A&P Mechanic

Hi All, I'm back!
How are you guys finding ways to mount the (only props available) APC props to an .020 Pee Wee engine?

I'm finishing a little project, and found that only APC has a prop for these small engines, now. It was very disheartening to see that they bored a 1/4" hole in the hub. Don't they realize that a Cox engine uses a small screw to attach the prop?

I'm trying to machine a spacer/reducer/bullet nose adapter for the thing. Any faster ideas from you experts?

Your comments will be fun to see. Thanks!

skaliwag

My Feedback: (1)

A piece of fuel tubing will work.
I have the Cox 4 X 2.5 props. PM or Email me.

Tim Wiltse-RCU

A&P,

The APC should have come with a set of spacers. These plastic spacers take up some of the slack. To take up the rest you can either use a piece of fuel line like Skaliwag said or like myself make up a small aluminum spacer.

LAter,
Tim

combatpigg

My Feedback: (3)

I would rather have them come with no hole.....1/4" inch hole is for a .40.

lukesp

Has anyone told APC how dumb this is ? Why would they put such a large hole on such a small prop ? [sm=50_50.gif][sm=confused.gif]

Andrew

The hole size is apparently tied to the manufacturing process -- since APC uses long fiber fiberglass rather than chopped glass and attempts to maintain fiber orientation along the blade axis, hole sizes that are close to the prop screw sizes that are used in 1/2A engines evidently are not easily produced.

I would suspect that if it were feasible from a manufacturing standpoint, they would do it. APC is pretty advanced WRT to prop aerodynamics and are well aware of 1/2A and small electric needs.

A&P Mechanic

Looks like you HAVE come up against this before. Sounds like I'm in the same boat. Let's keep trying. Thanks, Gus.

hogflyer

Andrew,

It would seem to me that using long strand glass fibers, the smaller the hole diameter, the better would be to allow the fibers to run straighter with respect to a straight line from tip to tip. Don't the electrics use a prop adaper that is much larger than the mounting screws of a .020 or .049? It could be that the designer(s)/engineer(s) of these props may not be familiar with the requirements of small Cox engines.

Hogflyer

Andrew

Hi HF --

I would think that prop design would be tied closely to engine performance -- given that COX and NORVEL users would be the primary buyers of 6" or less props (APC supported 1/2A sizes before the electrics became so popular), it would seem that they should be intimately familiar with the engines.

APC is injection molded. If the injection point is in the hub, there may need to be a large enough hole to allow the fibers to pass thru and maintain orientation. I had read an explanation some time ago regarding fiber length WRT to hole size, but do not remember the particulars.

I've emailed APC asking for their reason -- if I get a response, I'll post it here.

andrew

A&P Mechanic

Well cool, good idea to ask APC.
Also, for some reason, I don't know what WRT means. Oh, it just hit me-"with regard to". I was beginning to think it was some technical process used in the manufacture of props, but I guess not.

balsaworks

The Graupner Super Nylon 5� x 2� is a great alternative to the smallest Cox props.
Hobby Lobby seems to have them;

http://www.hobby-lobby.com/grprop.htm#2%20Blade

My props are a few years old now, but assuming they have haven´t been modified since the time I bought them, the centre hole is small enough to fit even the TD .010
The blades are narrow as well as thin and easy to cut down with scissors to suit your needs

They are stiffer than the Cox black type but don´t brake quite as easily as the Cox grey

I have used these on Cox engines & on some of my micro diesels that need prop sizes somewhere in between the Cox .010 & .020 – the small centre hole and the thin & stiff blades make them very easy to modify

MJD

My Feedback: (1)

That may be so, but they admitted to me directly on the phone that their drilling process is inaccurate - when I called to ask why their prop holes are often so grossly off center. Rifle through a pile of them at your LHS sometime. That is why it is crucial to use the molded recess on the back of the prop to index the propeller, rather than the through-drilled hole. In other words, don't fit a bushing to the through-hole, use one of the supplied inserts in the back of the prop hub, then shim that down to the required diameter. This is always important, but especially on small high revving engines.

Truthfully, I don't recall if the monster hole on the 4.2" props is molded or drilled out after molding, but beyond that I stick to my guns on the above comment. If it is molded on those particular props, well then, naturally it will work fine to bush down that hole as it will be accurately centered. UNLIKE the through-holes on most of their props..

Being a stickler for prop balance and centering never hurts!

MJD

Andrew

Hello MJD --

I believe the props are molded with a pilot hole, then drilled in a second step -- at least for the larger props. If they are not jigged during drilling and rely only on the pilot hole for centering then, as you say, it would be inaccurate.

The question I still have is why such a large hole? If the recess is accurate, a drill guide would be simple to turn and the through-hole could be drilled accurately and smaller.

Maybe I'll get an explanation in response to my email.

regards,

andrew

MJD

My Feedback: (1)

This is what has me puzzled too. I measured a few props and the diameter of the recesses varies somewhat. If those were kept fairly consistent during the mold design process, as in perhaps only a handful of sizes over the range of props, then making suitable fixtures to ensure accurate drilling would not be difficult at all. Stub length carbide drills in a mill and a decent fixture would drill with great accuracy and repeatabilty, and be little affected by the abrasiveness of the glass fillers. We do "stuff like that" on a daily basis here. So my only guess, without being overly negative towards APC, is that they simply have not expended the effort or money to set themselves up to drill accurately. I will be interested to hear what they say in response to your email.

MJD

peterburford

I injection mould my propellers with long-strand 'glass reinforcement.
The injection sprue is at the hub centre, so as to mould without any weakening weld line and uneven flow that occurs when the injection gate is at the side and the material must flow around a central core pin.
Of course, a core pin can produce the hole for free. Most props are moulded with a side, parting line gate. This side gate also easily permits multi-cavity moulds that make more than one at a time.
The 90 degrees from injection sprue to blades must have a smooth, as large as possible, radius so as to align the reinforcement and not shatter the 'glass during the high speed, high pressure injection process. Otherwise you just get glass-powder fill!
This whole scene means that the injection is around the inside of a large (For an .020.) centre hole that is then accurately punched out. (Not drilled.)
There is a recess in the hub that locates the propeller on the engine drive washer.
I have pics on my site, wwwpeterburford.com.au One shows a translucent moulding where the reinforcement is visible.

APC must have a reason for what they do. It would not be through ignorance, but for production economy.

Peter

andrew b

Try telling them that making a hole smaller is easier and cheaper!

If some one / a group doesn't buy stuff from an individual / company then whoever is selling the stuff should be made aware of the reasons why no-one is buying it. I would like to think that most company's that produce stuff would like to sell a high quality product, that is to say a product that they believe in and that thier product is the best availabe in the market!

Fixing stuff up to work with bits of fuel pipe is OK, but why do we have to? we are an individual market and that manufacturers needrecognise that they HAVE to produce a product that WE would like to buy, not mess with sheet that we have "make do" with stuff that doesn't fit, time to show them that the dollar makes the goods, not the goods "bend" the dollar.

MJD

My Feedback: (1)

p.s. yes, that is exactly how they are made, forgot to confirm that. The problem lies in inaccuracy of the fixturing and drilling process. My problem lies in the fact that doing so accurately is not a big deal. However, perhaps it is not cost effective to do it differently, but I'd rather have a $4 prop that has an accurate center hole than a $3 prop that does not. Besides, why put all the effort into the prop design and manufacture then bugger it up in the last manufacturing step? Makes no sense to me.

MJD

peterburford

We want the quality and just maybe we could possibly be prepared to pay a little more (Of course, providing it is only a 'little'.) to receive it.
I am sure that APC know how to make it right.

But the way the system works, the end customer doesn't get to choose what is on the shelf.
Just look at the products you are offered and how the promotion and packaging offers them.

I remember enthusiastically describing the virtues of one of our engines, only to be stopped by our European importer.
He patiently educated me.
Unless my price met that of the lowest offered, he would not import them, because his distributors would not buy them, because their retailers would not buy them, because their customers would not buy them.
The quality was certainly a contract between me, as manufacturer, and you as the user, but unless the price was low, the product would never get to market.

I really don't know what we few can do to change way of the majority in the world.

Peter

Schu

My Feedback: (5)

Peter.
Sadly as a home builder I have come to the same conclusion- almost everybody thinks only of price and then expects only perfection of the end result.
If one builds the best w/ the best materials all you hear is : ya thats great, but your 30,000 more then the other guy- they don't get it.
My 2 cents

Andrew

I received a reply from APC yesterday WRT the hole size. I'm including my original email and their response.

************
Hello --

Can you give me a quick explanation why APC props in 1/2A sizes have a large hub hole. I'm inclined to think it's tied to the long fiber injection process, but really would like to have a verified reason.

Thank you,

andrew batts
************
In a message dated 9/30/2006 8:59:59 P.M. Pacific Standard Time, [email protected] writes:
.....to the long fiber injection process

That's exactly right.

Regards,

Fred Burgdorf
************

Well, it's pretty cryptic, but I suppose their manufacturing technique (production and cost containment -- thanks to Peter for insight from a manufacturer's point of view) dictates the hole size. Given the size of the 1/2A market, it may not be feasible to add .50 or a 1.00 to the cost.

MJD

My Feedback: (1)

Well, a simple solution to APC prop hole woes with .020's would seem to be a nicely turned lightweight bushing that pops in the molded recess in the rear of the hub and that is a close fit to a 3-48 screw.

One can look at the reduced rotating mass as a benefit, if the hub is strong enough and there is no reason to believe it is not. This of course is only true if the bushing is lightweight and does not replace the mass missing in the large hole.

A while back I measured up a bunch of pylon props and sport props in the 9-10" range, with the intention of machining a few precision bushings to fit my various .50's. Hmm.. found out that even those vary all over the place (between props with otherwise identical hubs, so tell me how much precision is used in the mold design or construction), so I ended up with a lot longer of a list of OD's than I had counted on. Since I wanted these to be a size-on-size fit to the hub I am not content with just making some that are close and fit all the props. Ever since I got really anal about prop balance and centering, lo and behold I sure have a lot fewer vibration and foaming problems, not to mention the engines appreciate the treatment and plus you don't needlessly waste energy. And nowhere am I more finicky than on 1/2A's and smaller.



MJD