Thanks Claudio Pedrazzi sir for your valuable information about stress-stiffening and spin-softening.

Hi

Both answers are correct.

A standing wave pattern forms from wave interaction that superimpose and reinforce.

When dealing with rotating systems, one deals with vibration shapes that act with- or against- the rotation direction, i.e. the waves that create the mode pattern must propagate with or against rotation. On top of this, gyro forces are at work.

Such modes are referred to as forward- or backward- rotating modes.

https://en.wikipedia.org/wiki/Rotordynamics

See Figure 6 here where the BW rotating modes is at ~10 Hz at stand still

http://qringtech.com/2017/05/20/torsion-vibration-measurement-and-analysis-of-flue-gas-fan/

It is a bit like the Doppler phenomenon. Backward and forward modes merge into a single mode shape at zero rotation.

The basic situation is similar when there is flow present and wave motion is superimposed.

/Claes

Hi, Claudio Pedrazzi and Claes Richard Fredö. I wish to understand rotor dynamics better. Can I say that the stress stiffening is due to the centrifugal forces and spin softening is due to extension of the blade? Can you people suggest some good reading material for the basics of rotor dynamics. I have tried reading some of the online material but I am still clueless about forward and backward whirls. Thank you in advance.

Hi

This might help?

https://dyrobes.com/wp-content/uploads/2016/04/Introduction-to-Rotor-Dynamics-Critical-Speed-and-Unbalance-Response-Analysis-E.-J.-Gunter_linked.pdf

/C

Thanks for the attached PDF, Claes Richard Fredö but I couldn't find any reference to Campbell diagram or backward and forward whirls in this article.

Take a look below (2.7-4). They call it critical speed as this is of practical interest for rotors.

/C

Thanks Claudio Pedrazzi