The PD-14 engine noticeably differs from the American PW1400G and the Franco-American LEAP-1A already in appearance — in fan size. At first glance, it seems that the Russian engine is losing the race for efficiency.
"Pervy Tekhnichesky" explains why a smaller fan is the result of a specific engineering compromise and there is no need to worry.
Large Fan — Not Just Economy
The PD-14 fan diameter is 1900 mm, while the PW1400G is about 2057 mm, and the LEAP-1A is approximately 1980 mm.
The higher the bypass ratio, the more air the engine passes through the outer duct. This allows for more efficient thrust generation at cruise speed. The PD-14 has a bypass ratio of 8.5, the LEAP-1A — about 11, and the PW1100G/PW1400G — up to 12.5.
However, increasing the fan size means an increase in the diameter of the nacelle, the mass of the power plant, and drag. The aircraft has to provide greater ground clearance to the runway, which affects the design of the landing gear and wing.
Why the PD-14 Has No Gearbox
Pratt & Whitney used a gearbox so that the fan and turbine could operate at different optimal speeds. The PD-14 is built according to the classical scheme with direct fan drive.
For the Russian engine, this allowed avoiding an additional complex unit and maintaining a relatively compact nacelle.
At the same time, the PD-14 compensates for the lower bypass ratio with an efficient gas generator: the total pressure ratio reaches approximately 41.
What the MC-21 Got
The compact fan allowed the PD-14 to be placed under the MC-21 wing without excessive increase in nacelle dimensions. The engine itself develops up to 14,000 kgf of thrust and is approximately 10–15% more economical than the previous generation of Russian civil engines.
However, comparing only the nominal bypass ratio is not enough: actual consumption depends on the flight mode, engine mass, nacelle aerodynamics, and the entire aircraft.
Is This a Lag or a Calculation?
The PD-14 indeed did not become a leader in fan diameter. But its task was not to set a record, but to create an engine for the serial MC-21 with acceptable mass, consumption, reliability, and operational characteristics.
As a result, 1900 mm is not a random number, but a compromise between efficiency and dimensions. The PD-14 received a less extreme bypass ratio, but a classic gearless scheme and a compact power plant.