Last time, I explained engineering ethics and the development of the reciprocating engine in WWII.
This time, continuing from last time, we'll introduce the technology of reciprocating engines up to the end of World War II.
Further improvements to reciprocating engines for higher altitudes
In the previous explanation, we explained that exhaust gas-type superchargers can take in more air than mechanical superchargers.
However, the more you compress air, the higher its temperature becomes and the more it expands, thus decreasing its density. Consequently, there is a limit to the amount of air that can be compressed and taken in.
Furthermore, hot air can cause abnormal combustion in reciprocating engines (in the worst case, the engine will break down).
So the great geniuses of that time thought...
So, with an exhaust gas type supercharger...I thought that if I cooled down air that had been compressed at a high temperature, I could compress it even more.
Compressed air cooling system and water-methanol injection system
To address this, compressed air is cooled by a cooling system (intercooler) before being introduced into the engine.
The mechanism is very simple: it just cools compressed air by blowing cold outside air onto it.
When this mechanism is incorporated into the engine, the layout will be as shown in the diagram below.
The atmospheric temperature at altitudes where airplanes fly (around 8000m) is quite low (around -20°C), so simply exposing the intercooler to the atmosphere was quite effective.
This mechanism allowed for even greater air compression before it was drawn into the engine.
Another technique involved injecting water or methanol into the supercharger during takeoff or combat to cool it down by evaporation (latent heat of vaporization), temporarily increasing the boost pressure and boosting engine output (mainly used with mechanical superchargers).
This mechanism is simple: it involves attaching a device similar to a fuel supply system to inject water-methanol into the supercharger (various forms exist).
Since vaporized methanol is an alcohol, it burns in the engine, thus supplementing the gasoline (water does a little work by expanding into vapor, but basically it's just wasted).
Because there is a limit to the amount of water-methanol that can be carried, it can only be used during takeoff, emergency climbs, or when high power from the reciprocating engine is required during combat.
The concept is similar to the NOS (Nitrous Oxide System) in a certain Fast & Furious movie, where the power is temporarily boosted (the power-up mechanism is completely different).
Even with all these efforts, the competition in war is fierce and quickly reaches its limits.
This represents the cutting edge of aircraft engines in World War II.
On the other hand, one problem that hindered high altitude and high speed was the mechanism of the propeller.
Let me briefly introduce the mechanism and technology of propellers.
Propeller technology evolution
Surprisingly, although often overlooked, the propeller is actually just as important a component as the engine.
It's no exaggeration to say that propeller performance is crucial to determining an aircraft's performance, and there have been numerous technological innovations in this area.
In the early days of aviation, propellers were simply attached to the engine and rotated.
Early propellers in aviation
Initially, they were made of wood and typically had two propeller blades.
As airplanes became faster, the number of propellers increased to three, then four, and so on.
The materials also changed from wood to brass (a copper alloy), and eventually to aluminum alloy.
Up to this point, only the propeller has developed.
As airplanes became faster, know-how accumulated, and it was discovered that the appropriate propeller angle changes depending on the aircraft's speed.
So the great geniuses of that time thought...
Invention of variable-pitch propellers
I came up with the idea of adjusting the angle called the propeller pitch angle for each speed.
The mechanism developed for this purpose is called a variable-pitch propeller.
As you can see from the picture, by adjusting the rotation angle of a pair of gears, you can basically adjust the propeller pitch angle in a stepless manner (although it's not actually that simple).
Yes, you can enjoy Okesa Persimmons in the form ofUnlike a constant-speed propeller, stepless adjustment is possible with just a pair of gears, resulting in a significant improvement in performance and compactness.
There are two main types of propeller pitch adjustment mechanisms: electric and hydraulic. Roughly speaking, the two main types were the American hydraulic system (Hamilton Standard) and the German electric system.
At the time, Japan was unable to develop propellers on its own and had to pay the American company Hamilton Standard to manufacture them in Japan (Yamaha Motor).
With the realization of this mechanism, constant-speed propellers quickly became obsolete, and the mainstream approach shifted to using variable-pitch propellers after first changing the rotational speed using a gearbox (reciprocating engine).
By changing the propeller pitch, the aircraft's acceleration performance, cruising speed, top speed, and the fuel efficiency of its reciprocating engines were dramatically improved.
This was the dominant propeller design in the early stages of World War II.
Invention of the constant-speed propeller
So what was developed wasA mechanism was developed in which a transmission was attached to an airplane's reciprocating engine to adjust the propeller speed according to the flight speed; this is called a constant-speed propeller.
While this constant-speed propeller dramatically improved the speed performance of airplanes, it also had its drawbacks.
that isThe more you increase the number of gear ratios for the propeller's rotation speed, the larger and more complex the reduction gear becomes.
Airplanes need to be as small and compact as possible, but they faced the problem that the faster they went, the larger the gearbox had to become.
Therefore, the geniuses of that time tried to adjust the propeller to be more efficient.
The ultimate propeller of WWII
War is a terrible thing, and propellers continued to evolve during World War II.
The biggest problem with propellers at the time was that the propeller, being a heavy object, was rotating at high speed.As a reaction to this, a large force acts on the aircraft in the opposite direction to the propeller's rotation (counter-torque).
Therefore, in order to fly straight, airplanes of that era required the control stick to be tilted in the opposite direction to the propeller's rotation, rather than being set in the middle.
This isn't a big problem during simple cruising flight, but it becomes very cumbersome during sudden maneuver changes or in high-maneuver situations during aerial combat.
Furthermore, tilting the control stick during cruising means that the aircraft's wings are angled in the direction of travel, which increases flight resistance.
To solve this problem, I devised a mechanism called a contra-rotating propeller.
With this systemBecause the propellers cancel each other out, no unnecessary force is applied to the aircraft, resulting in significantly improved handling and reduced air resistance during flight.
However, this alsoIt has a drawback: the mechanism itself is very complex and large, making it quite heavy.
Furthermore, the incorporation of variable-pitch propellers makes it even more complex and larger.
These counter-rotating propellers were extremely difficult to manufacture, and during World War II, only Britain and the United States were able to adopt and mass-produce them.
The propeller structure has also evolved significantly.
At the start of World War II, propellers were made from a single piece of aluminum, but hollow propellers made of steel (an alloy of iron) were developed to be stronger and lighter.
This made it lighter and more robust, allowing the propeller to spin much faster.
This level represents the pinnacle of propeller technology in World War II.
The performance limits of the propeller's mechanism
As explained earlier, propellers underwent tremendous evolution during World War II, much like dinosaurs, resulting in significantly improved performance.
As propeller performance improved, rotating the propellers at higher speeds led to a major problem that had never occurred before.
Yes, it's the sound barrier.
Technological advancements have led to higher altitudes and higher speeds for aircraft.As the propellers became larger and their rotational speed increased, the speed of the air at the tip of the propeller broke the sound barrier.
Behavior of air at the speed of sound (compressible fluid)
Here's a brief explanation of what happens when air exceeds the speed of sound.
Further details will be explained in the fluid dynamics section of the Mechanical Design course.
First of all, air is basically,The theory was based on the premise that the density would always remain constant.
Well, in reality, the density changes only slightly, but it's within a range that doesn't significantly affect the theory.
The fact that density is constant means that, fundamentally, airflow follows the shape of the object.
When air exceeds the speed of sound, its density changes depending on the location, and its behavior becomes completely different.
To put it bluntly, without fear of misunderstandingDue to the speed, some of the air is compressed and becomes denser, creating a large density difference between it and the rest of the air.
This difference in density causes a significant change in the behavior of the air that was flowing along the shape of the object.
Because of thatBeyond the speed of sound, air does not flow along the shape of an object.
A well-known example of this phenomenon is called a sonic boom, which is a shock wave generated by a difference in air density.
Incidentally The term Mach (M) is used to express the speed of sound, but in reality, it is a dimensionless quantity and not a value that indicates velocity.
So, what is it? It's the ratio of the speed of a fluid flowing through an object to the speed of sound, expressed in Mach (M).
The reason we go through all this trouble is that the speed of sound in a fluid varies depending on the type and state of the fluid, so it cannot be used as a standard.
For example, in typical air, at 15°C and atmospheric pressure (1 atm), the speed of sound is approximately 340 m/s, which is about 1240 km/h.
At an altitude of 11000 meters, the speed of sound drops to about 1062 km/h.
Furthermore, in water, the speed of sound is 1480 m/s at 20°C and atmospheric pressure (1 atm).
This is why, from a common sense perspective, sound travels faster underwater.
in this wayBecause the speed of sound varies depending on the type and state of the fluid, it cannot be expressed uniformly, so the ratio Mach (M) is used.
It's better to think of it as a term that indicates the degree of compression of a fluid, rather than simply as a measure of speed.
This Mach (M) is used to describe the state of the flow.
The state described earlier, where the density of a fluid is always constant, holds true in the range M < 0.3 in Mach (M), and is called an incompressible fluid.
The point at which the effect of fluid density changes becomes significant (a sonic boom occurs) is approximately 0.7
Returning to the topic of propellers, since they generate thrust based on the theory of incompressible fluids, they cannot generate thrust when they enter the realm of compressible fluids.
このWhen an aircraft enters the supersonic range, the airflow around the propeller changes, and the thrust that was previously obtained is no longer available.
The high-performance propellers, without us even realizing it, had entered the uncharted territory of supersonic speed.
I think this is probably the first time humanity has faced the problem of the speed of sound.
As we would later discover, the top speed of an airplane that obtains thrust from a propeller is not determined by the engine's output, but rather...Due to the physical properties of the propeller, the speed will be roughly below subsonic (around 900 km/h).
In other words, no matter how high-performance the reciprocating engine or propeller becomes, the use of a propeller will limit the maximum speed.
This is an undeniable fact.
This represented the most advanced technology available at the end of World War II.
Modern propellers
Just because propeller speed limits were reached during World War II doesn't mean propellers disappeared from the world.
It remains in use today for the following reasons.
- Using a propeller allows for the use of relatively inexpensive and safe existing reciprocating engines.
- At speeds below a certain level (around 900 km/h), the aircraft is more efficient in flight, resulting in better fuel economy.
• It is relatively easy to manufacture.
And many more
For the reasons stated above, it remains in active service as a military aircraft.
The ultimate propeller today is a variable-pitch contra-rotating propeller with 16 blades (8 blades x 2). Furthermore, the propeller's shape is curved to maintain a constant airflow velocity at each point on the propeller.
This is how propellers continue to evolve and are adopted in various applications.
A familiar and unpleasant example, though somewhat dated, is the Soviet (now Russian) Tu-95 bomber, which used four engines and contra-rotating propellers to achieve a top speed of 950 km/h, the absolute limit of propeller speed.
The reason this feels familiar is that during the Cold War, the Tu-95 frequently violated Japanese airspace over Hokkaido (and perhaps still does).
At its worst, he came almost every day.
They are a rather unpleasant presence close to me.
A comparison of reciprocating engines for aircraft from various countries at the time (1940-1945)
Japan
Incidentally, while Japanese technology at the time was impressive, the Homare engine, with its single-stage, two-speed mechanical supercharger, was the limit (whether it could have been mass-produced is quite questionable). The practical altitude limit seemed to be around 8000m, and the top speed was about 644km/h. The propeller was a hydraulically operated variable-pitch propeller with four blades (it's unclear whether it had a hollow structure). Representative aircraft that adopted it were the Army's Ki-84 Hayate and the Navy's Shiden-kai.
America
The US had the incredibly famous B-29 with the Wright Cyclone R3350 engine, a monstrous machine with a two-speed, two-stage exhaust-gas supercharger and intercooler – like a fully loaded bomb. However, it was a finicky engine prone to breakdowns, and its success was only due to the sheer volume of American aircraft. The propeller was a hydraulically controlled variable-pitch propeller with a four-bladed hollow structure. It looks similar to Japan's, but the blade size is orders of magnitude larger. Its practical altitude limit was around 12000m and its top speed was about 587km/h.
Germany
Germany's Jumo 213A, with its two-stage, three-speed mechanical supercharger, was impressive (I personally really liked the DB601 series). Its practical altitude was limited to around 8000m, similar to Japan's, but its top speed was an impressive 732km/h. The propeller was an electrically operated, variable-pitch propeller with three blades, likely a hollow structure. The aircraft used was the FW190 series.
United Kingdom
The British used the Griffon engine, an improved version of the famous Marine engine, with a two-stage, three-speed mechanical supercharger—a truly monstrous engine. The propeller was the ultimate hydraulic contra-rotating propeller, with six blades (3x2), likely hollow in structure. Its practical altitude limit was around 10000m, and its top speed was about 720km/h. The aircraft that adopted it was the Spitfire Mk. 18 (up to the Mk. 24).
When you compare them, it looks roughly like the following diagram.
Comparison of aircraft performance in various countries
Looking at this alone, the B-29 might seem slow, but that's because only the Americans were carrying bombers. The P-51D Mustang, America's masterpiece of a propeller-driven fighter, was a monster, with a top speed of 784 km/h that was far superior to the others.
Furthermore, even after the war, Britain had the ultimate reciprocating engine, the Napier Sabre engine, which had a mechanical supercharger with a 2-speed, 1-stage system, but employed a somewhat complex mechanism called a sleeve valve engine. This engine, even after the war, was said to have produced 5500 horsepower at full boost.
For reference, the exceptionally well-functioning Homare has approximately 1800 horsepower, the American Cyclone typically has about 2400 horsepower, and the British Griffon typically has about 2000 horsepower, so 5500 horsepower is truly incredible (all figures are for maximum output at altitude 0m, pressure 1atm, and full boost).
To put it even more broadly, a higher horsepower engine results in better acceleration and higher speed, so the time it takes to reach maximum altitude, the time it takes to reach maximum speed, and the maximum speed itself are at a disadvantage with Japan's 1800 horsepower engine (although this also depends heavily on the aircraft).
This is a pretty tough situation even in Japan. Apparently, even the Homare engine was plagued with problems and quality issues, so getting around 1500 horsepower was considered good.
Furthermore, Japan's failure to develop variable-pitch propellers and its subsequent licensed production of the Hamilton-type hydraulic variable-pitch propeller from the American Standard Company during the war, which it was only able to manufacture throughout the war, was a significant setback.
Even just looking at the engine's auxiliary equipment, including the propeller, there was this much difference.
The gap in fundamental industrial capabilities, such as engine performance, mass production capacity, materials technology, and resources, had become so large that it seemed like an insurmountable barrier.
Even so, Japan, the United States, Germany, and Great Britain were practically the only countries capable of mass-producing reciprocating engines and airplanes, so Japan was a remarkably impressive and capable nation.
I think it's wonderful that there are so many Japanese engineers who are truly admirable.
The theory and even small prototypes of this type of reciprocating engine technology already existed in the 1930s, but the war was a tremendous era in which everything was realized and mass-produced all at once.
Just before World War II, reciprocating engines were producing around 1000 horsepower, but by the end of the war, just five years later, they were easily producing 2500 horsepower.
Unlike academics, technology has no value simply because a theory has been elucidated or an experiment has been successful.In this world, value only emerges when products are mass-produced, reach the hands of users, and prove useful.
The jet engine will finally be invented in the next installment (Germany had already achieved it).
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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