So far, we've explained the jet engine and pressurized cabin, which are essential technologies for the Comet's high-altitude flight.
The Comet, the subject of this article, was developed as a national project starting in September 1946, made its first flight in July 1949, and was completed at an astonishing speed, with mass production units being delivered in January 1951.
As someone who became an engineer in the 21st century, this speed is unimaginable. It's incredible.
To give an analogy, imagine a new type of transportation equipment using a power source with virtually no prior experience in civilian use, being mass-produced in just five years, and then available for anyone to use as long as they pay for it.
To give a more familiar example, basic research on the maglev train began in 1962, and commercial operation is scheduled to begin (this is just a plan) in 2027, so it has taken approximately 65 years from research to mass production.
While a simple comparison is difficult due to differences between trains and airplanes and the different historical contexts, it's not that the maglev is slow, but rather that the Comet is simply too fast.
Despite the significant challenges already described, the product was mass-produced in less than five years, supposedly having been resolved.
The Comet, the world's first jet passenger aircraft, carried the prestige of the British Empire and finally began commercial flights in May 1952.
Comet's commercial flights and reputation
When the Comet began commercial flights, it was probably met with a storm of praise.
However, being the world's first, there were certainly various difficulties in its operation.
Let's first consider the innovative brilliance of the Comet.
The brilliance of Comet
First of all, because it's a fully pressurized cabin, the comfort level of the cabins in previous propeller-reciprocating engine aircraft has improved exponentially.
The level of comfort is probably about the same as that of current passenger aircraft.
Next, jet engines are significantly quieter than propeller-driven reciprocating engine aircraft.
The sound of the propeller cutting through the air and the engine noise are quite loud.
Because the tip of a propeller rotates at a high speed (not rotational speed), it generates high-frequency noise, while an engine, which rotates at a nearly constant speed (maintaining an efficient rotational speed), generates low-to-medium frequency noise.
Even today, helicopters, though they don't use gasoline engines, make a pretty loud noise from their rotors when they fly nearby.
Moreover, with the reciprocating engine passenger planes of that era, engine noise was also a factor.
On the other hand, while jet engines are by no means quiet, they basically only produce high-frequency noise because they consist of multiple turbines (impellers, windmills) rotating at high speed.
I suspect the noise level of the Comet will be not much different from that of current jet passenger planes.
moreoverIt was the most powerful passenger aircraft, flying at speeds orders of magnitude greater than previous propeller-driven passenger planes.
It could fly at approximately 1.5 times the speed of the DC-6, the most advanced passenger aircraft with a reciprocating engine at the time.
As expected, it's state-of-the-art.
On the other hand, with such high performance, it might seem that delivery and operating costs would be high, making it only accessible to a select few wealthy individuals. However, one of the major advantages of jet engines that I haven't explained yet is that they can use low-quality fuel, so the longer the range, the lower the operating costs become, and it seems they could be operated at a reasonable cost.
In a gasoline reciprocating engine, the air-fuel mixture is compressed and burned for efficient combustion.
Therefore, gasoline (expressed by octane rating) with the contradictory properties of being highly volatile (easily mixed with air) and not easily self-ignited (not spontaneously burning during compression) is essential.Jet engines are quite omnivorous engines because all they need to do is burn fuel (to put it extremely, anything that burns will do).
fundamentallyJet fuel is diesel or kerosene (more precisely, kerosene-based fuel which is considerably cheaper) which is far cheaper than gasoline.
Moreover, even if the quality is poor, it's perfectly fine.
Of course, it was extremely popular with all airlines, with over 50 back orders (reservations). Among them was what is now JAL in Japan.
The British Empire and the development and manufacturing companies are very proud of themselves.
On the other hand, being the world's first, it presented operational challenges inherent to jet aircraft.
The difficulty of operating the Comet
While the initial launch was excellent due to these characteristics, the fact that it was a world first meant that it faced the following difficulties in commercial flight.
If we only list the major features:
1. Because it was a very early jet engine, it didn't have much power, and the aircraft was fully pressurized and quite heavy, resulting in a low power-to-weight ratio.
2. Jet engines have a drawback: poor responsiveness, making them difficult for pilots at the time to handle.
3. Although not explained previously, the swept-back wings resulted in insufficient lift at low speeds, making aircraft control difficult (poor stall characteristics).
These are its main features.
First, there were some unique circumstances in Britain that contributed to the insufficient power output of the jet engines.
As mentioned in the previous article on the birth of the jet engine, please remember that at the time, centrifugal jet engines were the mainstream in Britain, while axial-flow engines were the mainstream in Germany.
After the war ended, the usefulness of jet engines was well understood in all countries, so a fierce competition arose to recruit German engineers who had worked on German jet engines.
Unfortunately, Britain suffered heavy losses because the war was fought near its own territory, and lost the battle for engineers to the United States and the Soviet Union (there is also a theory that the Soviet Union forcibly took them away).
Although Britain recognized the potential of axial-flow engines, they were in a difficult situation where they had no choice but to adopt the centrifugal jet engines they had on hand in order to make the Comet the world's first jet passenger aircraft.
Although Rolls-Royce had begun developing an axial-flow jet engine, it seems they were unable to meet the Comet's deadline.
Therefore, the jet engine adopted was the de Havilland Ghost engine, a centrifugal jet engine developed and manufactured by de Havilland (an advanced version of the jet engine used in fighter jets).
It might have been sufficient for fighter jets, but even with four engines, it seems it was a bit of a struggle for passenger planes.
Furthermore, because centrifugal propulsion engines have a large frontal area, the engines were embedded in the wings, which made manufacturing and maintenance more difficult, in order to reduce the aircraft's frontal area as much as possible.
This ultimately led to the distinctive shape of the Comet.
Next, skipping the second point, I'll briefly introduce the third characteristic, the swept wings.
A swept wing is simply defined as an aircraft where the angle at which the main wing is attached to the fuselage's centerline is less than 90°.
A key difference between these swept wings and existing wings is their low air resistance at high speeds.
This effect is particularly noticeable in the subsonic range.
This has a significant impact on the aircraft's speed and fuel efficiency.
Instead, it generates less lift compared to existing wings, and lift is significantly reduced, especially at low speeds.
The Comet adopted this swept-back wing design.
In addition to the lack of lift at low speeds due to the swept-back wing design and the low engine output, the second characteristic is the poor responsiveness of the jet engine.The aircraft has become extremely difficult to take off and land.
Furthermore, as it was the world's first passenger aircraft to use jet engines, there were naturally very few (probably none) civilian pilots with experience flying jet engines, so it is likely they faced considerable difficulties.
These difficulties can be described in terms of specific characteristics,
To take off, the engine power is relatively small for the aircraft, and because of the swept wings, it has less lift at low speeds, so a long runway is required.
Furthermore, immediately after takeoff, if the aircraft angle is increased relative to the ground in order to climb, the swept wings cause a sudden drop in lift, and the aircraft will suddenly descend (poor nose-up capability).
To put it simply, without fear of oversimplification, the mechanism of lift generation on an airplane wing is that the air velocity VU over the upper surface of the wing is faster than the air velocity VL over the lower surface of the wing, causing the air to flow rapidly over the upper surface and very little air to flow over the lower surface, thereby creating a density difference in the air.
Because air density tends to remain constant, a force, or lift, is generated from the underside of the wing to the upper surface of the wing.
It's actually due to the pressure difference, but I thought density might be easier to understand, so I used density.
The angle between the wing and the ground is called the angle of attack, and when this angle of attack increases, the speed difference between the upper and lower surfaces of the wing decreases, and the lift also decreases.
Landing is much more difficult.Furthermore, because the aircraft has little lift at low speeds when approaching the runway, it cannot sufficiently reduce its speed, making it difficult to control.
Based on my research, although it's not mentioned anywhere, I think the design of the brakes, which are a surprisingly important but often overlooked aspect, was quite challenging.
Brakes are basically a mechanism that stops an object by converting kinetic energy into thermal energy.
The resulting thermal energy is then released into the atmosphere.
Kinetic energy is basically proportional to the mass of an object and proportional to the square of its speed (velocity).
Therefore, an object weighing 1000 kg (about the weight of a lightweight compact car) has kinetic energy at 50 km/h and 100 km/h. At 50 km/h it has 0.1 J, and at 100 km/h it has 0.39 J. So, even though the speed doubles, the energy increases by approximately four times.
In other words, simply put, the distance required for this object to stop when braking is about four times greater at 100 km/h compared to 50 km/h.
Because the Comet is heavy and has a high landing speed, even with high-performance brakes, it requires a considerable distance to come to a complete stop.
I apologize for this being just my rough estimate, but the Comet's approach speed to the runway was roughly the same as modern jet aircraft, around 250 km/h, while the DC-6, the most advanced propeller-driven passenger aircraft at the time (with almost twice the crew of the Comet), approached at around 170 km/h, so it's safe to assume that the Comet had a considerably difficult time approaching the runway.
Well, there are probably brakes that use air (brakes that use wings), but they're just a bonus; the main brakes are regular brakes (basically the same disc brakes as in modern cars).
Incidentally, current airplanes and bullet trains all use large disc brakes.
Extending the runways would solve the problem, but there are already a fair number of airports, and it's simply not feasible to renovate all the runways just for the Comet (the infrastructure development would be too expensive). Therefore, it's necessary to make it possible to take off and land on the existing runways designed for propeller and reciprocating engines.
These are quite strict conditions.
As will be explained in more detail later, it appears that the Comet aircraft was unable to stop completely on the runway on several occasions and went off course.
Furthermore, even when attempting to abort a landing, the aircraft's poor nose-up performance due to the swept wings and poor engine responsiveness made it difficult to regain altitude, resulting in a design that was not conducive to retries.
Despite these difficulties, they managed to keep the aircraft in operation by adhering to strict operational manuals and making some minor improvements to the swept wings (adding leading-edge slats and a type of high-lift device).
Comet plane crash
The Comet was an aircraft that possessed both the brilliance of being the world's first and the difficulties that come with being the world's first. Despite this, it managed to continue commercial flights until the crash that everyone knows about finally occurred.
But before that, there were definitely signs that it would lead to a major accident.
A precursor to the Comet plane crash
Three previous accidents had already occurred before the Comet crash.
The first incident occurred in October 1952, when a Comet was attempting to take off at night in bad weather. After takeoff, the pilot felt the aircraft gaining lift and raised the nose, but the poor stall characteristics of the swept wings caused the aircraft to drop sharply and hit the runway. Fortunately, the pilot aborted the takeoff quickly, resulting in only two minor injuries.
The second incident occurred in March 1953 when a Comet, fully fueled, attempted to take off. For some reason, the pilot hastily raised the nose of the aircraft, causing it to drop sharply, just like in the first accident. It landed on the runway but couldn't stop before reaching the end of the runway, resulting in an overrun. Tragically, all five crew members and six engineers from the Comet development and manufacturing company who were on board died.
The third incident occurred in June 1953 when a Comet aircraft failed its landing approach and went off the runway (overran). Fortunately, there were no injuries.
In all cases, the cause was basically attributed to the pilots' inexperience with the world's first jet passenger aircraft, and it was determined that there were no major problems with the aircraft itself.
in this wayAlthough there were three accidents in just one year from the start of operations in May 1952, the responsibility was basically placed on the operators.
I'll use a snake diagram to summarize the timeline up to this point for easier understanding. I used it quite a bit at work. It's a convenient way to visualize timelines.
this is,BrushBased on my experience, I believe that "no accident is 100% the fault of the pilot or the operation," and that there are often underlying defects in the equipment to some degree (essentially, it's difficult to use, so an accident will eventually happen).
Furthermore, if problems occur sporadically and persistently, it is often not a manufacturing issue but rather a problem with the specifications (a design issue).
Unfortunately, in large corporations like the one I used to work for, unless there is conclusive evidence, they deny responsibility and mostly dismiss it as the user's (customer's) problem.
The reason is that many large corporations have a culture of high pride and a strong desire to avoid any form of criticism, which prevents employees from seeing the facts with an honest eye (even when it comes to profit and loss).
In this kind of constitution and climateFor engineers, the ability to look at the facts is crucial and is closely related to engineering ethics.
This accumulation of events leads to the accidents caused by the cover-up of product recalls, which you are all familiar with.
Unfortunately, it seems the company that manufactured Comet had a similar corporate culture.
RatherPerhaps we are simply not currently applying the true lessons learned from the Comet plane crash.
Next time, we'll take a closer look at the Comet tragedy: the series of plane crashes.
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.
Comment: