Kyrillian, thanks.  I will check out this app.  I think a good place to put the iPhone would be to clamp it directly to the copilot's floorboard.

I have never had alarming vibration like Steve Colwell cites, so maybe we are talking about two entirely different situations.

I cruise at 2300.  Continental has had some crankshaft problems with engines that were cruised at 2200, but I forget the details and I'm not even sure they were 550s.  On Jul 26, 2013, at 5:22 AM, Kyrilian Dyer wrote:

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:
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DFT (note spike at around 16 Hz)
<image.png>

Setup (one of several sub screens)
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Sent from my iPhone