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#11
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Prop Indexing
nrp wrote: I was in a 172M club that had a bad first order shake appear after an engine overhaul. It turned out the prop had to be indexed to 45 degrees @ TDC to get rid of it. The orientation requirements are in the Cessna manual, but there is no explanation. As an ME with some background in dynamics, this is the only reason I can see for this: All 4 cylinder horizontally opposed engines with offset (in contrast to directly opposite) cylinders will have a second harmonic yaw motion about their CG due to the short connecting rods. Without a double crank speed counterbalance shaft system, there is no way to eliminate this. This causes a yaw vibration or swinging of the engine at 80 Hz at 2400 rpm for example. I think it is also the reason the vibration environment on the front of a Lycoming engine is so severe. I think this is called a "rocking couple". It's an issue with aftermarket Harley engines designed to use offset cylinders so as not to need female (fork) rods. When Colombo laid out the "short block" Ferrari V12 he wanted to have the bores on centers and fork/blade rods but Enzo said since Packard did not to so with the Twin Six it was unnecessary. A little bit off topic, as always. Certified a/c are stuck with LyCon but homebuilders who use 'em are idiots. They belong with the OXX-6, Pobjoy, Menasco, Hirth, Siemens-Halske, Guiberson and Caminez on a museum wall. |
#12
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Prop Indexing
I have checked a number of engine/prop combinations with prop balancing equipment and found the overall rock/yaw movement of any Continental or Lycoming 4-cylinder sensitive to prop index. There are 6 bolt holes in the crank flange and the prop will cover two. The most perpendicular to TDC is the worst position. The best is the position of a blade following TDC, the classic 10:00/4:00 suited for handpropping. Even the intermediate position with a blade leading TDC will cause more yaw on the aft of the engine. My layman's mind attributes this to the firing order. The reaction and rebound of a individual cylinder firing moving the crankcase. I'm not sure but some of it maybe also be attributed to camshaft and valve spring pressure. Valve train forces are much higher than you might first suspect. Most fixed pitch crankshafts do not have a particular index bushing that constant-speed models have. Yes, Lycomings have a index bushing for the ring gear position but its not for a prop necessarily. One can install a fixed pitch prop in any of three axis. 6 cyl Cont or Lyc cranks have a prop index bushing. Two choices, zero or 180 different. I know there has to be one but I have never seen a fixed pitch prop on a 6-cylinder myself. I bet if the prop was indexed to the middle two throws the engine would suffer from more yaw. Offhand, I know of several instances when manufacturers recommend a change in this reindexing of constant speed prop index. The first 300 serial numbers of the Mooney M20J can be converted from the IO-360 A1B6 to A3B6 by moving a prop bushing one hole counterclockwise which aligns it more closely to TDC. There is a Lycoming service bulletin advising about the same for some Aviat Huskeys. ANd there is a change thru the evolution of Grumman American service manuals. Oddest instance to me was hearing that some Europeans conducting tests on aircraft with split exhaust systems have found the sound signature is less when the prop is indexed a particular way, to be out-of-sync of the exhaust note coming out the pipe. Kent Felkins Tulsa |
#13
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Prop Indexing
I have checked a number of engine/prop combinations with prop balancing equipment and found the overall rock/yaw movement of any Continental or Lycoming 4-cylinder sensitive to prop index. But is there any evidence that performance can be impacted by prop indexing? |
#14
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Prop Indexing
In rec.aviation.owning Fly kentf AT entergate.com wrote:
: 6 cyl Cont or Lyc cranks have a prop index bushing. Two choices, zero or : 180 different. I know there has to be one but I have never seen a fixed : pitch prop on a 6-cylinder myself. I bet if the prop was indexed to the : middle two throws the engine would suffer from more yaw. Some Cherokee 235's have a fixed pitch prop. All Pawnee 235's have a fixed pitch prop (PA-25-235), at least all that I have ever seen. All O-300 Skyhawks have a fixed pitch prop, except for the very last geared ones, called "Powermatic". Every O-300 Skyhawk I've seen has the 10:00/4:00 prop positioning. Is it correct? Is it optimal? Don't know... -- Aaron C. |
#15
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Prop Indexing
Thanks for the reminder Aaron. Your're right.
Boy, that was a dumb statement I made. Heck I was working on a Skyhawk O-300 today! Kent Felkins " In rec.aviation.owning Fly kentf AT entergate.com wrote: I know there has to be one but I have never seen a fixed : pitch prop on a 6-cylinder myself. Some Cherokee 235's have a fixed pitch prop. All Pawnee 235's have a fixed pitch prop (PA-25-235), at least all that I have ever seen. All O-300 Skyhawks have a fixed pitch prop, except for the very last geared ones, called "Powermatic". Every O-300 Skyhawk I've seen has the 10:00/4:00 prop positioning. Is it correct? Is it optimal? Don't know... -- Aaron C. |
#16
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Prop Indexing
and someone worked out that the stresses in a crank due to
straight torque are I had reached that conclusion in an earlier posting - and maybe others. These things are buried so deep in the technical organizational underbrush that they never seem to reach the light of day. I sure wish someone in the real know would post to this group. Your two blade/three blade thing may be correct. Obviously your observations are. I have been looking at things from only a theoretical basis. |
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