The new Russian PD-35 engine received a fan with a diameter of 3.1 m. Its direct competitor, the American GE9X, reached 3.4 m. The difference is about 30 cm, or almost 8%.
At first glance, this looks like an advantage for the American engine. The larger the fan, the more air it can pump.
"Pervy Tekhnichesky" found out how the PD-35 with a smaller fan managed to achieve a higher bypass ratio than the GE9X, and whether size really matters for an aircraft engine.
30 cm turns into almost 20% of the area
As we said above, the difference between the fans is 8% in diameter. But the area of a circle depends on the square of the radius. Therefore, a fan with a diameter of 3.4 m has a cross-sectional area of approximately 9.1 m², while a fan with a diameter of 3.1 m has an area of about 7.5 m².
It turns out that approximately 20% more air potentially passes through the GE9X's inlet section. That's why the American engine looks so huge. Its 134-inch diameter fan even required a special solution during flight tests.
GE used a Boeing 747 as a flying laboratory. To install the record-breaking large engine, engineers modified the pylon. The mounting was made in such a way as to raise the GE9X higher and provide the necessary ground clearance.
Why the smaller PD-35 achieved a bypass ratio of 11:1
The bypass ratio shows the ratio of air passing through the outer duct to the air passing through the engine's inner duct. For the PD-35, this figure is stated at 11:1. For the GE9X, it's about 10:1.
That is, the Russian engine with a smaller fan diameter achieved a higher flow ratio.
The fan size itself does not determine the bypass ratio. The entire engine, the dimensions of the gas generator, the airflow through the inner duct, and the fan parameters are important.
The GE9X has a huge and powerful gas generator. The operation of the inner duct requires significant airflow. Therefore, the record-breaking 3.4 m diameter fan did not automatically turn the engine into a record holder in terms of bypass ratio.
PD-35 took a different path
The GE9X was created specifically for the Boeing 777X. The American engine develops thrust of approximately 47–49 tf depending on the mode and version.
_(blurred).jpg){kind=link}
The PD-35 belongs to a somewhat different category. Its basic calculated takeoff thrust is about 35 tf, and solutions created using its technologies can be used for further development of the family. Therefore, directly measuring the technological level of engines with a ruler is fundamentally incorrect.
The largest fan does not guarantee the highest bypass ratio. The PD-35 shows that this indicator depends not only on the fan diameter but also on how the designers calculated the entire gas generator and distributed the airflow inside the engine.