# 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? Jan 23, 2018 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. Jan 23, 2018 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. Jan 23, 2018 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. Jan 24, 2018 at 0:23
• Ah yeah, fixed it. Jan 24, 2018 at 0:26