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Whirlwind propeller shake

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Upsidedown
MPF Student
Posts: 5
Joined: Sun Sep 24, 2017 8:57 am

Re: Whirlwind propeller shake

Post by Upsidedown » Fri Dec 01, 2017 5:03 pm

Shipchief-exactly what I was thinking except below 2450 there was no smooth spots at all. It was better when I rotated the prop but still unacceptable with any power or airspeed combo below 2450/2430ish.
I was going to try all the other positions on the crank, with Whirlwinds blessing, (Jeremy at Whirlwind has been great to deal with), but I think I found the problem, or a problem anyway. I’ll have info in a coupIe days. I had to leave on a trip and will do some measuring Monday.

Upsidedown
MPF Student
Posts: 5
Joined: Sun Sep 24, 2017 8:57 am

Re: Whirlwind propeller shake

Post by Upsidedown » Fri Dec 08, 2017 11:08 pm

So the cause of all this may be that the face of the crankshaft was not machined straight. Unfortunately I won’t know if this was the issue until the spring but I will be sure to report back. I’ll tell the story so it may help someone

I purchased this engine with a very minor prop strike. It dialed good but I knew I would feel better if I send the crank out so I tore down an engine with 60 hours on it. The shop checked the runout, crack checked it and did the crank gear. That’s it. It all checked perfectly so yellow tagged and back in my hands I built up the motor. A year or two later when I installed it on the airframe I remembered noticing a slight rock to the flywheel, very slight like less than 1/16 inch of movement. If you held the flywheel at the 9 and 3 o’clock positions it had a very slight walk as you put more pressure on one side vs the other. If you held it on with pressure in the center it seemed fine. I assumed I was rocking it off the edge of the crank and still had thousands of hours left to finish the project so that clue went unnoticed. Now when I was getting ready to reinstall the prop the other week, I noticed the rocking again and it occurred to me that this may be the cause. At first I thought the flywheel was hanging up on the radius of the crank snout, there looked to be some marks that indicated this. So I cleared some material off the flywheel and no luck, still rocking. I borrowed a different flywheel and it still did it. With a straight edge I determined that the face of the crank was ground with a slight taper. Now it’s hard to measure because you can’t go across the center of the crank and with a micrometer you can only measure the 1st inch but it appears that the flange is 10-12 thousands thinner at the edge than in the center. Witness marks show the most of the contact between the flywheel and the crank is at the center. I put blue ink on the flywheel and pushed it on square and it showed the same.

This is what I think was going on. When I installed the prop initially the first bolt I probably tightened was at 12 o’clock, easiest to get at. The prop being horizontal at the time, this rotated the prop up, around the blade axis, which is why it measured fine in tracking. The blade angles would have been very slightly different but nothing I could measure. The math shows that the prop dome would have about a .030 runout in this situation. When I flipped the prop around I must have snugged the bolts more evenly and the vibration got better. I do remember one bolt being extremely tight during removal but was much easier to undo when the rest were loosened. So the clues were there I just didn’t think it was possible to have this problem.

Since this discovery I have gotten more sets of eyes on this for confirmation. I have decided that the motor is going out to a reputable shop for tear down and proper machining on the crank. Maybe some HP mods. This was the plan all along as I didn’t want to break in a new engine on a new airframe so at least it did it’s job. Hopefully this solves the problem and I will report back.

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