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Sr. Member
Posts: 171
Reply with quote  #1 

Do you notice that the vibration at the airplanes relatively high compared to what you might accept in plants?

Well, I do not travel a lot but I notice this especially with large planes when the handbag compartments are available in the center of the plane (to the right, at the center and to the left). I could see the vibration with my naked eye. So I assume it is more than 500 mil pp.

Also, the seats vibrate during take-off and landing. I have a strong desire to take vibration measurements when I see this.

Regards- Ali M. Al-Shurafa

Sr. Member
Posts: 1,918
Reply with quote  #2 

Jet engines are a class worse balanced compared to aero derivatives operated on ground at least the RR Avon.

Since the mounting are so much more "flexible" in the air is the only reason I got and I for sure believe that.
Humans are sensitive for lo freq. vibration. I have seen people running out of hydro turbine plants at 3 mm/s but low frequency that trigger some escape mechanism I think.

Some designs are obviously passing shell or frame resonances at take off and again I think it is rather low freq. and then you can see it and feel it and yes I guess amplitudes are
pretty hi. I notced that mostly in older Airbus versions, they improved in the newer I think but only subjective it may be influenced by the seating. It may or may not involve the reason for cracks in the beam system holding the wings to the fuselage "pickle fork beams" in quite a few 737NG that was found recently when converting them to cargo versions rebuild certification and quite a few was found at the alert inspection performed on all those flying at only like 50% expected lifetime. Not to mention the classic Hawaiian 737 that got cabbed as a section of the upper fuselage behind the pilots cracked and flew off. If you see thru the wall beside the door when entering the cabin, alert the staff or leave...... That was the case on that one.

You can see nice ODS demos of wing and tail flapping in one of the classic ODS software demos, looks quite nice.

I heard a nice story from CSi sales back in the day where they pulled out their toy 1900 and measured on the DC 9 engines
at their mount from the inside while flying if you were placed in that bad seating you could get a good reading from the aux gearbox on maybe the left engine
and they were as I believed quite different, some did have a not healthy noise if you sat in the seat close. So I tried to avoid that area. 


Good Vibrations since early 1950's, first patented vibrometer 1956 in the US.

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Posts: 1,810
Reply with quote  #3 
I'm more freaked out by the movement of some overpasses when I find myself stopped on one in the car.  I always wonder if they are designed to be "flexible" so as not to fall victim to cyclic stresses, or if they are designed to be rigid and just aren't. [confused]
"The trend is your friend"
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