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Zoom › Serious tail vibs???
11-05-2004 09:36 PM  13 years agoPost 1
ammo

rrVeteran

Singapore

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I am running 4200 on 9t. whenever i get pass 65% throttle(yes i did calibrate my radio atv to match the esc), the whole tail will get into serious vibrations that go with a loud buzz and shakes the whole tail at an extremely high frequency, feels like the whole tail is going to explode. and the gearcase is rotated due to the vibs.
It runs fine,smoothly all the way up to 100% without the tail blades. but i have balanced my tail blades to the best i can visually see and once i put them on, the problem comes back...

Has anyone experienced this?

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11-05-2004 11:26 PM  13 years agoPost 2
8AxleEd

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San Jose, CA

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How did you balance your tail blades?

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11-05-2004 11:44 PM  13 years agoPost 3
ammo

rrVeteran

Singapore

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Just like how we balance the mains. CG and static. a rod just nice for the screw holes and supported on the sides for free rotation.
Did i do anything wrong?

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11-05-2004 11:52 PM  13 years agoPost 4
BlakeMcBrayer

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Georgetown, Ky

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As for the tailcase rotating, you should pin the case. Center up the case and drill a hole thru the case and tail boom. This hole should be just large enough for a set screw to thread into. Make sure the set screw threads into the boom. They have a habit of becoming loser and looser if you dont pin them. If you are running a brushless then the pinning in almost required.

I had EXACTLY the same vib problem that I chased for weeks. Mine also started at 65%. I changed EVERYTHING, case, stock output shaft, blades, bearings and so on. It turned out to be the output shaft. It would check on glass perfect. But a slight imprefection in the shaft or blade balance would cause it to get vib at higher throttle setting.

Are you running belt drive? If so install a precision output shaft. I made mine from 2mm machine tool stock. I can run at 100% with zero vibs. I just got in 10ft of the shaft stock and will be selling stock length shafts for $5.50 mailed in the USA. If you want one, I probably get one to your country for $8.50 total.

Also Heli-Hobby and Sparrow product sell them as well.

Don't come off the throttle untill you see your GOD!

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11-05-2004 11:59 PM  13 years agoPost 5
ammo

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Singapore

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It sounds good to have a precision shaft for that fast running purpose.
Let me know how and when i can contact you for that.

I thought my friend and i are the only 2 peple having the same problem....

If that thing doesn't buzz like it does now, the gearcase stays in position well......

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11-06-2004 12:43 AM  13 years agoPost 6
BlakeMcBrayer

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Georgetown, Ky

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Send me a PM and we will work out the details.

The case can still rotate even if the vib is gone.

Don't come off the throttle untill you see your GOD!

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11-06-2004 01:04 AM  13 years agoPost 7
8AxleEd

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San Jose, CA

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ammo,

If it didn't vibrate when you removed your tail blades, then the problem is not the shaft.

It is a balancing issue, because it doesn't vibrate when the tail blades are removed.

The two tail blades need to have the same mass, and the same lengh-wise balance point.

Your method for doing this, while may be good enough with large blades doesn't seem to be working in this case.

The biggest problem with that method is that you can't get the blade centered exactly the same on the bolt easily. The blade bolt is a "lead-lag hinge". It is designed to swivel during flight. If the blade is swivelled on the bolt, even a slight amount, you have introduced an error in your system. It is best not to do it that way on such a small blade with such a small mass. It will be very sensitive to small errors.

I use the xcell balancer and a tripple beam balance for my blades.

Some have taken some tape, added it to one blade to see if it gets worse or better. If it gets worse, they switch the tape to the other blade. Once on the correct blade, they take the tape and move it in or out along the blade until the vibration goes away. It is a crude way of doing it, and probably only masks the problem. However, if you don't have the proper tools, it may work better than what you are currently doing.

In this type of a system, there are drivers and amplifiers. In this case, the driver is the torque imbalance on the tail rotor. The amplifiers in this case are going to be something like the tail boom, and the tail case link to the boom, things that vibrate because of the tail imbalance. There can also be dampeners, something connected to the driver and amplifier that absorbs vibration. Moving tape on a blade can either remove the driver, or it can set the vibration frequency to something such that the other parts of the system can absorb it. Because of the crudeness of this method, that is probably what happens.

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11-06-2004 01:21 AM  13 years agoPost 8
BlakeMcBrayer

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Georgetown, Ky

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I had my blades perfectly balanced both via a magnetic balancer and micro lab scales. Mine would not vib with the blades of but would with. Replacing the stock shaft with a precision shaft solved the problem. Seemed to work on the other 4 Shoguns I fly with.

Don't come off the throttle untill you see your GOD!

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11-06-2004 01:38 AM  13 years agoPost 9
8AxleEd

rrApprentice

San Jose, CA

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I'm glad that the new shaft worked for you BlakeMcBrayer. If you will note, my explanation wasn't completely about balancing, it was about dampening and balancing. I spoke of dynamic balance, drivers, amplifiers, and dampeners. They can be the same thing, but they can be orthogonal to each other and still allow your system to work smoothly.

A perfectly dynamically balanced system is preferable, but not always necessary, and in some cases not achievable.

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11-06-2004 02:02 AM  13 years agoPost 10
BlakeMcBrayer

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Georgetown, Ky

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8AxleEd,

I wasn't trying to knock your idea or anything. I was simply stating what had worked in my case and a few others.

I feel your ultimatly right in a balance issue. There must be (or have been) a slight error in my t/r confige that probably sets up an osicilation in my stock shaft. The stock shaft seems very weak. The hardened shaft I made is not nearly as prone to flex out of center due to small balance issues and begin to oscilate.

Don't come off the throttle untill you see your GOD!

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11-06-2004 02:18 AM  13 years agoPost 11
8AxleEd

rrApprentice

San Jose, CA

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Sorry BlakeMcBrayer, I didn't intend to sound confrontational.

I hadn't thought about shaft flexing, which is a real possibility. If that is so, then what I said could compensate for that too. I guess you could call shaft flex to be an amplifier at a resonnant frequency.

I've been looking at the helicopter as a bunch of control systems maybe too much lately! Dynamic balance with amplifiers and dampeners are like poles and zeros in an electrical system. If they are in the wrong spot, you get an oscillation. It is the same as a circuit with no phase margin one decade in frequency out from the 3db point of a filter. The feedback at that frequency makes an oscilator.

The whole helicopter works like that. The flybar is a mechanical gyro. The govenor mode and the motor modeled with the load attached is a control system. The gyro with the sloppy links, servo response time, and tail output torque are another system.

It is very intrieguing to see how the mechanical and the electrical are inter-related. They talked about how mechanical and electrical were the same in a dynamics class I took, but the helicopter is a perfect example to show it.

I'm done contemplating it all now for the moment. I promise!

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Zoom › Serious tail vibs???
11-06-2004 02:34 AM  13 years ago •• Post 12 ••
BlakeMcBrayer

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Georgetown, Ky

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I think we both have good points! I never took your replys as confrontational. My wife is the only one I consider to be "confrontational". For some strange reason she is always that way after the UPS or FedEx man leaves a package for me. Or maybe it's just because he left without her, heck i don't know so I'm just going to keep flying and having the occasional confrontion with gravity

Don't come off the throttle untill you see your GOD!

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11-06-2004 02:38 AM  13 years agoPost 13
engwerda

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Melbourne, Australia

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I thought i'd post here rather than start a new thread. I get serious tail vibrations but only when i'm spooling up and (so only at low rpms). When i get to flight headspees the vibrations have completely disappeared. I'm not sure if it even a problem but i thought i'd check.

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11-06-2004 02:49 AM  13 years agoPost 14
8AxleEd

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San Jose, CA

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That is not a problem at all engwerda. All helicopters have a resonnant frequency in which they will shake around (not just the tail). It is a really good thing if this happens at a low RPM.

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11-06-2004 03:49 AM  13 years agoPost 15
BlakeMcBrayer

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Georgetown, Ky

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8AxleEd is right. As long as it is low rpm, fly on. Even my nitro bird shakes alittle on spool up.

Don't come off the throttle untill you see your GOD!

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11-06-2004 06:04 AM  13 years agoPost 16
teebok

rrNovice

Pico Rivera, CA

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I had the same tail vibrations. I changed blades, tail shaft, tail blades and main shaft. vibrations still present. then i balanced everything twice, still stong vibrations on tail. I then put the heli in front of the fan on high & spun the blades to take a closer look. I felt the heli moving up & down in my hand, thats when i noticed the main rotor grips moving up & down. after removing the blade grips i noticed the spindle shaft was bent. replaced with new one, no more vibrations!!!!!!

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11-06-2004 06:29 PM  13 years agoPost 17
hobby bob

rrNovice

E. Northport N.Y.

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sometimes those low rpm spool-up vibes are simply from the blades finding thier "happy spot"...when you unfold your blades, and set the heli on the ground, the blades will vibrate while spooling, and the whole machine will shake around. Does it do it on the next spool-up if you don't touch the blades?

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11-06-2004 09:13 PM  13 years agoPost 18
8AxleEd

rrApprentice

San Jose, CA

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I had the same tail vibrations. I changed blades, tail shaft, tail blades and main shaft. vibrations still present. then i balanced everything twice, still stong vibrations on tail. I then put the heli in front of the fan on high & spun the blades to take a closer look. I felt the heli moving up & down in my hand, thats when i noticed the main rotor grips moving up & down. after removing the blade grips i noticed the spindle shaft was bent. replaced with new one, no more vibrations!!!!!!
If you've got a problem with a bent head spindle, or bent screws in the tail rotor hub, or a bent tail output shaft, or a bent main shaft, that is something that you had better fix!

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11-07-2004 10:35 AM  13 years agoPost 19
engwerda

rrNovice

Melbourne, Australia

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hi hobby bob,
I tested it today but even after landing gently and spooling up again i still had the same problems. I noted that when it was low rpm, the centre circle on the top of the rotor head was wobbliong a bit to either side, but when in high rpm it was staying perfectly in position to the point where i couldn't tell if it was spinning or not from just looking at it alone. Could this be a possible slight bend in the shaft, i already had to replace the stock one with a hp main shaft.

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11-07-2004 01:58 PM  13 years agoPost 20
BlakeMcBrayer

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Georgetown, Ky

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It's possible that it is a abent shaft of spindle, but I dought it engwerda. Mine has always done that low RPM shake and nothing is bent in mine.
The easy way to check the shaft is to pop off the link balls from the swash plate, remove the jesus bolt and entire head assembly. Make sure you don't have any loose links left on the swash, if you do and spool up, well lets just say something will go bang.

Anyway, spool it up on the bench (hold it good so t/r won't make it spin) and check the main shaft.

As for the spindle, it can be removed and rolled on glass or a mirror to check for straightness.

If either of these were bent, you would get vibs after you have lifted off. Like I said, mine does it all the time at low RPMs even after landing and going back up.

Don't come off the throttle untill you see your GOD!

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