Prop clocking

[SpecialT]

Can anyone share some propeller clocking wisdom? 60 degrees? 11 o'clock? 180 degrees?

Ok, it's been a couple weeks since I was able to fly, and I was not able to fly it after changing the oil and re-torqued my prop last time I went out. I had decided to rotate the prop according to these instructions (see attached)for a metal Sensenich on my O-320.

"Rotate the crankshaft by hand, until the engines number 1 cylinder is at TDC (Top Dead Center)... Locate the propeller on the engine flange with the number one blade at 11 o’clock."

Previously it was at about 1 o'clock and was installed by someone else. I did it because I noticed some slight vibration at about 2200 rpm in cruise and thought maybe it would help. When I flew the plane today, on the take off roll when I hit a certain speed right about before take off (if I remember correctly), I got some pronouced vibration and then it went away. I'm not sure if it's from the clocking or the fact that one of my tires was a little low on air. I am going to fly it some more hopefully in a couple of days and do some more investigation.

[Guest]

Re: Prop clocking

I would just follow the manufacturer's directions. Also track the prop.

Perhaps it's out of balance?

Dave R

[mikerice]

Re: Prop clocking

The clocking is most important on starting. We pulled our prop for maintenance, and forgot to mark it properly to the hub. The result was as we were throwing the prop to start (after re-install in the best guess position), the impulse on the mags clicked with the blade at noon - 6 o'clock. This is a most uncomfortable/dangerous position because you need to pull from about 2 o'clock to get the mags to fire as you pull through. We tested the clicking of the mags and decided we wanted them to fire at about 9:30, so removed the prop again and re-clocked it at 11 o'clock so pulling through about 20-25 degrees fired the mags on the down stroke. It starts on the first throw now.

As for the prop itself, each prop has a #1 and #2 blade, usually stamped on the hub. Convention is to put the #1 blade at the 11 o'clock position, but the #2 blade can be used to try to alter the balance if you are getting a vibration. In other words, flipping the blades 180 degrees in the direction of rotation might adjust your prop balance slightly.

As Dave suggests, the prop may need to be balanced at a prop shop or a dynamic balance. We are going to try a dynamic balance to see if it does anything... 4 bangers are inherently not as smooth as 6 cylinders so we may have to live with a little vibration.

[PA1195]

Re: Prop clocking

Track and dynamic balancing do help. I've had it done on several 4 and 6 cylinders engines. Knowing the prop pitch is correct before balancing is important as even from the factory they can vary from pitch and distribution specs and between blades.

On the small Continentals the weight can be installed under one of the prop bolts. Bolt length can be adjusted to accommodate washers for weights. On mine now they decided to use a piece of flat strap chromoly under one bolt and bent it towards the prop center under the skull cap. It was made longer and therefore heavier than the dynamic balancer's computer call-out clock weight because the bend moves the overall weight mass inward. Bending and moving the overall mass inward towards the center of the prop hub reduces the balancing effect of the strap's added weight.

Going from a 1/16" track and no weight to added weight eliminated some minor but still felt rudder pedal/upper panel/side window vibration at cruise rpm. It can and did make a difference especially in flight.

Gary

[SpecialT]

Re: Prop clocking

Thanks for the input. Well I was thinking I moved the prop to the "right" position, but is 11 o'clock the position when looking at the prop from the front when mounting, or is is when sitting in the cockpit looking out the front window? Maybe I moved it from 11 o'clock to 1 o'clock. I think I'll try the 180 deg. swap next to see what I get.

[PA1195]

Re: Prop clocking

is 11 o'clock the position when looking at the prop from the front when mounting

In my experience yes. Designed to fire before the prop goes vertical on the down swing.

Gary

[mikerice]

Re: Prop clocking

All of my descriptions are based on standing at the prop looking aft. Gary said the same thing - that the mags fire as you pull the prop through 9 o'clock or so. The prop has 6 bolts evenly spaced, so 360/6 is 60 degrees as you move the prop one bolt hole at a time. That means you only have 3 holes to index to, as the other 3 would be the #2 blade in the same relative positions. You will find one position allows the mags to click (fire) on the downward swing. Then you can rotate the prop 180 degrees, and that would put the opposite blade in the correct firing position, if you wanted to see if the balance is affected.

[Guest]

Re: Prop clocking

It is possible that the manufacturer is specifying a prop position based on the prop's location relative to the crankshaft throws for the purpose of managing engine power pulses and dynamic balance considerations.

It is also possible that the manufacturer is considering the above and hand propping at the same time and he is giving the best compromise.

I would just follow the manual.

I am not sure what you gain with a 180 degree swap, it is a 2 blade, correct?

[PA1195]

Re: Prop clocking

Sometimes and somehow the prop's track between blades can be improved by 180* swapping. Probably a combination of the crankshaft being slightly out of round but within manufacturer's tolerance (https://www.cubclub.org/samples/web_con_maintenance.pdf) in combination with a propeller that isn't perfect either...one blade is slightly deformed relative the other. Swapping "may" both increase (make it worse) or decrease stacking tolerances (make it better) and improve blade tracking (http://okigihan.blogspot.com/p/propeller-vibration.html).

Prop balancing link: http://okigihan.blogspot.com/p/prope...unbalance.html

Gary

[Guest]

Re: Prop clocking

ahhh

Thanks Gary, I had not realized that tracking was a problem to be solved and what you said makes perfect sense for that.

Dave R

[N96337]

Re: Prop clocking

It is possible that the manufacturer is specifying a prop position based on the prop's location relative to the crankshaft throws for the purpose of managing engine power pulses and dynamic balance considerations.

It is also possible that the manufacturer is considering the above and hand propping at the same time and he is giving the best compromise.

I would just follow the manual.

I am not sure what you gain with a 180 degree swap, it is a 2 blade, correct?

I totally agree with this and with what Gary says.
John

[PA1195]

Re: Prop clocking

Here's an example of a balanced prop - Field Approved M76AK-2-40 on my Taylorcraft. This was done by a local prop repair station using an electronic balancing system at about 2400 static/typical cruise rpm. Vibes went from a bit over 0.2 IPS to well below that value. The weight is just under 50 grams, but as I note above would be less providing all the weight could have been placed and centered under a bolt head with room for prop dome installation. Extending the 4130 tab allowed dome clearance and met the balance requirements called for at that clock location by the test instrument.

In the left pic the prop is at the 11 o'clock position that allows for magneto impulse release before going vertical.

http://www.propellerman.com/dynamic-balancing.html

Gary

[Guest]

Re: Prop clocking

very cool weight scheme!

before going vertical? or horizontal? I don't follow

[PA1195]

Re: Prop clocking

Hi Dave. The weight scheme was and is an experiment in longevity and effectiveness. The Repair Station first tried washers under that bolt head with a longer bolt to maintain full thread penetration in the crankshaft's flange. The added weight of the longer bolt and washers (they need to be small diameter to fit inside the dome) was insufficient to fully balance the engine. They next tried fastening the weight to the dome bracket support leg that's attached to that bolt. Centrifugal force soon partially wear cracked the leg of the bracket the weights were attached to but the dome didn't depart the aircraft.

This is the latest iteration. A piece of 4130 chromoly strap heat bent to stay as close to the bolt head and required balance weight location. The thickness, width, and length protruding forward determines the balance weight. No other shapes or material thickness were tried.

Once installed the dome was attached and a balance run done. The weight was made longer and heavier to begin with. The balancer machine calls out the clock position and weight it predicts will balance the assembly. By trimming the forward angle (see shiny cut) the final weight versus indicated balance via sensor movement in inches per second was established (the sensor is installed vertically on top of the upper crankcase split).

It's now about 50 grams - more than if it had been just washers because a portion is closer to the prop center and has less effective rotating mass. The closer to the hub's center the more weight is required to achieve balance.

We inspect it periodically and no problems have been noted. It's now a smoother engine than without it installed.

Gary

[SpecialT]

Re: Prop clocking

We've been fogged in this whole time and not able to fly. I might rent a car and drive the kids to Oshkosh before I get a chance to turn the prop and test it out. But I found some more interesting discussions about prop clocking at the 9/3 o'clock position, which is not an advantageous position for hand propping but seems to reduce 1/2 order vibrations.

http://mstewart.net/super8/propclock/index.htm

I guess the 9/3 position (#1 TDC and #1 blade in line with the #1 cylinder) is the one advocated in the Sacramento Sky Ranch manual for smoothness because it's in line with the plane of the front crankthrows. Since I have a starter on the engine this might be a viable option. When I get a chance I'm going to try it and report back.