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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
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