JonFirst thing I used to do was check each blade statically on the bench (p
rop not assembled) Jerry Davis used to offer a description of how to do thi
s, basically=C2- mock up a wood support for the blade, the fulcrum at one
end represents the centre of the prop. (I knocked in 2 nails on a line at
right angles to the blade span) The blade is then set on the rig so that it
's root end is at the equivalent distance from the centre of the assembled
=C2- prop (you need to measure this carefully).=C2- A small block on th
e rig near the fulcrum gives a way to butt the prop against at it's correct
spanwise position.
A third nail roughly 2/3 of blade span knocked through the rig acts as the
datum.
Now measure the weight of each blade at this datum point when placed on the
rig. This weight will be proportional to the moment of the blade and all t
hree need to be equal. Use an accurate digital balance.
It is not satisfactory to simply compare the actual weight of=C2- each bl
ade. The moment, which is what's important, can be different even if weight
is equal.
Arplast used to adjust the moment with lead shot dropped into a spanwise ho
le down the centre of the blades.You should find it is about 8 or 9 inches
deep.
Theoretically it should be possible to achieve good balance this way withou
t running the engine but there are more than enough variables affecting dyn
amic balance, as you have not doubt realised.
Graham.
On Friday, 25 March 2016, 15:09, italianjon <jon.catilli@gmail.com> wro
te:
Dear All,
I need some help in understanding the information I have gathered on the di
agnostics of my propeller.
I used the Smart Avionics PB-3, the configuration for the setup is illustra
ted in the diagram. The PB-3 was mounted on the port side of the manifold a
nd the pick up reflective strip was mounted on blade 3. The red block shows
the PB-3 vibration sensor and the blue line highlights the x-axis orientat
ion.
Forgive the handwriting, I did it with a mouse :)
Ignore the axis in grey. I have used an illustration from the Rotax Install
ation manual to give context to my diagram, and are not used for my purpose
s.
Point 1 is my starting reference.
Point 2 had 1 washer (ca. 4g.) removed from blade 3.
Point 3 had the washer returned to blade 3, and 1 washer from blade 2 to bl
ade 1.
Point 4 had a washer from blade 1 moved to blade 3
Point 5 had 2 washers removed and moved 1 to blade 1 and 1 to blade 2.
In total blade 2 ended up 3 washer down on the start, blade 3 ended up 2 wa
shers up on the start and blade 1 ended up 1 washer up on the start.
As far as I understand the PB-3 0=C3=82=C2=BA is the alignment in the X-axi
s, when is X-axis mode on the PB-3 app. Plus it registers the out of balanc
e when the force pulls it in the X-direction. Therefore the propeller has t
o rotate between the 225=C3=82=C2=BA and the 284=C3=82=C2=BA for the out of
balance to be register (Very simplistically speaking).
>From my first diagram, and given 120=C3=82=C2=BA between blades, then blad
e 2 sits at 240=C3=82=C2=BA=C3=A2=82=AC=C2 or basically where I under
stand the imbalance to be if I have interpreted my data correctly.
With so much weight removed from the blade 2 I would have expected the imba
lance to move inwards, but instead it remains in more or less the same spot
. Leading me to conclude that a =C3=A2=82=AC=C5=93bigger=C3=A2=82=AC
effect than a mass imbalance is at play.
Blade 2 is also the one that I previously mentioned, in another post as hav
ing more play than the other blades.
I know the Arplast PV50 has play in the blades when it is at rest. This tim
e I attempted to measure the at rest play. Blades 1 and 3 are fairly consis
tent with each other and approximately 1=C3=82=C2=BA of play. Blade 2 has a
pproximately 4=C3=82=C2=BA.
I think what I am looking for, and the reason for this post, is for someone
else to think about my workings to sanity check if I have missed anything;
or also to check the conclusion is genuine and I am not just =C3=A2=82
=AC=C5=93looking for evidence of blade 2 because that is my apparent proble
m=C3=A2=82=AC.
Thanks all
Jon
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=C2- =C2- =C2- =C2- =C2- -Matt Dralle, List Admin.
S -
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=C2- =C2- =C2- =C2- =C2- -Matt Dralle, List Admin.
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