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The free "dynamic balancing" by only prop cloking = myth verified

Antoine.S

Member
Hi,

I eat frogs so please once again excuse my english in my threads,

Two month ago i've tested a myth..

THE MYTH:

"You can reduce the vibration under at least 0.3IPS like a "dynamic balancing",
by only cloking your propeller horizontaly (3h/6h if prop mount was a clock) when 1# cylinder is at Top Dead Center" :eek:

Lot of you guy's will respond as me before testing:
"BULL****! the prop act's like a flywheel!
If balanced changing clock position can't change anything!" :p

I tested it anyway!
Wanting real numbers, not only "siou" feelings,
I Take my RV to a freind helicopter workshop, he have a hight dollar dynamic balncer that he use for dynamic balancing rotors. And planes sometimes.

There is a graphic chart of the vibration you can experience
in any light aircraft.
Good to look at to speak the same language:
http://www.expaircraft.com/PropBa1.gif


First value with the prop cloking on standrad position (11h / 5h) with no extra balancing. (Like lot of club airplanes).

At 2250 RPM the dynamic balancer indicates 0.7IPS... Very Rough vibration
(what they call Very Rough is not enormous, i'would better say "really feel-able vibrations, you can "see" some tiny vibrations in the cockpit during flight ")

Ok we got a reference number!

Now time to "clock the prop horizontal (9h/3h) and test it,
(i've put the N?1 blade in the front on the 1# cylinder)

Same 2250 RPM... 0.02 IPS!!!! :eek: Huuuu somthing wrong with the balancer???
"inside during the static run up I feel clearly a noticeable difference, but is the number real..?" :confused:

We checked all the sensors,mounting plates,wires etc..
And tested it again:

2250RPM... 0.03IPS...
"Whow! It seems to be true!....." :rolleyes:

My freind to be sure of the results, putted a little scew/nut in one of the starter plate hole to unbalnce it just a bit, and check his dynamic balancer..

2250RPM... 1.2IPS..

He removed the screw/nut and try's again..

2250RPM... 0.03IPS! :cool:

"Ok so it's real! Now time to test it in flight! Cause it will not be better with a real dynamic balancing!"

The flight was a clear, "no sensor needed" response to the Myth:
Smoother engine RPMs every where, especially at 2350RPM witch delivers no vibration at all, only the smooth explosions of the o-320 in front of us,
no hight frequency vibrations anymore (they use to be feeled with my nail pick directly on the metal inside fuselage, now i feel nearly nothing, like it been divided by 2)
It feel's like the motor gain RPM faster than before, and i feel somthing like the plane is climbing better than usual but it can be the oat or somthing else..
(when you stop the engine, a prop clocked horizontal 9H/3, the prop will stops absolutely vertical like a "I" on your hood 12h/6h)

I run like that for 10h now and like it so mutch!

I'call sensenich and lycoming to be sure it's not an issue to clock the prop like that, both for prop or engine.
They both said that it is totally safe! No problem!
The lycoming guy said "between word" that they know it's gives good result with vibration but they prefers to claim 11H/5h standard cloking
to security for hand starting or proping. (anyway i will never try to start my 160hp by hand!)


A friend, after i tell him my little experiment,
try the horizontal (9h/3h) position on his airplane a kind of big "b?b? Jodel",
Same improuvments (feelings) on vibriations and smoothness!
He keep it like that, and don't want to go back to the 11h/5h standard position...


Now time to try to understand why it works so well!

I think it's due to harm force of the prop blades in that position that is avoiding the crankshaft to bend on eatch piston push, so it's limitating vibration.
In the same way the disc of the prop stop shaking allowing better efficency.
(The metal prop wheight is for sure an adventage in this explanation)

My idea to explain the feeling of little more power is that the blade is now passing by the air intake at his maximum piston delivey force..
Maybe it's helps power with a kind of blast at the valve open time?
Or somthing due to a different positioning maximum piston force delivery on the blade disk, maybe passing by the crowl?

Need the quantic phsysitians of the VAF forum to answer the "why?" ;)

but
Even if i'm not sure to know "WYH?"
in my opinion:

"MYTH VERIFIED!"
 
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Impressive

Antoine,

That's an interesting bit of work.

Someone, such as Walt, with many observations (& associated data) could give empirical background for verifying or debunking the myth.
 
Very interesting experiment. I confess I needed a few minutes to realize that "cloking" or "clocking" referred to un-bolting the prop, rotating it, and bolting it back on.


I've hand prop'ed many airplanes and for that reason will stick with Lycoming's recommendation. I also desperately need to get my propeller+engine combination dynamic balanced.

I am hoping Walt will contribute his experience to this thread. I consider him a sage source.
 
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propping start is still possible

precision, i forget to mention (i will add in my the 1st post):
i've put the N?1 blade in the front on the 1# cylinder...

propping start is still possible but you will place yourself on the right of the prop and lift the blade by a movement of the harm from down to up, staring to walk back in the same time... as the engine start...
(the compression will be passed when blade cross horizon)

ONLY A POSSIBILITY! can may be dangerous!
 
Prop position

I think Sacramento Sky Ranch book also recommended 3/9 o'clock position for prop for lowest vibes.
 
Mike clocked the prop on his -8 years ago and was very pleased.

Best,

The data on this activity can be found here.
Important to note that I was not interested in the difference in dynamic prop data, but rather the airframe vibrations. "My Butt Feel". There is very good data and information in there.
 
Cylinder #1

I believe props coming from the factory or OH are notched/marked to align with #1 Cylinder.
 
I think the OP just got lucky with vib level after re-clocking his prop, changing the relative position of things obviously re-distributes the mass imbalance and at some point you may get lucky. Of course trying to balance every prop this way would be expensive and extremely time consuming, with a CS prop you would have to re-index flange bushings for each position as well.

I posed this prop clocking question to the experts at Hartzell, here was the response I received from Les Doud, senior engineeer at Hartzell (same guy Kahuna worked with above):
(I bolded the 2nd paragraph of his reponse as a warning to those tempted to go this route)

"There have been a few reports written by engine companies or airframe manufacturers that document testing they’ve done with propellers mounted in several indexing positions. I did some testing myself on a IO-470 Cessna 180. My experience helping customers (OEM, modifiers, and EAB) determine vibration levels has provided a folder full of spectrum data. The indexing position that worked best on several applications was the propeller in-line with the front crankpin on the crankshaft. Keep in mind the industry standard indexing for decades has been to facilitate hand-propping which is different from the one that results in lowest vibration. This in-line indexing seems to lower the ½ order vibration level in the cabin. However, on some applications the propeller indexing has little or no influence. I think the lesson learned is, if an airplane is experiencing unexplainable high ½ order vibration, then propeller indexing should be considered. The only way to determine that for any particular application is to measure the vibration levels with the propeller in several positions.

One word of caution, the propeller manufacturer may only approve one specific indexing due to propeller blade stresses. That indexing may be different from the position that offers the lowest cabin vibration level. So it is always important to check with the propeller manufacturer before people start playing with different indices.

Don’t hesitate to contact me directly if you have any questions."

Respectfully,

Les Doud
 
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Reclocking the prop is an approved modification on some Certified aircraft with the 540/2 blade combination. The reason for the "modification" is to reduce vibration. I flew a Rocket that was once a real paint shaker - and then the owner did some Web research and re clocked the prop. Ran nice and smooth after that.
 
The indexing position that worked best on several applications was the propeller in-line with the front crankpin on the crankshaft. Keep in mind the industry standard indexing for decades has been to facilitate hand-propping which is different from the one that results in lowest vibration. This in-line indexing seems to lower the ½ order vibration level in the cabin.

Explanation:

http://www.vansairforce.com/community/showpost.php?p=155608&postcount=39
 
There may also be confusion regarding the "clock face" that is used. :)

Most aircraft things are specified using the direction of flight, such as left/right.

In this case I would assume that the clock digits are for a clock face looking aft from in front of the spinner.
 
If re-clocking a prop beware that some engines have mounting lugs in the flange of the crankshaft, in specific positions to index the prop to the crank flange and re-clocking the prop may involve re-indexing the flange mounting lugs or you will push them out toward the rear of the engine when mounting the re-clocked prop!
Merry Christmas,
Mahlon
 
Back when I had my 76 Warrior, the shop pulled the prop to replace a cracked spinner bulkhead. During my first flight out of the shop, the plane felt rough during the entire flight. At shutdown, the prop stopped at 12:00. Bingo!!

I checked with the service manual..... 12:00 for 160hp and 10:00 for the 150hp. Moved the prop to 10:00 standing in front of the spinner...... Back to a smooth running Warrior.
 
i like to be sure so..

I've just calling sensenich for some re-ptiching infos..
(no relation with our thread)

but i take the time to ask them again to answer the question of the safety about our subject :

"is it possible to index* horizontally (9h/3h) the very special propeller 70-CM7 witch is the RV metal propeller designed for O-320 that got the 2600RPM limitation without over stress it due to crankshaft vibration absorption?"

The answer given by Mr Griffe (general manager aluminum propeller division):
His words:
"There is absolutely NO restriction about indexing*, on every of our RV fixed pitch metal propellers, you can index* them any way you want!"

:) good news!


*read "clocking" it's the same! ;)
 
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