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HomeAircraftHelicopterBergen R/C Helicopters › Intrepid - Almost a $5000 disaster........
11-07-2004 12:19 AM  13 years agoPost 21
JamesC

rrNovice

Melbourne, Australia

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Well.........I must say I'm impressed with the volume and quality of responses to my problem, from forum members, Noel at Radiocontrol Australia and Bergen!

Gyan, thanks for the engineers report. Sounds more and more like something got out-of-whack/balance in the instant prior to the failure. The general tone of responses to my post seem to further support this.

So, when I get it back together, I'll make extra sure its silky-smooth. Of note, (Galifrey) I checked the tail rotor pitch links........they are 20 mm as opposed to the 18mm recommended in the 'Malorie' manual you linked to. (why does the shipped manual not mention this?). BUT, at 20mm they were not binding anyway (just answered my own question!).

Also, Chris, the hub is not bent - looks fine on testing the way you suggested.

Finally, SHYKMASTER.........if you use 2 grub screws in 180 degree opposition, then the only tractive points are the tips of each grub screw. This is pretty unsafe and prone to failure. 2 only work if they are spaced at 135 degrees or less.

What I suggest is:

1) Use only one screw and if there is not a hole or dimple to accomodate it in the shaft, make one with a high speed drill

2) Knurl or 'ridge' the end of the grubscew/s for better 'bite' into the shaft

3) Use threadlocker

Hope this helps.

James

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11-07-2004 02:59 AM  13 years agoPost 22
shykmaster

rrApprentice

New York

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Thank YOU!!! I was worried I wouldn't get an answer before tommorow, flying sunday! I bought the heli used and only saw one grub screw installed so I figured the other one flew out.
Again Thanks for your quick response. I love my friends at runryder. To date they have never let me down

josh

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11-07-2004 12:47 PM  13 years agoPost 23
Malorie

rrElite Veteran

Paw squared, MI

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Tail Blade tracking.

It is important to track your tailblades when adjusting the TR pitch linkages. An out of track condition could very well have been responsible this trouble.

The best way to track them in is to turn the blades in the grips 90 degrees (either clockwise or counterclockwise). Then move your pitch slider so that as the tail rotates, one of the tail blade tips just touches the tailboom. Now carefully rotate the tail (without moving the pitch slider) and see if the other tip touches the tailboom in the same manner. If not, adjust one of the pitch linkages to achieve equal pitch on both blades.

This process takes only a couple of short minutes and will assure that the tail blades track nicely and the tail runs smooth and true.

Best of luck,
Malorie

Life's a journey, NOT a destination.

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11-08-2004 07:45 AM  13 years agoPost 24
JamesC

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Melbourne, Australia

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Thanks Malorie,

I'll add that important step to my reassembly as soon as the parts arrive.

Cheers

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11-08-2004 11:42 AM  13 years agoPost 25
Malorie

rrElite Veteran

Paw squared, MI

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Normally I don't look at the head, but if after a rebuild you're still chasing a vibe, it's worth looking at.

Life's a journey, NOT a destination.

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11-14-2004 09:27 AM  13 years agoPost 26
Al Magaloff

rrMaster

12,199 Posts- Enough Time Wasted. See Ya!

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Josh, opposing set screws are bad engineering. When using one set screw, the hub is pulled into the shaft, making full contack, when the screw is tightened. When using two opposing screws, the hub is pushed away from the taishaft, as the screws are tightened. The screws then being the only contact to the tailshaft. "Better" engineering, is two screws at 120 degrees

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11-14-2004 09:28 PM  13 years agoPost 27
cmartin

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Kouts IN

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Grub screws
I have been using 2 grub screws for a while now and have not had any problems. I tightened the grubscrew that has the indentation in the shaft first then tightened the opposing side just tight enough to hold. I dont see how with the first screw tightened how the second could pull the hub away from the shaft, and even if it did , I would think it would be so minute that it wouldnt matter. There would probably be more of an imbalance in the tailblades than from the offset tail hub IMHO.

Chad

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11-14-2004 10:05 PM  13 years agoPost 28
Al Magaloff

rrMaster

12,199 Posts- Enough Time Wasted. See Ya!

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The hub is put up on two pivot points. This is documented, not just with helicopter tail hubs. I believe the one set screw advisory comes right from Bergen.

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11-14-2004 10:16 PM  13 years agoPost 29
cmartin

rrVeteran

Kouts IN

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Seems like with one side already tight it would not be able to lift the hub off the tail shaft on to those two points. But I could be wrong.

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11-14-2004 10:23 PM  13 years agoPost 30
Al Magaloff

rrMaster

12,199 Posts- Enough Time Wasted. See Ya!

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Agree, it's just bad engineering. Also, most people get carried away with the tightening thing. They crank the driver, until they see a puff of smoke, then go a little more!

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11-14-2004 10:26 PM  13 years agoPost 31
JamesC

rrNovice

Melbourne, Australia

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Guys,

Al is right on it.

With a single grub screw, torque loads are dissipated across a wide area because the rotational force vectors are intersecting, having originated from a (theoretically) infinite number of points across the crescentic contact surface opposite the screw.

With 2 grub screws in 180 degree opposition, all torque is concentrated across 2 parallel vectors, running in opposite directions. This applies a huge turning moment across the tips of the screws and results in failure. Less of a problem if both screw tips are 'sunk' but still not ideal. So even a 'minute' difference will matter. Look at the through bolt on a main shaft / rotor head after a bit of abuse and note where the subtle bends occur ( No, I'm not advocating grub srews for rotor heads!! )


Cheers

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HomeAircraftHelicopterBergen R/C Helicopters › Intrepid - Almost a $5000 disaster........
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