The turbojets of modern civil airplanes are characterized by a large bypass ratio, i.e. a ratio of bypass flow divided by main flow that is greater than 5, and that may be as great as 9 or 10. This leads to a reduction in the transverse dimensions of the body of the turbojet between the fan and the turbine (the “wasp waist” effect), and this reduction in transverse dimensions leads to a reduction in the bending strength of said body.
Bending deformations of the turbojet body in turn lead to deformations of the casing around the rotor, with the casing ovalizing, which reduces the clearance between the casing and the rotor at some locations while increasing the clearance at other locations (the “casing distortion” effect).