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Trouble with a Big Mig

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Post  roddie Mon Dec 16, 2013 9:39 am

Hey Rusty, Sounds like a great day of flying! I'm envious... as I've only flown alone... and haven't flown for a long LONG time! That's one of the reasons why I love this forum so much.

Now that the troublesome Norvel is off the plane.. and you can disassemble it to check for any internal damage that the crash may have caused. I'm wondering if the crank got bent... (The Streak went into the ground; hard enough to separate a fuse doubler right?) Even if the crank is slightly bent... it could be causing the front bearing to heat-up and start to bind... which would cause the symptoms you've been experiencing. I don't know how you'd check the cranks' "run-out" at home... maybe by holding a straight-edge against the shaft and slowly rotating it with a light source behind it.

I'm not sure (because of my admitted lack of knowledge...) if this engine in question has a threaded prop-shaft or takes a prop-screw? If it was the latter; was the screw long enough to almost "bottom-out" before tightening the prop? I've read that having a longer prop-screw can add a considerable amount of strength to the crank. If the screw was particularly short; a forceful-enough impact could have bent the crankshaft.

Rusty, I apologize if this is way off-base.. I know you're frustrated with this engine. I'm just trying to mention things you may not have thought about yet.

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Post  RknRusty Mon Dec 16, 2013 10:24 am

Roddie, thanks for thinking about it, I appreciate the input.
That's actually the track I'm on, but in a way I'd have never thought of. A knowledgeable buddy says that an impact sometimes causes the front crank bearing to rip off a bit of aluminum from the case. It becomes welded, or at least bound to the steel causing it to tighten as it heats up. Some serious crank polishing might be in order. When I take it apart I'll see if there's any evidence either in the case or on the crank. If that's the problem, the case may be a nose bleeder for the rest of its shortened life, and it would have been better if I hadn't run it 15 times since it happened.

I use long black oxide hex screws to attach the prop on all engines. The Norvel screws are incredibly short. I have no idea why they would design them like that, but I changed them all out. I have three Big Migs. It will probably stay in the drawer until after the Tach Race. I've got more on the table than I have time for already. I'd forgotten how fast the little suckers were. Shoot, I thought my flying was just getting better. lol! 
Rusty

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Post  Surfer_kris Mon Jan 27, 2014 5:43 am

@RknRusty wrote:I knew the outcome so I played with the needle. The longer it ran, the more I had to richen it. Bit by bit, up to a half turn by the time it used all the fuel. So next was some Sig 35. Same results but had to set the needle back to the original starting point to get it cranked. Then bit by bit, richen it up to the same half turn to keep it going.

This sounds like it is overheating, unless it is a fuel delivery problem. You could try adding a head shim or two on the same fuel and see if that improves.

The rather slow heating effect suggest that you might have a little too tight fit between the crank and crank-case. This will eventually go away, but you can speed up the process by lapping the crank to the case by using some fine polishing paste.

What prop did you use to get 28,800 rpm?
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Post  RknRusty Mon Jan 27, 2014 7:20 am

@Surfer_kris wrote:

This sounds like it is overheating, unless it is a fuel delivery problem. You could try adding a head shim or two on the same fuel and see if that improves.

The rather slow heating effect suggest that you might have a little too tight fit between the crank and crank-case. This will eventually go away, but you can speed up the process by lapping the crank to the case by using some fine polishing paste.

What prop did you use to get 28,800 rpm?
That 28k engine is another one, and I don't know it's history. I put an APC 5x2.5 or something in that range on it when it bench ran that speed. That one now takes the place of my problem engine.

The troublesome one will be in the drawer for a while. It is very well broken in with many hours of flying time, all full speed on a bladder. I have opened the venturi and spraybar for inspection and cleaning several times since this problem started, forced brake cleaner through it, epoxied the venturi to the case to eliminate the possibility of air leaking, changed fuel, plugs, gaskets, head clamps, and back plates. I've thoroughly cleaned the internals, but have not inspected the crank for my latest theory, an aluminum deposit bound to it causing stiffness. I will not lap the crankcase, because any tightness is not natural. Whatever it is happened suddenly after a year of regular flying. The only clue I have left is that it's been doing this ever since my neighbor plowed it into the grass. I'll follow up on it after I get my other projects out of the way.
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Post  Surfer_kris Mon Jan 27, 2014 11:40 am

Okay, the engine may well have ingested something unhealthy...

I would check the crank and case for damages then. I haven't had any problems with the Norvel propbolts myself, and I guess it depends on the prop used too. Those APC a really thick around the hub. With cox engines it is important with a long screw so the crank end is not sheared off at the splines. I haven't had that problem with the Norvel engines, but the prop bolt is a regular M3-thread so it is very easy to find a longer replacement (at least here in europe).
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Post  RknRusty Mon Jan 27, 2014 2:57 pm

@Surfer_kris wrote:Okay, the engine may well have ingested something unhealthy...

I would check the crank and case for damages then. I haven't had any problems with the Norvel propbolts myself, and I guess it depends on the prop used too. Those APC a really thick around the hub. With cox engines it is important with a long screw so the crank end is not sheared off at the splines. I haven't had that problem with the Norvel engines, but the prop bolt is a regular M3-thread so it is very easy to find a longer replacement (at least here in europe).
Yeah, I want to tear it down again and get a good look under bright light and a magnifying glass. Last time I tore it down, I wasn't looking for anything specific, just making sure it's clean. I'll polish the crank with some 1000 grit and oil.

I do use APC props when I fly on my short field, a 6 x 2. With long lines I use a Master Airscrew 5-1/2 or 5-3/4 x 3. The prop bolt thing is just a carryover from my Cox habits. But long bolts make me feel better, so I do it.

Thanks for your interest, Kris. You've piqued my curiosity again. I'd like to have it back in action, or at least know for sure what's got it down. I need some engine Zen now and then.
Rusty

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Post  fit90 Sun Sep 13, 2015 10:08 pm

Hey Rusty,

How hanve you been? I was just going through some old posts and came across this one. Did you ever figure out what was wrong? I have heard of similar problems being caused by the crank to case fit being too tight and requiring some crank polishing. I am not sure why this would have happened to a broken in engine unless something was bent in the crash, Just wondering if you found anything.

Bob
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Post  RknRusty Sun Sep 13, 2015 11:20 pm

Hi Bob,
I'm still doing lots of flying and having a great time learning PA. But I still play with the small ones too. This is the engine you gave me. I don't think it's bent the shaft, it runs great and smooth as ever until it warms up and binds. I outfitted another older Big Mig with a Cox 128 tpi NV and have been using it on that same Streak.

Just a couple of weeks ago I finally polished the crank. I am pretty sure it had some aluminum fused to it. I still need to reassemble it and run it, and I think it will run like new again. I took a pause from that project when I wrecked my Oriental, but I hope to get back to it in a couple of weeks, as well as a trio of Stuntman23s I'm building. I will probably bolt it back onto the same Streak It was powering before.

How about you, how are things going. Healthy and having fun I hope.
Take care,
Rusty

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