Data
Hello David, I'm trying to buy a prop balance unit. I already spent my fundage on 30,000lb capacity aircraft scales, so I'm going to have to work the balance unit on investments. If I get a couple more people together for a group buy at 250 a piece... I get the machine, they get their prop balanced... and you get the data...(If it can be recovered from the machine. I'll bend over backwards to help you get the data) I've done a TON of Track and Balance on helicopters in the service.
Hi Brian,
If the data is easy to extract, that'd be great. Pls don't bend over backwards and break your back - I wouldn't want to interrupt any flying.
May I ask what you are doing with the data? Maybe fatigue analyzing?
No, nothing quite that fancy - far from it, actually. There were a couple of objectives. First, I'm trying to learn a new programming language called python. Problem is, I'm a perl, Unix/Linux shell, C kinda guy - so this python stuff is a little different for me. Anyways, to learn something new, I've got to put it into practice and not just read and do the examples in the book(s).
Secondly, I've also had an idea for interfacing to somewhat of a unique piece of h/w - of which python libraries are available on Linux, Windows, and some internet tablets. Unfortunately, some further research has shown that the h/w may not be up to the task. Oh well. I've other things I can apply python to - and at work, too!
Anyways, this prop balancing thing has had me a bit perplexed. What I couldn't figure out is how 'IPS' could describe a vibration. I mean, "inches per second" is a velocity, not an acceleration. I eventually found the answer at Sacramento Sky Ranch:
IPS defined (http://www.sacskyranch.com/vibm.htm)
IPS is just an acronym for "inches per second". When measuring a vibration, the velocity is the speed the object reaches as it passes through the center of the range of displacement. Similar to the highest speed a pendulum reaches at the center of its swing.
Velocity is arrived at by simply integrating the signal that an accelerometer produces. If you integrate a second time, you get displacement.
It has been found through experience that velocity is a better unit of measurement to use if you want the numbers to track with how severe a vibration is independent of RPM. In other words, a 1 IPS vibration is pretty severe no matter if the object is turning at 200 RPM or 200,000 RPM. This is not true for units of acceleration or displacement.
Now, for harmonic oscillation, the acceleration is just omega times the
velocity; for, say 2400 rpm (fundamental frequency of 40 Hz), your vibration of 1 IPS corresponds to an acceleration of 20.9 ft per sec^2 or about 0.7 Gs. That does seem a fairly healthy vibration for the front end of an engine with propeller.
It takes on average 65 grams to correct for a 1 IPS vibration. But this value
varies greatly from ship to ship. From a low of 20 to a high of 130.
Ok, so this got me thinking (dangerous, I know). I envisioned, very simplistically, a pencil attached to the tip of the spinner that writes on a sheet of fixed stationary paper in polar coordinates. If the prop were unbalanced, the pencil would draw a circle. That seems easy enough to integrate. I realise this doesn't tell me where the imbalance is, just the magnitude.
You can PM me if you want. A guy in my unit used to take the sensors and place them all over the aircraft BY HAND! to sense vibration. He had the SMOOTHEST helicopter out there.... Not surprisingly, all his components seemed to last a lot longer and a lot less sheetmetal repairs were done on his aircraft. He had a publication that explained all about vibrations and how they interacted and what would happen. It dealt with gearing and ratios and all sorts of stuff. I send all my customers to get it balanced every two years or so. The guys down in Sandersville did my Grumman a long time ago and it made quite a difference. I'll admit, it's been hard to get my customers to let go of a couple hundred bucks... but they were always so complimentary after it had been done by who I sent them to. I hear the guys in Gainsville, GA do good work too. Is there anybody I can contact for you to help you get some data? What format do you need it in?
Best
Brian Wallis
interesting about the helicopter and its maintenance record. As you can see/read this is just for my curiosity and entertainment. Seriously though, if the data is
easily retrievable, I'd appreciate it. if not, that's Ok, too. As for format, I'd think I could accept anything - as long as the encoding is known.
Thank you,
