A geared engine can save fuel: the turbine rotates quickly, while the huge fan rotates slower, at its optimal speed. This is the scheme Pratt & Whitney used in the PW1000G family.
But for the Russian PD-35, a different path was chosen. Alexander Inozemtsev, General Designer of ODK-Aviadvigatel, explained that when transmitting power of about 50 MW and above through a gearbox, a problem arises that cannot be solved by simply increasing the size of the gears.
"Pervy Tekhnichesky" explains "in simple terms" why a huge amount of energy has to be transmitted through a gear train, and the inevitable losses turn into heat.
The gearbox separates speeds
In a conventional turbofan engine, the fan is directly connected to the turbine. However, different speeds are beneficial for these components.
A large fan is more efficient when rotating slower – this improves thrust efficiency and reduces noise. The turbine, on the other hand, operates more efficiently at high speeds.
The gearbox allows these modes to be separated. Pratt & Whitney used this scheme on the PW1100G, PW1400G, and other engines of the GTF family.
However, for the class of super-heavy engines with thrust from 35 tons, the geared scheme becomes technically much more complex and may prove to be irrational.
The problem starts with tens of megawatts
The PD-35 belongs to a completely different weight category. The engine is designed for thrust up to 35–40 tons, and its fan diameter is about 3 m.
When transmitting power of about 50 MW, even the high efficiency of the gearbox does not eliminate losses. Part of the energy inevitably turns into heat.
And here begins the main problem. The gears and oil heat up, and heat must be constantly removed. The higher the transmitted power, the more energy has to be dissipated. This requires a more complex cooling system, increases mass, and complicates the design.
Therefore, a direct fan drive without a gearbox was chosen for the PD-35.
Why the PD-35 is similar to the GE9X
The American GE9X, an engine for the Boeing 777X, is built on the same principle. With a huge fan about 3.4 m in diameter, General Electric also abandoned the gearbox.
At the same time, the PD-35, without a geared scheme, achieved a high bypass ratio of 11:1. For the GE9X, this figure is about 10:1.
A high bypass ratio usually helps reduce fuel consumption in cruise mode: most of the air passes through the outer duct and creates thrust with the fan.
This results in an interesting engineering dilemma. Pratt & Whitney used a gearbox to make the fan and turbine operate at different speeds. The creators of the PD-35 and GE9X chose a direct drive scheme for giant-class engines.
For the PD-35, abandoning the gearbox was not a simplification for the sake of economy. With tens of megawatts of power, extra "gears" turn into a heat dissipation problem. And sometimes the most difficult engineering choice is to decide which mechanism is better not to have in an engine at all.