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Thread: O-360 in Super Cub...multiple cracked exhaust cracked engine mount, two new mufflers??

  1. #1

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    O-360 in Super Cub...multiple cracked exhaust cracked engine mount, two new mufflers??

    I just took over managing our soaring club towplanes. Pawnee and Cub.

    our cub has 2200 since overhaul..good compression and running strong. BUT...when I reviewed the logs in detail from last overhaul..I was Suprised at the number of times our exhaust had been sent out for welding ..plus two new mufflers and several repairs..and multiple spinner assembly repair ..way too much.

    so I figured vibration..dialed crank..fine, balanced prop, just required very minor trim up. So now I'm trying to figure out how to tackle this apparent problem.

    engine has hollow crank. Yes, I am aware that O-360 has harmonic issues..we have a yellow arc from about 2100-2300 RPM..

    wondering if our descent profile contributes..we hold 80 mph, 2000 rpm, full cross control...

    Or heat? We run 380-420 CHT in summer

    Anyone have similar very costly issues with O-360 in towplanes..or otherwise.

    brand new to forum..but not to airplanes..not an A&P.

    john Stewart
    Soarboulder

  2. #2

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    From a exhaust standpoint I would suspect you have a pretty high EGT when you are climbing. Everyone says EGT "DON'T MATTER", but, in this case of rapid heat cool cycle I think it could lead to metal fatigue on the exhaust system.
    DENNY
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    PerryB's Avatar
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    When you say "balanced the prop", was this dynamically with a Dynavibe (or similar) instrument? That's the right way to do it, because you're balancing the entire rotating mass (engine and prop) as one assembly.
    The cross controlled descent could be a factor, think of all the prop noises you hear when running up in a crosswind. There's some pretty jinky aerodynamics going on with the prop in those conditions. Do you really have to get it out of the sky THAT fast? What prop are you running?
    After Monday and Tuesday, even the calendar says WTF !

  4. #4

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    We have many Towpilots...and I'm also chief Towpilot..I teach running a bit Rich..but pilots tend to have a mind of their own...I'm planning on reinforce running a bit Rich..keeps CHT and EGT down...I'm looking for as many ideas and experiences to try to keep the tug going without breaking the bank..thanks for your reply..

  5. #5

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    Prop was dynamically balanced after crank dialed and tach calibrated...

    towplane utility is a function of turn times..if you have 3000 feet to descend rapid descent increases the number of tows one plane can do..this is similar to what I did when flying jump planes..but I looking for any I puts..thank you for responding..
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  6. #6

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    That full cross control with thrust (2000 rpm) for that long of a time is brutalizing that engine installation. Think about where the relative wind is striking the prop and depending on which way you are slipping you are pulsing that installation each blade swing. Then you got gyroscopic precession happening with each pulse and that thing is getting torqued every which way on every blade swing. You got lot of precession because it is spinning at 2000 rpm. It makes my head hurt just thinking about all the forces acting on it. I would also be worried about what you are doing to the vertical surfaces on the airframe.

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    skywagon8a's Avatar
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    Quote Originally Posted by Soarboulder View Post
    ..I teach running a bit Rich..but pilots tend to have a mind of their own...I'm planning on reinforce running a bit Rich..keeps CHT and EGT down.
    What do you mean "running a bit Rich"? If you are climbing you want to be full rich unless you are operating at very high altitudes. 3-4,000, even 5,000 feet is not high altitude for this purpose.

    Is your exhaust system close to any fixed parts which it is hitting on continually? It will not be touching when the engine is stopped and you are looking. Look for little rub marks.

    Is the exhaust tail pipe properly secured with braces to the engine?

    Have any of the exhaust ports been resurfaced? If so, this places stresses on the tightened down exhaust system which can cause cracking. You haven't told us where the cracks are located. Same place each time?

    "Sent out for welding". Does the welder have a fixture to clamp the pipes in while welding? A little distortion during this process will contribute to a poor fit on your engine which will cause cracking.

    Are the cracks along the edge of a weld, or across a pipe somewhere? Stainless exhaust pipe is prone to cracking where a repair is done if not done properly.

    Jut a few thoughts from out here in the ether.
    N1PA

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    We operate at 5300 msl..density alt usually 6000/7000 or more. Thanks for the input

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    Which engine mount, (dynofocul mounts can shake like a wet dog on start up and shut down which can cause breakage issues) also how old are the lord mount thingies?
    Remember, These are the Good old Days!

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    New engine mount .dynfocal..last year..issues extend beyond that time..will reexamine mounts. Thanks.

  11. #11
    skywagon8a's Avatar
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    Quote Originally Posted by Soarboulder View Post
    New engine mount .dynfocal..last year..issues extend beyond that time..will reexamine mounts. Thanks.
    2200 hours in just one year? If you changed to a dynafocal mount last year you must have changed the engine at that time?
    N1PA

  12. #12

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    Quote Originally Posted by Soarboulder View Post
    New engine mount .dynfocal..last year..issues extend beyond that time..will reexamine mounts. Thanks.
    Watch your throttle cable connection methodology. There’s a thread around here somewhere on such...
    Remember, These are the Good old Days!

  13. #13
    Steve Pierce's Avatar
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    Crossover exhaust? Pictures of exhaust repairs would help. Sutton exhaust has much less mass to create stresses and crack.
    Steve Pierce

    Everybody is ignorant, only on different subjects.
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  14. #14
    Cub Builder's Avatar
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    If it was mine, I'd change to an unbaffled exhaust (perhaps the hot rod muffler?) and a composite spinner. The composite spinner doesn't have any residual stresses from forming, so with the same type of mounting is less prone to cracking.

    -Cub Builder
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  15. #15

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    Yes..I've been investigating different mufflers..thanks for the input..

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    Charlie Longley's Avatar
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    We (Evergreen Soaring) run two Pawnees and used to have a 180 HP Super Cub. The Cub went away in a ground handling accident. We’re down at sea level so full rich on the mixture most of the time unles we’re aerotowing somewhere. I know there’s a lot of different methods of descending. We tend to just put the nose down after release and run around 2400-2450 rpm. Maybe one deorbit 360 if needed. I am the A&P that’s been doing the maintenance for the last 4 years. We’ve had zero cracked cylinders looking at the log books.

    The Super Cub was auctioned off by the insurance company. I heard the airframe wasn’t useable in the rebuild due to multiple cracks in the longerons. The Pawnee is a much better airplane for towing but harder to check pilots out in. It’s a lot easier to fly but you need a high performance sign off and a two seater tail wheel for the check out.

    I know what you mean about tow pilots having a mind of their own!
    Last edited by Charlie Longley; 02-11-2018 at 09:19 PM.
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    aktango58's Avatar
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    Quote Originally Posted by Charlie Longley View Post
    We (Evergreen Soaring) run two Pawnees and used to have a 180 HP Super Cub. The Cub went away in a ground handling accident. We’re down at sea level so full rich on the mixture most of the time unles we’re aerotowing somewhere. I know there’s a lot of different methods of descending. We tend to just put the nose down after release and run around 2400-2450 rpm. Maybe one deorbit 360 if needed. I am the A&P that’s been doing the maintenance for the last 4 years. We’ve had zero cracked cylinders looking at the log books.

    The Super Cub was auctioned off by the insurance company. I heard the airframe wasn’t useable in the rebuild due to multiple cracks in the longerons. The Pawnee is a much better airplane for towing but harder to check pilots out in. It’s a lot easier to fly but you need a high performance sign off and a two seater tail wheel for the check out.

    I know what you mean about tow pilots having a mind of their own!
    What does a guy need to do to get a check out? Would be fun to come fly a Pawnee for a weekend here and there...


    I used to fly a 180 for jumpers, and was able to manage to keep the 225 j model in tact coming down 10,000 feet. I could often be down and picking up the guys as they made the 50 yard walk to the road...

    Spirals. Keep power on, plan on some speed and toss in the bank and you can come down fast without abnormal stress.

    The Atlee muffler was nice as one less AD inspection. Also, more power.
    I don't know where you've been me lad, but I see you won first Prize!
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  18. #18
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    Charlie, whats the usual altitude for glider release?
    Cessna Skywagon-- accept no substitute!

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    Charlie Longley's Avatar
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    Quote Originally Posted by hotrod180 View Post
    Charlie, whats the usual altitude for glider release?
    3,000 agl
    I usually only do one 360 if we’re operating to the north.

  20. #20
    Charlie Longley's Avatar
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    Quote Originally Posted by aktango58 View Post
    What does a guy need to do to get a check out? Would be fun to come fly a Pawnee for a weekend here and there...


    I used to fly a 180 for jumpers, and was able to manage to keep the 225 j model in tact coming down 10,000 feet. I could often be down and picking up the guys as they made the 50 yard walk to the road...

    Spirals. Keep power on, plan on some speed and toss in the bank and you can come down fast without abnormal stress.

    The Atlee muffler was nice as one less AD inspection. Also, more power.
    Apply to Evergreen Soaring as a tow pilot. You’ll need to provide a two place airplane for the checkout. Or we could go in my PA-12 provided you buy the gas and beer!
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  21. #21
    aktango58's Avatar
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    Quote Originally Posted by Charlie Longley View Post
    Apply to Evergreen Soaring as a tow pilot. You’ll need to provide a two place airplane for the checkout. Or we could go in my PA-12 provided you buy the gas and beer!
    How much beer per hour does a 12 burn? Thinking a 4 place taildragger would work for the checkout, and you will teach the 'glider' part of the flying?

    I will look into Evergreen Soaring. Thanks!
    I don't know where you've been me lad, but I see you won first Prize!
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    Thanks Charlie..I used to work for Hans training in Alpha Jets at KAWO! Thanks for the reply. We check guys out in the Pawnee after they have towed a while in the Cub..our cub has over 15000 hours since new!! Trying to figure out if the slipping descent has something to do with exhaust cracks..also looking at different exhausts...no conclusions yet but lots of ideas...almost got around to joining your club..but the operation there was slowing down..thanks again for the reply..
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  23. #23

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    Overhaul was 14 years ago...long trail in logbooks

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    There must be something in the Air...I have a PA-12 too! Got O-320 with high compression heads..160 hp. I fly it in the mountains around Boulder and fly the Wave too!

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