rrweather

My Feedback: (7)

Got a quick question...

I purchased a used corona from a guy on RCG. I am finally getting it ready to fly (crash) and have a few questions. When I first got it, I spun it up and noticed there was a lot of vibration. It was so bad that at half-throttle, the corona would start to bounce. I originally thought the main shaft was bent. I took it apart and the main shaft is good to go. I actually ordered a replacement before I took it apart and realized I don't need it yet. Yesterday I messed around with it again. When I originally took the main shaft out, I noticed that the bolt holding the rotor hub onto the main shaft was almost completely smooth, as in no threads. I also noticed that the rotor hub bowed out (or bulged out) around where the bolt went through it. When I reinstalled the shaft, I noticed that it was impossible to tell where the flat notch in the shaft was to it could line up with the bolt. I also noticed from the day I got it that the main rotor could rotate without rotating the shaft, gears, or motor shaft. Obviously there was slippage somewhere. So back to yesterday...I was messing around and at half throttle, the vibrations was just too much. I gave it full throttle and the motor was screaming but the rotor wasn't spinning fast enough to even make it light on the skids. So my thought is that the main shaft is basically spinning within the rotor hub. Every time the flat spot in the shaft hits the bolt, it creates massive vibrations. This would explain why the threads of the original bolt were worn smooth.

I ordered a replacement rotor hub. While I'm waiting on it, I wanted to get some feedback from anyone on here as well. Since parts seem to be harder and harder to come by, I'd like to make sure there isn't something I'm missing that would explain my problem. Thanks for the help. Hopefully my description is clear enough.

FlyingFish

My Feedback: (17)

I would say a new mainshaft and new rotor hub should fix the problem. I've been playing around with these coronas for about 3 years now. Their pretty simply to repair since there fix pitched. The vibration should be mostly gone after replacing those two parts, but heres a tip, if you still get some vibration in the rotor head, everyone thinks right away that the blades are unbalanced, but that's not always the case. Sometimes it's just a matter of making the blade grip bolt tighter or looser (I'm talking about the bolt that holds the Blade grip to the rotorhead, NOT the bolt from the blade grip to the blade)

ViperRR

What I do is not to use the stock bolt that holds the hub to the main shaft (called the Jesus bolt by most 120 users). Reason being, the threads on that bolt contact the shaft's flat spot act like a file and wear the bolt and the shaft. What you need to do is find a new bolt that is only threaded 1/2 way on the bolt, so there is non-threaded smooth rod contact to the flat.
I'll see if I can find the package (I believe it's Great Planes) and give you the size & part#. You'll have to dremel the threads a bit, for the bolt is longer than stock.
Vipe

ViperRR

Buy the Great Planes 4-40 X 1" socket head screws (part# GPMQ0316). There is enough non-threaded area at the base of the screw head to avoid the filing action of the fully threaded stock bolt. You'll have to dremel off a 1/2", as the stated size needed in the 120 manual. I never had any worn hub play or chewed up main shaft flats from this method.
One thing to add: DO NOT over tighten the bolt! It will deform the plastic hub. All the bolt does is index the hub to the shaft (kind of like an indexing pin). Just tighten the bolt to where the nut just starts to contact the hub. Your deformed hub needs to be replaced if you tightn too much.

ViperRR

Hey rrweather,
Did you ever get the bolts and a new hub and/or shaft?