How does a Frahm damper work?

[chris pochari]

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

[Eric Hunt]

https://www.google.com.au/search?q=bell+407+frahm+damper&safe=off&espv=2&tbm=isch&imgil=UD5fKJHqQOzfEM%253A%253BLT8IfTgTybjV6M%253Bhttp%25253A%25252F%25252Faeroparts.cadorath.com%25252Findex.php%25253Froute%2525253Dproduct%25252Fproduct%25252526product_id%2525253D1732&source=iu&pf=m&fir=UD5fKJHqQOzfEM%253A%252CLT8IfTgTybjV6M%252C_&usg=__yDY6Js3FPjhzxpSfBpXtoI8_Bos%3D&biw=1722&bih=1193&dpr=1&ved=0ahUKEwiPjb-6uarTAhVIn5QKHf3HAEcQyjcINA&ei=Cyn0WM_bLsi-0gT9j4O4BA#imgrc=UD5fKJHqQOzfEM:

[chris pochari]

https://www.google.com.au/search?q=bell+407+frahm+damper&safe=off&espv=2&tbm=isch&imgil=UD5fKJHqQOzfEM%253A%253BLT8IfTgTybjV6M%253Bhttp%25253A%25252F%25252Faeroparts.cadorath.com%25252Findex.php%25253Froute%2525253Dproduct%25252Fproduct%25252526product_id%2525253D1732&source=iu&pf=m&fir=UD5fKJHqQOzfEM%253A%252CLT8IfTgTybjV6M%252C_&usg=__yDY6Js3FPjhzxpSfBpXtoI8_Bos%3D&biw=1722&bih=1193&dpr=1&ved=0ahUKEwiPjb-6uarTAhVIn5QKHf3HAEcQyjcINA&ei=Cyn0WM_bLsi-0gT9j4O4BA#imgrc=UD5fKJHqQOzfEM:

so basically those springs absorb vibration?

[iChris]

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.freepatentsonline.com/0989958.pdf

 

 

 

Edited April 17, 2017 by iChris

[chris pochari]

 

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.freepatentsonline.com/0989958.pdf

 

 

 

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

[Guest pokey]

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.

[iChris]

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.

 

 

 

 

'>https://youtu.be/f1U4SAgy60c

Edited April 18, 2017 by iChris