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HomeAircraftHelicopterKasama Srimok 90 › Srimok Vibrations
10-29-2011 02:52 PM  6 years agoPost 41
luca bianchessi

rrApprentice

bergamo, italy

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so how can I check the frequency on the logger?

thanks!

Luca

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10-29-2011 03:28 PM  6 years agoPost 42
rexxigpilot

rrProfessor

Florida

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Click on the Spektrum Analyzer buttom, the green graph at the upper right of the Others Live field. This will show you the frequency and RPM.

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10-29-2011 09:07 PM  6 years agoPost 43
luca bianchessi

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bergamo, italy

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perfect, would a bench test be enough?

thanks

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10-30-2011 05:13 AM  6 years agoPost 44
rexxigpilot

rrProfessor

Florida

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Maybe, but It would be best to be in the air to eliminate ground resonance. Take off both main and tail blades and bench test. It can't hurt. If you don't see an obvious high vibration frequency, put blades on and hover it. A bad or improperly installed thrust bearing will show up only with blades on.

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11-02-2011 09:38 AM  6 years agoPost 45
TomKreuzberger

rrNovice

Bischofshofen, Salzburg, AUSTRIA

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I would mount the vbar sensor with a rather hard mirror tape.
We have done it that way and it works perfectly.

Nice greetings from Austria
Tom

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12-17-2011 01:43 PM  6 years agoPost 46
luca bianchessi

rrApprentice

bergamo, italy

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so finally in my gallery there is a picture of the vbar vibe analyzer during a bench test. no main blades.

what do you think about this? How do you read this kind of graph?

thanks!

Luca

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12-17-2011 04:18 PM  6 years agoPost 47
rexxigpilot

rrProfessor

Florida

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Luca,

What head speed were you running when the analysis was done? Also, what gear ratio are you using. Your highest vibration has a frequency of 11,232 RPM or 187.2 Hz. This is caused by a moving or rotating part on you heli that is moving or rotating at this frequency or some integer divisor of this frequency.

You will typically see harmonics of this frequency that are multiples of it that will have a much lower amplitude with each order. However, I don't see that in your graph.

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12-17-2011 05:32 PM  6 years agoPost 48
luca bianchessi

rrApprentice

bergamo, italy

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thank you for your help!

the gear ratio is the standard 8:25.

I thought 11,232 rpm was the RPM of the engine....so I don't know what the headspeed was, but I know I was about half the stick and without main blades.

so do you think this vibration is giving me problems? could it be the source of all my problems? or my situation is normal?

thanks!

Luca

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12-17-2011 06:28 PM  6 years agoPost 49
rexxigpilot

rrProfessor

Florida

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The vibration could very well be your engine if you were at about half stick since 90 engines are at around 15K-15.5K RPM running at full throttle. BTW, I did see from one of your previous posts that you run your heli at a 1950 RPM head speed. Your 8.25 gears will give you an engine speed of over 16.1K, which reaches or exceeds the maximum operational speed of the OS .91 and 1.05 engines. This will cause the engine to vibrate badly. I have the 98T main gear (8.17 GR) to run a higher head speed.

The vibrations in your graph are very strong and will cause a problem with the V-Bar IMO. Have you adjusted the dampeners on the engine mount yet?

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12-18-2011 10:18 AM  6 years agoPost 50
luca bianchessi

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bergamo, italy

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that's what I thought too, 11k could only be the engine.

I will lower the headspeed down to 1900, you are right, 1950 is a little too much....
yes I have andjusted them but nothing changed.

what could it be in the engine? I changed bearings, piston, and piston sleeve....

thanks!

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12-18-2011 02:37 PM  6 years agoPost 51
rexxigpilot

rrProfessor

Florida

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Exactly which engine are you running? If you have the pumped OS91HZ-PS, that could be your problem.

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12-19-2011 06:22 AM  6 years agoPost 52
luca bianchessi

rrApprentice

bergamo, italy

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I have a yamada 91 sr with the hatory pipe....

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12-19-2011 04:15 PM  6 years agoPost 53
rexxigpilot

rrProfessor

Florida

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Is it running smoothly? I had to change/convert the engine on my T-Rex 700N when I went FBL to get a smoother running model. The was using a G-T5 FBL unit, not V-Bar.

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12-19-2011 08:03 PM  6 years agoPost 54
luca bianchessi

rrApprentice

bergamo, italy

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well I am not really sure about it since I have not been really able to test it because of these vibrations.....the only not new part on the engine is the crackshaft......could that be the problem?

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12-19-2011 10:39 PM  6 years agoPost 55
rexxigpilot

rrProfessor

Florida

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If you rebuilt the engine, maybe you got some ST parts in the SR. Just a thought.

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12-20-2011 06:32 AM  6 years agoPost 56
luca bianchessi

rrApprentice

bergamo, italy

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the only parts I got are from the yamada 3ds, but yamada itself told me I could do it with no problems.....

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12-20-2011 08:35 AM  6 years agoPost 57
Mercuriell

rrApprentice

Sunshine Coast, Queensland, Australia

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That vibe is undoubtedly from your tail which is the commonest cause of problem vibes on the Srimok. Try running the heli with the tail blades off and I would bet you dollars to doughnuts the peak will disappear.

I have summarised an approach to tail vibe busting elsewhere but briefly you can try a pair of Radix 95 tail blades, check the hub is square (look for out of track tail blades when it is running) - also make sure the gearbox and crown/pinion backlash is correct.

John

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12-20-2011 12:33 PM  6 years agoPost 58
luca bianchessi

rrApprentice

bergamo, italy

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Yesterday I tried without tail blades and the vibe was still there. today I will try to run everything without the whole tail boom and I will check the analyzer.

why are you so sure it comes from the tail??

thanks!

Luca

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12-20-2011 02:37 PM  6 years agoPost 59
luca bianchessi

rrApprentice

bergamo, italy

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today I fully realized it must be the engine. in fact, the vibration is present also at lower rpms. also, I spooled the heli up (with blades) while it was hooked to the computer and the analyzer was on. as soon as I hit the Throttle hold, the vibration was gone even if blades and tail blades were still spinning fast....

could it be the engine shaft??

thanks

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12-20-2011 02:49 PM  6 years agoPost 60
rexxigpilot

rrProfessor

Florida

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If that engine was involved in a hard crash, it could be the crankshaft.

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