Prop balance stand

Props, Spinners and McDowell Starter issues.
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shorty
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Prop balance stand

Post by shorty » Sun Sep 25, 2011 21:03

This is not exactly Aeronca related, but mabe it is.

I recently built a balance stand as My C-150 has a nasty vibration which is clearly comming from th' prop.

Well, here's what I got:
(many thanx to Ian for his most excellent work on the cones.)

Now on to the interesting stuff.
The balancing jig worked fine on the "dry runs" at home. So off to the airport to get busy with the real thing.
Pulled the spinner off, marked the prop and flange for indexing purposes, removed prop, installed in jig. spent the next few minutes admiring how easy it spins, you can actually blow on it and set it in motion.
Finally got down to business, marked the tip of one blade (in order to differentiate the two) and gave it a light spin. I expected mabe two or three complete revolutions, but it just continued to spin for a while. I went to the sody machine and got a drink. Got back just in time to watch the last few pendulum swings as the heavy blade came to rest at the bottom.
Just to be certian that it wasn't a fluke, I gave it a light shove in the opposite direction. Same blade came to the bottom.
Grab the file and get to work. But wait a minnit. It came to rest with the blades at the 1:00 and 7:00 position, instead of 12 and 6.
give it another spin, same result, same blade. Hmmm. mabe shaft or cones are off. rotate shaft and cones 180* in relationship to the prop, spin, same result, rotate one cone 90* spin, same result, rotate shaft 90*, same result,
heavy blade at 7 and light blade at 1. so the cones and shaft are good.
Also checked track, and found it to be dead on. (need a light gauge to measure th' difference, feeler gauge won't do it)

Question is why does it stop at 1 and 7, every time. rather than 12 and 6?
Don' wanna do any filing untill I get this rectified.
Also if ya sneeze in th' direction of it, yer in for a long wait for it to stop spinning. ;)
Thanx
Shorty

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DELCOM
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Re: Prop balance stand

Post by DELCOM » Sun Sep 25, 2011 21:29

I do not believe that filing a prop with a hand file is a good idea. For me if I was to even suspect that the prop is not right I would send it to a prop shop. I will be very interested to see what everyone else says. I would hold off until you are really satisfied that this is the way you want to go. Just my two cents

Good question though


Del
N84029 7AC conv. #2716 Completed and flying
N83059 7BCM #1715 fully restored and sold

Al Hatz
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Re: Prop balance stand

Post by Al Hatz » Mon Sep 26, 2011 07:48

Question is why does it stop at 1 and 7, every time. rather than 12 and 6?
Because all the weight isn't just at the outer part of the blade it's also in the hub and inner blades. That's why when you dynamically balance a prop you will find two weights attached. They find the point of imbalance and split the difference.
Balancing a prop by just weight is common practice when it comes to wood props but for a metal prop I would spend the money and have it dynamically balanced instead of filing on it.

seaheli
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Re: Prop balance stand

Post by seaheli » Mon Sep 26, 2011 08:00

If you look at the rear of the hub of the propeller there are balance holes in which to pack lead.

If a owner decides to paint their prop and add the tip markings, over the years the weight does not balance from the painting.

Also, lead will sometimes dislodge during the removal of the propeller.

I would not recommend filing the prop, have an approved repair station remove the old paint, inspect, paint and balance!

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shorty
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Re: Prop balance stand

Post by shorty » Mon Sep 26, 2011 18:11

Thanx for the answers ya'll.
Here's what I decided to do.
This morning I dropped off th' prop at Sensenich, and they'll gimme a call when it's done.
This prop has had nothing done to it other than dressing a few nicks since 1969, so no wonder it's outta balance.

But I also found that the balance rig works just fine, and when a Known good prop is installed it will stop spinning where it wants to, and there's no tellin' where that is untill it stops. And it don't "rock", it just slows down to a full stop. (unless a gust gets it)
Thanx
Shorty

Dennis
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Re: Prop balance stand

Post by Dennis » Tue Sep 27, 2011 21:34

Shorty,

I was at our local prop shop to day and I asked them about you problem with statically balancing your prop. They said that a prop should be balanced to stay in both the horizontal position and the vertical position. If the prop will not stay in the vertical position, then you file on the outer half of the prop on the leading or lagging edge of the outer 1/2 of the blade depending on where the prop stops. If the prop will not stay in the horizontal position, then you file on the inside 1/2 on the leading or lagging edge. The static balance should be followed with a dynamic balance on the engine.

This is most important, they said this type of work should only be done by someone who knows what they are doing. This is because too much filing in the wrong place could make your prop unairworthy. They also pointed out that to do the balance is almost an art. Removing metal from one spot may help in the vertical position but hurt in the horizontal position. Bottom line, this is one of those things that should not be tried at home by the inexperienced.

Dennis

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shorty
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Re: Prop balance stand

Post by shorty » Sat Oct 01, 2011 20:16

Good info.
I've used a balance stand in the past, for the purpose of equalizing the weight between the blades, when blending out nicks. Before dynamic balance was available for less than the purchase price of a new 182.

The purpose of building this stand, and hangin' th' prop in it, was to convince my A&P/IA that th' vibration was indeed caused by th' prop, and not th' one cylinder with low comp. after sitting for 2 years.
This is th' first time I've seen this crazy stoppage.

And after finding that th' stand works so well, I have a few airknocker buddies that may want to use it. After all, an extra coat of varnish on th' light blade may be all it takes to smooth out yer Middletown vibrator. :wink:
Thanx
Shorty

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shorty
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Re: Prop balance stand (update)

Post by shorty » Thu Oct 13, 2011 21:08

When I dropped off the prop for repair, I was told that it would take about 7 days.
On the 15th day I gave 'em a call to find out the status of the repair. First words outta th' ladys mouth were:"has it had a propstrike?" I hadda tell her that I had no idea as I just bought th' little thing, and have logged a whole .1 in it. The logs don't mention anything major since overhaul, some time ago. (not runout yet but OH'd anyway) Hmmm.

Anyway she went on to tell me that it was screwed up as a can of worms. But repairable,
and just commin' outta th' grinder, next step is re-twist, and paint.
Based upon what I was told, the profile on one blade was off quite a bit, and the back side of that blade had more waves in it than the north Atlantic on a stormy day. the blades were pitched differently, and it looked like it had been "repaired" some time in it's life.

But I was re-assured that it will be set to new limits, and any vibration caused by th' prop will be gone. Now I'm waitin' on th' call to come and get it.
Thanx
Shorty

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skyking3286
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Re: Prop balance stand

Post by skyking3286 » Fri Oct 14, 2011 11:45

Aren't you GLAD you sent it in?!!! :D
Mark Peterson
Harvey Field, WA
A copy of my old Chief website is preserved here:

http://www.reocities.com/mrpeters.geo/index.html

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shorty
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Re: Prop balance stand

Post by shorty » Sat Oct 15, 2011 22:00

Most definately! :D
Thanx
Shorty

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