Hohn,
I doubt that the setup is wrong.
My concerns on vibration is two fold.
First is crank shaft alignment. Having a love hate relationship with
all things airplane, I am not criticizing the Rotax, but it is possible
since it is a 5 section crank shaft or so, the shaft may have slipped as
it is only pressed together. This is highly unlikely unless the engine
was bought used and had a prop strike that was undisclosed. In this
case the engine vibrates is over a wide range of RPMs. I had a 914
with this problem once. It never balanced right.
Next is that the cracks indicate to me that the failure is due to
lateral movement of the silencer, or attempted movement, which is
restrained by the pipes. This side to side stress means the harmonic
could be coming from the piston induced horizontal vibration of the
boxer engine. This could be from a lack of proper harmonic balancing of
the crank which is corrected by use of a rotating weight or other means
such as crank shaft lobes to dampen the harmonics. Since many LSA
aircraft fly with the 912S, I doubt that they have recorded exhaust
problems we can draw from so we are in the blind right now. I know the
Ran's aircraft 912S installations have no exhaust pipe problems to speak
of, but they do get failures at the weld points occasionally. I have
never studied the exhaust pipe springs and silencer movement closely on
the ground engine run-ups in our aircraft, and probably should.
Frans correctly noted that the exhaust needs support or exhaust pipe
dimensions changes, to affect changes to the harmonic. Problems arise
with supporting the exhaust as it does move with heat, and can only be
supported to the engine such as on the 914. Perhaps some stainless
brackets supporting the silencer to the frame may be appropriate. An X
type support comes to mind on the aft pipes to stabilize the silencer.
Even a .041 stainless safety wire may work. The 912 has no problems
with this cracking problem in my shop, the 914 is supported extensively,
and only the 912S seems to have the issue. So must I assume it is an
engine specific problem. Probably not an accurate assumption as
usual...
My welder inspected the crack as did Chris Piper at CKT and the type of
stainless may be the problem, but the jury is still out on that. There
is not enough data on this problem. Chris at CKT, has solved the
horrible exhaust silencer failures of the past and beefed up the
previously anemic down pipes and I thought we were all right, then this
problem started to creep up. We are considering the pipe bend and weld
when under heat stress is the problem. Changing the material and heat
treating may solve this if it is not an engine related problem. Extra
support as Frans suggests may be necessary.
The Warp Drive and other props too, do vibrate under compression braking
during a de-acceleration but it is slight as you noted. This vibration
doesn't bother me as it is not constant and is only momentary and
changes in its harmonic (as measured by the giggle of my calf muscles
which are not registered with the bureau of standards). So if the
run-up and full power dynamic balance of the prop is fine, I am not
worried about the prop causing this problem.
More will come out. In the mean time, I will try a safety wire X brace
and see what happens.
Regards,
Bud
----- Original Message -----
From: JonSmith<mailto:jonsmitheuropa@tiscali.co.uk>
To: europa-list@matronics.com<mailto:europa-list@matronics.com>
Sent: Monday, August 27, 2012 6:51 PM
Subject: Europa-List: Re: XS Exhaust System Cracks
<jonsmitheuropa@tiscali.co.uk<mailto:jonsmitheuropa@tiscali.co.uk>>
Hi Bud, Many thanks for your input on this. Interesting to hear about
the problems you've had too. Just to give you some data from me, my
problem has been both the rear pipes cracking, ie cyls 3 and 4. I first
found them both cracked together after about 15 hours flying on my new
engine installation. They were both cracked about 75% around the
circumference inside the tightest part of the bend. They may have been
like that for quite a while as they are almost impossible to spot
without a mirror when in situ. It was taking no. 3 off to fit the new
HD starter I quite by chance found it cracked. I had them both welded
up. After about 2 more hours flying I found no. 3 cracked again,
following almost exactly the same line as the first time (but not quite
on the weld repair - it was fresh metal that was failed rather than the
repair itself). I had it welded again. Since then I did just a quick
20 minute flight and have found no. 4 has cracked again, same pattern.
So each re!
ar pipe has cracked twice now. Both front pipes have been ok to
date. The cracks do seem to run through where the spring attachment
loops are welded on as my previous picture shows. As far as I am aware
my springs are stainless - as supplied by Europa (they look stainless to
me!). The engine runs very smoothly - as you say it can be a bit clunky
on start/ stop but not excessively. My propeller is a BSW (Bog Standard
Warp!) ground adjustable 64" non tapered blades warpdrive fitted to my
912 ULS. I always keep the carbs well sync'd. I do notice sometimes a
slight vibration during slowing down and/ or descending - if flying fast
at a high power setting when I reduce power at high speed, at a certain
point during the power reduction - usually when the throttle has been
brought a fair way back a slight vibration/ resonance can sometimes
suddenly kick in but it's only ever very slight, a slight tremble than
an actual vibration. More of a slight change in sound. I've never t!
hought anything of it. I don't really know where to go with t!
his now
as it seems to be an ongoing problem I have and I can't really fault
anything else in my setup.
Cheers, Jon
--------
G-TERN
Classic Mono
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