength.
trulySince the aircraft's maximum strength limit is unknown, acrobatic flight (extreme maneuvering flight) is meaningless.
It is likely that the actual test flights were conducted with the latter margin of safety in mind (based on the fact that no problems occurred), so no matter how many acrobatic maneuvers were performed, there would absolutely be no problems with the aircraft.
A study of fatigue testing during Comet's test flights.
Looking at the test flights from the perspective of fatigue testing, it's surprisingly true that no problems were found during the tests.
I apologize for judging from the results, but to briefly review, there have been about three crashes, and for the aircraft whose flight counts are known, the flight counts were 1230 and 900 respectively. In RAE's reproduction tests, damage occurred at around 3060 flight counts.
If this malfunction occurs after this number of flights...It wouldn't be surprising at all if a crash (mid-air disintegration) had occurred during the test flight.
Moreover, if fatigue tests are properly conducted through test flights, achieving a flight count of 1000 is not an impossible number at all.
Further investigation reveals that the test flight period spanned 1 year and 4 months, from July 1949 to January 1951, before mass production deliveries began, so it was easily possible to achieve 1000 flights.
Even if it did not crash (break apart in mid-air),There is no doubt that cracks leading to destruction would inevitably appear in the corners of the windows or around the antenna holes.
The crash wouldn't have occurred if all parties involved, including the development and manufacturing companies, as well as the government, hadn't overlooked the cracks that appeared during this test flight.
As mentioned in the explanations of the fatigue tests during the previous ground tests and this test flight, a world-first machine like the Comet attracts considerable attention, but at the same time, it ventures into uncharted territory, so development is usually carried out with great caution.
Therefore, prototype aircraft that have been flown for long periods, long distances, and many times during test flights are always disassembled and thoroughly checked.
The disassembled parts will be put on display for many people to see.
While it's unlikely that every corner of the Comet's windows (counting from the photos, there were over 20) or the antenna hole has cracks, cracks have definitely appeared in multiple locations on the same parts.
It's simply unthinkable that we would overlook this.
Conversely, even if a crack had been discovered, it's inconceivable that one would conclude there was no problem simply because it didn't lead to complete destruction.
In any case, it's normal to take some kind of action or countermeasure.
Normally, this would cause a huge fuss.If no errors are found after reviewing the ground test data, methods, and conditions, and conducting a thorough check of the design drawings, then there is no other option but to accept the actual performance, implement countermeasures, and repeat the test.
In other words, whether it was due to poor test flight conditions, inadequate management or checking systems, or something else, we have no choice but to conclude that the test flight was completely meaningless.
As introduced last timeJust like in the ground tests, a considerable number of errors occurred during the test flights, making the crash an almost inevitable accident.
Based on the test flight details,It's hard to believe that the problem wasn't even about the engineering issues of stress concentration and fatigue failure, but rather that there were major issues with the testing check system, judgment criteria, development system, organization, governance, and ethics.
Up to this point, we've already made numerous major mistakes during crucial ground tests and test flights, putting us in a very precarious situation.
It is incomprehensible to me that all those involved, including the development and manufacturing company, as well as the government and other related parties, did nothing to point out or stop this situation (although it is possible that even if someone pointed it out, they simply ignored it).
This concludes our discussion of the problems encountered in the development of the Comet.
Next time, we'll finally start thinking about how to respond to a plane crash.
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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