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700 Class Electric Helicopters 700 Class Electric Helicopters manufactured by Align, Tarot, SYMA, Airhog, Chaos, HK and similar. |
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09-02-2014, 03:29 PM | #1 (permalink) |
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What happened to the trex 700 tail hum?
I'm looking at buying a new heli and the new trex 700L top looks attractive to me. But i was wondering if there still is an issue with the tail humming and shaft snapping.
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09-02-2014, 03:43 PM | #2 (permalink) |
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They changed the shaft last year sometime to one without a step in the shaft where it enters the hub. You should be good to go.
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09-03-2014, 04:05 AM | #3 (permalink) |
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Thanks. Haven't been following the issue. Was wondering what happened.
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09-04-2014, 11:44 PM | #4 (permalink) |
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You won't snap a tailshaft, but there is still a hum issue. Some have it, some don't.
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09-05-2014, 12:35 AM | #5 (permalink) |
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I just built the 700L. I don't notice any hum. I really liked they way it went together. At first I had some tail wave but it turned out to be my fault; too much Loctite in the tail dog-bone screws causing binding.
I was reading up about 700 tail stuff this week and I am not sure I could tell what caused the hum. One theory seemed to be blade balance or blade weight. I put my tail blades on my DuBro balancer and they seemed dead on. A pair of CY 105's I also have on hand were the same: well balanced.
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09-05-2014, 05:23 AM | #6 (permalink) |
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Yeah i was reading some on the topic too and couldn't find decent explanation aswell. That's why i decided to ask and see if i could clear some doubts up.
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09-05-2014, 11:01 AM | #7 (permalink) |
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After a few more test flights with different head speeds, this is going to be my go-to heli other than my 550E.
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This is your brain on helicopters! Trex 700L, Trex 700N DFC HB gasser,Trex 700N V2 HB gasser, Trex 550X. Spirit or BD3SX |
09-05-2014, 12:36 PM | #8 (permalink) |
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i have one 700DFC from the first batch, no hum , but when i touch the boom it has vibes , yesterday replace the stock tail blades with a RAIL's 105 and no more vibes.
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09-05-2014, 01:54 PM | #9 (permalink) |
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Seems to be one of those heli related mysteries :-)
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09-05-2014, 04:02 PM | #10 (permalink) |
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Mine has the hum at higher headspeeds... But my primary HS it is not there...
I got one of those boom support braces, hoping that takes care of it!
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09-05-2014, 11:34 PM | #11 (permalink) | |
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Quote:
I don't think its anything wrong with the trex, I've had other heli with this same tail resonance at certain RPMs. logo500 is a good example.
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09-05-2014, 11:43 PM | #12 (permalink) |
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Harmonic tube vibrations. Caused by resonant harmonic vibrations by torque tube. Usually caused by evenly spaced torque tube bearings in the tail boom. NEVER EVER evenly space them. Always randomly space your TT bearings. Just gotta shuffle them around until the vibes are down to a minimum. Try it I guarantee it.
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09-06-2014, 03:39 AM | #13 (permalink) | |
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Quote:
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09-06-2014, 08:53 AM | #14 (permalink) |
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The tail hum itself wouldn't bother me too much. But does it result in the breaking tailshaft? Or do these two issues have nothing to do with eachother?
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09-06-2014, 09:53 AM | #16 (permalink) |
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Can you tell me the positions in the TT where you place the 3 bearings thanks in advance
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09-06-2014, 11:06 AM | #17 (permalink) | |
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Quote:
Resonances have a width. The can get excited a range of speeds around a central point. The width is called "Q" in electrical engineering. So if you make the sections different but only different by a small amount, they can still all resonate. I am just guessing but 20% difference between any two sections is probably more than ok.
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This is your brain on helicopters! Trex 700L, Trex 700N DFC HB gasser,Trex 700N V2 HB gasser, Trex 550X. Spirit or BD3SX |
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09-06-2014, 11:52 AM | #18 (permalink) |
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Evenly spaced bearings cause counter weighting harmonics. If the bearings are evenly spaced leaving equal exposed lengths of TT then the equal lengths with counterbalance each other causing a harmonic vibe. Look at it like a teeter totter. One equally weighted fat kid on each side with counter weight each other causing a even movement. Now make on side of the teeter totter plank shorter and it throws off that counter weighting. 2-3 bearing id try like this. ______|__|_____|___________ . Make sure to use a higher rated abec # rated ceramic bearing. The higher the abec rating, the tighter (less play) it is. Lose stock bearings are vibe feeders. I use boca abec#7 rated and it's weirdly quiet. Even the other guys I fly with ask about it all the time.
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09-06-2014, 11:55 AM | #19 (permalink) |
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Another cool trick is to spool it up with only main blades off to running rpms. Use your hand to feel the portion of the TT That is vibrating the worst and slide a bearing in that direction. TT vibes occur more often towards the center and end of tail
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