Hi Gyozo
to try and brush up on stuff i learnt 30 years ago i asked dr google.
he/she was very forthcoming with a lot of information. some good and
some could have been found scribbled on a public toilet wall in black
felt tip pen.
what i had overlooked was that this is a resonant vibration and when
its in action (vibrating or shimmying) it is part of a tuned system(ie
harmonics through the fuselage, wings, tyres, cables, tail plane, main
gear etc etc.) so in operation when landing, if the tyre hits something
as big as a pebble or even a grain of rice really, it can initiate a
small movement in the wheel which can excite a movement or wave in the
fuse (or elsewhere)and transfer to the wings and then back again to the
wheel moving it in the other direction which again creates a wave or
movement and so on and there you have a self perpetuating resonant
shimmy. and then after a while as tom said your tyre may have worn or
changed shape and may continue to be the initiator of the shimmy(sorry
if i didnt explain that very well. at least the picture was clear in my
head.) Any part of the system can add to or negate this effect by
bringing it into or out of "tune" with the rest of the system. things
such as loose cables, tight cables, worn bearings, new bearings, odd
shaped tyres, under inflated tires overinflated tyres... you get the
picture.
now after all that if youre still on board my question is, did this get
gradually worse or did it just appear one day?
it it just appeared overnight i would look for things that would change
suddenly like things cracking or breaking (or bulging in the case of the
tyre) but if it just got worse over a period of time them i would look
to things such as gradually softening tail wheel rubber mounts, springs
stretching etc etc.
my moneys still on the rubbers though :-)
cheers
ric
On 26/09/2020 4:18 am, Tom Preisser wrote:
Hi Gyozo,
Be advised that after you have replaced your shock mounts, you should
also replace your tire. A tire that has suffered prolonged shimmy will
acquire wear that will sustain the flutter - the only fix is to grind
the tread or install a new one.
Tom
On Fri, Sep 25, 2020, 13:40 Győző Horváth <[email protected]
<mailto:[email protected]>> wrote:
Hi Richard,
thank you for the photo.
I put on some angles and a distance which can help the
quantification of the problem. Illustrated picture is attached.
As I see the kingpin angle of your Dimona and mine is very similar
96 degree. However the the kingpin-fork angle in yours is only 15
degree and in mine it is 36. The rake angle and trail distances
are different as a consequence. It can be due to your solid rubber
shocks. Mine rubbers are worn and soft. It is visible on the
pictures that yours keep wider distance. This relates to Ian’s
comment with the washers.
What do you think?
I do not know what is the relation between the shimmy and rake
angle or trail distance. Should those be smaller for avoiding the
shimmy?
On the basis of John’s comment and experience it can be so.
Does anyone know the theory behind?
It would be interesting and useful to collect the numbers of these
angles and distance from several Dimonas. Unfortunately the
manufacturer’s data are not published or I am not aware of it.
BR
Gyozo
2020. szept. 25. dátummal, 15:58 időpontban Mister Sutton
<[email protected] <mailto:[email protected]>> írta:
Hi Gyozo
i went a hunting. i was looking for some photos i had taken of
mine a short time ago but instead found this one (attached)
notice how the wheel axle(bolt) is only marginally trailing
behind the "kingpin" (castoring axis) in the attached image where
as the in the image of your aircraft lit looks to be almost an inch.
let us know what you find
cheers richard
On 25/09/2020 3:50 am, Győző Horváth wrote:
> Hi,
>
> Hereby I attach some photos of rear gear.
> Perhaps somebody can see some disorders on it.
> The rubber shocks are worn, however evenly.
> Any further comments are welcomed
>
> Gyozo
<IMG_1789.jpg>
<IMG_1791.jpg>
<IMG_1794.jpg>
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