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Thunder Power RC ProModeler
mcfast

Key Veteran

Quebec Quebec Canada

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I had the same problem with two of my Predators, high frequency buzz in the tail, real hard to see, but the gyros did not hold, once I figured it out, it took some time, I installed 4 bearing supports, now they are smooth as glass, now the gyros hold great and it fly's real nice now!

P.S. sorry for the spelling or the grammar I am dyslexic!

10-03-2016 03:11 PM
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RM3

Elite Veteran

Killeen, Texas - USA

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As stated above the TT bearings should be unequally spaced to avoid the sine wave vibration thing. Are the tail boom AND TT BOTH straight ? Are the TT bearing supports off center ? Are the TT ends screwed up ?
thats the creepy part...

last month I spent an entire week completely rebuilding the entire tail section... I have a surface plate so I checked the boom and the TT... both were fine. the bearings were excellent, the bearing mounts good and the spacing was unequal between them as well as the ends, so no unsupported TT section length was equal to any other. The ends I machined to fit dead center and fit smooth with the TT drive ends, and the dogbone ends where machined axially symmetric.

so harmonics here came into play despite the unequal spacing of the bearings... this is why I was able to feel a vibration right in the center of the boom, but not at the ends... apparently the resonance frequency of that Section was in tune with the speed of the main rotor which caused the vertical tail shake... If I would have moved the bearing support it would have put it out of resonance but would have put another section into tune. With three bearings the resonance frequencies are now higher in each section.

showing a preference will only get you into trouble, 90% of everything is crap...

10-03-2016 04:00 PM
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Gearhead

rrMaster

Vt

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sine wave
I know about that, the wife get's that way now and then

Jim
Buzz Buzz Buzz

10-04-2016 04:26 AM
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icanfly

Elite Veteran

ontario

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you said they were evenly placed, DEATH KNELL, you're not supposed to place them evenly because that you get the jump rope effect, resonant frequency. I think an unspoken rule using two bearings is one at 75% and the other between the larger space. Or, you have a boom of 100% you place on at 1 25%, and 66%, you mix 1/4 and 3rds with two bearings remember, Got 3, then place them 30, 55, 80 assuming the tt length is 100% and yes don't put a bearing inside the boom support location. Those locations don't matter forward or backward in the boom.

The torque tube shouldn't be resonating. I've heard some real awefull noise in one youtube vid of an align product, just horrible.

10-04-2016 11:15 PM
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RM3

Elite Veteran

Killeen, Texas - USA

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I also said +- an inch... they are not all exactly 8 inches apart (i said approximately...not exactly).

besides...
I think an unspoken rule using two bearings is one at 75% and the other between the larger space.
the section that has the bearing "between" the other space is now susceptible to a standing wave as each side is equal in length...

I think by going to three bearings I now have four sections that none can have a resonant frequency near what the previous sections did when it had only two bearings.

more bearings = shorter sections = higher resonant frequencies... what does it matter if the TT now has a much higher resonant frequency that the system cant reach?... the idea here is to make sure that no structure on the heli has a resonant frequency low or high that the model normally produces...

Mcfast already stated he added four bearings on his model and its now smooth as glass...

that gets me thinking... on a belt tail, the tension itself would set the resonant frequency to ONE specific point (like a guitar string)...

that situation alone would break the general rule... it does have a resonant freq no matter how tight or loose it is...

showing a preference will only get you into trouble, 90% of everything is crap...

10-04-2016 11:41 PM
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icanfly

Elite Veteran

ontario

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I see, you effectively tuned out the frequency of the rotors. On the matter of belts, I'm now a firm believer in a sprung tensioner as it doesn't require resetting tension manually to adjust for use and temperature without it.

I agree on the +/- an inch on each successive bearing in the boom, as long as their are no two identical measurements of bearing placement it'll , SHOULD, be ok.

How much do 4 bearings and grommets weigh?

10-05-2016 02:12 AM
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Four Stroker

Elite Veteran

Atlanta

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Well we tuned out some stimulus but we don't really know what it was. Since this was a nitro (or gas) I suspect the engine. "decided to run it without the main rotor" What the tail vibration fed back into is a mystery.

10-05-2016 02:25 AM
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RM3

Elite Veteran

Killeen, Texas - USA

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How much do 4 bearings and grommets weigh?
an ounce and a half maybe?...each bearing and grommet "mount" together weigh similar to two quarters... I have three bearings now...not four (Mcfast had four on his)

no doubt that the engine is the source of the initial vibs. gassers, particularly the PUH format engines are known for this. The RC format with its larger fan magneto smooths out more of the torque pulse.
What the tail vibration fed back into is a mystery.
I could probably dig into the physics (being a recent graduate with a minor in mathematics) and find an answer. But my theory is that the tail vibs in that section were likely a fundamental frequency to the main rotor. At lower HS the frame was likely absorbing the resonance... but at higher headspeeds the frame couldnt cope with the amplitude.

I plan on burning a gallon this weekend provided good weather... and post a vid if I can get a cameraman (aka son) to go with.

showing a preference will only get you into trouble, 90% of everything is crap...

10-05-2016 04:38 AM
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Tyler

Elite Veteran

Chicagoland area

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Notice in your drawing where the tail boom support is clamped to the tail boom. Perhaps this is directly transferring engine vibrations to the closest boom bearing and energizing the TT through the bearing.

Maybe relocating that rear bearing could solve the problem, with some experimenting of course.

Good problem solving endurance on your end.

Enjoy things that money can buy IF you don't lose the things money can't buy.

10-05-2016 01:40 PM
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Four Stroker

Elite Veteran

Atlanta

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Rubber TT bearing mounts ? Make some delrin TT bearing mounts. These soft mounts are part of the problem. The old style delrin mounts were a hand press or slip fit in the boom and had a tiny (1/16" or 2mm) O-ring in a groove to hold the mount in place. You greased them up or spit on them and pounded them into place. The new rubber mounts are cheaper and compensate for cheap bent parts. They certainly change the resonant frequency.

Don't trivialize these dynamic problems. I read a 400 page book on helicopter flight dynamics in 1985 that took some guy 40 years to write - summarized his entire career.

But really if you fixed it don't worry about it anymore.

10-05-2016 01:56 PM
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RM3

Elite Veteran

Killeen, Texas - USA

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Notice in your drawing where the tail boom support is clamped to the tail boom.
thats just a rough drawing (NOT TO SCALE) to illustrate WHERE the vibs are in relation to the other bearings and where vibration was not felt... i used MSpaint and 2 minutes...

please dont start putting your rulers up to screen either...

showing a preference will only get you into trouble, 90% of everything is crap...

10-05-2016 04:26 PM
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RM3

Elite Veteran

Killeen, Texas - USA

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Make some delrin TT bearing mounts. These soft mounts are part of the problem
I thought about that... but not worth it considering the inconsistencies of the boom ID and added complexity in parts...plus the additional weight I add to the boom may "tune" it back toward resonance with the rotor... my model already has an ideal CG... I dont want to add dead weight to the front of the heli to compensate.
But really if you fixed it don't worry about it anymore.
exactly!

showing a preference will only get you into trouble, 90% of everything is crap...

10-05-2016 04:32 PM
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