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Prop Indexing

Steve Pierce

Steve Pierce

BENEFACTOR
Graham, TX
So I have seen this twice on 180 hp Super Cubs recently. There is a proper prop indexing forced by the machining of the prop hub and the bushings in the crankshaft.
Note the two short prop bushings at 3 and 9 o'clock. This puts the prop at the perfect spot for hand propping when installed correctly.
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Is your point that the prop is indexed at 9-3, rather than the typical 10-4?

We have the same indexing on an O-360 in an RV4 with a constant speed Whirlwind prop. The WW prop manual says to index at 10-4, but the prop will only fit on 9-3 (horizontal).
 
One of the really annoying things that Aviat did some years ago was to re-index the engines on Huskys from 10/4 to 12/6. Try hand propping one of those when on skis in deep snow.

Turns out Lycoming says there is a very slightly reduced vibration on that engine with the prop at 12/6. We moved the indexing back to 10/4 for hand propping, based on Lycoming Tech saying it's approved, and the Husky data listing both indexing.

MTV
 
You moved a constant speed prop to 10-4? The choices are 11-5 or 9-3. There’s no in-between. My cub is at 9-3 and given the height that’s a suitable compromise.
 
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stewartb said:
You moved a constant speed prop to 10-4? The choices are 11-5 or 9-3. There’s no in-between. My cub is at 9-3 and given the height that’s a suitable compromise.
Click to expand...

What is the condition at which this position is measured? Is it where the prop usually stops? I have not attempted to measure mine but I estimate it stops at 10-4. Trailblazer carbon CS prop on an IO-360.
 

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frequent_flyer said:
What is the condition at which this position is measured? Is it where the prop usually stops? I have not attempted to measure mine but I estimate it stops at 10-4. Trailblazer carbon CS prop on an IO-360.
Click to expand...

Yep, ALL early Huskys came with prop indexed at 10/4. Every 170/172/175 with Avcon conversion I’ve seen has same indexing. That is standard indexing on O-360s, with some exceptions.

MTV

Every IO-520 Continental I’ve met with a two blade was indexed at 10/4.
 
To re-index one of these props, you have to move the indexing bushing. It’s possible to simply remove two bushings and swap them, but it’s possible to damage a bushing in the process, so good to have a spare on hand in case.

But, check engine manufacturer’ data to verify the indexing is approved. In this case “approved” implies that engine and prop have passed a vibration survey at that indexing.

Looks like someone took a shortcut on the one in photo.

MTV
 
Apparently Hartzel makes two different prop flanges. That surprises me but I don’t know anyone with a Hartzel CS prop. Everyone I know with big engine Cubs uses Whirl Wind and has the same 11-5 or 9-3 index choices I have, and most use 9-3. With my engine’s bore and compression I hope I never need to hand prop it. :)

Having Hartzel bolts aligned directly under the blades must suck to install and remove props. My offset bolts are bad enough!
 
stewartb said:
Apparently Hartzel makes two different prop flanges. That surprises me but I don’t know anyone with a Hartzel CS prop. Everyone I know with big engine Cubs uses Whirl Wind and has the same 11-5 or 9-3 index choices I have, and most use 9-3.
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If 9-3 is one available position then 11-5 is possible but not 10-4. 6 bolt mounting only allows rotation in 60 degree (2 hour) increments. Doesn't the availability of the availability of 9-3 depend on how the prop hub is machined? If the hub mounting flange is offset 30 degrees than the available positions would be 8-2 and 10-4.

I'm no expert on CS props. I have flown a few but the Trailblazer is the first one I have owned and maintained. I have tried to collect all the manufacturer's related data and found this prop flange data in a quick search:
 

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frequent_flyer said:
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That data is interesting. Do they give any clues as to the applicability of the different positions?

As Stewart has noted, the Whirlwind on my Cub's IO-360B also is indexed at 9-3. Totally useless for hand propping when on floats. I don't choose to relocate the bushings as they are in the position which Lycoming placed them, which was originally intended for a Hartzell C/S prop. Also at my age, I have no business hand propping on floats.
 
skywagon8a said:
That data is interesting. Do they give any clues as to the applicability of the different positions?
Click to expand...

I decoded my prop part number index as "0 and 180 degrees, non-counter bored holes, R flange". Earlier I posted an image of how the prop stopped.

What is bothering me about this indexing discussion is that I don't understand why the place the prop stops should define the indexing. Why wouldn't indexing be relative to a much more specific datum such as top dead center?

From memory I think my PA-28 and my FX-3 have the prop blades at about 1-7 at TDC.
 
frequent_flyer said:
What is bothering me about this indexing discussion is that I don't understand why the place the prop stops should define the indexing. Why wouldn't indexing be relative to a much more specific datum such as top dead center?
Click to expand...

Lycoming Service Instruction SI1098Q seem to show that the indexing reference is number 1 crank pin at 12 o'clock. If the engine stops half way between compression strokes then stopped position would seem to be the indexed position not TDC.
 
I have a friend who told me the 9-3 position is best because if you know you’ll nose over and have the presence of mind to stop the prop first? No prop damage. That’s flying beyond this boy’s comfort zone, but it’s a true story.
 
When Aviat changed the indexing of the props on Huskys, I enquired of the factory why they did so. The response was that Lycoming told them that the 12/6 indexing had better vibration properties on the O-360 with a Hartzell 76 inch prop.

So, I called Lycoming Tech support and asked them about that. They said that was indeed true, that the improvement was minimal, but measurable. I asked if they recommended against re-indexing back to the 10/4 position to facilitate hand propping. They said no problem, so we re-indexed the prop back to 10/4. Soon thereafter, we switched to composite props, which have much better vibration properties in any case.

Having to hand prop one of these engines with the prop indexed wrong could mean a night out at -30 or colder.....I hand propped that Husky with the prop at 12/6, a challenge at best. We also started replacing the miserable aerobatic gel batteries in our Huskys with Odyssey batteries. Never HAD to prop one again.

MTV

MTV
 
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