Prop balancing problem

[Terry Caron]

I don't see a Prop Balancing sub-forum, so I'll put this here. 

I have 2 10-6 APCs that settle the blades at 9-3 such that the 3:00 blade can swing almost to 9:00 but still settle clockwise to 3:00.
Sanding blades to balance is easy enough but do these have hub problems?
Holes look concentric and centered to the eye.
$6 is $6 and I have lots more time than $$. 
What to do?

Thanks.

Terry

[jim gilmore]

my guess is that there is too much weight on 1 side of the hub. It may not actually be at the hub but could be through out the blades towards the side they settle to.      Not real sure if there is a problem with a prop like that. Hope somebody whom knows more answers.

[Steve Helmick]

APC molds the prop with the counterbore, but no hole (which makes the counterbore into a tooling dimple, I guess). They have a guy drill the holes with a fixture of some sort. And that's the source of the problem. It should be possible to do a search for correcting the hole location and read how it's done. I've not done it, and probably never will. An end-mill or router cutter should be used, IMO, because a drill will always follow the existing hole, even through a drill bushing. After the hole location is correct, then a bushing is made to correct size.  Steve   

[Terry Caron]

Hi Jim -
They do vibrate my bench mount noticeably.
I should have mentioned earlier, but assuming a hub prob, I carved away nearly half the hub backside on one with no change.

Steve -
I've read somewhere of redrilling/bushing APCs, seems like maybe a post by Randy Smith, but at the time it wasn't important enough to pay much attention.
I'll try the search and see what comes up.

Thanks guys!

Terry

[john e. holliday]

I was told one time and it worked on wood blades, if the prop stops every time with same side of the blade down, loosen it and rotate 180 degrees.  If vibration is just as bad or worse discard it. 

[Bill Johnson]

APC has technical article on balancing and mounting:

https://www.apcprop.com/Articles.asp?ID=259#balance

That said, I have the same issue as far as the prop balancing in the manner you described but haven't measured any significant issue with vibration, which I measured on the stand using an application on my cell phone.

In fact, I've noted the same result on a variety of wood props, wanting to settle at 3 and 9 but always with a certain blade oriented one way. I balanced 20+ blades yesterday and I bet almost 1/3 or more displayed that result. I've spun the blade 180 on the shaft, moved cones around but it doesn't change the result.

It certainly calls for more investigation. Having balanced many full size props, I've never seen this before.

One other thing, though. Static balance does not guarantee dynamic balance. After static balancing full size props, I balance them dynamically and they usually call for some correction. As I noted, I didn't see any issue with one of those props running so there's a possibility there's something else going on. Do you have the ability to measure blade angles?

[frank mccune]

    Hi Guys:

    What am I missing here? If the blade stops at 9 and 3, it must be balanced.  I can turn my props 180 degrees on the balancer and I still get the same result.  If they stop at 9 and 3, are they; not balanced?

                                                                                                                            Be well,
                                                                                                                           
                                                                                                                            Frank

[Douglas Ames]

APC's have been around a while.
There are a lot better propellers out there that don't have such a crude manufacturing process.
I don't care for them because of the mold seam around the blade edge, rounded hub face (needs rework/ finishing) and constant re-tightening.
They really need some kind of gasket washer to keep the prop nut tight.

[Allan Perret]

   Hi Guys:

    What am I missing here? If the blade stops at 9 and 3, it must be balanced.  I can turn my props 180 degrees on the balancer and I still get the same result.  If they stop at 9 and 3, are they; not balanced?

                                                                                                                            Be well,
                                                                                                                          
                                                                                                                            Frank

Not necessarily.  To say it is statically balanced any one blade would have to stop at all the clock positions, 3, 6, 9, and 12; in other words both horizontal and vertical.  Most props when the blades are initially balanced will stop horizontally but not vertically because the hub is offset.  The solution is exactly what the manufacturers tell you not to do.  
Spend a little more and buy a better prop, you will have less headaches.  I am liking Falcon and Xoar wood props right now.  Both have higher than average percentage of props good to go right out the bag..

[jim gilmore]

As I was trying say in the first answer I gave.
I'll be a little more involved here....
If you were to divide a propeller into 4 cuts from the mounting hole.    +  the blades divided in halve and then the hub cut in halve 1 blade each side of the hub. The prop is balanced with 1 blade on each side of the hub....But in the other direction which splits the blades  in half. the front of one blade and the back of the other blade are NOT in balance. wither this is because the hole is out of alignment or the blades are just heavier I cannot say. But you could refine the entire length of the heavy side to bring the propeller into balance. How much work might be involved I could not tell you.
But spinning the prop is the easiest way to see how out of balance it is....

[Terry Caron]

After a bit more checking, I find that, despite APC's claims, the molded holes in these two props are not necessarily centered in the hub.
Dial caliper measurements show the backside molding (only) to be ~.010" different at the 9:00/3:00 points (with the blades vertical).
And the thicker side turns out to be the one I carved on in the above pic.
I checked 2 other props that balance well and they had essentially no variation.

And Bill, I don't have a pitch gauge.

Terry

[Bill Johnson]

Great explanations, gentlemen!

Frank, what you are missing is that if you number the blades #1 and #2, #1 blade always stops at 9:00, even when I put in the 3:00 position, it swings around to 9. That's what's happening to Terry's props also.

I could use a good prop protractor too. This is the one I currently have but it's a little ungainly for use on a  10-12" prop. It's made for one in the 70" plus range.

[Terry Caron]

Hey Bill -

That looks exactly like the one I don't have!     

Terry

[Tim Wescott]

I have a very low-friction balancer, and a six-second rule: if the prop is still seeking one direction or another, but the overall period of oscillation is six seconds or longer, then I declare it balanced.

So far, I haven't shaken any planes apart.

If you have a big enough lathe and enough patience, the best way to center up one of the APC props would be to chuck it up in something that allows adjustment up and down and in and out, get the molded-in hole centered using a dial indicator, and then bore it out with a single-point cutter.  If you're that good, you can probably just arrange to work that much time at a machine shop and get enough money to buy several dozen props.

[Terry Caron]

Hi Tim -

Mine is a homebrew magnetic balancer, ajustable so only one spindle point touches a magnet, and the oscillation time is short, involving at most a 3/4 back-turn from 9:00 to 6:00 then return to static 3:00.
But a long time ago, I was that good on a lathe.
I just had to raise kids instead of buy props then.  

Terry

[Motorman]

Terry, The way you carved away the driving pad of that prop is very bad, I would not use that prop. If you have a prop that sits sideways you can try to punch the center out then use one of the bushings that comes with the prop. You can also remove material from the leading edge of one side and the trailing edge of the other side to some extent.

MM

[Terry Caron]

I've retired both those props MM.
"Safety Third" as the Nasa guys said.

Terry

[Darkstar1]

Try this,
It worked for me with just about any prop.

http://www.horizonhobby.com/content/resource-center-video-render-horizonhobby-channels

How to balance props. Scroll down page to Horizon Hobby Products. Click on How to's Air Click the button to the right
Until U find how to balance props. They changed the pages since I last bookmarked it.

[Terry Caron]

Good deal - found it, and the portion on hub balancing look like just the ticket.
I'll try it on the one I haven't carved up and see if I can save it.

Thanks, Darkstar.

Terry

[frank mccune]

     Hi DarkstarI

     Can you give me directions to enable me to get off of the ad for the electric plane.  I can not see any area called products to click on. Where is that found?

                                                                                                                                                 Thanks,


                                                                                                                                                 Frank

[Steve Helmick]

Terry, I kinda thought I recalled that you were a machinist (also). So, I thought I'd give you an idea. IF you can get or make a plug/drill bushing to fit the counterbore in the front side of the hub, a stubby & flat ended drill should do a nice job of correcting the hole location, or perhaps a Trident-Point sheet metal drill. An end-mill would be best, of course. It could still be "exciting", so be careful.

Lately, I've been reaming holes in CF props. A stepped carbide reamer in the drillpress makes it easy and mucho safer than a drill.  Steve

[Terry Caron]

As to that Steve, I can shorten/regrind a drill, turn a guide with the drillpress and files, rig up clamps to secure the prop (wish I had a drillpress vise) - may give that a try.
While hardly worth the effort for a couple props, I'll likely end up with others in the future.
Or I could go into the APC rebuild business for forum members. 

Terry

[john e. holliday]

Being a computer illiterate I couldn't find it.

[Terry Caron]

Frank & John -

On the left below the main video screen, "Horizon Hobby Products" is a dropdown box.
Hover over it and scroll down to "How-tos - Air" - click that and a series of video thumbnails comes up; scroll with the arrow on the right (like seeing all the pix on an eBay listing) to the 4th page; the "How to - Prop Balancing" video is in the middle.

Terry

[john e. holliday]

I want to thank you very much.  In all the years of balancing props never thought of the vertical balance.   I have a balancer similar to what is shown that I have used a lot.   I have learned a lot watching that video.  Again thanks.

[Steve Helmick]

Scroll down page to "Horizon Hobby Products". Click on "How to's Air". Click the "> button" to the right
Until U find "How To - Prop Balancing".

I cleaned up the instructions a little and it may help, Doc. It wasn't particularly easy to find, so don't feel bad about your confuser skillz. If anybody has a connection to Horizon, they might suggest that this sort of thing should be easier to find, like in a link from the "Resource Center" button (look across the top of the Home Page). Seems way more logical to put another button "How To" or something.

Anyway, I watched the whole video and found it worthwhile and I learned some new tricks. Thanks for posting the link, Darkstar1 !!!! Steve

[Dick Pacini]

[Bill Johnson]

They say a picture's worth a thousand words, so here's a graphical depiction of the issue:

[john e. holliday]

Still learning some thing new each day.  Thanks for the APC video.

[David Hoover]

Either I must be missing something here or I'm a complete numbskull.  In the video about balancing APC props I completely get the concept of balancing the prop in two planes.  But in that example, it seems to me that they've ended up with a prop that is nicely balanced about the rear counterbore but is completely out of whack with the hole which is what sits on the shaft it's mounted to, except for some of the electric motor adapters which have a larger diameter boss on the drive hub.  Moving the hole to be concentric with the accurately located counterbore on the back of the prop takes a significant piece of equipment and a fair modicum of skill, not something everyone has in their back pocket.  All of which raises the question, "Why the bleep did APC design their props in such a bass-ackward manner?

I look forward to my elucidation.

[Douglas Ames]

Great explanations, gentlemen!

<snip>I could use a good prop protractor too. This is the one I currently have but it's a little ungainly for use on a  10-12" prop. It's made for one in the 70" plus range.

We used to use them at Eastern Air Lines to check the throws of the spoilers on 727's and DC-9's. I'm sure they were of the `Connie vintage.
Very nice aircraft tool! At American we use pendulum protractors.

[jim gilmore]

Just a thought Not saying this will work 100%..
1. drill hole in propeller 1 size larger....
2. with a bolt same size as propeller shaft coated with Pam (spray lube)
stuff the hole in the propeller , with the bolt heavily coated with jb weld and large washer at each end and force the bold towards the direction tat needs to be closer to the hub edge to move the center of the hole where you want it.
let dry and remove bolt and clean . You should be able to readjust the hole where you want it like this...

[Target]

Either I must be missing something here or I'm a complete numbskull.  In the video about balancing APC props I completely get the concept of balancing the prop in two planes.  But in that example, it seems to me that they've ended up with a prop that is nicely balanced about the rear counterbore but is completely out of whack with the hole which is what sits on the shaft it's mounted to, except for some of the electric motor adapters which have a larger diameter boss on the drive hub.  Moving the hole to be concentric with the accurately located counterbore on the back of the prop takes a significant piece of equipment and a fair modicum of skill, not something everyone has in their back pocket.  All of which raises the question, "Why the bleep did APC design their props in such a bass-ackward manner?

I look forward to my elucidation.

Ohh, ohh, pick me, pick me! I can answer your question!!!
I don't even hardly fly powered planes, just started with C/L planes recently.

APC props are not the evil you think they are. What they are, is, able to be fit to a variety of shaft sizes. If you read the instructions, they tell you that if your shaft is smaller than the back counter bore bit bigger than the front hole, you drill out the front hole SMALLER than the back counter bore.
Essentially, you are doing this for clearance. The motor shaft doesn't need to touch the sides of the front hole at all. You then slip in your proper sized shaft adapter ring, and viola, a perfect (?) fit to a variety of shaft sizes (all the sizes of the adapter rings).

IF you happen to have a motor shaft larger in diameter than the back precision counter bore, then you must drill the prop out from the backside, using the counter bore as a pilot hole.
I don't relish doing this, but if I had to do it, I may be tempted to use the same size as the counter bore, drilling from the backside, then flip over and final drill to the bigger size.

In reality, I find that its rare that the counter bore is smaller than the shaft size used.

I hope this is understandable and helpful.

APC things did this way so that most people could just slip in a ring to the counter bore without doing any prop reaming at all, and also for their production needs. Not very evil, if you ask me, sort of clever, to effect one size fits many with minimum effort.

Regards,
Chris  

[Chris Wilson]

Still trying to get my head around the fact that a perfectly balanced prop is best on a perfectly imbalanced engine.

Anyone else see the irony in that?

[Terry Caron]

Not so ironic Chris, just preventing a concantenation of evils.
If we must have unbalanced engines, minimize other sources.

Terry

[jim gilmore]

I would think an electric motor would be closer to be being balanced from the start.

[Chris Wilson]

I would think an electric motor would be closer to be being balanced from the start.

I do get what you mean but having worked with high speed 12v electric engines for years I can assure you that the only reason that an electric engine is even remotely balanced is by a deliberate and expensive added factory action.

The point that I am trying to make is that with a single cylinder I.C. engine the 'balance' must be holistic - if a degree of discreet unbalance in the prop fosters an overall improvement then why not go with that?

Its a shame that we don't have a method that shows the overall drive train oscillation, of which the prop is only a part of.

[jim gilmore]

I fully understand that a single cylinder engine  would not neccessarily be a balanced movement.
I was just asking if an electric motor would by the nature of its shape be closer to being  balanced is all...
I realize that armatures can be balance and doing so is an expense in it self.

[Darkstar1]

The APC video and drawing hits it on the head. I've been doing this over the last few years now and occasionally with
the larger props the hub can sometimes be still a bit unbalanced.
When I balance a hub I use lead. I can glue on one thin strip most times its a bit more so I can file
to suit. The lead is the thin flashing that's used around roof vents. I got plenty when I had my roof replaced a few years ago. After that I can work the blades until they are perfect.

[Allan Perret]

Just a thought Not saying this will work 100%..
1. drill hole in propeller 1 size larger....
2. with a bolt same size as propeller shaft coated with Pam (spray lube)
stuff the hole in the propeller , with the bolt heavily coated with jb weld and large washer at each end and force the bold towards the direction tat needs to be closer to the hub edge to move the center of the hole where you want it.
let dry and remove bolt and clean . You should be able to readjust the hole where you want it like this...

MAS has a balancing/drilling jig that relocates a larger hole and then you can use a standard bushing for the engines shaft size.

[Terry Caron]

I haven't tried APC's method yet but it makes sense - flat face to flat face is how the prop/washer/nut is on an engine.
One problem I forsee is that the drawing they have of a washer in use doesn't show/caution that the washer hole should be as close a fit as possible to the shaft size, else the washer itself will cause an imbalance.

I'll report back on the attempt to save my clunkers.

Terry

[Lauri Malila]

MAS has a balancing/drilling jig that relocates a larger hole and then you can use a standard bushing for the engines shaft size.

Allan,

I have seen that device and I think it's quite stupid. A prop has to be statically balanced AND as symmetric around it's center axis as possible.
If you cannot find the asymmetries with a micrometer, vernier or other measuring devices,  and rectify them, it means that there are too big variations in material density. That kind of props should not be used.
I'm talking mainly about wooden props but I've seen quite hopeless carbon props, too.
When I make my props, I allways spend some time choosing the wood density & grain, and weight the blades after machining, before I join them together.

Lauri

[Allan Perret]

A prop has to be statically balanced AND as symmetric around it's center axis as possible.
Lauri

Ideally yes.  
But if the hole is 10~15 thou off from a balanced location, and you make a new hole displaced by the same amount to get it  balancec, is that really a significant amount of asymmetry ?

[Terry Caron]

I put the carved-up prop (shown in an earlier post) in my fixture as shown in the videos, flat to flat, and it will set at any point I place it.
I ran it to 10+k and it induced no appreciable vibration.
My thought is that I had luckily removed the proper weight to balance it.

I then mounted the 2nd prop in the same way and it showed it's previous hub imbalance.
I placed a small mount of clay (5.5 grains; 437.5 grains/oz) in the back cavity and it now also sets at any point.
While it might work just fine, a short test indicates clay maybe not completely fuel-proof; might migrate while running too.
Six #8 lead shot will replace it.

So, other aspects of the discussion aside, I'm happy to say that the procedure shown in the video works for me and these (previously) "junkers".

Terry

Addendum - only 3 #8 shot + superglue does it and it also runs w/o undue vibration.