Resonance...?

Mike,
could you recommend a vibration analyzer apps? I have a Dynavibe to balance my rotor but at fix frequency, it don`t have spectrum analyzer.
 
Arco
With what confidence do you say that the ELA blades are balanced at 25%??

For a spectrum analysis apps, I can't really "recommend" one because I haven't done any serious testing (I'm the spoiled owner of a PB3 and a PB4 that probably cost less than your Dynavibe) the one on my smartphone is this one:
https://play.google.com/store/apps/details?id=de.bremen.btm.movalyzer&hl=en
I found this pretty good because it goes down to the low frequencies (5 - 6 hz) we're interested in. It's Android (a disadvantage for some) and another disadvantage is that you can't record data, you have to keep looking at the screen (not too clever when flying) and since I've downloaded it on my new phone it doesn't work in all three dimensions. Look at it and see if you can find a better one, especially if you're an Apple guy, I'd be interested to hear about a better app for Android or Apple.

Mike
 
Thanks for the info, I will test the apps. Also, I downloaded all the apps available in google play related to vibration, so in summer time I will compare all of them.
Before I bought my Dynavibe, I contacted Smart Avionics to buy the pb3, but it was too early for them. They said that PB3 is no tested yet in gyroplanes and they refuse to sell it, even I said that I will take the risk.
About ELA blades data, I am 100% sure, I have very good relationship with the factory and I have access to the drawings.
 
Apple guy here...I have used an app on iOS for serveral years that I can recommend. It is called of all things "vibration". Samples from 10 to 100 Hz internally, and 1 kHz to 16 kHz with microphone and USB accelerometer. Sensitivity of the internal accelerometers is on the order of 0.02 g with max at plus/minus 2.0 g. You can set the sample length, strart delay, number of times to repeat. Stores the results, and email to another computer if you want.
3 channel accelerometer acquires and displays time series data, removes DC bias, applies a Hamming window and performs FFT on each channel. Input is via internal, microphone or external $$ Digiducer USB accelerometer. Cost is $5

Dave
 
Unfortunately, 360 rpm only is 6 Hz for the 1/rev vibration, Dave.
 
No Title

[FONT=arial, helvetica, sans-serif]I guess I need to be clearer. The sample rate for the internal accelerometers is actually 1 - 100Hz. I meant to write 1.0, not 10. Apple actually limits the max sample rate to 100 HZ. That does not mean that you cannot resolve a 6 Hz vibrational mode. You can set the sample rate satisfy the Nyquest criterion for the frequency you need to measure. I just did a test on a variable speed rotating platform that I have on my bench, and could easily see 5.0 Hz vibration mode. I also held the iPhone against my ceiling fan, just above my head, and can see a 0.39 Hz vibration mode. Another handy feature is that you can use your Apple watch to trigger the sampling. So you can rigidly mount the iPhone wherever necessary, and trigger the sample, remote from the phone. Also use an app called "inclinometer" It also can also use the Apple watch as a remote display. Used to use this on large telescopes, to measure angular rotation of instrument located 20 ft up in the air.

Cheers,

Dave
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After much testing and head-scratching, I'm starting to realize that the stick shake of my ELA may have nothing to do with a resonance, and that the vibration may be originated in the rudder, oscillating in the propeller blast. Yesterday, I observed that, flying s/l, the vibration gets stronger when the rudder is strongly deflected. Besides, and irrespective of the airspeed, the shake is absent when gliding, and present with power on. The cables controlling the rudder are (in my opinion) adequately tensioned, but perhaps some adjustment could be advisable...

Any comments will be welcome...
 
Try stepping hard on both rudders in flight to change tension.
 
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Consult the maintenance manual for the proper rudder cable tension, it does warn of a possible high speed flutter if not correct.

Also, because of how the ELA rudder pedals are linked to the nosewheel stepping hard on both pedals has no effect on rudder cable tension.
 
Rdalcanto, I have used the iPhone for vibration measurments on many instruments, but not a gyro. I would recommend that you choose a location that specifically reflects what you are trying to measure. That is kind of a vague answer. I am sure that Jean-Claude would have more insight into a suitable location. Reguardless of location, it is important that the phone be firmly mounted so that the measured movement reflects the vibration from that component. I want to make clear the "Vibration" app I use on the iPhone measures on three axis, and you can calibrate the axis. It does not provide you with, for example, radial vibration modes, not give you a solution as to mass and radial location for balance. The data is exportable, so you could solve for such a solution.

Cheers,
Dave
 
I think I have discovered the cause of the annoying shake in my gyro. While flying yesterday evening, in very still air, I discovered that the shake gets noticeably stronger when yawing to the right, and diminishes when yawing to the left... I knew already that the shake was absent when flying with the engine at idle, either in vertical descent or when gliding at any speed...

I believe now that the shake is caused by some sort of oscillating turbulence caused by the interaction of the prop flow with the (big) fixed trim tab installed on the rudder. It gets transmitted to the body, and from there to the control rods, so it's felt in the stick...

Any comments are welcome...
 
Lots of really good information,many thanks to all or you.

I noticed that after looking at the ELA gyros that the rudder is not balanced,unblanced control surfaces will cause all sorts of problems, a balanced rudder will

have a portion of it forward of the hinge line,that arangement allows balancing and the forward portion makes the rudder earier to push on.

However at the slow speeds gyros fly unbalanced control surfaces are usually good and do not cause problems,but the rudder on gryos are

guite large and could cause the vibration.
 
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As I understood it, the rudder vibrates not because of intrinsic flutter but because it is exposed to vortices shed by some upstream structure. Balancing the rudder will not help in such a situation.

-- Chris.
 
It's called buffeting. I think the upstream swirl of the prop may assist keeping the flow attached to the right side of the fuselage while promoting unnattached flow on the left. The vortices or bubbles that are shed then react with the rudder downstream. I had a similar problem on my aircommand CLT with a wide modified fairing at around 60mph cruise. Also the original Xenons with their wide cabin and bluff rear would not so much buffet but had improved climb when flown with some some slip maybe because the prop was now seeing cleaner flow because it interacted favourably with the swirl. All this is speculatiion but might explian what's going on. If so vortex generators on the left of the fairng should help The power settings may also have an effect as they change the angle of attack of the fairing on some gyros.
 
Xavier
I've just seen a newish ELA in a hanger where I'm working I couldn't talk to the owner but remember he complained about the rudder buffeting the last time I was thereHe even talked about cutting the rudder and tail to make a balanced rudder.
The guy I was working with told me that ELA had supplied him with a new rudder that had some sort of offset hinges. I looked at it and couldn't see anything that was obvious to me but did see the old rudder in the corner of the hanger so that part of the story is true.

I was also told that the owner found a crack in the welds in the tail boom and had to weld the strengthening piece that ELA are quietly offering to anyone who finds cracks in the tail boom. We've had at least one fayality in France with the tail coming off due to bad welding.

I left a voice mail with the owner asking him to ring me, if he does I'll let you know what I find out.

It seems to me that ELA have a problem, they know about it and have a solution but aren't telling anybody about it. This seems to be typical of them hiding problems from their customers. I've given up trying to work with them, they're the worst vendor I've come across so far and I wouldn't recommend them to anybody.

You speak Spanish so should be able to communicate with them ask them about the ELA at Lens in France. They speak very little English and non at all once you have a problem they just stop answering your emails.

Mike G
 
Mike G;n1127815 said:
Xavier
I've just seen a newish ELA in a hanger where I'm working I couldn't talk to the owner but remember he complained about the rudder buffeting the last time I was thereHe even talked about cutting the rudder and tail to make a balanced rudder.
The guy I was working with told me that ELA had supplied him with a new rudder that had some sort of offset hinges. I looked at it and couldn't see anything that was obvious to me but did see the old rudder in the corner of the hanger so that part of the story is true.

I was also told that the owner found a crack in the welds in the tail boom and had to weld the strengthening piece that ELA are quietly offering to anyone who finds cracks in the tail boom. We've had at least one fayality in France with the tail coming off due to bad welding.

I left a voice mail with the owner asking him to ring me, if he does I'll let you know what I find out.

It seems to me that ELA have a problem, they know about it and have a solution but aren't telling anybody about it. This seems to be typical of them hiding problems from their customers. I've given up trying to work with them, they're the worst vendor I've come across so far and I wouldn't recommend them to anybody.

You speak Spanish so should be able to communicate with them ask them about the ELA at Lens in France. They speak very little English and non at all once you have a problem they just stop answering your emails.

Mike G

Whoa !!!! Tail came off due to bad welding? You sure?
We had resonance issue in testing when we had lightened our tail and also changed tubing vendor to a US mill manufactured tubing. For some reason when the tail got side gust load its frequency matched a harmonic of the welded frame tail boom. We obviously caught it and changed both internal structure of the tail in manufacturing and tail boom stiffness to change their frequencies so it would not resonate. Just this week one of the customers compliant about vibrations in his new gyroplane that was being test flown, trying to work with the rotor wasn't helping much and we noticed that vibration is very high frequency and you could feel it even on the ground. Eventually we figured out that one of the engine mount rubbers was not inserted into the ring all the way. Of course by this time we had dynamically balanced a prop already which is good but probably was not completely necessary (0.28 IPS down to 0.08 IPS). Its always interesting when some of these happen what turns out to be the reason.
 
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Lots of thanks to all of you for the very valuable comments and suggestions. I'll try some some possible solutions and hope to report back soon. Anyway, I have a pending appointment with the people at the ELA factory in order to solve the problem. It means, however, a 400+ km flight, and I'm reluctant to do that in the winter. Perhaps next spring...
Concerning the cracks in the tail boom, my gyro was the first ELA factory-reinforced for that contingency. The delivery was delayed for some months because the civil aviation authority had first to approve the mod.
Mike: please give my e-mail to that ELA pilot at Lens. I'm very interested to exchange opinions with him, as it appears that we have exactly the same problem. He may write in French. I know the language well enough.
 
XXavier, when was your ELA manufactured? Also, if it’s not too much trouble could you post some pictures of the tail boom reinforcements the factory installed, I would like to compare them to a ELA we have here.
 
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