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Book Review:
Mechanical Vibrations by J.P. Den Hartog

Sitting around discussing my balancer/balancing problems with my mechanical engineer friend at the end of a long weekend party, he wearily suggested and loaned me a copy of Hartog’s Mechanical Vibrations, a textbook by an MIT Professor Emeritus of Mechanical Engineering.
I have to repeatedly thank the ole’ codger because this book continues to provide major ‘Aah Hahs!’ for me. While the author assumes the reader is an engineering student, he does provide explanations of the results of mathematical derivations. His description of the affect of critical speed of a rotating shaft was critical to my understanding the phase angle information coming from an electronic balancer being used on my tail rotor.

I’ve discussed his statements on this subject with a variety of people having loads of experience in the rotary wing field and haven’t found anyone saying: “Oh yeah. That’s obvious to the least tutored in the field”.

While calculus is often used for derivations, the results are explained in an easily understood choice of words. For example, see the highlighted quote in the article on balancing elsewhere in this issue (pg 6).
Included are some startling examples of the use of a good understanding of mechanical vibrations. If calculus doesn’t frighten you, by all means get a copy of the book. If it does leave you cold, at least talk to some one that has one and see if they can find explanations in there that are useful with your helicopter.

Yeah, I hear some of you saying that all this fuss is just about resonance and a cool pilot like you can tell when the ship starts shaking real hard. But let me tell you from experience you cannot feel all there is to the resonance and what you cannot feel is very important.

Prof. Hertog also describes the cause of a secondary resonance occurring at a lower frequency, than the normal resonance frequency, in shafts that lack total symmetry; like shafts with keyways or flats, (like most tail rotor shafts).

You say you don’t have any vibration in your helicopter? It flies totally smoothly? Whoo-ee!! Don’t fly over my house. I’ll repeat the old codger’s statement: “If you are in a helicopter and don’t feel any vibration, feel free to step out and walk away”.

Stu Fields

Email: eh@iwvisp.com