Why Jet Engine FAN BLADES are Loose?

If you have seen the airplane engine's Intake Fan, you probable have noticed that there is a little play or clearance. Do you ever try to know, why blades are loose?

Let's Understand this.

I hope you guys already know Inertia and Center of Gravity (CG).

We must first understand what happens if fan blades are not flexible but joint rigidly.

The shaft rotates in a housing and there is a little clearance between housing and shaft, in which oil is filled to lubricate the shaft. In rigid attachments, fan blades are directly attached to the shaft with the help of flange or special joiners.

Suppose the fan assembly rotates anti-clock wise, then the inertia of each blade tend to keep the blades as it was, which generates a force which is opposite of direction of rotation. because each blade has its own center of gravity the moments changes at the root of the blades and these forces transferred to shaft which result in wobbling of shaft, means shaft is no longer rotating on its axis, this is called Engine Vibration. This engine vibration reduces the engine efficiency and may lead to engine detonation and failure.

But When these blades are flexible (loose)

When blades are loose means they are not connected rigidly to the shaft but each blade is inserted in a hatch called fir tree, now there is a gap between blade and fir tree. When fan assembly rotates, inertia tend to resist the blade moment resulting in opposite force at the tip of blade, when moments changes after the blade CG, now this moment is not transferred to the shaft but blades adjust their moment in the gaps and shaft is free of those external force hence reducing or damping engine vibration.

Turbine blades are flexible because those blades undergo high temperature and the material expands, so there must be some vacant space present for that material to expand freely.

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