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T-REX 450 › Tail Vibration
09-11-2007 11:18 AM  10 years agoPost 1
Darren Lee

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Woodstock, GA

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I'm trying to track down a nasty vibration that started a few flights back. No crash, nothing has been changed out. This is a T-Rex 450S with the new tail slider, plastic tail grips and batman tail blades. 430XL with 12T pinion.

It's a vibration that becomes apparent only at high headspeed, such that I can fly around in normal mode or ID1, but when I switch to ID2 it gets really noisy and the vertical fin is blurry. This morning, I took off the main blades and spun it up on the bench. Bad vibration starting around 75% throttle (half stick). Took off the tail blades and tried again, smooth as silk!

So, I thought it must be the tail blades out of balance, even though they've never touched the ground and I've been running them for many flights with no vibration. I tried two other pairs of tail blades and got the same result each time. Nasty vibration starting at half stick. With no blades on, the heli runs smooth all the way up to 100%. The tail shaft and hub appear to be running dead true. What could be causing this? I really don't think it's the blades because I've never had a problem with them being out of balance.

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09-11-2007 12:09 PM  10 years agoPost 2
Boidman (RIP)

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One screw loose, and/or outboard bearing wearing? Even assuming perfectly balanced blades, and the same weight removed from each side with the blades off, it may require that extra weight to make the problem obvious. The thing is that that should show up on the horizontal fin, usually.

Something odd going on with loose linkages even though they are loaded up almost all the time? Tail blades going just slightly in and out of track could cause the vertical fin to vibrate.

Your report leads right to the tail grips, but that vibration location confuses the darn deal.

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09-11-2007 12:21 PM  10 years agoPost 3
Darren Lee

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Woodstock, GA

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Well, I am running 2mm CF fins and the horizontal one is very stubby. So, I doubt the horizontal fin would ever show obvious deflections.

The blurry vertical fin was visible in flight, but on the bench there is another indicator. The aft end of the rudder pushrod where it connects to the bellcrank shows severe deflection (up and down).

Also, there is a good bit of slop in the slider linkage. I've already been through 3 of these darn things, they just tend to get loose over time. Maybe I'll just replace the slider, grips and hub.

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09-11-2007 12:38 PM  10 years agoPost 4
menace

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Sydney, Australia

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http://www.wattsuprc.com.au/product....asp?pid=HS1200

are u using these tail parts?

because i have found the tail slider to wear a bit on the tailshaft and sort-of groove and give a tail vibration after about 20 or so flights (dep. on flight style)

!boo!

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09-11-2007 12:40 PM  10 years agoPost 5
Darren Lee

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Woodstock, GA

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are u using these tail parts?
Yes.

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09-11-2007 01:12 PM  10 years agoPost 6
TomRex

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West Palm Beach ​Join Date:​12-28-2005

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How about re-seat the tail hub, you know, R&R it. When you tighten the set screw, tug on the hub to the outer part of the flat on the shaft and lock it down. Remove your Main Blades, spool up to check for vibs in the tail/blades, balance with tape. Also heres the slop/dampener mod for the V2 slider.

Ignorance is the absence of facts.Stupid is lacking the intellectual capacity to comprehend the fact

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09-11-2007 01:17 PM  10 years agoPost 7
Darren Lee

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Woodstock, GA

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How about re-seat the tail hub, you know, R&R it. When you tighten the set screw, tug on the hub to the outer part of the flat on he shaft and lock it down.
Never heard of this. What exactly are you trying to accomplish by pulling the hub out? I've already done the fuel tubing mod, it helps with slop but only so much.

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09-12-2007 12:00 AM  10 years agoPost 8
T-Rex-Flyer

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Panama City, Fl

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TomRex

You're picture is not clear, can't tell what you did for you're slop/dampener mod. Also why are you missing parts to your tail case.

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09-12-2007 01:08 PM  10 years agoPost 9
TomRex

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West Palm Beach ​Join Date:​12-28-2005

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Beef, More throw! That micron of extra distance will help. If you push on it when tightening you effectively shorten throw.

Rex flyer, On the V2 slider only, ad a piece of fuel tube 3/16 long to the slider ball.
The parts are not missing they're thrown away! not needed

Ignorance is the absence of facts.Stupid is lacking the intellectual capacity to comprehend the fact

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09-12-2007 02:53 PM  10 years agoPost 10
helimatt

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Lafayette, IN

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My experience with vibration in the tail has almost universally been traced to the tail hub being bent or one or both of the screws that hold the tail grips on being bent. Typically this results from a crash. However, not always, and the earlier versions of the hub which don't have an integral stub boss which extends into the grip bearing rely on the screw to handle all the side loads. If you do quick stops from fast pirouettes, a lot of side force is put on the blade grips which in my opinion could bend one or both of the screws. The grips don't run true, and you get lots of vibration difficult to balance away. A slightly bent hub or screw may be compensated by putting tape on one of the blades to somewhat reduce vibration, but it'll never be quite right. The newer version is shown in Boidman's tail hub section picture.

Remove the tail hub from the shaft with the grips still attached (no blades) Hold by the grips alone, and put a lateral twisting force on them to see if the hub spins one way or the other- if so the hub or screw or both are bent. Its much easier to demsonstrate than to explain, so if that's not clear let me know.

Never, ever, ever, ever give up.

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09-13-2007 12:47 PM  10 years agoPost 11
Darren Lee

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Woodstock, GA

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I am very frustrated now!

I've replaced the tail shaft, bearings, hub, grips, pitch slider and belt! Still vibrating!

I've checked the balance of the blades mounted to the hub, looks great!

I've removed the entire head to make sure it's the tail.

Same thing happens no matter what I replace. Up to half stick it's fine, then BAM! Nasty vibration if it spins any faster.

On a side note, are the metal tail grips designed to use a different hub than the plastic grips? I bought this part yesterday:

http://www.readyheli.com/HS1221_Tai...nk_p/hs1221.htm

and the screw bosses are too short for the plastic grips! If you try to tighten the screws, the grips lock up. What's going on here?

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09-13-2007 02:59 PM  10 years agoPost 12
TomRex

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West Palm Beach ​Join Date:​12-28-2005

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I see your gyro is right up front next to the motor, a big no no!

Ignorance is the absence of facts.Stupid is lacking the intellectual capacity to comprehend the fact

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09-13-2007 03:02 PM  10 years agoPost 13
Darren Lee

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Woodstock, GA

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That's not my current setup. This is how I have it now:

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09-13-2007 03:13 PM  10 years agoPost 14
TomRex

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Ah Ha! he says! Try and move your esc to the side frame if not up there on the lower toungs away from the gyro, seems like something is screwe. Maybe some rf or emf from the esc. I had some wierd stuff untill I got it sorted. Your on 2.4Ghz I'm on pcm and gotta be on the ball or get locked out.

Ignorance is the absence of facts.Stupid is lacking the intellectual capacity to comprehend the fact

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09-13-2007 03:22 PM  10 years agoPost 15
Darren Lee

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Woodstock, GA

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It's not an electrical problem. My esc is under the battery mount. I have also tried unplugging the tail servo and disconnecting the slider linkage to eliminate as many variables as possible.

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09-13-2007 03:58 PM  10 years agoPost 16
TomRex

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Your saying it a physical vibration not a wag. Did you remove the main blades and try the tape trick?
Dynamic balanceing is how I learned to balance tires with the old dreaded clip on tire balancer.

Ignorance is the absence of facts.Stupid is lacking the intellectual capacity to comprehend the fact

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09-13-2007 04:56 PM  10 years agoPost 17
Boidman (RIP)

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The Home Stretch

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I'm going to go from the original, "No crash, nothing has been changed out," and also the, "It's not an electrical problem."

Your two statements mean that the only possibilities are wear and movement.

Some boneheads once wanted to know why I took longer to build one of these bastards, then the next guy. Someone in our gang was having problems with the SE he had built. With a couple of models I had built, plus the problem one, some tests were done.

About a 2 foot length of maybe .060" music wire was tie-wrapped centered across the horizontal fin, and a 1 foot piece was attached mostly above the vertical fin.

The 3 ends vibrated with maybe a 1-2 inch amplitude when the head was above maybe 24-2500 rpm, on the problem heli. On the other 2, the amplitude varied up to about 1/2 inch and then less, as parts of the wires and helicopter moved in and out of harmonics.

Increasing belt tension increased the vibration. With this setup, less tension always netted less vibration.

My favorite point and his problem was the ol' main gear rub the nub dealiwhop.

He also had a way too tight mesh between the autorotation gear and its mate in the tail drive assembly.

I believe some component of the vibration was transferred down the tensioned side of the belt, as well as through the boom.

The location of the holes in the frame are not always accurate, and my experience is that there is always enough slop to allow the main and tail drive shafts to be cocked front to rear, between the sides.

The first issue that usually occurs is an overly tight belt on a helicopter that is flown hard, pulls the front pulley to the rear, slipping the screws and 2 ears on the upper bearing case, back. That can be the cause of a belt beginning to ride hard against a pulley flange, or sometimes even improving the tracking.

Hard elevator maneuvers can really load up the main shaft and bearings, fore and aft. I have found more than once that the upper and lower main shaft bearing brackets have shifted in the frame, even though there had been no crashes. The shifts were enough to change front and rear, pinion and gears, meshes and clearances.

If you know that your new and old parts are true and good at the tail, without any, "but they must be" type thinking, then start looking hard at the other end of the boom. Even though you removed the rotor head to test for vibration origin, this does not preclude the front end from being where the problem originates.

Y'all is about to develope a nasty little pseudosexual realtionship with that there toy, and properly handled, you should both be fully satisfied when all is said and done.

(just because I type more, don't ignore the other suggestions)

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09-13-2007 05:59 PM  10 years agoPost 18
Darren Lee

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Woodstock, GA

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Thanks for the insight Boid. Do you have any advice on the HS1221 hub issue? I wasn't able to use that hub, used another one I had in the spare parts box. It's not new, so there is a chance its tweaked as the original one may have been.

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09-13-2007 06:38 PM  10 years agoPost 19
Boidman (RIP)

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The Home Stretch

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Without checking, that hub looks like what I think is the current design. I thought the S and SA had the same current design. I upgraded my SE to that one, but it was packaged with a different group of parts.

It seems you have an old and a new hub, besides the one in the heli. My guess is you have no micromillimetrical measuring tools. You could stand the old and new hubs up on end, maybe one or so inch apart, and set some type of straight-edge or even a spare boom across the top of them. With the correct close separation of the hubs, you should be able to see a lack of parallelity between your desk/bench and the ruler/boom, if there is even a small difference in length.

With coat hanger gauge wire, you could bend up a short piece until it just fits the length of a hub, then compare it to the other. The same could be done for an outside diameter comparison.

My suspicions are now leaning back towards the rear of your bird.

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09-13-2007 06:51 PM  10 years agoPost 20
Darren Lee

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Woodstock, GA

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My guess is you have no micromillimetrical measuring tools.

Not neccesary. The old (silver) and new (black) hubs are obviously different. The boss is shorter by .03-.06" on the new one.

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