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#1
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Prop Indexing
Why are fixed-pitch props on PA-28s indexed to the 10-4 position from the front? a) To help hand propping? b) To minimize vibration? c) To improve performance? My money is on (a) but others disagree. |
#2
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Prop Indexing
Airplanes without starters have the prop indexed for hand
propping, engines with starters have the blades on a two bladed prop aligned with the crank throw. -- James H. Macklin ATP,CFI,A&P -- The people think the Constitution protects their rights; But government sees it as an obstacle to be overcome. some support http://www.usdoj.gov/olc/secondamendment2.htm See http://www.fija.org/ more about your rights and duties. "Mike Granby" wrote in message oups.com... | | Why are fixed-pitch props on PA-28s indexed to the 10-4 position from | the front? | | a) To help hand propping? | b) To minimize vibration? | c) To improve performance? | | My money is on (a) but others disagree. | |
#3
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Prop Indexing
Jim, we learn something every day !
Is there a reason that the 2 blade prop is aligned with the crank throws ? -- Roy N5804F Piper Archer "I have had some bad landings but I have never missed the runway" "Jim Macklin" wrote in message news:y9nuf.38976$QW2.20013@dukeread08... Airplanes without starters have the prop indexed for hand propping, engines with starters have the blades on a two bladed prop aligned with the crank throw. -- James H. Macklin ATP,CFI,A&P -- The people think the Constitution protects their rights; But government sees it as an obstacle to be overcome. some support http://www.usdoj.gov/olc/secondamendment2.htm See http://www.fija.org/ more about your rights and duties. "Mike Granby" wrote in message oups.com... | | Why are fixed-pitch props on PA-28s indexed to the 10-4 position from | the front? | | a) To help hand propping? | b) To minimize vibration? | c) To improve performance? | | My money is on (a) but others disagree. | |
#4
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Prop Indexing
I recall something about harmonics and power pulses. For
hand propping, the blade angle should be such that you bring the piston to TDC with the blade above the shoulder so you can pull it through as you step back, when hand propping a plane with a starter, the piston is at TDC with the blade angle lower and the person is more likely to be in a position to fall into the prop rotational plane. Of course, I could have made it all up or simply remembered some details, IF I cared I'd look up the answer in one or more texts/web pages from a reliable source. -- James H. Macklin ATP,CFI,A&P -- The people think the Constitution protects their rights; But government sees it as an obstacle to be overcome. some support http://www.usdoj.gov/olc/secondamendment2.htm See http://www.fija.org/ more about your rights and duties. "Roy Page" wrote in message ink.net... | Jim, we learn something every day ! | Is there a reason that the 2 blade prop is aligned with the crank throws ? | | -- | Roy | N5804F Piper Archer | "I have had some bad landings but I have never missed the runway" | | | "Jim Macklin" wrote in message | news:y9nuf.38976$QW2.20013@dukeread08... | Airplanes without starters have the prop indexed for hand | propping, engines with starters have the blades on a two | bladed prop aligned with the crank throw. | | | -- | James H. Macklin | ATP,CFI,A&P | | -- | The people think the Constitution protects their rights; | But government sees it as an obstacle to be overcome. | some support | http://www.usdoj.gov/olc/secondamendment2.htm | See http://www.fija.org/ more about your rights and duties. | | | "Mike Granby" wrote in message | oups.com... | | | | Why are fixed-pitch props on PA-28s indexed to the 10-4 | position from | | the front? | | | | a) To help hand propping? | | b) To minimize vibration? | | c) To improve performance? | | | | My money is on (a) but others disagree. | | | | | | |
#5
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Prop Indexing
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. A two blade propeller has to present the same yawing moment of inertia in response to the 80 Hz engine swing. It can do so only if it is phased such that it is oriented 45 degrees to the plane of the crank throws. If it is phased in the plane of the crank throws, the engine swinging will present different yaw inertias to alternating second harmonic inputs ( i. e. every other one), causing a first order response of the system. This first order response will appear as a shake at crankshaft frequency. This would not be a problem we had three blade propellers, or 6 cylinder engines. Anybody want to challenge this? I'd be interested in further discussions on this topic |
#6
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Prop Indexing
Jim Macklin wrote:
Airplanes without starters have the prop indexed for hand propping, engines with starters have the blades on a two bladed prop aligned with the crank throw. Some do, some don't. A four-cylinder engine will stop with all of the pistons very close to half-way down the bores. If the prop is aligned with the crank throw, it will stop in a vertical position. My Maule did this, my C150 did not. It is not possible to align a two-bladed prop with all the crank throws of a six cylinder engine. George Patterson Coffee is only a way of stealing time that should by rights belong to your slightly older self. |
#7
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Prop Indexing
This is a very interesting discussion.
I have never had my prop off the Archer. Is it possible to fit a prop on a PA28 indexed incorrectly or is there a definite radial locator ? -- Roy N5804F Piper Archer "I have had some bad landings but I have never missed the runway .... yet" "George Patterson" wrote in message news:37xuf.19929$z45.8314@trnddc02... Jim Macklin wrote: Airplanes without starters have the prop indexed for hand propping, engines with starters have the blades on a two bladed prop aligned with the crank throw. Some do, some don't. A four-cylinder engine will stop with all of the pistons very close to half-way down the bores. If the prop is aligned with the crank throw, it will stop in a vertical position. My Maule did this, my C150 did not. It is not possible to align a two-bladed prop with all the crank throws of a six cylinder engine. George Patterson Coffee is only a way of stealing time that should by rights belong to your slightly older self. |
#8
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Prop Indexing
it will stop in a vertical position. My Maule did this, my C150 did
not. It is not possible to align a two-bladed prop with all the crank throws of a six cylinder engine. My 172M (a different one from my earlier posting) stops with the prop @ 45 degrees, and is at TDC @ 135 degrees. I don't think a 6 cyl engine has any reason to be sensitive to prop orientation when it comes to smooth operation, although I can appreciate that they might for some other unknown reason. |
#9
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Prop Indexing
I'd be interested in further discussions on this topic Me, too! My original post came as a result of a discussion I'm having elsewhere. My assertion was that since most service manuals specify 10-4 as the right position, they are simply repeating "old lore" with respect to hand propping. My interlocutor feels that whatever the manual specifies is as a result of wanting to get the best performance from the airplane. My response to this is that performance won't be impacted, and that if anything, vibration would be the other issue, but that this would be better addressed with a 12-6 positioning. |
#10
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Prop Indexing
Sorry guys -
In reflecting my earlier post, I now think my earlier reasoning is invalid. Yes there is a second harmonic (80 Hz) yaw moment generated within the engine, but the rotating propeller will also change the reflected yaw inertia at 80 Hz. Therefore, now I don't see how this coupling could make a 40 Hz shake. Can somebody shed some more light on this? I know the orientation is critical on a 172M, and I've seen a couple of homebuilts that had similar prop orientation problems too. |
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