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Why are aero engine blades designed as loose structures?

Posted by: Fymicohuang 2021-07-20 Comments Off on Why are aero engine blades designed as loose structures?

Aerospace engines are known as the jewel in the crown of modern industry. Their output power is amazing. Aero engines weighing only a few tons can propel passenger planes weighing dozens of tons and hundreds of tons. Generally speaking, the manufacturing cost of aero-engines accounts for about 30% of the total manufacturing cost, and the importance is self-evident.

The internal structure of aero-engines is very precise and complex, and the connections between parts should be tightly connected. But when passing by the aircraft engine, a rattling sound can be heard from inside. Could it be that the fan blades inside the engine are loose? That’s right, it’s loose, but don’t be afraid, it was originally designed like this.

The aero engine completes the compression and expansion of the gas, and generates powerful power with the highest efficiency. The blade that drives the aircraft forward is a special part with a large number of complex shapes, high requirements, and difficult processing.

The curved configuration of aero-engine blades is extremely complex, which transmits considerable torque and extremely high combustion temperature. Therefore, the material of the blade is generally made of high-temperature resistant and high-strength materials. The initial material was a polycrystalline nickel-based superalloy. Later, a polycrystalline blade with oriented grain boundaries was made. The crystalline direction was consistent with the force direction, and the performance was improved a lot. Finally, it developed to the widespread use of titanium alloy materials. Integral blade processing technology.

As the load-bearing component of the engine, the blades of these fans are not firmly fixed on the engine, but are stuck in the tongue and groove of the fan disc through the tenon. There is a gap between the tenon and the groove. When the fan rotates slowly, under the action of gravity, each blade will slide toward the axis when it is close to the twelve o’clock position, and when it is close to the six o’clock position, it will move towards The axis slides out in the opposite direction. As a result, the fan blades collided with each other in the process of slashing to and fro, making a rattling sound.

The mortise and tenon structure connecting the fan blades and the fan plate is called a “vertical tree” mortise, because it looks like a vertical tree and a fir tree, showing a toothed V-shaped tenon and a groove between the tenon and tenon. The obvious gap can facilitate the free sliding of the tenon within a certain range.

Why are aero engine blades designed as loose structures?

When the engine fan rotates rapidly and exceeds the critical speed, the rotating shaft begins to approach its geometric center due to its elasticity, and then crosses the geometric center and approaches its imbalance. At this time, the direction of the centrifugal force received by the blade starts from the new rotating shaft and passes through the position of the blade. The component along the tangential direction of the turbine disk points to the opposite direction to the unbalanced position, which makes the blades deviate in this direction, thereby re-adjusting the unbalanced position so that the center of gravity is close to its geometric center, thereby reducing vibration.

This process is dynamic, so the vibration of the fan at any high speed can be dynamically reduced, and the angle of the blades relative to the tangential direction of the turbine disk changes periodically, so the blades need to swing left and right.

In order to allow the blades to swing freely to a certain extent, the blades are not completely fixed on the turbine disk, but are left with gaps, and the shoulders cannot be completely close to the adjacent blades, which is also to allow the blades to swing. It is certain that not only the blades of the fan are loose, but the blades of the entire compressor are all loose.

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