5.7.4 Methods of vibration control
It is not practical to 'tune' blades to avoid resonances above the eighth engine order. The harmonics lie too close together and natural frequencies cannot be calculated or controlled to sufficient accuracy. Fortunately, excitation forces at these high frequencies are small and are incapable of exciting complex modes.
If a wheel mode and a harmonic of rotational frequency coincide below the eighth engine order, selected modes may be 'tuned out' by adjusting the blade mass, particularly near the tip. The geometry of tip ties or coverbands can be altered, or material added to/removed from shrouding, or the material of these components may be changed. Altering the position of the shrouding may change the natural frequency of certain modes and it may even be possible in certain circumstances to adjust the blade profile. Experience has shown that a continuous lacing or tip tie arrangement is less likely to form complex modes of vibration than blades connected in groups or packets, and is more predictable. Complex modes are likely to be excited by high frequencies and are almost impossible to 'tune out'.
The damping afforded by loose' pins, tip ties, lacing wire, etc. and the choice of blade material, minimises the effect of random or broadband excitation which is difficult to quantify. Damping reduces the response to resonances as erosion or impact damage changes the natural frequencies of particular modes and bring these natural frequencies into closer coincidence with harmonics of rotational speed.