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
|