# What is the range of amplitude of vibration in a helicopter?

Vibration in helicopter can be caused by many different reasons. I am interested to know what is the range of amplitude of vibration?

I am interested to know how amplitude varies with frequency of vibration? I am looking for an answer in terms of distance from a reference or equilibrium or stopping condition. For example does the maximum distance (amplitude) from the reference is 2 cm or less or more when helicopter moves?

I understand that it would depend on the helicopter type. So any reference to such analysis would be appreciated.

This is similar/extention to the question What is the range of frequencies of vibration in a helicopter due to the main rotor alone? where it asked about frequency of rotor.

• Seems like the answers to your previous question answer this? Or at least would be a starting point? – fooot Jan 23 '18 at 22:46
• @fooot The previous question is not the same. I am in desperate need for some information. This site helped me (really big way) and now I need more information so i have asked. It is not that I did not search the web, but I could not find any proper answer so I am asking. – Creator Jan 23 '18 at 22:53
• I just thought it would be good to mention your previous question here, given that it's related and also provides the frequency vs. amplitude graphs that should be relevant here. – fooot Jan 23 '18 at 22:57

The amplitude of the vibration depends on the frequency and the acceleration: at a given acceleration, only at low frequencies do we get any significant displacement. If displacement is taken as the intensity indicator, the frequency plot is weighted towards low frequencies - the higher frequencies have a higher energy contents, that is why often a logarithmic relative scale is used (dB).

This document gives good information and background on measuring th severity of vibration. It can be done with an accelerometer: integrating acceleration yields velocity and displacement as well. From the document:

Experience has shown that the overall RMS value of vibration velocity measured over the range 10 to 1000 Hz gives the best indication of a vibration's severity. A probable explanation is that a given velocity level corresponds to a given energy level so that vibration at low and high frequencies are equally weighted from a vibration energy point of view. In practice many machines have a reasonably flat velocity spectrum.

The vibration frequency plot in the previous answer is in dB acceleration, an earlier slide shows the reference. If it is displacement of a particular frequency in the plot you are after:

• Read dB acceleration at the particular frequency
• Convert into m/s$^2$ to get acceleration $a$
• Amplitude = $\frac{a}{(2 \pi f)^2}$

• The link to the document is not working. – Creator Jan 24 '18 at 0:23
• Ah yeah, fixed it. – Koyovis Jan 24 '18 at 0:26