Thorp Air Command - T18.net

While repairing P-leads and checking timing I noticed my prop is indexed horizontal when #1 is at TDC. Since I know Sensenich has published procedures to index the number 1 blade at 10 o'clock when the engine is at #1 TDC (I believe the Cessna manual calls out the same) and I have seen unofficial references to 12 o'clock, I was wondering if anyone knows of anything that would require the Sterba to be different. I didn't see it addressed on the Sterba website. The Lycoming manuals say to check with the aircraft manufacturer but they also have S.I. 1098 which calls out specific bushings in specific locations and warns of excessive stresses if the prop is not indexed correctly but I can't check those I unless remove the prop. If I am correct in thinking my prop is installed in the wrong position, it could easily be the source of the vibration I was feeling on Friday but I wanted to see what the group had to say before I actually removed the prop.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Remove the prop and while you are at it, check the runout on the crank flange. One never knows.

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Bob Highley
N711SH
SN 835
KLAL

If no one wants to address the indexing question, one thing I could use is the torque values for a Sterba installed with 1/2" bolts. He doesn't list anything over 3/8" on the website. I need those even if I don't remove the prop so I can do the periodic retorque. It hasn't been checked in 3 years.

BTW Bob...the timing was set at 30° BTDC and I need rudder cables...don't condition inspections get done on your airplane??? Or maybe homebuilt aircraft aren't required to be inspected every year. I haven't fully delved into the regs on this yet.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Did a quick search on indexing and it deserves my further attention . I do a conditional every 12 months and use the EAA Conditional Inspection worksheet (free) . As far as your bolt torque values you might be able to send a telegram to Sterba and see if they have that info . If not you can try these sites/references . Pretty sure I use 36ft/lbs for my Almar Demuth , but I would have to check .

Performance Propellers Maint. Sheet PDF
FAA AC Acceptable Repairs and Practices . FAA pub is available from Spruce or any good pilot shop .

RB
BOO YA !

Thanks Rich, AC 43.13-1B is available for free download on the FAA website and I get a copy every month with my AD subscription but I don't guess I ever looked at the wood prop section. I had checked the Sensenich wood prop instructions (23-27 foot pounds if I remember correctly) and sent an email to Sterba to get care and feeding instructions for the prop beyond what is on their website. I also asked them about indexing the prop. It may just be a trial and error thing.

After I fixed the P-leads I decided to run the full annual checklist. Mostly just finding the same stuff I would find on any other 40 year old aircraft. I can't think of a better way to get to know it though, short of building one anyway.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Specified proper torque values can vary quite a bit among various wood propellers. Most of that is based on the type of wood used, with the orientation of grain, lamination details, etc. also factoring in. Don't guess - check with the manufacturer. Too tight can permanently ruin a prop. The issue is wood crush. With 3/8" bolts, the old Pacesetter 200 props used to specify 25 ft. lbs. Aymar/Demuth specified 18 ft. lbs. and that's what I use. They are, or at least mine is hard maple, and it can probably stand a little higher values, but it shouldn't be necessary.

I've never known of any issues with indexing a prop on the commonly used engines on a T-18. We've always just installed it so that it stops and is on a compression stroke for easiest/safest position to hand-prop if necessary, with the blades being more horizontal than vertical (in the case of 2-bladed props) in case of an engine-off emergency landing.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Good points John . I went and checked my AD prop paper work and it is 26ft/lbs . Not sure where I got 36 ? Probably a "Golden Moment ?"

RB

My prop is mounted so that when the engine shuts down the blades are at 10 and 4 o'clock when looking from the front. I don't know how it is indexed from TDC. This is the position my A&P insisted that it be mounted, mostly I believe for hand propping which I don't ever intend to do if I can help it.

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Bruce Finney
N18JF T-18C #262
Yelm, WA USA

I've hand-propped to start my engine and other's several times over the years. It's no big deal with an O-320.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Ira always indexes his prop, and it makes a difference on his 290, Smooths things out....Dan

I have been told that the relation of prop blade to the crankshaft throws makes a difference in vibration. and as others have said it puts the prop blade in a better position for hand propping.

Phil
118BC

I had the Cessna indexing wrong. Instead of calling out clock positions the Cessna manual says to line one blade up with the TC mark on the starter ring then rotate the prop to clockwise to the first available mounting position. This makes the mags snap when the blade is down around the knees when hand propping which is terrible for hand propping. I asked a Cessna engineer about it and he said not to change it because that position is directed by the engine manufacturer to counter forces within the engine. The Skyhawk that had me looking into it is O-320 powered. I wish now I had kept the original email but I still have access to the text that was in it.

Since Skyhawks only come with heavy metal propellers, what I don't know is if a wood or composite prop weighs enough to matter. Especially since the instructions for a wood Sensenich gets installed in a position that favors hand propping. Sensenich does not appear to specify any index position for their composite or metal props.

In the end, my guess is there is nothing wrong with how my prop is indexed. It just surprised me based on my past experiences which is primarily production airplanes with metal props.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Instead of weighing the airplane today I attempted to balance the prop. The first run was .52 IPS which (for those who don't know) is rough. The balancer recommended 20 grams of weight to be installed. I couldn't find anything convenient to install that weighed 20 grams even for a test run so I indexed the prop in accordance with Sensenich instructions and re-did the first run. The new IPS was .27. Still not great but much improved.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

I had a guy check the balance of my AD prop . He hooked everything up and the initial run gave us .123 IPS . He said he could get it down lower by adding an AN bolt/washers to the flywheel and do another run . As the initial run was well below the acceptable threshold we called it a day .

RB

I was going to add a bolt with washers to the flywheel but the manual for balancing said that is a no-no on installations using long prop spacers. While I don't know it for a fact, I would guess four inches would be considered long as far as spacers go. At a minimum, I still need to change the front bulkhead on the spinner and I am considering changing the complete spinner assembly so I am not looking for perfection at this time. I just thought it was interesting that moving the prop one position reduced the vibration by half on my installation.

My spinner is attached with #8 machine screws with lock nuts. It would take 32 stainless washers split between 2 screws to test what the balancer is now telling me I need to do. The screws I have on hand will hold 4 washers each and those 8 washers brought it down to .17 IPS. The alternative is to start drilling holes and installing bolts in the rear bulkhead with only the hope it will be correct in the end.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson