First time balancing a prop. I sanded the back of the heavy end of an APC 11 x 5 which was hanging at 6:00. Now the original light end (OLE) is doing some weird things. I positioned the prop at every point of the clock and here is what the OLE moved to:

Starting Position Ending Position
12:00 1:00
1:00 1:00
2:00 4:00
3:00 4:00
4:00 4:00
5:00 4:00
6:00 4:00
7:00 4:00
8:00 4:00
9:00 4:00
10:00 1:00
11:00 1:00

Then I spun the prop clockwise three time and the OLE ended up at 4:00, 2:00 and 10:00 respectively. When I spun the prop counterclockwise the OLE ended up at 4:00 all three times.

What is going on and what can I do?

Perhaps your hub is out of balance a bit.

A prop that only moves very slightly a short distance is telling you something.

The perfectly balanced prop won't move at all when placed at any position. And obviously won't move no matter what that position is.

If the prop moves only very slowly and only a short distance, it's very close to perfectly balanced. And if that's somewhat random, you have an imperfect balancer.

I would suggest that your balancer is not perfectly free of friction, or the prop isn't being held uniformly or not securely.

and.........................

It is important to check props for balance, but it's actually not overly important to have them perfectly balanced.

If I were you, I'd check over that balancer closely. And I'd try it with a couple of other props. And I would have no reservations using that prop if it was only moving slowly such short distances. But veryify that the balancer can and will find an out of balance prop dependably. Put a bit of masking tape on one blade of that haunted prop and see if the balancer dependably points out the taped blade. Test the balancer to prove it works dependably and go from there.

Most of today's props are going to be pretty good for balance. But it's "expertly sensible" to check them. And same with the balancer. Check it out.

It sounds to me that your prop is pretty well balanced right now.
The random ending spots show that a fair balance has been achieved.

A couple notes on balancing. Props tend to have a little flashing at the edges of the center hole. If the prop has been reamed, the flashing may be more pronounced. The flashing (rough edge) will not let the cone of the balancer to set true to the hole and it will give you a false balance.

To correct this, you need a stepped prop reamer. Ream the prop to the proper size hole, then do a 1 turn cut with slight pressure with the next step. This will cut the flashing off and leave you with a very slight bevel on the edge. Do this on both sides of the prop. This will give your cones the best chance of setting true to the bore of the prop.

Second, when you install the cones and shaft on the prop, spin each cone a couple times to make sure any dust or stray balsa chips are not interfering with the seting of the cone. Keep the cone clean, wipe them down with a rag with solivent once in a while. If you can see anything on the cone, it is to dirty to do it job correctly.

A lot of times, you will find a prop balances tip to tip, but has a heavy side. IE once it is balanced, turn the prop 1/2 turn and see what happens then. If it stays in place, you are good to go, if it roll 1/2 turn then settles down to a balance, you have a heavy top/bottom and it needs to be resolved. One of the old timers at our field skuffs up the hub of the prop on the light side and applys a ribbon of epoxy on the hub. Leave it hanging with the epoxy hanging down so it forms a nice bead. Use more than enough to do the job. Then when the epoxy is set up, you can grind off the excess to balance top to bottom.

One other note on reaming. The APC props have a stepped bore. Make sure you start the ream from the large side. This keeps the pilot of the reamer tracking the orginal bore. If you start the reamer from the small side, you will loose alignment about 1/2 way through the prop and it is easy to get an egg shaped hole due to the wobble of the reamer. The best approach is to ream the prop on a drill press with a setup that keeps the hub exactly alligned with the reamer. IE you don't wnat the prop setting at an angle, but exactly perpenducliar to the axis of the reamer, in all directions. I found my big drill press's table was off by about 1/4 degree and I was getting wobbly props. My small drill press is dead on, and that is the one you want to use. I also don't turn the drill press on, but hand turn the drive pully, watching to keep your fingers out of the belt. If a reamer isn't turning exactly true, you will get an oversized hole under power, but driving it my hand lets it follow straight.

Don

Edit. I had to add one more thing, NEVER drill a prop. You can't get a drill to follow the hole with any accuracy. The stepped reamer is the only way to go

I would like to add one more thing to Don's good advice. Whenever I buy an "electric" APC prop, I get this little piece of paper in the bag with the prop explaining that the small hole through the hub is a non-precision hole, and the larger hole on the other side of the hub is the precision surface. I find it impossible to balance my electric props, as the cones from my balancer have to engage both surfaces (one on each side). Although I don't recall ever seeing this explained when buying a prop for a glow engine, I am highly suspicious that this might be the case as well. In short, this might be an explaination for your (and anyone elses) problems with trying to achieve a proper balance on a prop with an off center hole.

Lm

Actually, you don't have to orient both cones into the prop. Just place the two cones facing the same direction on the shaft. One goes into the prop and centers it. Since APCs say that one side is not precisely centered, face the imprecise side away from ththe cone going into the hole. The other cone's flat face will provide perfectly adequate pressure for both cones to hold the prop accurately and firmly.

This is done all the time with props that are too thin to allow both cones to project into the hole without interference.

da Rock

Yes you are correct, and I have actually done this many times. I guess I didn't take the time in my first post to be as precise as I should have been. The actual problem is that the hole that passes all the way through the hub is the non precision hole. The precision "surface" is the larger hole on the front of the hub, which only enters maybe one third of the way into the hub. If I put the balancer cone on the back portion it will engage the non precision hole. Now if I reverse this, and I put the flat face of the cone against the back of the hub, and the cone into the front of the hub, I still have this problem. The cone will pass through the precision hole without touching it, and still engage the front portion of the non precision hole. There seems to be no way to center the balancer on the precision hole, short of turning my own adaptor on a lathe. Sorry about the long harangue, hope this makes more sense.

Lm