The International Cessna^® 170 Association

Reviewing page 45 of the 170 book which discusses prop position on the crank.

The instructions say to line up TC with the crank case halves. Then place the number one blade in the 11 o'clock position for smoother operation. The original position should have been the number one blade in the 1 o'clock position.

Should the TC be on top or bottom?
Should the number 1 cylinder be on compression at TC or doesn't it matter?

The instructions don't specify either.

A few annuals ago I asked an A&P/IA about that wirte up. He explained that with the prop in the 11 O'clock position it makes the airplane easier to start when hand proped. According to him smoother opeation refers to making hand propping easier, not a smoother running engine.

Engine vibration can be reduced significantly by dynamic balencing the prop. Small weights are added under the spinner to reduce the vibration, it makes a substantial difference on some engines.

Way I understand it, you put the trailing edge of the #1 blade on the top dead center mark of the crankshaft flange.

I've indexed my prop the 170 way and the "Swift" way and it
didn't make a hill of beans worth of difference..... It's been awhile
since I dug the info up, but I think the only difference with the Swift
way is the blades are positioned 180 degrees from the 170 way
(not the biggest deal in the world with a 2-bladed propeller....).

Either way, the prop is in the same "spot", and both ways make it easy
to hand-prop it (which, if I'm not mistaken, is the primary reason for
indexing it the way they suggest).

Bela P. Havasreti
'54 C-170B N170BP

I appreciate the answers but both missed the point of my questions which I've had answered by a friend.

First the TC mark should be at the bottom of the engine lined up with the split in the case. That places the number one cylinder either at TDC on compression or exhaust.

Second, it doesn't matter which TDC your on when indexing the prop.

I don't know whether "smoother operation" meant easier hand proping or less vibration. I thought discussion along time ago was about less vibration. Anyone have any comments on this issue.

Place your #1 cylinder at TDC, (compression), then put the prop at 11 o'clock. I may be all wet, but I have done this when my engine seemed to vibrate more than usual, and found the prop was not in this position. This was after I had done some engine work. It may seem strange, but it does work. The part about hand proping is obviously correct, it places the blade where it is the easiest to pull through the compression stroke. although I have opted for a n 0360 Lyc. on our plane, the C145/ 0300, is by far the best running engine I have flown with, just not the power I wanted.
Jon

I had the same experience as Jon with my C-145. Clocked the prop like he said and it smoothed out.

It should not matter if it is on the compression or exhaust stroke, the prop will be in the same alignment relative to the crankshaft either way.

You're right that #1 or #2 blade shouldn't matter but it might help to experiment with that also as the prop might not be perfectly balanced.

I think the point was the 11 oclock position seems to help.[/quote]

The the crankcase split should be aligned at the Zero on the bottom of the prop flange, which will place the #1 piston at TDC. The number one blade of the prop should now be at the 11 o'clock position (when facing the cockpit as if you are about to "prop" it for starting) ....or when viewed from the cockpit (like everything else should be) that #1 blade will be at the 1 o'clock position.
Why would we want the blade at that position? Because when starting,...the prop is at the best location for hand-propping. (In other words, the TDC position ISN'T where the engine normally fires when running. The mags are timed to fire 28 degrees, and 26 degrees BEFORE TDC when the engine is running. When starting, the impulse couplings RETARD the spark considerably.) But what about the rest of the cylinders? There are cylinders firing 3 times for each rotation of the crankshaft....one at the 11 o'clock position, ...one at the 7 o'clock position,...and one at the 3 o'clock position, during start. This action is all advanced 26/28 degrees (approx. 1/4 turn) after the engine passes 300 rpm or so approximately to the postions of 12, 8, and 4 o'clock,...which smooths out vibratory impulses to that designed into the crankshaft counterweights.
Why all the fuss about the #1 blade? In theory it makes no difference, but it provides a uniform installation in the fleet.

I don't have the books at home with me right now, But I do recall reading an early S.I., S.L., or S.B. about the reclocking of the prop blades for smoother ops. i will look it up tomorrow, to get the wording correct.