The fan of the promising PD-35 aircraft engine is one of the largest in Russian engine building: its diameter is 3.1 meters, and its outer radius is approximately 1.55 m. The main load here is borne by 18 wide-chord blades, each about 1.4 m high. The total mass of the set is about 300 kg, meaning the average mass of one blade is approximately 16.7 kg.
3.1 Meters Turn RPMs into Monstrous Force
At maximum RPMs, the tip of the blade moves at hundreds of meters per second. At such speeds, even a relatively small mass possesses enormous kinetic energy — comparable to the impact of a projectile. This is why blade failure is one of the critical scenarios for an aircraft engine: the design's task is to prevent fragments of a broken blade from damaging the aircraft.
At such RPMs, even this mass at a radius of 1.55 m creates enormous centrifugal load. The centrifugal force acting on the blade is calculated by the formula F = mω²r: the larger the radius and the higher the RPMs, the more the blade is literally 'pulled' outwards. For the PD-35, this load is colossal: this is why not only the blade airfoil itself but also its attachment point to the disk is critically important.
The Lock: Not Just a Fastener, But a Force Chain
The blade is held in the disk by a lock connection — this is not a bolt or a rivet, but a high-precision assembly through whose contact surfaces the centrifugal force is transmitted from the blade to the disk. At the same time, the lock perceives not only a constant load from rotation but also variable influences: aerodynamic forces, vibrations, dynamic peaks.
Engineers calculate not individual parts, but the entire force chain: blade — lock — disk — shaft. A weak link anywhere in this chain can lead to cascading failure. Therefore, the design of the lock includes calculating contact stresses, wear resistance, fatigue strength, and the behavior of the assembly under variable loads.
How the Engine Protects the Aircraft: Blade-Off Tests
To ensure that the engine can withstand blade failure, special tests are conducted with an artificial blade-off on a running rotor. The goal of such tests is to verify that the engine casing will contain the fragments within the power plant and prevent them from penetrating the walls and damaging the aircraft.
Thus, the fan's lock connection is not just a fastening assembly: it transmits enormous loads from the blade to the disk, and the engine casing is designed as a 'trap' for debris in case of an accident.