|
It sounds like accelerometers mounted in suspect
locations with the engine running at the RPM indicated by the spike shown by the
Apple app. would lead you to the culprit.
Wolfgang
----- Original Message -----
Sent: Friday, July 26, 2013 12:47
PM
Subject: Re: [LML] Re: Potential
Problem-Engine Vibration
Kryilian,
I too have this vibration phenomenon in VH-XTZ – I spent hours and hours
of flying time trying to isolate it, trying different power settings,
different aircraft loading, different angles of attack, you name it – I could
not find a pattern. Mostly its there, but occasionally nothing. Visually, it
manifests itself with a very slight port wing tip / aileron quiver;
physically, you can mainly feel it ever so slightly in the rudder pedals.
Having said all this, others fly in the aircraft and don't know what I'm on
about. But, I have now found that running at about 2540rpm is quite
smooth and all but gets rid of the vibration. Its noticeably worse at say 2400
– 2450rpm.
So – endeavouring to quantify things, I employed the iPhone App you have
mentioned. The attached pics are from way back in 2011; these should
give everyone an idea of what can be produced. Below my comments at the time
wrt to attached samples.
As one of our other forum contributors mentioned, vibration is very
subjective without the use of measurement devices… so can I suggest that those
whip have access to iPhones go flying and capture outputs in the same manner
and with same engine settings – how about with iPhone resting on centre
console and engine set at 2500rpm, 19" and leaned 28degC (50degF) rich of
peak…. Or as anyone else might suggest otherwise. Then we can all
collect data that should be legitimately comparable, and perhaps identify if
there are any substantial differences in the vibration characteristics.
Anyways, just an idea….
Comments from tests in August 2011:-
Captured
three outputs with 2400rpm and about 19” MAP. First sample with iPhone
on the centre console shows a spike at ~20Hz – that is approx. equivalent to
1200rpm, which exactly half the engine engine rpm. For other two samples,
iPhone on the dust cover in the middle and then a bit to one side; these show
the same 20Hz spike, and also a spike around 38 - 40Hz – this would seem to be
close to the engine rpm, but 38Hz is equivalent to 2280rpm, which does not
make sense as there is no way the engine rpm varies by that much. So, I’m not
sure what all this means. I’d love to get the iPhone onto the rudder pedals,
but that’s kind of tricky! The iPhone doesn’t measure above 50Hz, so I can
detect prop beat or exhaust beat.
Regards,
John
John N G Smith
Tel / fax:
+61-8-9385-8891
Mobile:
+61-409-372-975
Email:
john@jjts.net.au
If you'd like to measure the frequency and amplitude of relatively low
frequency vibration I'd recommend an iPhone app called 'Vibration' ($4.99
from the App Store). It's certainly not high end but it may help in
characterizing some of the vibrations that you guys describe. The only
problem I see is that it only samples up to 100 Hz, which gives you
frequency data up to 44.9 Hz (~2690 rpm). This won't give you any data
at propeller frequencies. But it may help tell if its a function of
RPM at 1/rev, or if it's asynchronous with the engine (and doesn't correlate
with engine speed, such as buffet or flutter).
A few screenshots (recording of an out of balance car tire at 70 mph)
are shown below. Note that the app can output data to a .csv file, which can
be reviewed/analyzed later. I do this sort of thing with proper
accelerometers, recorders and analyzers on helicopters in my day job, so I'd
like to help if I can. If anyone has luck with the iPhone app please
send me the data files. Perhaps we could then compare between
aircraft.
Regards,
- Kyrilian
L2K-236
Time display:
DFT (note spike at around 16 Hz)
Setup (one of several sub screens)
Sent from my iPhone
I have a smooth running Legacy RG IO-550 with a three blade Hartzell
scimitar prop, HC-J3YF-1RF/F7391D-3/SM7.
According to Hartzell's website, this is the recommended three blade
scimitar prop for the Legacy RG with IO-550 engine. It is a 72"
diameter prop, not the 78" prop mentioned in Bob R's posting. I
thought all the scimitar props on Legacys were the same as mine.
I consider mine to be "smooth," but that is a terribly subjective
concept. However, I can say for sure that I do not feel vibration in
the stick and don't see any aileron vibration or wing tip
vibration. Another Legacy builder and pilot commented on how
smooth mine is, but maybe he was just being polite.
Unless there is a way to quantify "vibration" and "smoothness," I
don't see how we can accurately communicate about this. One person's
rough could be another person's smooth. Maybe if someone had a
video of stick shake or aileron shake? A glass of water sitting on
the glare shield?
Dennis
Legacy 625 hours
|