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Thanks for your input. Just to get the information up to date about
my taxi tests of the shake, it should be noted that the taxi tests were done
without the main gear wheel pants installed.
Jim
----- Original Message -----
Sent: Monday, August 27, 2007 12:03
AM
Subject: [LML] Re: ES Strut issues
What I take from the points below leads me to focus on the main
gear more than the nose. As Fred so well described, resonance occurs
when the modal frequency of a system coincides with the frequency of a
vibration source. If there is little damping, then the source can be
very small but still result in potentially catastrophic motion/damage.
Perhaps I'm misunderstanding, but it seems that:
1. Vibration has
been observed with various nose strut designs
2. Braking (on the
mains) has an influence on the vibration
Though I'm not arguing that
the nose strut does not have exhibit resonance itself, I wonder if the mains
act as an amplifier, that if removed would relegate a nose strut vibration
mode to an infrequent or non-existent issue.
I proffer that a small
imbalance (due to the wheel or rotor CG not being precisely on the rotational
axis), or uneven brake application (due to a slightly warped brake rotor) may
excite a longitudinal mode of the strut/wheel/pant. The energy of one or
both wheels/struts/pants moving longitudinally may cause a yawing motion of
the fuselage that either simply loads the nose strut laterally, or also causes
the nose strut to shimmy (energetic nose wheel yaw to strut roll
interaction). A modal impact (rap) test could be used to determine the
natural frequency of the strut and damping of the structure (which I'm
guessing is very low), and a simple calculation would show at what wheel speed
an imbalance would coincide with this. Removal of the wheel pants will
reduce the mass, thus driving the natural frequency up, and potentially out of
range of normal landing speeds.
I have no personal experience with
this, and offer the above solely as conjecture. If someone is willing to
provide the bird, I may be able to assist with said rap test. It would
have to be in south FL, after October.
Cheers,
-
Kyrilian
Skip Slater <skipslater@verizon.net>
wrote:
<snip>
I don't have a taxi speed that I
avoid, though I can get a shaking if I try to brake lightly at low taxi
speeds. I don't get any shimmy at all when taxiing with the brakes
off.
<snip>
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