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Name: Cesar M.
Status: educator
Age: 30s
Location: N/A 
Country: N/A
Date: 12/20/2004


Question:
I will like to know the concept of harmonic vibrations. I am a flight instructor and I know for example that the hydraulics pumps driven by the transmission rotate at different speed to reduce harmonic vibrations, undesirable because failure and cracks of components and structure of the helicopter will occur. Please explain to me how this phenomenon occurs.


Replies:
Physical structures -- buildings, bridges, automobiles, or aircraft -- can vibrate. Like a violin, piano, or horn there are certain vibrations that reinforce one another. These are called resonances. They can cause the structure to vibrate with large and/or increasing amplitudes. This flexing puts stress on the same locations of the structure and the moving back and forth in the same place eventually weakens the structures at these flex points resulting in damage or failure. In order to obtain a quiet ride in an automobile designers engineer the suspension to have many different vibrational modes. Then they tend to cancel one another. If you have ever seen a car with bad shock absorbers, if you push up and down, the car will begin to rock in "harmony" with your pushing and will continue to rock rather than damp out. I'm not sure that it is true, but anecdotal reports tell that troops marching across a bridge break marching cadence because if everyone is stepping at the same frequency this harmonic motion can cause the bridge to begin to vibrate and perhaps collapse.

Vince Calder


I am a structural engineer and we also have to deal with vibrations. Perhaps approaching from our view may help. Bridge designers often incorporate large springs within the structure to deal with the problem, trying to dampen vibration, sort of like shock absorbers in a car. With bridges, mainly the problem is wind. The classic is the Tacoma Narrows bridge ( I think Verazona straits, try a Google) where there was no thought to vibration, the structure just designed for wind loading. In , I think the fall when the wind whipped up, it whistled thought the cable suspension system ( the bridge is similar to the Golden Gate). Anyway, those vibrations carried down to the deck, the deck began oscillating, the waves getting more extreme, till the bridge literally tore apart. The harmony being the addition of like waves. A phenomena occurs, sometimes even employing signs in applicable regions, an Army troop marching at full cadence is ordered to stop marching and walk across, at their own individual gaits. The" in time" stepping produces vibration, the multitude of soldiers adding to the vibration. So, sound waves being not visible, to be understandable think of other waves. When trying to think of waves, as sound waves, picture other waves, like waves in water. Opposing waves can cancel each other, or like the troops, if in the same direction , same shape, can add, I guess, hence the word "harmony" trying to explain that addition, or extra strength.. Hope this helps.

James Przewoznik


Vibration in general is bad for materials. It can lead to fatigue of parts and eventual failure. But, if you know the extent of the vibration, you can properly design the component(s) to live longer than the you expected to use the component. That is why planes and helicopters have limited life. There are two types of vibration: free and forced. Free vibration is when a component vibrates under the action of forces inherent in the system, specifically mass and stiffness of the material. Every system has this property and it is called natural frequency. Forced vibration occurs due to external forces causing vibration of the system. This is like a helicopter blade rotating and causing vibrations throughout the whole helicopter. Now if a forced vibration coincides with the natural frequency of a component, then large oscillations can result, tearing the component apart. One of the most famous examples of this was the Tacoma Narrows Bridge. Oscillations started due to the wind (aeolian harp). These oscillations corresponded to the natural frequency of the bridge, which led to excessive force on the bridge which caused it to collapse. I am sure there is video on the web of this disaster. To limit vibration problems, analysis is done to minimize vibrations occurring around natural frequencies of components, hence your mention of the pumps. We can determine the frequency that will be transmitted by the pumps and do things to mitigate the problem, i.e. dampers, running at different speeds, etc. Different materials can be used (different mass) or a different component shape can be used (different stiffness) to also limit natural frequency problems of the component that may see these vibrations.

Hopefully this helped. Thanks for using Newton.

Christopher Murphy, P.E.


Basically, all materials are slightly elastic or "springy." It might not seem like it, but steel beams, pipes, buildings, bridges, and tuning forks can all be made to vibrate in different ways.

Like a stretched guitar string that makes a certain musical note, materials and structures tend to vibrate at certain natural frequencies. If you could push on a building really hard, it might vibrate back and forth at a frequency of 1 cycle per second, or Hz. That's its natural frequency. (Actually it is more complicated - the building might vibrate sideways at 1 Hz and up and down at 3 Hz.) A drinking glass might vibrate at 500 Hz if you hit it with a spoon.

If the item is "driven" by a force at the natural frequency (forced oscillations), vibrations can keep getting larger and larger. Sort of like pushing someone on a playground swing. If you push correctly, the person can swing very high.

Loud sound can shatter a drinking glass if it is at the right frequency. See http://www.acoustics.salford.ac.uk/acoustics_world/glass/glass.htm. The right kind of earthquake, if it lasts a long time can topple a building if it gets into "resonance".

Vibrations are a big problem in machines that rotate. If you speed up a car engine you may hear buzz and vibrate at different engine speeds as things in the engine compartment start to vibrate. Sometimes these vibrations can cause damage.

So, in light of the resonance problem it is reasonable that a hydraulic pump would have a rotation speed chosen so that it does NOT induce any vibrations in other parts on the helicopter.

Bob Erck



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