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APC prop balancing (for XS airplanes of course..)

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APC prop balancing (for XS airplanes of course..)

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Old 10-12-2005, 02:21 PM
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MJD
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Default APC prop balancing (for XS airplanes of course..)

Interesting - last night I was sitting at the bench with a "refreshment", and had decideed it was time to balance a few new props. I was working on a variety of 8" - 9" APC props for my Magnum and Whiplash aircraft (hence the relevance to this forum, didn't see a "propellor" forum but engine-heads know best anyways right?). I found several that stubbornly refused to balance without hub sanding (arghh don't say that!), as it was obvious the blades were in balance but the props were out of balance quite a bit in the other axis.. so I eyeballed a few and noticed several that with the naked eye seemed to have the hole off center by a few thou. So I got out the vernier caliper and measured from the hole ID to the hub OD in a few locations around the hub, and indeed I measured as much as 0.015" eccentricity. Lo and behold, in each case the long-side measurements agreed with the location of the heavy side of the prop. This indicates to me that there is some inaccuracy in the fixturing for post-drilling the prop holes.

Since the molded-in recesses in the front and rear of the hub should be bang-on due to CNC machining of the molds, I am left wondering why the holes could not be more accurate. The holesa re visibly off-center realtive to the molded recesses. The eccentricity seems a little excessive.

Anyone else notice this? Or am I whining about something that everyone knew about already.

So hmm, how do I add weight to the light side to bring the prop into balance. Or do I sift through them and reject the ones that are way off? That doesn't seem right.

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Old 10-12-2005, 03:44 PM
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Default RE: APC prop balancing (for XS airplanes of course..)

MJD,

Interesting. I have noticed this (imbalance) too, but never explored further as you have. Sanding the heavy side is OK. AMA competition rules allow it, and you'll only need to take off about 1/16" at most. There is plenty of material left, and plenty of strength -- never had a failure with the APCs. But stay out of the prop arc anyway.

Have you noticed that the more "refreshments" you drink, the harder it is to get them to balance?
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Old 10-12-2005, 04:45 PM
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Default RE: APC prop balancing (for XS airplanes of course..)

machine a bushing to fit in the hub recess and ream the hole way oversize.

doesn't someone sell those? perhaps I need to make a few..
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Old 10-13-2005, 08:48 AM
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Default RE: APC prop balancing (for XS airplanes of course..)

I am going to do just that actually. Good lunchtime project in the machine shop. However, it is a bandaid fix for a bit of sloppiness at the manufacturer's end and thus bothers me that it is needed.

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Old 10-13-2005, 08:51 AM
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Default RE: APC prop balancing (for XS airplanes of course..)

ORIGINAL: DHG

MJD,

Interesting. I have noticed this (imbalance) too, but never explored further as you have. Sanding the heavy side is OK. AMA competition rules allow it, and you'll only need to take off about 1/16" at most. There is plenty of material left, and plenty of strength -- never had a failure with the APCs. But stay out of the prop arc anyway.

Have you noticed that the more "refreshments" you drink, the harder it is to get them to balance?
Not falling off my stool usually becomes the bigger problem...[sm=greedy.gif]
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Old 10-13-2005, 09:02 AM
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Default RE: APC prop balancing (for XS airplanes of course..)

Yeah, Ive seen the holes off a bit too.

With the racing props, several folks do what Swiss noted there... the counterbore in the back of the prop is properly tooled - and is on center. The shaft hole is not always exactly on center (2 piece tool or for some reason?) Who knows. But if you noticed it after consumption of refreshments, it is worth fixing

Anyway, the bushings work well. I think you can get prop bushings from Jett or from www.darrolcady.com just make sure to get the 1/4" dia bushings - the ones for the Nelson are 5/16".

Or you can make them yourself.
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Old 10-13-2005, 01:26 PM
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Default RE: APC prop balancing (for XS airplanes of course..)

I checked one out under a stereo microscope in the lab just now, and from what I can see those holes are drilled after molding. There are small turnings attached to the rim of the prop shaft hole, and I also see evidence of cutting tool contact off center to the hole.

If this is the case, I would say their fixturing needs some revision. One hole measured 0.015" off center, which is plainly visible to the naked eye. A decent holding fixture for drilling should be able to do much better than that. We drill several hundred molded parts here a day with better accuracy than that with little effort.

I can spin off a few bushings here in the machine shop, no sweat. Shouldn't have to though.

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Old 10-13-2005, 01:46 PM
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Default RE: APC prop balancing (for XS airplanes of course..)

p.s. I just wrote an e-mail note to APC regarding my observations, so will post any replies of interest on the subject.
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Old 10-13-2005, 02:25 PM
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Default RE: APC prop balancing (for XS airplanes of course..)

I've seen them off here and there for some time. I can almost gaurantee that they will replace them if you bring it to their attention.
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Old 10-13-2005, 02:31 PM
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Default RE: APC prop balancing (for XS airplanes of course..)

I'm sure they would, but that's a hassle for both parties. I live in Canada, so I would have to send them down via international mail, they would have to ship back, it all costs time and money. I work for a hobby product manufacturer (not RC) and if we had to start replacing products for any reason other than a very occasional random defect or material flaw, we'd take a hard look at our production methods to reduce warranty problems. It is not a tough problem to design tooling or fixturing that would center the holes properly. Anyow, I'll see what they say about it when they have a chance to write back. In the meantime I will eyeball props at the store a lot more carefully. That way you get 'em, not me...
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Old 10-13-2005, 04:51 PM
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Default RE: APC prop balancing (for XS airplanes of course..)

I agree MJD, I know Fred at APC pretty good, and I believe the problem has been fixed. Have those props been hanging on the hobby shop wall for some time, or are they new stock?

Personally, I've been reaming them out with a tapered reamer and using the sleeve over the Nelson Shaft to align with the molded portion on the back of the prop.

When you check the hole, flip the prop over and look at it at several different angles. The sunken hole in the front can cause shadow issues that make the offset look worse than it is.
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Old 10-13-2005, 06:54 PM
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Default RE: APC prop balancing (for XS airplanes of course..)

I suspect the main reason for the hole position error is the drill "walking" from wear. I have machined a lot of glass reinforced plastics and carbon reinforced PEEK and they both chew tools up like mad, even PCD diamond coated tooling. In short order tools look like the remote after fido took his wrath out on it. just fubar. dull chipped to snot drills tend to walk like mad.

they need to use a waterjet or a laser [laser may be problematic] to cut that hole, no tool wear problems .

thats my .002

JD
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Old 10-14-2005, 08:40 AM
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Default RE: APC prop balancing (for XS airplanes of course..)

I got an informative reply from Fred Burgdorf at APC this morning. As I suspected due to mold design issues they are forced to post-drill the prop shaft hole, and he indicated that it is a source of headaches at their end. He also said that he would replace any props that were obviously out of whack. I can't ask for more than that.

As to tool wear, yes I know all about abrasive materials and tool wear and those headaches! We machine silica-filled (read, full of powdered sand) phenolic compounds on a daily basis here, and they are total pigs on cutting tools. Solid carbide only, with routine regrinding. The glass fibers in the APC prop nylon are extremely abrasive as well and eat tools for lunch. Solid carbide tools will last a fair amount of time, but all tools will wear in that environment in production volume applications. I am sure APC knows all about that.

As for me, as mentioned I am going to machine a few bushings for the various recess sizes and shaft sizes I have, and be done with the problem. I can then center the prop with the cone of the balancer in the bushing, and drive the flat side on the other side of the prop to ensure centering equivalent to what I would get on the engine shaft.

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Old 10-14-2005, 10:59 AM
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Default RE: APC prop balancing (for XS airplanes of course..)

MJD - doesn't APC make prop hub spacers for different size crankshafts? Did Fred mention anything about that?
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Old 10-14-2005, 12:03 PM
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Default RE: APC prop balancing (for XS airplanes of course..)

No he didn't. I know I get bushings with my electric props, but I am not aware of anything for glow engine props. I'm not too worried about making bushings, we have a fully equipped machine shop I can use anytime I want.

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