Prop Balancer and Balancing

rdw777 Sat Apr 22, 2023 10:25 am

Bench running one of my .020’s and noticing the vibration on the test stand I thought we can do better than that…I had cobbled up a balancer when I started playing with gas engines again so I modified it to get a little better results….It’s made with a couple of wooden yard sticks and ply base that is tapped for 8-32 screws for leveling points…. The razor blades were leveled on a sheet of glass and calibrated to the base by another small level…. The base levels were originally in one unit but I cut them apart to work for this application…. The balancer ended up being very sensitive but easy to adjust…

I have eleven Cox .020 4.5 x 2 props that are a mix of old and new, Black nylon and hard grey plastic…. When checked on the balancer I found that ALL of them behaved like the hub was heavy on one side, Or not exactly center drilled, Or a combination of both…. The other odd thing is that they ALL hung in the same relation on the balancer, Horizontally with lettering on the face in the same orientation to each other…

So I read up what I could on prop balancing I decided to go at it from the hub rather than the tips in this application….I used bits of clay on the outside of the hubs to get an idea then used lead filings from a fishing weight packed into the appropriate void(s) on the backside of the hub then sealed with a drop of CA…. Some props required more lead than others…. One prop balanced particularly well so I use it as my baseline when running an engine to see how the others compare…. MUCH smoother that the way before …. On some of props it’s looking like they may need a combination of hub weight and tip adjustment….

Happy .020 Small Cox Logo

andrew Sat Apr 22, 2023 12:44 pm

Very innovative approach to balancing, Robert. I have shied away from bar balancers because of the difficulty in assuring the razor blades are parallel and level. Your use of glass to set the blade orientation is a great idea.

With such small props, any friction can nullify the effort. I've been using the magnetic style balancer for both Half-A and electric props, but your design is also excellent for the lightweight props. I would have never thought about using the cavities in the hub for weight pockets -- just keep an eye on the CA; I don't know how well it stands up to heat, vibration and oil.

a--

gkamysz Sun Apr 23, 2023 8:55 am

A big factor is the assembly on the engine, how square the prop ends up on the drive hub after you tighten it. I hadn't ever really given it much thought with Cox engines. While making some of the hubs and prop nuts I designed, I was looking at how they fit various components. Sometimes you tighten and it runs true, sometimes it's out by an alarming amount. It's almost impossible to see with a cylinder on the engine, I was doing the tests without one, just a crank and case. Didn't matter what I used for a screw or prop nut. APC props seemed better than others and I think's it's accuracy of the prop's hub.

Ken Cook Sun Apr 23, 2023 9:30 am

I true the faces of the hub with a shaft that fits the hole properly. Seeing I use a bushing for most of my APC hubs and others, the shaft is the diameter of my bushing hole which is close to 3mm. I have two circular steel discs which I knurled the perimeter of. The discs are 3/8" thick and are about 1 1/4" in diameter. I glue sandpaper to the face of the disc and slide the assembly onto the shaft. I have one on the face and one on the back of the hub and simultaneously twist them back and forth which squares the faces to the prop hole. Unfortunately, while this works very well, it doesn't square the hole to the prop itself only the hole to the faces of the hub. If the prop hole is out, by the time this telegraphs out to the tips of the prop, the prop might be balanced but it's really running out of concentricity. I've had many Grish black Tornado props which were so far out, the prop was not even useable. When the spinner hit the hub, you could visibly see the gap created where one side would touch the hub prior to tightening. For this reason, I don't care for mold lines or raised lettering which designates manufacturer or size in this area due to it not enabling the prop washer or spinner to sit effectively.

rsv1cox Sun Apr 23, 2023 9:39 am

Your getting down to the nitty gritty Robert. Quite important in the models that you fly and crafted for very light weight.

Being a backyard flyer I have never given props much thought and thinking back to the way I abused them gives me shivers. I would think the manufacturing process of nylon/plastic props would give them the edge over wooden props.

Love your balancer! More nitty gritty. Smile

aspeed Sun Apr 23, 2023 1:37 pm

I grabbed a couple handfuls of props to check for balance from my prop drawer. Every plastic one was balanced well. The fiberglass and wood ones needed balancing. I scrape the flashing on the edges before using and balancing. Hate flipping a prop and getting cut. I was really surprised that so many props were balanced from the factory. I used the magnetic one. The APC props say on the instructions to use the spacer ring on the back, as that will be square to the back of the blades and the face that sits on the drive hub. If I drill the hub like some Master Airscrew props need, I take a smooth flat file on the back of the hub to flatten any flashing first, and rest the prop on a machined ring that is about the diameter of the hub face and use a drill press. Push down real hard on the prop so it does not spin and be ready to let go if it does. The drilled hole should be square to the back face this way and pretty much centered. Revved a blue million RPMs like this over the years.

**944_Jim** Sun Apr 23, 2023 5:23 pm

RDW,
I love your solution. If I didn't have a magnetic one, I'd be making a clone of yours!

I appreciate the others' remarks. Gentlemen...thanks. This thread is bookmarked now.

sosam117 Sun Apr 23, 2023 8:01 pm

I had one of those manufactured "magnetic" prop balancers. (Top Flite Power Point Prop Balancer)

I check the rod and "cones" that tighten to the prop and that alone was out of balance.

I marked the cone part with a black mark and watched as the cone part rotated, without a prop and it was not towards the black Sharpie mark.
Tried to balance the rod/cone part that the prop slips onto.
After a while I gave up and gave it to a friend and I went back to my "old" Dubro prop balancer.
Downside of my Dubro balancer is that it really doesn't do small props very well.

GallopingGhostler Sun Apr 23, 2023 11:07 pm

A perfectly balanced prop will stop at any position without rotating backward or forward. Left and Right side symmetry will be in perfect balance no matter the angle. If it rotates back even though horizontal, the prop hole may be off center, or the prop is out of balance laterally.

I found the Pee Wee's ran best with a perfectly balanced prop. It is tedious to get there, but well worth it. I always thought of it as a well placed investment of time.

balogh Sun Apr 23, 2023 11:20 pm

I have the Top Flite magnetic balancer whose rod is too thick for 020 and 010 size props. I took the spoke of a bycicle weel and wound 2 layers of scotch tape around its center for the 010 props to fit snugly. The end of the scotch tape compromizes circular symmetry a bit but not enough to falsify the 010 prop balance. Turning the balanced prop a quarter on the scotch tape "bushing" will leave the balance unchanged..

010-s are even more sensitive to prop balance than 020-s and the 010 engine will be very grateful for a well balanced prop..

Levent Suberk Mon Apr 24, 2023 12:39 am

Magnetic prop balancers are not good, because pointed ends of prop shaft must be centered exactly on center of round magnet. If center of magnet is missed, then a balanced propeller is thought to be an unbalanced propeller! All of these related to magnetic flux.

rdw777 Mon Apr 24, 2023 7:16 am

Thank you Gentlemen for your ALL your replies, I was hoping to learn a little more and I’m not disappointed!!!

I had not paid much attention to the face of the hub and from your prompts thought I better look into it … I put together a PW .020 lower end based on gkamsyz’s suggestion, Tested a few of the props (Cox grey) for fore/aft tracking, And yep, the faces need some attention as well….They’re not terrible but I see room for improvement….The face truing tool tool that Ken mentioned makes good sense, I’m going to try and make one of those!!….I like the ring tool for center boring described by aspeed as well … Those are old school, Experienced suggestions, Thanks!!

I’m learning more about the balancer as I get better acquainted with it…. The levels on the base are a good starting point but the balancer itself will tell when it’s close to optimum….A prop that is close will want to slowly roll to one side or the other… I tweak the screws on the base then turn the whole thing 90 degrees and check it again… This is repeated until the near balanced prop will not move…. This is done setting on the sheet of glass….I suppose a balanced wheel of some sort might be good to calibrate the gadget?…. I may try and find one of those kids gyroscopes for that?

A lot of fuss for a seemingly simple task for a tiny engine but I like the tinker factor and the continued learning of our little gas fired friends….The result should be better efficiency and less wear and tear…. Will report back as this progresses…. Thank you again TEAM

Robert

**944_Jim** Mon Apr 24, 2023 6:43 pm

Sigh...I guess I'm looking for either a 50 Gigawatt flux capacitor to overwhelm the magnetic balancer's imperfections, or I'm building a razor blade balancer.

Oh, BTW, this is also how I balance motorcycle wheels (ABEC 7 bearing-supported on factory axle). But that balancer is almost knee high and too big to use on .010 props. lol!

706jim Mon Apr 24, 2023 8:48 pm

No question that a balanced prop is a good thing for any engine. However i thought I'd relate one of my Pee Wee stories just to show what you can get away with.
I crashed a Lil Stinker powered with a Pee Wee 0.020 and the end of the crankshaft broke off at the prop hub splines.
(What to do what to do.....)
So I disassembled my wounded Pee Wee and shortened the crankcase with a file. Then took the crankshaft and ground a crude taper on the end where the spline broke off. Then used a countersink to put another crude taper into the prop driver, all with just a hand drill motor. Screwed the prop on and hoped for the best.
I installed this on a Berkeley "Baby Yank" free flight and headed off to a farmer's field for a test flight.
Started my crudely repaired Pee Wee and launched it with a full tank of fuel not concerned in the least as the engine sounded like it was going to quit.

And THAT'S the one that flew away never to be seen again 51 years ago!

getback Tue Apr 25, 2023 6:21 am

From what i have read and seen from ads. for balancers to people /forums ect. everybody has a different way to do it . I have not that i recall had much of a problem with this issue and it sounds like a real PIA of coarse the commercials will prove that wrong Laughing

May \be some day but not today seems facing /drill the hub just to gat started on the tips if needed would take ME a good time and frustration... I like your Idea Robert and alot of good advice and story's too from the gang Huh...

.

Surfer_kris Tue Apr 25, 2023 10:56 am

Levent Suberk wrote:Magnetic prop balancers are not good, because pointed ends of prop shaft must be centered exactly on center of round magnet. If center of magnet is missed, then a balanced propeller is thought to be an unbalanced propeller! All of these related to magnetic flux.

I have no idea what you want to say there, but the axis (made from a soft ferromagnet) will self-center as it magnetises.

Levent Suberk Tue Apr 25, 2023 11:52 am

Surfer_kris wrote:
Levent Suberk wrote:Magnetic prop balancers are not good, because pointed ends of prop shaft must be centered exactly on center of round magnet. If center of magnet is missed, then a balanced propeller is thought to be an unbalanced propeller! All of these related to magnetic flux.

I have no idea what you want to say there, but the axis (made from a soft ferromagnet) will self-center as it magnetises.

Pointed tip of propeller shaft must be on center of disc magnet. If pointed tip of shaft is off center of disc magnet then propeller swing to left or right. I tried this with a propeller balancer that used neodymium disc magnets.

Levent Suberk Tue Apr 25, 2023 12:07 pm

I meant this. Sorry for sketching.

andrew Tue Apr 25, 2023 12:25 pm

rdw777 wrote:..............
A lot of fuss for a seemingly simple task for a tiny engine but I like the tinker factor and the continued learning of our little gas fired friends….The result should be better efficiency and less wear and tear….
Robert

And that, Robert, is the driving force for many of us involved in this hobby. Very Happy

GallopingGhostler Tue Apr 25, 2023 1:17 pm

I've always thought of balancing props as more of an art than a science. Razz

From Mike Reed's no longer available blog, Flame Warriors.

My balancer is similar to Robert's with various sized steel dowel shafts to fit prop holes teetering on two razor blade edges mounted to an aluminum bracket to a wood base. But it is not elevated to allow a prop to teeter to vertical. Compact, got it in 1972, still works like intended.

Just saying, day after community band concert. Stereo

Levent Suberk Tue Apr 25, 2023 2:15 pm

Austin Craft propeller balancer:

Surfer_kris Tue Apr 25, 2023 2:25 pm

Levent Suberk wrote:I meant this. Sorry for sketching.

I still do not understand what you are after there. The only requirement is that the axis is straight and that the prop is geometrically centred on that axis. Then if the centre of mass aligns with the prop axis, the propeller will not rotate. If the centre of mass is off from the axis then the whole system will rotate towards the centre of mass being below the prop axis.

The prop axis should also be made from a soft ferromagnet, that way there is no preferential rotation of the prop axis relative to the magnetic field. Each end of the prop axis will self-align to be in the part with the strongest magnetic field.

It is easy to test the axis on its own, or one can also rotate the prop (e.g. 90° or 180°) into different positions on the axis and check the balance again.

Surfer_kris Tue Apr 25, 2023 2:32 pm

Here is my set-up for small props:

GallopingGhostler Tue Apr 25, 2023 2:47 pm

Levent, that is similar to the one I have, maybe the same manufacturer, but is on a different wood base. I may have just bought the balancer itself and mounted it on a scrap piece of wood (51 years ago, hard to remember details now). But, has same number of steel dowels.

It was very simple, but good for balancing half-A props. Regarding comments on the magnet bearing balancers, should not be a problem with the half-A props because they are lightweight. Even with heavier props, still shouldn't be much of a problem, because of their heavier weight, balancing is proportional to weight, so there is greater margin for measuring error, that would not be good with the half-A's. They (unless the racing version engines using racing props) don't have the higher RPM's the half-A's do.

One can tell whether a prop is well balanced or not. The small Bee's complain. Surprised

Being picky on balance has immeasurable benefits. Very Happy

Levent Suberk Tue Apr 25, 2023 2:54 pm

Surfer_kris wrote:
Levent Suberk wrote:I meant this. Sorry for sketching.
Each end of the prop axis will self-align to be in the part with the strongest magnetic field.

Not every time will self align. If the tip of shaft that touch to magnet is mistakenly off centered -such as placed to right of magnetic disc center or placed to left of magnetic disc center- then one tip of propeller goes to down. I tried it. Prop shaft must be exactly on center of disc magnet.

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