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Thread: Prop fell off!!

  1. #1
    Bluebird19kf's Avatar
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    Default Prop fell off!!

    So today I was flying my kf3 and after takeoff at 300 the engine reved up uncomanded. There was no thrust. Turned towards the airport and made the field. I shut down the engine and the propeller was still spinning. After it stopped I pulled on it and it came off with the gearbox shaft. It's a b box. Anyone ever heard of this?

  2. #2

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    Default Re: Prop fell off!!

    That’ll get your attention. Post pics when you get the box pulled apart....interested to know what failed. Glad you and the plane made it down safe

  3. #3
    Senior Member efwd's Avatar
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    Default Re: Prop fell off!!

    Damn! how did it not separate from the engine? Glad your safe.

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    Default Re: Prop fell off!!

    If the bolt holding the drive gear on the crankshaft came loose and the gear loosened on the tapered crankshaft, it would probably do what you describe. Guy I know had his prop fly off when the keeper that holds the driven gear on to the prop shaft came off. The prop went flying across the yard then. Good thing he wasn't flying it then. JImChuk

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    Default Re: Prop fell off!!

    Quote Originally Posted by avidflyer View Post
    If the bolt holding the drive gear on the crankshaft came loose and the gear loosened on the tapered crankshaft, it would probably do what you describe. Guy I know had his prop fly off when the keeper that holds the driven gear on to the prop shaft came off. The prop went flying across the yard then. Good thing he wasn't flying it then. JImChuk
    Possible, but for the prop shaft to come out of the housing the bearings would have to completely disintigrate. If the crank gear bolt is intact upon inspection, the likely culprits are: badly out of balance prop, prop strike or lack of lubrication of the bearings or combination of those.

    In another thread you mentioned 190 hrs on the engine and stuttering in midrange. Obviously something was wrong for this to happen on a 190 hour engine. Could the stuttering actually have been vibration? If so, the problem might have been one of the above all along.

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    Bluebird19kf's Avatar
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    Default Re: Prop fell off!!

    I took the gearbox apart. Nothing appears to be wrong although I haven't cleaned up everything and inspected closely. The bearings feel fine. The previous stuttering problem ended up being mis-adjusted choke valvle. I am running the 68" 3 blade Ultralite Ivo prop. Is that not good with the b box? Is it really just the split washer that fits in the shaft groove holding it in? Seems crazy it is that easy for that to fail. Also, I don't think the gearbox has been inspected since new and has like 350 hrs. on it. Is it likely that the spring washers got weak and that causes the split washer thing to come out of the shaft groove?

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    Default Re: Prop fell off!!

    Quote Originally Posted by Bluebird19kf View Post
    I took the gearbox apart. Nothing appears to be wrong although I haven't cleaned up everything and inspected closely. The bearings feel fine. The previous stuttering problem ended up being mis-adjusted choke valvle. I am running the 68" 3 blade Ultralite Ivo prop. Is that not good with the b box? Is it really just the split washer that fits in the shaft groove holding it in? Seems crazy it is that easy for that to fail. Also, I don't think the gearbox has been inspected since new and has like 350 hrs. on it. Is it likely that the spring washers got weak and that causes the split washer thing to come out of the shaft groove?
    I must be missing something here? The front bearing inner is pressed onto the shaft and the outer is pressed into the housing. I don't see how the bearing could be intact unless heat from free spinning (between shaft and bearing) caused the inner to expand and free itself from the shaft, allowing the shaft to come out without the inner half of the bearing. I'd look there for damage to - shaft wear, inner race wear.

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    Default Re: Prop fell off!!

    I was just looking at a YouTube video of a guy overhauling a b gearbox. Itsbeen a year or more since I had one apart, and I was questioning my memory after another post was put up on this topic. Watching the video, you can see that if the keepers are removed that hold the large gear on, the propshaft can then move forward. When he reinstalls the prop shaft on reassembly, he taps it a little to get it in place, but it's not pressed on. Interesting side note on the video, is the reason for overhauling the gearbox is the prop had hit something while cruising, and all the teeth were knocked off the small gear. That would also allow the prop to freewheel, although it's not your problem. Were you able to figure why it freewheeled when you tore your gearbox apart? JImChuk

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    Default Re: Prop fell off!!

    Danzer, just reread your last post, and have to take issue. The front bearing is pressed in from the inside of the gearbox case. Its held in place by a snapring. Then the propshaft is pushed in from the front. I would post a link to the video I mentioned, but I'm using a tablet cause my PC is messed up. Just search for b gearbox overhaul on utube.. It's about 16 minutes long. JImChuk

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    Default Re: Prop fell off!!

    Hi Jim,

    Just looked at what I think is the same video. If you look at about 6:38 he puts the front half of the gearbox in a press to drive the shaft out of the front bearing.

    Then at 11:38 he puts it back in the press to put the keepers in - that is also where the shaft gets pressed into the bearing - that pressing is to 3600 Lbs. Point is - it is a sufficiently tight fit for the bearing to do the work and the shaft not to spin in the bearing.

    Of course the keepers would have had to come out or disinigrate for the prop gear to come off of the shaft too the shaft to come out of the case.

    https://www.youtube.com/watch?v=B_pn4XGnaHU

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