# What is the range of frequencies of vibration in a helicopter due to the main rotor alone?

Vibration in helicopter can be caused by many different reasons. I am interested to know what is the range of frequencies of vibration due to the main rotor alone?

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

The vibration from the main rotor (and of the tail rotor) are harmonics of the Blade Passing Frequency: number of blades times rotational velocity. This site contains some data on the Blade Passing Frequencies of some helicopter types, for analysis purposes for military helicopters which must comply with MIL-STD-810F.

MIL-STD-810F, Method 514.5, paragraphs 2.2.6 and 2.3.3 are concerned with the vibration levels that helicopter components and cargo must withstand. The helicopter vibration environment is a combination of many sinusoidal components due to the main rotor, tail rotor, gearbox, and engine.

Since the vibrations are also imparted on the surrounding air and then onto the fuselage, acoustic measurements are a good indication of the mechanical vibrations as well. Above graph shows acoustic measurements of seven Hueys flying by, and shows harmonics at intervals of 12 Hz. From the referenced article:

The first notable peak occurs at 24.0 Hz. A series of harmonics occur at 12 Hz intervals thereafter. These are the blade passing frequencies of the main rotor.

The main rotor blade passing frequency is: 5.40 Hz x 2 blades = 10.8 Hz

The Doppler shift increases the apparent blade passing frequency to 12 Hz. The theoretical speed of the helicopters was 76 mph based on this shift.

An interesting factoid is that the highest peak is not caused by the main rotor but by the tail rotor: the one at 123 Hz.

The highest main rotor vibration peak occurs at 60 Hz, five times the main rotor Blade Passing Frequency. There are still noticeable 12 Hz peaks at up to 300 Hz sound frequency = 270 Hz frame vibration frequency.

EDIT

This presentation shows vibration measurements taken off of an unnamed helicopter. The rotor frequency is detectable as a small peak, the blade frequencies are much more dominant.

• The links discuss acoustic frequency, will the spectrum be different if we place a sensor at the body of the helicopter to measure movement/vibration of the structure? Jan 16, 2018 at 4:56
• @Creator The acoustic vibrations have the same source as the mechanical vibrations. There will be a difference in the amplitude of the different frequency ranges when measured directly at the frame, as acoustic propagation has different damping characteristics than mechanical propagation. But the general trend will be similar: harmonics of the BPS of main rotor, tail rotor, transmission gear wheels, turbine & compressor BPS. Jan 16, 2018 at 5:01
• Does the acoustic spectral peaks depends on the number of blades? I assumed it does. Is the structure vibration would be dependent on rpm alone? I understand your point that the source is same only the medium of the wave is different (air and surface). Would you help to clear my confusion? Jan 16, 2018 at 17:07
• Good question. The structural vibration would also be a function of number of blades: if you follow a blade throughout a revolution it tilts up and down, lunges forward and back. The acoustic vibration mainly picks up the blade lift pulse as it passes a ground observer, these are also imparted on the fuselage, through the air in between the rotor and fuselage. Mechanical vibration would look different for different hub layout, a teetering rotor like the Huey has does not allow for individual blade lead/lag motion. But still the blade passing frequency would be visible on mechanical vibration. Jan 16, 2018 at 20:44
• Thank you. I am interested to know the lowest spectral peak (like resonance not harmonics) at the structure of an helicopter for rotation of the main rotor. Can I assume it is approximately proportional to the product of number of blades in the helicopter and RPM of the main rotor? Jan 16, 2018 at 20:54