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Jim,
You have provided a most thorough and complete set of
information related to your shake/vibration, and based on what you sent, I’ll
offer an explanation—and just a hint at what you might consider to
moderate, but probably not completely correct, the problem.
I believe your shake is caused by excitation of the natural
harmonic resonance of your main gear/wheel/brake assemblies. Every object
subjected to an acceleration force has one or more natural resonance
frequencies; most of the time it’s not such a nuisance/bother.
Specifically, your main gear legs have a natural resonance frequency, and one
or more pulsed inputs are acting on it at this frequency—generating the
shake you observed. The excitation(s) likely are present during all ground
taxi/rollout, but the frequency is harmonic with the gear legs themselves for
only a very small band of taxi/rollout speed, so it doesn’t cause the
gear to shake most of the time. The reason I’m not optimistic that you
can completely cure the shake is that it’s almost impossible to totally eliminate
the source—no matter what it turns out to be; and the only way to change the
natural frequency of the gear assembly by more than a small amount is to
redesign it.
You covered most of the likely inputs, but it’s worth noting—and
checking—them again. Tires out of balance and/or out of round are an
obvious and logical source. Ditto for the brake rotors being out of round, or
the rotors being rusty/dirty/defaced in such a manner that the brake pads don’t
exert an equal resisting force though 360 degrees of rotation. All of these
should generate the same frequency—based on tire rotation speed—so
any, or a combination of these could cause a shake at a harmonic of the natural
gear leg resonance frequency. This means that it’s possible to
experience this when the excitation input occurs at the harmonic frequency, or
at ˝, or twice, or other multiples of the gear assembly harmonic, although co-frequencies
normally generate the strongest resonance (severe shake).
Loose wheel bearings can amplify this, as can any other loose
components in the gear/brake/wheel assembly. The fact that changing brake
pressure affects the amplitude indicates to me that out of round/uneven rotors
are at least contributing to the shake.
The next thing you might consider is the wheel alignment—toe
in/toe out. Without going through the details, either situation allows the
wheels to track together/apart until the tire adhesion fails and the tire “slips”
just a bit on the pavement. This “unloads” the turning force a bit,
causing the gear to act like you were pulsing the brakes. This allows the wheel/gear
leg to move forward relative to the rest of the airplane; then the cycle starts
again, slowing and dragging the wheel back until it slips again. If the
frequency of this matches the harmonic frequency of the gear assembly, the
amplitude of vibration/shake builds up until you move away from this input
frequency by accelerating or slowing. Applying the brakes can change the speed
at which this occurs, and your note that the gear shakes on takeoff—at a
different speed—indicates to me that this component is contributing.
One thing you can do to check to see if harmonic resonance is
causing this is to change the natural harmonic of the gear assembly a bit by
removing the wheel pants. This changes the mass just a bit, and should result
in the noted shake occurring at a slightly different speed(s). The difference
will be rather small—equivalent to the percentage difference by which you
changed the total weight of the gear assembly by removing the pants—but it
will be detectable. This test is useful to confirm that the shake is caused by
harmonic vibration—but you still don’t have a solution.
If you decide that you are dealing with harmonic vibration, one
solution is to re-design the gear leg assembly. This is probably impractical, and
any you are likely to build will still have a natural harmonic that could be
better or worse than your current gear. Given your trouble shooting to date, I’d
start an alternative “fix” with the wheel alignment. Automotive
shops have elaborate machines for this, but if you can find a “skid pad”
that was used in the good old days, this should be adequate. Lancair sells
tapered shims for the axle mounts to correct alignment. As with all trouble-shooting
of this type, it’s important to re-test each time you make a change like
this—to confirm or eliminate this as a source.
Then go back through the wheels/rotors/tires for balance and
roundness. Do the best you can on each item. There is probably no “standard”
answer here, but the closer you can get to perfect in all items, the better.
Best analogy is the difference between a blue-printed and balanced airplane
engine and a “stock” one… the difference in vibration and
smoothness is remarkable…ditto for your landing gear.
Hope this helps…
Bob Pastusek
From: Lancair Mailing
List [mailto:lml@lancaironline.net] On Behalf Of Jim Scales
Sent: Thursday, September 06, 2007 6:47 PM
To: lml@lancaironline.net
Subject: [LML] ES shake (apparently not strut related)
Well, I'm back. This time with some new information, though,
so bear with me. The last thread about my ES shake issues revolved around
the nose gear strut and its role in the problem. The latest tests do not
support that position.
I have video taped the landing gear of my ES during slow speed
taxi, high speed taxi, light braking, heavy braking, normal take off and normal
landing. I taped the nose wheel, LH main wheel and RH main wheel
individually. I now have a pretty good idea of what the "shake"
looks like and where it is. Of course, this is not the same as knowing
what is causing it and how to fix it.
The nose wheel is rock solid with no shimmy, shake or shudder at
any speed. This surprised me. It also occurs to me that the
"shake" that I have felt through the years has not changed. That
leads me to believe that the nose strut has probably not been a part of the
problem on my airplane at all.
The video shows the main gear wheel pants exhibiting a light
shudder, moderate shake or what appears to be a fairly violent shake at various
speeds, conditions and brake application.
I did quite a number of tests and taped the following events
(the left main exhibited somewhat more severe shake than the right main, but
the events happened under the same conditions and speeds):
-Slight but definitely noticeable shake at steady taxi speed of 12
knots. One or two knots either side of that number and the taxi is
smooth.
-High speed taxi tests were conducted by accelerating to 50-60
knots, reducing power and braking as in a normal landing and
rollout. In every test the shake showed up at around 35 knots and
went away at about 30 knots when decelerating. The degree of the shake
was directly related to brake force. Harder braking, harder shake.
Light braking, light shake. No braking, very little or no shake.
Shake lasts somewhere in the 2-3 second range. It is felt in the seat
somewhat but is more notably felt in the stick as a rapid, short stroke, front
to rear movement.......think "rattle". The impression is also
there that the tail is jumping. Don't know if it is, but plan on taping
the tail during an event and see.
-Interestingly, the video showed that the mains shake moderately
for a second or two during acceleration, somewhere around the 40-50 knot
range. I believe this has been present for quite some time and that I
usually dismissed it, when I noticed it, as runway related. The runway at
my home airport was just resurfaced so I am now convinced that this shake is related
to issues I am trying to address.
If this were a car and exhibited these symptoms I would suspect
any or all of the following issues:
-Out of round tire, flat spotted or faulty ply.
-Severe out of balance tire/wheel assembly.
-Warped or wavy brake disc.
-Worn or defective suspension part.
At the present time, the condition of the wheel/tire/brake
assembly is: brand new discs, no visual defects or deformity in the
tires, balanced on static fixture.
I do realize that tire balance can be relative to speeds and that
a static balanced tire does not necessarily mean that the tire remains balanced
at all speeds and forces. The part of the puzzle that is most difficult
for me is the consistent speed relation to the shake. The current tires
are Goodyear Customs, as were the previous set.
Before that, I had McCreary. Before that was Michelin. I don't
remember what the original brand was that came with the kit. In all instances
the tires were statically balanced. In all instances, when the
shake occurred,
it was at the about the same speeds. As I write this,
though, I seem to have the impression that when I installed new tires the shake
would diminish somewhat. The speeds would remain the same but the
severity of the shake seemed to be less. I don't really have any
specific memories to support this.
The main gear has been aligned per factory specs. I checked
the alignment again during the recent annual inspection and found no problem.
I have run a variety of air pressures and can not really see any
difference in how the airplane acts. If there is an effect on the plane
it is certainly small. I currently run 60 in the mains and 45 in the
nose.
The discs and wheels show about a .015" run out. This
was the same variance that I found with the old discs. Evidently the
wheels are slightly out of true. The wheels and discs are the Cleveland
numbers that were with the original kit. However, it
would seem that if the disc were the problem that the shake would
occur at all speeds when brakes were applied.
I am trying to draw conclusions from this exercise and am having
trouble reaching the definitive answer. It would seem on the surface to
be a balance issue. That is, until I throw in the braking and the fact
that every set of tires has been balanced and every set has exhibited the
shaking to various degrees at about the same speeds and under the same
conditions as every other set. This makes me think it might be an
airframe related problem with harmonics and interaction of components, etc.
I wouldn't even begin to know how to chase that down.
If I were not so closely connected to the issue and were analyzing
someone else's problem I would probably point to the tire assembly balance as
being the problem and suggest that the tires and tubes be replaced and the
assembly dynamically balanced rather than statically balanced. I would
suggest the wheels be replaced or rebuilt to true so the run out as a possible
contributor would be eliminated. I would assume that when the tire has a
tendency to shake due to imbalance or tire deformity the application of brakes
magnifies the problem and gets the violent shake going. The person whose
problem I was trying to solve would probably point out that I was being too
simplistic and that the problem was much more complex than that. Just
thinking out loud............
I have been in touch with Lancair and they have been very
interested in helping solve the problem. They have asked some very
specific questions and offered some suggestions. I expect to continue
that communication as well as posting this on the LML site for any additional
input. I would appreciate any input. I apologize for the length of
this post but there was just too much information.
If I can figure out how to upload the video I will do that, too.
Thanks in advance for your comments, questions,
solutions.................
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