4th Jul 2012, 06:51
I am looking for a portable G meter to be used to measure airframe vibrations perceived in the cockpit on a composite a/c. I have read in other discussions that something was on the market in the recent past, but I could not locate the products... probably alredy discontinued; somebody suggested me to use an iPhone or iPad with specific software, but since I don't own any of these hi-tech stuff I would prefer to purchase something that does not need to be replaced as soon as a new mobile technology becomes available... may I ask for your support?
Genghis the Engineer
4th Jul 2012, 07:12
A g-meter is much too coarse a device to get anything meaningful about vibration problems. I'd also be surprised if the accelerometers in an iphone give a good enough rate - although it would be interesting to have a go.
I'd suggest high rate electronic accelerometers, mounted appropriately, run into some form of data logger.
Running the data through an FFT analyser should then give you useful data about magnitudes and frequencies - and if you've mounted the sensors appropriately - the vibration mode also.
Almost certainly it is related to an engine speed, so you ideally also want to be logging engine speed although I have got away with doing that manually against the clock in a light aeroplane test programme where I was looking at pretty much what you're describing.
4th Jul 2012, 08:48
Genghis, thanks for your reply, I was really counting on getting some feedback from you!
Indeed I realize that a G-meter is not the ideal instrument for a professional measurement. My idea to use it was related to the purpose of this (non-professional) testing: while flying an aeroclub owned composite a/c I noticed some airframe vibrations which in my understanding are not supposed to be there. I discussed the thing with a colleague working at the manufacturer and he asked me to take some quick measurements suggesting to use the iPhone integrated G-meter. I will give him the logged data and depending on that he may decide to trigger a deeper investigation.
From that the decision to go for non-orthodox instruments....:\
Genghis the Engineer
4th Jul 2012, 10:14
I'm not an apple user, but a quick search shows:-
10 Creative Ways to Use the Accelerometer [iPhone] (http://www.creativeapplications.net/iphone/10-creative-ways-to-use-the-accelerometer-iphone/)
Look at the last item starting "context logger" - can't be any harm in trying one or more of these apps.
The FFT analyser however is the biggie to get meaningful analysis. Whether any of those packages include such capability, I've no idea but if you have an iTunes account, I'm sure you can find out quickly enough.
My experience is that you are likely to find that the main resonances are at frequencies which are a factor of the engine speed. So, for example if you are running the engine at 2000rpm, which equates to 33Hz, you are likely to get modes at 33, 66, 99, 132Hz, and possibly 16 and 8. It's the magnitudes that are important.
I did some work on this with the Jabiru engine some years back, after an entertaining flight test incident where a CFRP propeller spontaneously combusted. If you PM me an email address I'll see if I can find a copy of the report, but in a nutshell the factors were:
- Tip Mach number (anything above 0.8 became problematic, and vibration was substantially less below 0.7)
- Fuel type: mogas giving a similar pattern, but much larger resonance peaks than 100LL. 91UL wasn't around then, so I can't offer an opinion about that.
- Propeller rotational inertia was also a significant player. The low inertia 2-blade wooden props that for example is standard fit on the Jabiru aircraft provide much smaller resonance peaks than the popular modern high inertia composite props such as the Arplast or Warp Drive (possibly the best brand name ever for a propeller!)
Charles E Taylor
8th Jul 2012, 16:33
Perhaps not exactly IRIG-106,
But for nothing. Try this.
App Store - xSensor (http://itunes.apple.com/us/app/xsensor/id345145166?mt=8)
Then you might want to overlay the data on Video for Context.
A programme like this cheap and very useful, you can trial it for free.
Have lots of fun.
9th Jul 2012, 11:08
...it looks like I really need to invest in a iPhone or a iPad...
Sigh! on the point to give up...:}
10th Jul 2012, 10:34
Try one of these:
GCDC X16-1C Usb-Accelerometer 3-axis Data Recorder (http://www.gcdataconcepts.com/xlr8r-1.html)
10th Jul 2012, 13:42
Meanwhile somebody from a test pilot school suggested me this complete system:
Comment was that setting it up is not very user friendly, but it looks like this PFDR incorporates most of the basic parameters needed during classical flight testing...