jsharkey

jsharkey

Well Known Member
At the weekend I changed the oil and rotated the propeller 60 deg so that the leading blade was at a +30 deg high position to make hand propping a practical if unlikely possibility. Prior to that it sat naturally at a -30 low position when the engine stopped. Since making the change the engine/prop seems to be running "rougher". Does that make sense or am I imagining things? The engine is a YO360-A1A with a Sensenich 72/85 FP aluminum prop. Tach time is about 23 hours.

Jim Sharkey
RV6 - Phase 1
 
The prop on my 0320 Piper was removed to replace the backing plate. They then proceeded to install the prop as a 160 hp instead of the 150 hp that my Warrior was.

As soon as I got it up to cruse, I knew that something was wrong. Prop was at 12:00. I removed it and set it back to 10:00 per Piper service manual............

It's now smoothe as silk.
 
For propping, you don't want the blade to sit "naturally" at 30? high... you want it to come to the top of the compression stroke at 30? high.

Yes, your engine cares where the prop blades are. Why I don't know.

Hope the first helps.
 
I may have placed the prop at 90 deg to the crank plane. The other options are +/- 30 deg.

Does it make sense that it would run smoother when not at 90 deg?
 
Theoretically, I think the engine should not care. Like a big flywheel. But, they do care. Don't know if it's run-out, balance, airframe blocking or combination but it makes a big difference.
 
I previously owned a share of a Piper Archer, we had problems with slight engine vibration we couldn't track down. Had the prop balanced, checked everything...until a sharp-eyed A&P mentioned that the prop had to be "clocked" correctly on the crankshaft. Following his advice, we removed and reinstalled the prop at the right position on the crankshaft...no more vibration. Unfortunately that was so long ago I can't remember where he got his information from.

So the engine may not care, but in our case changing the prop's position on the crankshaft solved our vibration issue. And less vibration should be a good thing...
 
Just checked at airport

jsharkey said:
I may have placed the prop at 90 deg to the crank plane. The other options are +/- 30 deg.

Does it make sense that it would run smoother when not at 90 deg?
Click to expand...

Actually it was at 90 deg to the crank plane before. Now when I set the TC1 mark at 12 o'clock the prop is aligned 2-8. Smart thing is to put it back as before :)
 
Dumb question but I still want to know........how does clocking and a three blade prop have any correlation? Do you still need to do it? How about multiple cyllenders such as 4 vs. 8?
 
Data Point

jsharkey said:
Actually it was at 90 deg to the crank plane before. Now when I set the TC1 mark at 12 o'clock the prop is aligned 2-8. Smart thing is to put it back as before :)
Click to expand...

I returned the prop to its original position - Blade #1 at 12 o'clock with the TC1 mark on the ring gear also at the 12 o'clock position - and I think that it is running much smoother.

I believe that this means the prop is aligned at 90 degrees to the plane of the crank pins.

Just one more data point.

Jim Sharkey
RV6 - Phase 1
 
Conjecture!

'Just wund'rin'! I Don't deny that prop clocking has an effect on roughness because there are far too many who have seen definite results when they changed the prop's position. Here is my guess as to what's happening, for what it's worth. The slowdown of air in front of a cowling isn't circular in shape, but bears a definite relationship to the shape of the cowling and the presence and shape of the spinner ahead of it. Consider this: in front of the cooling inlets there should be much more slowdown than there is at the top of cowling, and even the bottom, since their surfaces slope smoothly back and offer less resistance to the airflow. That would seem to mean that the majority of slowdown will be in front of the inlets, especially if their openings are much larger than necessary to accomodate the engine's cooling flow. Peter Garrison has shown me the flow around the front of his cowling and this is basically how it is. Based on this, it seems to me that the torque reaction to the engine on a power stroke will be different, and less, when the blade is in the faster flow above and below the cowling than when it is passing in front of the inlets; the blade will see more resistance when passing the inlets. I would think that this effect would be even greater if the blade was located on the crank such that when passing the inlets, the crank is near 15 degrees ATDC at the point of maximum cylinder pressure and torque. I would also suspect that the effect would be less with a three-blade prop or a six cylinder engine, but maybe the three-blade prop/six cylinder combo would have an issue. 'Just wund'rin'!
 
Thermos said:
I previously owned a share of a Piper Archer, we had problems with slight engine vibration we couldn't track down. Had the prop balanced, checked everything...until a sharp-eyed A&P mentioned that the prop had to be "clocked" correctly on the crankshaft. Following his advice, we removed and reinstalled the prop at the right position on the crankshaft...no more vibration. Unfortunately that was so long ago I can't remember where he got his information from.

So the engine may not care, but in our case changing the prop's position on the crankshaft solved our vibration issue. And less vibration should be a good thing...
Click to expand...

Piper service manual..........
 
elippse said:
....it seems to me that the torque reaction to the engine on a power stroke will be different, and less, when the blade is in the faster flow above and below the cowling than when it is passing in front of the inlets; the blade will see more resistance when passing the inlets. I would think that this effect would be even greater if the blade was located on the crank such that when passing the inlets, the crank is near 15 degrees ATDC at the point of maximum cylinder pressure and torque.
Click to expand...

It is possible to introduce a forcing frequency into the rotating system by variation in load due to difference in prop inflow. The subject is well covered in the literature, in particular the cases of ship propellers and hull blanking, and twin engine aircraft prop tips swinging through disturbed air at the fuselage side.

However, in your theory the inflow location is very much inboard along the blade span. Not much leverage to that load variation, and I recall a certain prop designer who holds that the inboard blade sections on standard propellers don't do spit anyway ;)

We kicked around prop clocking quite a bit in a thread a few years ago. I think clocking is unlikely to change a torsional vibration signature, but would make a significant difference in the mechanics of conventional blade vibration:

http://www.vansairforce.com/community/showpost.php?p=155608&postcount=39
 
Dan

In the earlier thread I believe that it was you who suggested there was some "snaking" along the length of and in the plane of the crank, or close to it, due to the offsets of the cylinders/crank pins relative to each other. This results in "wagging" of the nose of the crank and propellor flange in that same plane. If the propellor is mounted perpendicular to that plane then its moment of inertia about the "wagging axis" is much lower than if it is mounted at + or - 60 deg to it, which are the only other options, so it doesn't tend to get flexed as much by the "wagging" forces.

Just a thought.

Jim Sharkey
RV-6
 
DanH said:
and I recall a certain prop designer who holds that the inboard blade sections on standard propellers don't do spit anyway ;)
http://www.vansairforce.com/community/showpost.php?p=155608&postcount=39
Click to expand...

Hoisted on the petard of my own words! Shame, sir! Don't you know that words, once spoken, can not come back to haunt the speaker?
But 'tis true, I have been known to make such vile claims. And to this day I still stand by them. But in a more serious vein, (vain, vane?) may I dissemble and say that maybe as a consequence of my previous innanities on the subject, that maybe herein lies the clue, or words to that effect, or maybe not! Plus I would not utter such a plebian phrase; more along the lines of expectoration!
 
Without getting into all the details of these clocking threads, I decided to test my seat of the pants feel on my engine the past few flights.

I first flew with the prop clocked I think the way it is suggested for best hand starting. Flew about 5 flights that way.

I then reclocked with the front TD mark at 12 and the prop at 12 & 6. That was worse than the original. (I know, I was using the wrong mark)

Yesterday I reclocked the prop with the rear TD mark at 12 and the prop at 3 & 9.

The seat of the pants (no electro gizmos doing fancy analysis) was that it was much smoother on the last setup. I am keeping it that way....
 
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Brantel said:
Without getting into all the details of these clocking threads, I decided to test my seat of the pants feel on my engine the past few flights.

I first flew with the prop clocked I think the way it is suggested for best hand starting. Flew about 5 flights that way.

I then reclocked with the front TD mark at 12 and the prop at 12 & 6. That was worse than the original. (I know, I was using the wrong mark)

Yesterday I reclocked the prop with the rear TD mark at 12 and the prop at 6 & 9.

The seat of the pants (no electro gizmos doing fancy analysis) was that it was much smoother on the last setup. I am keeping it that way....
Click to expand...

Now I'm confused - 6 & 9 - and - front and rear TD marks ???
 
jsharkey said:
- front and rear TD marks ???
Click to expand...

I believe he is referring to the timing marks on the flywheel.
The marks on the back side line up with the crankcase split at TDC whereas the ones on the front line up with a mark on the starter housing.

Not quite sure how he got the prop at 6 & 9. That would not be good for the prop. I'm hoping that was a typo.
 
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Ah Ha!

Mel said:
I believe he is referring to the timing marks on the flywheel.
The marks on the back side line up with the crankcase split at TDC whereas the ones on the front line up with a mark on the starter housing.

Not quite sure how he got the prop at 6 & 9. That would not be good for the prop. I'm hoping that was a typo.
Click to expand...

Timing mark differences would negate all of my earlier argument :)

More research needed.

JimS
 
Yep, The TD mark on the front of the ring gear for the starter housing and the TD mark on the back for the case split....I messed that up on my original attempt. I had the front one at 12 by mistake.

The 6 & 9 was a typo.... 3 & 9 is correct and I fixed the original post. 6 & 9 would be a challenge with a 2 blade prop. :eek:

jsharkey said:
Now I'm confused - 6 & 9 - and - front and rear TD marks ???
Click to expand...

Mel said:
I believe he is referring to the timing marks on the flywheel.
The marks on the back side line up with the crankcase split at TDC whereas the ones on the front line up with a mark on the starter housing.

Not quite sure how he got the prop at 6 & 9. That would not be good for the prop. I'm hoping that was a typo.
Click to expand...

jsharkey said:
Timing mark differences would negate all of my earlier argument :)

More research needed.

JimS
Click to expand...
 
jsharkey said:
Timing mark differences would negate all of my earlier argument :)

More research needed.

JimS
Click to expand...

OK - my smooth running prop is actually aligned with #1 blade at 4 o'clock with #1 piston at TDC.

The education and recreation continues :)

Jim Sharkey
 
DanH said:
and I recall a certain prop designer who holds that the inboard blade sections on standard propellers don't do spit anyway ;)
http://www.vansairforce.com/community/showpost.php?p=155608&postcount=39
Click to expand...
of
I had someone measure the chordline angles of a prop that had been of a recommended variety for the RV-6 and had been flown on one for several years. I calculated the helix angles for three radii of a propeller operating at 190 mph at 2700 rpm. These radii were chosen to represent three stations coinciding with the the cowling cooling inlets; 7.5" which is 1" outboard of the spinner, 10", and 12.5". The corresponding helix angles, which describe the path of a given radius in its rotary and forward motion, are 57.6 deg., 49.8 deg., and 43.4 deg. The actual measured blade angles at these three stations are 48.3 deg., 42.5 deg., and 35.4 deg. For these stations to be able to actually produce forward thrust, the incoming flow would have to be slowed down 80fps at 7.5", 63 fps at 10", and 69 fps at 12.5". Now keep in mind, that is for the average slowdown for the full circle of rotation. Since the majority of a slowdown will be where the cowling is most blunt, at the cooling inlets, and in the region above and below the cowling where it is the least obtrusive to the free-stream, to obtain this average slowdown it would have to be incredible in front of the inlets. Even so, with the inlets only accounting for a very small percentage of the arc area that blade sweeps through, it is easy to see that this particular example of propeller technology produced not only no forward thrust in this region, but most likely was guilty of actually producing reverse thrust which not only robbed it of efficiency but probably also slowed the cooling air entering the inlets. I had someone do an analysis of the slowdown of the air in front of the engine and cowling of a T-6, and only it even came close to having such a severe impact on the difference between the helical angle and the slowdown-necessitated chord-line angles, and that was for a slowdown that really was for the full 360 deg. path of the blades! This being the case, I still think that the clocking might be due to the variation in velocity ahead of a cowling, but probably only where the cowling is of a design that is not very streamlined.Someone who has experienced clocking-induced roughness and is of a curious nature might want to try putting his prop at all three available positions and see which position is best and which is worst.
 
hypothesis

Hi, Dan! I read the previous post that you gave me the link to, and that certainly may be one hypothesis. In the hypothesis of my posting, I mentioned that the pressure peak in the cylinder, with correct ignition timing, occurs about 15 degrees ATDC. But the torque peak doesn't occur until about 40 degrees ATDC due to the crank position not being at an angle at pressure peak to give much torque. It's interesting to note that in the region from about 25 degrees ATDC to 140 degrees ATDC when the torque is the greatest, if the prop was aligned with the crank pins, a two-blade prop would be passing through the regions above and below the cowling where the airflow is the fastest and smoothest! However, roughness from a two-blade prop passing through slower air during peak torque would give a one/revolution, not a one/two revolutions as in your response. I think that the use of the strobe which you mentioned might be a good experiment for some person or group to try to see if a more definitive explanation could be ascertained. It would also be interesting to see if this phenomenon is more prevalent with lighter wood-composite blades than with metal.
 
Engine torque

DanH: I got a torque curve from CF Taylor and was able to fit a pair of triangular-shaped curves to it with a fairly close match. The positive torque starts at dead-center, rises to a peak at 30 degrees, then goes to zero at 150 degrees ADC; its peak value is 2.85 times mean torque. The negative torque starts at 60 degrees after dead-center on the following cycle, peaks at 150 degrees ADC, then returns to zero at the following D-C; its peak is -0.65 mean torque. The combined effect from four cylinders starts at D-C, has a 2.85 MT peak at 3o degrees, descends to -.65MT at 150 degrees then returns to zero at the next D-C to repeat for the other cylinders. Does this fit well into your hypothesis?
 
Paul,
Gotta like a man with C.F.Taylor on his bookshelf.

Sorry, can't generate much enthusiasm for the issue right now.....on the home stretch with a new airplane.
 
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