Originally Posted by
KenV
OK. Let's try this: the Newtonian physics associated with the air acting on a separated fan blade are completely overwhelmed by the Newtonian physics of the moments of inertia acting on the blade. Kinda like a bullet in air-to-air combat. Sure, aerodynamics will affect the bullet's ballistic trajectory. But those affects are overwhelmed by the moments of inertia acting on the bullet.
Two points:
1. A fan blade that "rotates forward" will almost immediately result in a blade tip/case strike. The resulting forces acting on the blade will cause the blade to fail at the fatigue crack.
2. What would cause the fan blade to "rotate forward"? The fan is a compressor section. In other words, in a turbo fan engine the fan blade compresses the air, it does not accelerate the air like a propeller. Further, the huge tension forces resulting from the massive centripetal acceleration of the blade will overwhelm any aero force pushing on the blade. Consider a guitar string. It is under considerable tension. Can you blow on the string to cause it to bend? No. The tension on the fan blade is several orders of magnitude greater than the tension on the guitar string.
Mostly true. Compressor rotating section typically increases flow velocity, stator increases pressure. although depends upon the stage and design, and the degree of reaction gone into blade design.
Very true about tensile stresses, which far exceed bending stresses by a significant factor. This actually stiffens the blade considerably in the varying pressure field it operates. one would be very surprised, even at max chat, if a blade would make it outside the cowl without smashing into the edge, as the cetrepetal forces compared to aero forces are massive. My type, you can actually see the fan bending forward at the tips at Max, at least an inch forwards, due to the forces on the blades. The centripetal element is why blades stretch and the cowl has a frangiable seal which wears away as the blades stretch to maintain efficiency. Google turbine creep, the situation is far worse in the hot section.