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12-27-2008 01:05 AM  8 years agoPost 1
nrad2000

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USA

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This has been asked many times in different ways and the answers always the same - but perhaps maybe I'm over looking something - and thats where you guys come in.

I have a Century Hawk Pro converted to 50 size w/ os hyper and scale muffler (cool power 15%). It's sitting inside a funkey Bell 222 fuselage. For the last few months it has not flown because I'm trying to track this problem (which I'll tell you now).

When I increase throttle and the clutch engages it is fine. when it picks up a little bit of head speed (must be at couple hundred RPM) the rotor head wobbles side to side. As it picks up head speed it passes this wobble point and smoothes out. I can't go to full throttle because I'm testing this without main blades.

I changed the feathering spindle because that was slightly bent from a hard landing half year ago. I ran the motor without the head assembly and looked down at the main shaft and that was straight. no wobble in the main shaft. Engine accelerates nicely. no gurggles or sputtering on acceleration. Idles fine. I'm now thinking maybe it's the flex in the fuselage that the mechanics sits on. I have all the braces top and bottom.

I have a Raven 50 in a hughes 500 fuselage and that has no wobble in the head in acceleration. You know what - let me swap out the head and see what happens. But, if you guys have any other ideas please let me know.

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12-27-2008 01:24 AM  8 years agoPost 2
Gyronut

rrProfessor

Martinsville In.

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Of course you could rule out the basic stuff and check the head balance. Bent main shaft. Blade balance.

But for my money it's probably just ground resonce.

My TT MT would do it all the time.

I just spooled it up as quickly as I could and flew out of it.

Rick

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12-27-2008 02:19 AM  8 years agoPost 3
george0079

rrElite Veteran

USA

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What about paddles and flybar?

Hell... I can fix that.
Uh oh..
Nope.
It's ***ked!!!
RE-KIT!!!!!

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12-27-2008 03:43 AM  8 years agoPost 4
BarracudaHockey

rrMaster

Jacksonville FL

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Don't run a 50 unloaded without blades or without the head. You're going to tear something up.

Depending on your blade tightness it's not all that unusual, some helicopters pass through a period of resonance where they shake a little.

Like I said, no reason to run a helicopter w/o blades.

Andy
AMA 77227
http://www.jaxrc.com

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12-27-2008 05:32 AM  8 years agoPost 5
nrad2000

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USA

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I know I shouldn't spool up without the blades - so I didn't rev it. Just enough to see that at a certain point the wobble goes away - then spool down.

I swapped rotor heads with the other century heli. tomorrow morning I'll check one last time and see if their is a wobble (again without blades). I know what you guys are going to say (don' do it) but I just can't bear to watch my blades strike ground or boom strike due to severe chicken dance.

I put a post up regarding to the crazy chicken dance that my bell demonstrated a couple months ago. on spool up (the blades were just spinning around 10% speed) the bell would bounce from left wheel to right wheel and increase in tempo. I thought she was going to tip over - popped throttle hold and gave full positive pitch and it went away. while trying to figure out what caused the dance I took apart the rotor head and examined the feathering spindle - sure enough it was bent. I'm not sure if it was caused by the wild chicken dance or couple month before this I had a real bad landing in pod and boom form. I did get a new feathering spindle installed and I tested it today (without the blades and for the reason of this post) and their was wobble in the head(as described earlier in this post).

My flybar is perfectly aligned and centered - the paddles are aligned. The blades are balanced and rebalanced (close to perfect).

I'm going to re-examine the flex in the fuselage.

Maybe I just need to accelerate past that wobble speed as "Gyronut" said.

Strangely, (without the blades) during observation - the landing gear was steady as a rock during the test. I watched the tail and no shakes. I watched the side wheel well that protrudes out of the bell 222 (the small winglet that houses the landing gear) and no shakes. using the top of the fueselage where the rotor head comes out of as reference (the foramen magnum if that makes any sense) and the main shaft - watching the two I can see wobble. But, there is no bend in the main shaft.

The mech must be swaying side to side inside the fuselage - is my next guess. (even with all 4 bracings) Maybe I need to somehow re-enforce the side walls. That means more weight.

Well, tomorrow I'll test the other rotor head - see what happens.

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12-27-2008 09:09 PM  8 years agoPost 6
nrad2000

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USA

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The other rotor head did the same.

After lunch I'll put the blades on and just gun it to hover rpm.

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12-27-2008 09:13 PM  8 years agoPost 7
nrad2000

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USA

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i hear it over and over again - everyone tells me not to run the rotor head without the blades. why is that? I know I can over rev. But, can I really tear something apart in the heli?

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