Prop stopping vertically

Discussion in 'Maintenance Bay' started by Ryanb, Nov 10, 2019.

  1. Ryanb

    Ryanb Touchdown! Greaser! PoA Supporter

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    I have read somewhere in the past that a prop that repeatedly stops vertical on a 4 cyl engine, is very likely indexed wrong to the crankshaft. Is this true?

    Reason I ask is, one of the 172’s at the flight school seems to be this way. 9 times out of 10 it will stop vertical upon shutdown. It never occurred to me that it seems to happen more often on this particular airplane than on any of the others until I read about it, so it had me curious.
     
  2. Ravioli

    Ravioli Final Approach

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    I was told, rightly or wrongly, that the prop should stop in roughly the same position each time. Which position is based IIRC on how it was "clocked."

    so 9 of 10 in the same position sounds 'normal'.

    I have 3 blades and it's clocked so one is vertical. Minimizes my manual prop manipulations for the tow bar.
     
  3. Dan Thomas

    Dan Thomas Final Approach

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    In the 172 manual it tells the mechanic to rotate the crankshaft until #1 cylinder is at TDC, then hold the prop vertical and rotate it clockwise (as seen from the front of the airplane) until the prop's bolt holes line up with the crank's flange holes, and put it on. That will have the prop stop at the 10:00-4:00 position as seen from the front, and puts it in the best and safest position for hand-propping.
     
    Last edited: Nov 10, 2019
  4. Stewartb

    Stewartb Final Approach

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    A 4 cylinder Lycoming with a 2-blade prop should stop with the prop at 10:00 and 4:00 when viewed from the front.
     
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  5. Dan Thomas

    Dan Thomas Final Approach

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    You're right. My clock is out of whack. I'll fix that.
     
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  6. MooneyDriver78

    MooneyDriver78 En-Route

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    Why is that?


    Tom
     
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  7. DesertNomad

    DesertNomad Pattern Altitude

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    Out west I think they stop 3 hours earlier.
     
  8. Stewartb

    Stewartb Final Approach

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    It positions the prop for hand propping.
     
  9. Kenny Phillips

    Kenny Phillips En-Route

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    I thought it was to keep birds from roosting.
     
  10. Shuswap BC

    Shuswap BC Pre-takeoff checklist

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    Every plane I have owned has stopped in numerous places, and my twin never stopped with its props in sync.
     
  11. 455 Bravo Uniform

    455 Bravo Uniform En-Route PoA Supporter

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    It’s opposite in the Southern hemisphere.
     
  12. Bill Jennings

    Bill Jennings Touchdown! Greaser! PoA Supporter

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    This page intentionally left blank
    well, except for later model senecas
     
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  13. JAWS

    JAWS Line Up and Wait

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    And also has to do with aligning the blades with the connecting rods to minimize vibration.
     
  14. nrpetersen

    nrpetersen Pre-takeoff checklist

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    A 172M club I was in years ago majored their engine. When it came back it shook at crankshaft speed, to the point that they were considering opening everything up again until some sharp mechanic realized that the prop was oriented incorrectly.

    I am a retired mechanical engineer experienced in vibration and engine designs, yet I don't understand why this is only a 172 problem, and not on other 4 cyl aircraft. But I fully accept that it is.
     
  15. JohnAJohnson

    JohnAJohnson Cleared for Takeoff

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    Not all of them. Two-Bladed Cardinals (IO360 or O360) stop at nine and three.
     
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  16. biplanebob

    biplanebob Pre-takeoff checklist

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    It could have been a stack up of balance tolerances of the various rotating components on that particular aircraft...ie; the balance of the crankshaft, prop, ring gear , and spinner assemblies all lined up to a bad combination. A common cure to a vibration problem on 2 blade props is to rotation the prop 180 degrees in the clock position.
     
  17. Bell206

    Bell206 Pattern Altitude

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    FWIW: Can't comment on the consistency of where a prop stops, but most props have a specific index point called out during installation which is usually stated in the MM--and not are all the same. Lycoming also has a SB out there that defines correct prop flange bushing locations to ensure proper prop indexing. Don't know about the hand-prop requirement, but it has been my understanding the prop indexing was to reduce engine/prop vibrations as mentioned above. And when troubleshooting errant prop/engine vibrations every shop/OEM I've talked to always questioned if the prop was installed in the correct position. So to reduce issues prior to performing a dynamic prop balance I would usually check prop position among other things prior to start.
     
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  18. cgrab

    cgrab Pattern Altitude

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    My O-320 Cardinal stops like this
     
  19. James331

    James331 Touchdown! Greaser!

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    What if you’re left handed (wrong handed) and it’s day light savings?
     
  20. Ryanb

    Ryanb Touchdown! Greaser! PoA Supporter

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    I guess you’re s*** out of luck!

    ;)
     
  21. alfadog

    alfadog En-Route

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    Turn it by hand and see where the impulse coupler snaps. That's a pretty good indication of TDC. I wouldn't judge anything by where it stops when you turn it off.
     
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  22. DesertNomad

    DesertNomad Pattern Altitude

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    My 6-cyl Dakota stops all different places.
     
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  23. Lachlan

    Lachlan En-Route

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    Depends on what oil is in it. XC20W-50 with CamGuard will start on the first blade the first time, every time. AeroShell 100, without a flight plan, will require you to check with any traffic in the area, advise, get their advice, and the enter a right midfield downwind across the runway without any subsequent radio calls. None of this applies during Standard Time, so yank & bank til Spring!
     
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  24. geezer

    geezer Pre-takeoff checklist

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    First, a couple of humorous items.

    One of the members of the flying club always turned it to 9 and 3 when he finished flying, IT LOOKS SO MUCH BETTER!

    Airplane salesmen also turn the props to 9 and 3 so they can hang FOR SALE signs neatly on them.

    Another member turned the prop to noon, TO KEEP THE BIRDS FROM PERCHING ON IT!



    On a much more practical note, ALL of our 4 cylinder Cessna's stopped at 10 and 4, both Lycoming and Continental. In this position you have the best angle for hand propping the engine strongly and safely. I was taught proper technique half a century ago by an experienced CFI, and it is too difficult to accurately describe to attempt in a written form.

    I have propped 4 cylinders up to 240 HP, and 6 cylinders to about 280 HP. Oddly, taildraggers are easier to safely prop than trikes.

    6 cylinder engines stop at 3 different angles, 120 degrees apart, and the only position they should be propped from is 10 & 4. If you fail to get a start from the first try, switch off, turn carefully to 10 again, switch on, and try another time.

    Our Lycoming has a mark on the crank flange which aligns with a mark on the prop, to prevent 180 out installation, which is important to us, as we have had the engine dynamically balanced.

    I have assisted mechanics who removed and replaced props for us about a dozen times, and they usually put a mark on hub and prop with a magic marker or equivalent, to be sure they put them back on as found.
     
  25. Dan Thomas

    Dan Thomas Final Approach

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    CS props often have a recessed bolt hole at one spot on the mating face of the hub, and a larger bolt dowel in the flange fits into it to index the prop. There would have been reasons of balance considered in that case, and the 9 - 3 stopped prop is the result.
     
  26. Dan Thomas

    Dan Thomas Final Approach

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    The prop wants to stop in the easiest position for the crankshaft, which is determined by two of the cylinders with pressure in them. There will be one coming up on TDC on its pressure stroke and another going down on its power stroke, before its exhaust valve is open, so the crank will want to rest where the pressures in those two cylinders are about equal. On a four-banger there are two positions that look like one because they're directly opposite each other. The six has three positions, so a two-blade prop will pick one of those three, and a three-blade prop will look like it has one position.

    A hot engine sometimes stops fairly abruptly when shut down and all the above is just theory. Clearances are tight and oil is really thin when things are hot enough. Try moving that prop a few minutes after shutdown (don't get bit if it fires!) and then try again once it's cooled off. Big difference in friction.
     
  27. Salty

    Salty En-Route PoA Supporter

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    Mine stops at 7:42 and 1:42, but I think it must be Zulu time or something because it’s never the right time.
     
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  28. John in Hattiesburg

    John in Hattiesburg Filing Flight Plan

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    is this the same for 4cyl Lyc's and 6cyl Conti's or.....?
    asking cause I have an older 172 with the 6cyl Conti
     
  29. Clip4

    Clip4 En-Route

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    On the IO-550 on a Cirrus, the prop stops at the same position each time.
     
  30. geezer

    geezer Pre-takeoff checklist

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    John asked:

    is this the same for 4cyl Lyc's and 6cyl Conti's or.....?
    asking cause I have an older 172 with the 6cyl Conti

    The Cessna 6 cylinder Continental should be at 10 when at rest, or 120, 240 degrees more.

    The club had a 56 172, and in cold weather, I often hand propped it due to weak battery.

    The description of Switch off, rotate until 10 o'clock, was specifically that plane and engine.

    Those are very smooth running engines, and except for their famous tendency for carb ice, my wife and I enjoyed it for many cross country miles.
     
  31. James331

    James331 Touchdown! Greaser!

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    It needs to for a proper PLF ;)
     
  32. nrpetersen

    nrpetersen Pre-takeoff checklist

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    Three bladed props have the same transverse moments-of-inertia no matter what plane they are being shaken. A two blade prop moment-of-inertia will be grossly different in the two transverse axes. All this is is critical as a 4 cylinder opposed engine inherently swings in a yaw axis at twice crankshaft frequency.

    A 6 cylinder engine doesn't inherently swing. It is all a balance thing.
     
  33. Stewartb

    Stewartb Final Approach

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    If the average 4 cyl Lycoming has a prop vibration the first step is to reclock the prop 180*. Still 10 and 4 but with blade position reversed. Mine have benefitted from it. My CS prop also stops at 10 and 4, and while the prop flange does have bushings prop can be installed with any bolt in that position. I’ve re-clocked that prop, too. Always 10 and 4.
     
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  34. flyingbrit

    flyingbrit Pre-takeoff checklist

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    Are you referring to what is sometimes called a "rocking couple"? I thought this was caused by the opposing cylinders not being in the same geometric plane and thus not cancelling each other out perfectly. Why would this not be true of an opposed six cylinder also?
     
  35. nrpetersen

    nrpetersen Pre-takeoff checklist

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    Are you referring to what is sometimes called a "rocking couple"? I thought this was caused by the opposing cylinders not being in the same geometric plane and thus not cancelling each other out perfectly. Why would this not be true of an opposed six cylinder also?
    Yes - You are correct in that is caused by the cylinder pairs being offset rather than in a perfect H layout. I once thought the second harmonic cancels in a 6 cyl engine with L-R cylinder offset, but no way could I prove that! Now I'm not even so sure.... Any other engine geeks or nerds out there?
     
  36. Dan Thomas

    Dan Thomas Final Approach

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    Yup, as I said in post #26: The six has three positions, so a two-blade prop will pick one of those three, and a three-blade prop will look like it has one position.
     
  37. Grum.Man

    Grum.Man En-Route

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    A weak cylinder can also contribute to the prop not stopping in the same position.
     
  38. Timbeck2

    Timbeck2 Final Approach

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    And mechanics will position the prop at 12 and 6 to keep from walking into it.
     
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