JoeyG123

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I recently crashed my Hawk Sport (first time trying to fly nose-in) and had to replace a few parts - namely: main rotor blades, fly bar and tail boom. Before the crash the heli flew beautifully and as far as i could tell didnt have any of the vibration problems it has now. I havent really noticed any bad flight characteristics but I can easily tell that the vertical and horizontal fins are shaking/vibrating big time. The other thing that hints that there is some sort of prolem is that it makes a weird noise as I increase throttle and lift of the ground. If anyone has any ideas as to what I should check to fix this problem please pass them along to me. I havent met any pilots in my area and any help you more experienced pilots can provide is greatly appreciated.

Thanks a lot

Joey

JoeyG123

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one more thing, as soon as i lower engine power once on the ground the vibrations seem to go away - i dont know if that matters just thought id include it.

also, the blades i bought are supposed to be pre-balanced and i dont have a balancer - so if this is what you think the problem is- is there any way to balance them without buying a balancer??

thanks

davidg51537-RCU

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Even though they may say pre balanced, you should check them anyway. The hawk manual shows a way to balance them if you don't have a balancer.

Also, did you check that the main shaft or the feathering shaft is not bent? That happens alot of times when you take out the main blades and flybar. Check those too.


David

Cougar-RCU

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Take the shaft out and roll it on a piece of glass
look for day light while rolling ~S~
I hope that helps!
Jeff

JoeyG123

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okay, i have replaced the main shaft and tried running the heli without the baldes (i completely took of the head) and there is stiill vibration.
Also what should the needle mixture screw be set at; how many turns do you have yours set at??

davidg51537-RCU

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I usually hear between 2 1/2 turns to 3 out. But why are you asking that?

Have you checked your tail output shaft. Are the tail blades chewed up possibly causing them to be out of balance. How bad was the crash? Did you smack hard? Could you have tweakd the tail boom? Missing any teeth on the main/crown gear? This sound you speak of, does it sound like something is grinding?

Tell us more.


David

Cougar-RCU

My Feedback: (2)

I just had this happen to my Falcon, where the starting shaft
did not disengage from the engine, the collar slipped and was turning the whole time I had the engine running. talk about
a vibration sheesh. You might wanna push down on your starter shaft, and check that as well, I had to dremal a flat , now it works great , just a thought
-Good Luck-

JoeyG123

My Feedback: (1)

the crash wasnt too bad and i have replaced the tail boom already. When i replaced the boom the output shaft looked perfect. The tail blades werent chewed up at all and i dont think anything is wrong with them. The main gear is missing one tooth but i had read about someone not having any problems with four teeth missing; could that cause such relatively strong vibrations? the sound i hear happens at just about at hover and it sounds as if something is vibrating - i cant really describe it. Cant the engine tuning cause certain vibrations? any suggestions on that?
as for the start shaft i dont think there is a problem; i have to push it in to engage it and it comes out just fine - is that what you are talking about??

thanks

Cougar-RCU

My Feedback: (2)

Yep, that's what I was talking about..
You have Four teeth missing? on your MAIN gear?
I don't care if I had one missing. I would replace it.
MY option only..
good luck
Jeff

JoeyG123

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i only have one tooth missing. is doesnt seem to cause any real problems unless its responsible for the vibrations

Cougar-RCU

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I dunno what to tell ya.. I have no clue if its causing it or not bud!
I am sure others will tell you it's just fine that way I guess

JoeyG123

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just to make sure it wasnt the tail i took the entir tail assembly off. the vibration was still there. i can notice it now because when i was holding the the frames to hold the heli in place i could feel it. I am pretty sure now that it is something with the engine and things that are connected to it - clutch, fan. But before i go take a part more stuff can anyone tell me how much a heli normally vibrates, like just from the engine or something. should there be no vibration at all or is there always going to be some engine vibration just becaus eits an engine??

JoeyG123

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after further inspection i have found that on of the gears has a small amount of wobble to it. could this cause the vibrations i have been describing?? how can i fix this??

davidg51537-RCU

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yes, that could be the source of the vibration. Which gear? Since they are plastic, you will have to replace it I imagine.


David

JoeyG123

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its the counter gear, what a pain to get off. i just got done spinning it up on a dremel and it makes that vibrate so i think i may have found the problem. if only there was a way to fix it??

davidg51537-RCU

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Well the time and effort to balance it would greatly outweigh the cost of a new part. So I would not even fool with that.


David

JoeyG123

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as i said i took apart the heli to check out the counter gear, which has a slight wobble. Then i went to the century site and looked at their FAQ's only to find that they know the counter gear wobbles and they specifically say it isnt a cause of vibrations. Anyway I put the heli back together and noticed that the blades were much more difficult to move if i tried to spin them the opposite direction they spin. I then started it up and the blades were trying to spin with just the starter trying to start the engine. Once i got the engine running the blades would try to move much harder than they had before and if once they were spinning i tried to stop them by pushing on the head button the engine would stop. It seems as if the clutch isnt working right so i took everything apart again and rebuilt it according to the instructions - i couldnt find a problem though. what could cause the clutch (??) to tighten up like that?

thanks for all the help so far.

Joey

Culverson

My Feedback: (14)

Carefully check the output shaft again, I had my Hirobo belts to tight and it actually warped it just a bit, it caused a bad vibration. Also might want to remove the tail boom and inspect you tail wire guides and TW, on the Nexus I once had the TB wire supports to shift to the front and cause a herendous shake. There is a field rep that once flew at our field here in AL., look on the site (manuf.) and give hi a call, tell him a fellow from Jasper (Hay Valley) sent you. He can help on Century..
Info:
Alabama Brilliant Bruce Eddy (Sport) (205)465-2842 [email protected]
Terry taught him how to fly one of our field members..Bruce is a great fellow to request info from, plus he's in AL.

rotordoc

Joey,

Checking your original post I noted that you did not say you replaced the most obvious - the Feathering spindle. In a crash of any helicopter where the blades are damaged the immediate component to take absorb impact forces after the blades is the spindle, then the mainshaft. So if your mainshaft was bent I would bet your feathering spindle was also.

Any vibrations caused by the rotating main rotor (tracking not right, flybar paddle tracking not right, head imbalance, bent flybar, bent spindle, bent mainshaft) will be a low frequency vibration. Why? The main rotor turns at a low frequency (1400 to 1800)

A vibration caused by the rotating tail rotor (bent output shaft, imbalanced tail rotor, one blade grip coming loos, bent hub or retaining bolt, migrating drive supports inside the boom etc.) will cause a mid frequency shake and will be noted mostly in the vibration (buzzing) of the tips of the fins. Why? The tail rotor turns at a mid frequency (6500 to 9000)

A high frequency vibration (would cause fuel foaming) will always be related to the be engine (too lean, imbalanced components. misaligned start shafts etc). Why? the engine turns at the highest frequency (14000 to 18000).

Good points to remember when chasing down a vibration

UrcHobby

My Feedback: (2)

Joey Centurys gears can wobble here is the link to the site of centurys F&Q http://centuryheli.com/support/faq.html Please let me know how you make out I just got done chasing down a vib prob on my Hawk Sport It can really be a pain

JoeyG123

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Thanks for all the help everyone. I havent had time to try and fix the problem and i need some new flybar arms so I am just going to wait a little while to get the problem solved. again, thanks for all the tips -

Joey

bhelisN6

After reading all the responses, everyone has brought up the key areas that could cause vibrations.

Maybe we take a step back Joey,

Please describe carefully the type of vibration.

- Is the entire mechanics violently vibrating?
- Is only the landing gear vibrating?
- Does the vibration seem to pitch the helicopter front to back?
- Or left to right?

You say you removed the head, again where specifically was the vibration?

- Are you using training gear?

Generally, when a vibration persists, we continue to replace parts until the vibration is eliminated. More common though, the vibration is a result of a part that looks fine from visual inspection but actually is loose. Go ahead and carefully inspect all fasteners on the helicotper.

Also, I have flown Century Hawks from its first generation, the counter gear has never significantly caused problems (other than looking strange).

Andrew

hkIIIflyr

I have the hawk III and had similar problems. I found through researching that the plastic clutch bell was not perfectly circular on the inside where the clutch engaged. When the engine accelerated and the clutch engaged inside the egg shaped bell it shook pretty bad. My quick fix was to take a light sand paper and gently rough, very gently, the inside by placing the sand paper in between the clutch and the bell grit outward and spin the bell. After doing so I noticed the bell spun freely around the clutch thus a little more circular in shape. I immediately noticed a difference and it has worked fine ever since.

HkIIIflyr
Century also sells the aluminum replacement bell as well

JoeyG123

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hkIIIflyr-i found out that my clutch wasnt working right the other day. i took it apart to find out what the problem was with the blades always turning, even at idle, and found that my clutch bell was slightly mishapen, like you said. replaced that and havent had a problem since.

hkIIIflyr

Thats great man I'm going to order a new bell soon, because I know its only a matter of time before it gives me problems.

Good luck HkIIIflyr