The Russian PD-35 is being developed as a new-generation engine for heavy wide-body aircraft. Its take-off thrust is expected to reach 35 tons, and its concept largely overlaps with advanced Western developments.
However, in the main economic indicator of civil aviation – specific fuel consumption – the Russian project is still inferior to the “American” one.
"Pervy Tekhnichesky" explains why a difference of a few grams of kerosene per kilogram of thrust became the price for reliability and how Perm designers plan to close this gap.
Small but precious: what these grams of thrust mean
The declared specific fuel consumption of the PD-35 in cruise mode is estimated at 0.51–0.53 kg/kgf h, while for the General Electric engine this figure is lower – approximately 0.49–0.52 kg/kgf h.
Specific consumption shows how many grams of fuel an engine burns per hour to create one kilogram-force of thrust in cruise mode.
It is impossible to directly compare engine appetites in tons. The GE9X is a giant with a 3.4-meter diameter fan, which develops a maximum thrust of up to 47–49 tf. The PD-35 is more compact – its fan is 30 cm smaller. But if we look at the efficiency of a unit of thrust, the GE9X does its job cleaner and more economically.
"Perm Gambit" vs. American Heat
The secret to the extreme efficiency of the American engine lies in its “hot section.” To squeeze out maximum efficiency, General Electric increased the compression ratio to 60:1 and introduced innovative ceramic matrix composites into the design.
Russian engineers took a fundamentally different path. The PD-35 gas generator was deliberately made cooler for reliability. But reducing the thermal load comes at the cost of a drop in thermodynamic efficiency – hence those extra grams of consumption.
At the same time, the PD-35 partially compensates due to other solutions. Its two-shaft scheme is simpler and potentially more reliable than three-shaft analogues, and a high bypass ratio (11:1) helps to effectively collect thrust from the external airflow.
All that glitters is not gold
The difference in consumption between the lower limits of the indicators is about 4%. If we compare the upper design 0.53 kg/kgf·h of the PD-35 with the minimum 0.49 of the GE9X, the advantage of the American engine reaches approximately 8%.
But here lies the main reason for the pride of Perm engine builders. The GE9X is a triumph of extreme technologies, but its ceramic hot section is incredibly expensive to manufacture, and every repair turns into a complex and costly operation.
By making the PD-35 “cooler,” ODK designers will get not a record-breaking, but a balanced, “down-to-earth,” and maintainable engine.