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How does a Frahm damper work?

belll 407 frahm damper rotor hub vibration mechanics engineering bell rotor blades

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#1 chrispochari

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Posted 16 April 2017 - 20:28

I know it is designed to reduce vibrations but how exactly does it do that? If Any Bell engineers on this forum could explain it without complex calculus that would be awesome ;)



#2 Eric Hunt

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Posted 16 April 2017 - 21:33

https://www.google.c...UD5fKJHqQOzfEM:



#3 chrispochari

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Posted 16 April 2017 - 21:36

so basically those springs absorb vibration?



#4 iChris

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Posted 16 April 2017 - 23:55

I know it is designed to reduce vibrations but how exactly does it do that? 

 

The FRAHM damper is named after its inventor, Hermann Frahm. Its common application, with regard to helicopter rotor systems, is often referred to as a Tuned Mass Damper (TMD). It consists of an oscillating weight, mass, which is attached to the main rotor system, setup in a way that the number of degrees of freedom and therefore the number of resonances of the complete system are dissipative.

 

A weight is located on the rotor hub axis and is held by springs allowing it to vibrate in the horizontal plane and in the other directions. The weight/spring system is excited by the periodic cyclic loads on the rotor hub and responds at the excitation (tuned) frequency thereby counteracting the excitation, absorbing rotor hub vibration.

 

In other words:

 

Ideally, the frequencies and amplitudes of the damper system and the main rotor should nearly match so that every time the rotor pushes one way, the damper system creates an equal and opposite push the other way, keeping the rotor system horizontal displacement at or near zero. There’re a number of different system out there, the names and terms are many, this is just one of many. Examples of rotor hubs below are the AS350 and Bell 407. 

 

You can view Frahm’s 1911 patent drawing at: http://www.freepaten...com/0989958.pdf

 

AS350%201_zpsngcl3n9j.png

 

AS350%202_zpsowerq05b.png

 

Frahm%20Assembly_zpsqe7evmjl.png


Edited by iChris, 17 April 2017 - 00:30.

Regards,

Chris

#5 chrispochari

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Posted 17 April 2017 - 04:03

 

The FRAHM damper is named after its inventor, Hermann Frahm. Its common application, with regard to helicopter rotor systems, is often referred to as a Tuned Mass Damper (TMD). It consists of an oscillating weight, mass, which is attached to the main rotor system, setup in a way that the number of degrees of freedom and therefore the number of resonances of the complete system are dissipative.

 

A weight is located on the rotor hub axis and is held by springs allowing it to vibrate in the horizontal plane and in the other directions. The weight/spring system is excited by the periodic cyclic loads on the rotor hub and responds at the excitation (tuned) frequency thereby counteracting the excitation, absorbing rotor hub vibration.

 

In other words:

 

Ideally, the frequencies and amplitudes of the damper system and the main rotor should nearly match so that every time the rotor pushes one way, the damper system creates an equal and opposite push the other way, keeping the rotor system horizontal displacement at or near zero. There’re a number of different system out there, the names and terms are many, this is just one of many. Examples of rotor hubs below are the AS350 and Bell 407. 

 

You can view Frahm’s 1911 patent drawing at: http://www.freepaten...com/0989958.pdf

 

AS350%201_zpsngcl3n9j.png

 

AS350%202_zpsowerq05b.png

 

Frahm%20Assembly_zpsqe7evmjl.pngThank you very much, I couldn't ask for more, where did you get these diagrams?



#6 pokey

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Posted 17 April 2017 - 11:56

harmonics, vibrations, wave attenuation, cancellation. Very precise and often misunderstood science. Acoustics too----look at noise cancelling headsets. Pretty easy to see what is happening, but very difficult to control. Eric Clapton, comes to mind.... oh yes, guitar.  Some music is good, some hurts. Back on track now. "OK mom!"  lunch is ready.



#7 iChris

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Posted 18 April 2017 - 00:48

Thank you very much, I couldn't ask for more, where did you get these diagrams?

 

The simplified dynamic behavior of the airframe can be represented by a spring K and a weight M suspended from the rotor, which excites the airframe at a frequency of 3w (three blade system).

 

The response of the weight M to this excitation (load F1) varies with M and with the natural frequency of the spring, a function of M and the spring stiffness. Therefore, depending on the airframe dynamics (M and K), the vibrations will either be amplified or attenuated but they will always be present.

 

If a weight m is now added via a spring k to weight M (m < M), the vibration characteristics are altered. Weight m is excited by weight M but it counteracts and tends to reduce the excitation load F0. It can even cancel it out if the resonator's natural frequency is equal to the 3w excitation. If the resonator (damping system) frequency is equal to the excitation frequency, the airframe does not respond since the weight M does not vibrate, i.e. the damping system cancels out the vibrations.

 

The AS350 diagrams are from an old Eurocopter systems training manual. The older manuals contained more technical information while the newer manuals became more operational than technical. The Bell drawings were from the B407 maintenance manual.  A practical example of these TMD principles can be seen in the following YouTube video.

 

Screen%20Shot%202017-04-17%20at%2010.11.

 

 

 


Edited by iChris, 18 April 2017 - 01:13.

Regards,

Chris





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