Does a prop need to be "clocked"?

455 Bravo Uniform

455 Bravo Uniform

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455 Bravo Uniform
When building automotive engines, the rotating assembly is balanced (internally balanced with crank counterweights, sometimes externally with weights on the harmonic balancer and flex plate too).

So what happens when you hang a prop on the front of an airplane engine? Does it need to be clocked to match existing internal balancing? Or does it not matter as long as each blade is fully balanced with the other blade(s)?
 
It doesn't HAVE to be, but people seem to get good results from additional dynamic balancing.
 
455 Bravo Uniform said:
When building automotive engines, the rotating assembly is balanced (internally balanced with crank counterweights, sometimes externally with weights on the harmonic balancer and flex plate too).

So what happens when you hang a prop on the front of an airplane engine? Does it need to be clocked to match existing internal balancing? Or does it not matter as long as each blade is fully balanced with the other blade(s)?
Click to expand...

Yes,,,, read the maintenance instructions for the engine and the Airframe. For example. the 0-300 on a C-170 requires the engine to be on the compression stroke at TDC and the prop mounted at 10:00/04:00 number 1 blade up. Most of the Lycomings have a master stud that only allows the prop and flywheel to mount in one position.
 
455 Bravo Uniform said:
When building automotive engines, the rotating assembly is balanced (internally balanced with crank counterweights, sometimes externally with weights on the harmonic balancer and flex plate too).

So what happens when you hang a prop on the front of an airplane engine? Does it need to be clocked to match existing internal balancing? Or does it not matter as long as each blade is fully balanced with the other blade(s)?
Click to expand...
Some props and engine cranks have attachments so the prop can only be installed in one position.
 
mtuomi said:
It has to be installed to a certain position so true, that has to be done. I was thinking does it have to be balanced, no it doesn't.
Click to expand...
Every prop was static balanced prior to being delivered from the factory or prop shop. and will be with in limits of the Vib criteria for its installation. but many times, that isn't good enough. Dynamic balancing is always better.
 
Wooden props doesn't seem to make a difference where ya put them.

 
Also think hand prop requirements.

I just balanced a prop where the initial check was 0.01 IPS. I have never seen that even with a brand new prop. Must have been a Wednesday prop. :)
 
A 172M club I was in majored their O-320 E2D engine. Afterwards it shook so badly they were ready to remove it and tear it down again when someone discovered the prop was not clocked per the Cessna manual. That fixed the problem.

As mechanical engineer (retired) with a lot of vibration experience I can't explain it either.
 
455 Bravo Uniform said:
When building automotive engines, the rotating assembly is balanced (internally balanced with crank counterweights, sometimes externally with weights on the harmonic balancer and flex plate too).

So what happens when you hang a prop on the front of an airplane engine? Does it need to be clocked to match existing internal balancing? Or does it not matter as long as each blade is fully balanced with the other blade(s)?
Click to expand...
A 4 cylinder engine fires at 180* intervals. A 6 cylinder at 120* intervals. The prop is aligned with the crank position relative to a specific cylinder TDC, at least that's true of the planes I've owned. On an O-320 viewed from the front the prop should be at 10:00 and 4:00. Mine always stopped in the same place every time. My 3-blade is indexed to TDC of one of the cylinders (can't recall which). Even with a locater pin it would be simple to install it 180* out of position (on the exhaust stroke) so finding TDC on the compression stroke is important. Props are my least understood aircraft part. I enjoy talking to prop engineers but I quickly recognize how little I know about propellors.
 
I'm a big fan of balancing props, it's not too hard to find a guy with a dynavibe to do it, takes but a few starts.
 
If you don't know, you better find out because it might really matter.
 
As others have noted the prop should/must be installed as spec'd in the manual.

Dynamic balancing can be a very good thing too. The 'Kota was only really happy at 2400 rpm when I got it. After dynamic balancing it was happy at any normal operating speed. By only happy I mean the needles on gauges would be dancing so bad as to be unreadable when it was at 2300 rpm. Any time a gauge was pulled for maintenance the report would always note the jewel was flat spotted. The turbo inlet temp gauge and the tach had to be replaced and I am just about certain that vibration killed them.
 
Does a crank move smoothly at any given power setting, or does it speed up and slow down at rates our tachs cannot see?
If the latter, maybe it is pressure waves coming off the prop hitting the cowling, during a power stroke.
 
Pressure waves is what contributes to cracks in the old style stove pipe King Air exhaust stacks. Terrible design.
 
In a four cylinder engine the crankshaft accelerates at the moment of combustion, or at the top and decelerates toward the bottom of every 180* turn of the prop, so 4800 times a minute at cruise RPM. The bigger the displacement and higher the compression the more exaggerated it is, which is why 0-360 props are clubs compared to 0-320 props, so they can withstand the forces exerted on them. So explained my prop manufacturer when I asked. My IO-400 has a counter balanced crank, something that's becoming more common in high output 4 cylinder engines. The counterweights are to reduce twist, flex, and vibration. The engine is known for being turbine smooth. The prop engineers have more sophisticated terminology but that's it in a nutshell.
 
Tom-D said:
Yes,,,, read the maintenance instructions for the engine and the Airframe. For example. the 0-300 on a C-170 requires the engine to be on the compression stroke at TDC and the prop mounted at 10:00/04:00 number 1 blade up. Most of the Lycomings have a master stud that only allows the prop and flywheel to mount in one position.
Click to expand...

The Lyc's flywheel doesn't clock the prop. With the flywheel in the right position, the prop can be put on in any of three positions, and two of them are wrong.

I just hung a prop on a 172S's new IO-360 two days ago, and it was the same as all other 172s with Lycs: the manual says to get the flywheel set with the TDC mark on the backside straight up (which is the same position as the TDC mark on the front side lined up with the starter's timing index point), hold the prop vertical and then rotate it clockwise (as seen from the front) until the bolts line up with the hub nuts. That gets the prop in about a 1:00/7:00 position at TDC, which makes any necessary hand-propping much less dangerous as well as somehow minimizing vibration.

I think the least vibration comes with the prop (two-blade) near the horizontal when the engine fires at its usual advance point. That horizontal mass will damp any lateral rocking caused by differences in piston and rod weights. But that's just speculation.
 
Let'sgoflying! said:
Does a crank move smoothly at any given power setting, or does it speed up and slow down at rates our tachs cannot see
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Crankshafts accelerate and decelerate constantly because the force from the pistons is constantly changing. They also flex a lot more than you might expect - that is one of the problems with detecting misfires (OBD-II regulation) by measuring crankshaft acceleration - you get different results at each end.
 
nrpetersen said:
A 172M club I was in majored their O-320 E2D engine. Afterwards it shook so badly they were ready to remove it and tear it down again when someone discovered the prop was not clocked per the Cessna manual. That fixed the problem.

As mechanical engineer (retired) with a lot of vibration experience I can't explain it either.
Click to expand...

Harmonics, maaaan.....o_O
 
JAWS said:
Also think hand prop requirements.

I just balanced a prop where the initial check was 0.01 IPS. I have never seen that even with a brand new prop. Must have been a Wednesday prop. :)
Click to expand...

.01? Not too bad... I remember one time I did a tail rotor blade repair on a Blackhawk, and on installation, we couldn't get the vib down anywhere near an acceptable level. Myself and the Test pilot looked at what the line guy was doing, and he was chasing the vib all over the place, and had almost maxed out the balance weights in two quadrants. We suggested he strip ALL the weights off and we ran it again. After putting just a tiny little bit of weight on one quadrant, it came out as close to ideal as you would ever want. I basically accidentally balanced a TR paddle with Hysol 9309 and abrasion strip material.
 
Dan Thomas said:
The Lyc's flywheel doesn't clock the prop. With the flywheel in the right position, the prop can be put on in any of three positions, and two of them are wrong.

I just hung a prop on a 172S's new IO-360 two days ago, and it was the same as all other 172s with Lycs: the manual says to get the flywheel set with the TDC mark on the backside straight up (which is the same position as the TDC mark on the front side lined up with the starter's timing index point), hold the prop vertical and then rotate it clockwise (as seen from the front) until the bolts line up with the hub nuts. That gets the prop in about a 1:00/7:00 position at TDC, which makes any necessary hand-propping much less dangerous as well as somehow minimizing vibration.

I think the least vibration comes with the prop (two-blade) near the horizontal when the engine fires at its usual advance point. That horizontal mass will damp any lateral rocking caused by differences in piston and rod weights. But that's just speculation.
Click to expand...
O-320 has one larger lug on the prop flange, the flywheel and prop will only fit 1 way.
 
Tom-D said:
O-320 has one larger lug on the prop flange, the flywheel and prop will only fit 1 way.
Click to expand...

So why do the Cessna manuals have to tell you how to clock it?

That larger lug fits into the larger hole in the flywheel, but it does not stick out of the flywheel's front surface as the other five do, and so does not engage the prop or prop spacer. You can install that prop in any of three positions, and so Cessna publishes clocking info for doing it right.

I work for a Cessna dealer.
 
Dan Thomas said:
So why do the Cessna manuals have to tell you how to clock it?

That larger lug fits into the larger hole in the flywheel, but it does not stick out of the flywheel's front surface as the other five do, and so does not engage the prop or prop spacer. You can install that prop in any of three positions, and so Cessna publishes clocking info for doing it right.

I work for a Cessna dealer.
Click to expand...
I stand corrected, I thought it protruded thru to the prop.
 
Checkout_my_Six said:
Hint: something about how to clock a wooden prop....and moisture.
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The is about storing one. Clocking refers to the position on the engine/ crank.

Always store a wooden prop in the horizontal position so and moisture in it will not drain to the lower tip.
 
Tom-D said:
The is about storing one. Clocking refers to the position on the engine/ crank.

Always store a wooden prop in the horizontal position so and moisture in it will not drain to the lower tip.
Click to expand...
really?...Wonder what Sensenich sez?
 
nrpetersen said:
A 172M club I was in majored their O-320 E2D engine. Afterwards it shook so badly they were ready to remove it and tear it down again when someone discovered the prop was not clocked per the Cessna manual. That fixed the problem.

As mechanical engineer (retired) with a lot of vibration experience I can't explain it either.
Click to expand...

I got caught by that once. I put the marks at the wrong location and the prop shook badly. No excuse, but it was 5 Am and had been working all night and I was trying to help get more done. I should have just went home! :)
 
Tom-D said:
Clocking refers to the position on the engine/ crank.
Click to expand...

Thanks, I did not know that.

This reminds of a problem at my last annual at a shop in another state. They took off my three-blade constant-speed prop because it was leaking oil and sent it to a prop shop. When the prop came back, my shop had forgotten how it had been oriented, so they mounted it a random way on the Continental engine, sent me a photo, and asked me if that's how it used to be when the engine stopped. Luckily I had a photo, showing how it had been, which was completely different from the way my shop had just remounted it. They then charged me extra to take the prop off and put it back on the right way -- I refused to pay for that (and for quite a few other items, all between $100 and $500, that they billed me for without authorization). I won't be going back to that shop.
 
NoHeat said:
Thanks, I did not know that.

This reminds of a problem at my last annual at a shop in another state. They took off my three-blade constant-speed prop because it was leaking oil and sent it to a prop shop. When the prop came back, my shop had forgotten how it had been oriented, so they mounted it a random way on the Continental engine, sent me a photo, and asked me if that's how it used to be when the engine stopped. Luckily I had a photo, showing how it had been, which was completely different from the way my shop had just remounted it. They then charged me extra to take the prop off and put it back on the right way -- I refused to pay for that (and for quite a few other items, all between $100 and $500, that they billed me for without authorization). I won't be going back to that shop.
Click to expand...

When I put a prop on, I use a drop of lacquer on the parting seam. Because I can't remember which way It came off either.
 
NoHeat said:
Thanks, I did not know that.

This reminds of a problem at my last annual at a shop in another state. They took off my three-blade constant-speed prop because it was leaking oil and sent it to a prop shop. When the prop came back, my shop had forgotten how it had been oriented, so they mounted it a random way on the Continental engine, sent me a photo, and asked me if that's how it used to be when the engine stopped. Luckily I had a photo, showing how it had been, which was completely different from the way my shop had just remounted it. They then charged me extra to take the prop off and put it back on the right way -- I refused to pay for that (and for quite a few other items, all between $100 and $500, that they billed me for without authorization). I won't be going back to that shop.
Click to expand...

It is normal practice to mark parts that could be installed in more than one location. A 50 cent grease pencil or a line of paint can be your best friend when it comes to reassembly.
 
JAWS said:
It is normal practice to mark parts that could be installed in more than one location. A 50 cent grease pencil or a line of paint can be your best friend when it comes to reassembly.
Click to expand...

Learned that within the first week as an apprentice machinist.

Cheers
 
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