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Building Tips and technical articles. => 1/2 A building. => Topic started by: Tim Wescott on May 18, 2010, 10:20:19 AM
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Last week I talked the LHS owner into buying some APC props in a variety of sizes suitable for 1/2 A -- 6x2, 5.7x3, and 5.5x2.5.
They're c**p! Or at least the mounting holes aren't at all suitable -- they're all too big, and in the case of the 6x2 they're counter bored such that the surface for the hole is only about 70 mils high.
So I have three thoughts:
1: Don't buy APC props in these sizes any more.
2: Hand make adapters to make these work
3: Ask where to get decent props for 1/2 A!!
For thought 2, it looks like both the 6x2 and 5.5x2.5 props have 6mm holes that go at least part way through, without much strength to be lost if I just finish drilling the hole and then ream to 1/4". Then I just need to make a bushing that's .250 on the outside and .125 on the inside (this is for Cox engines). If I had a 6mm reamer I'd just use that, with a matching bushing. (I kind of need to get a 3mm reamer anyway, to match my lone Norvel engine. Granted, a 3mm screw in a 1/8" hole doesn't give the prop much opportunity for misalignment, but who wants small when they can have none?)
The 5.7 x 3 props use the "regular" APC bushing, which looks like it's 7.5mm stepped down to 6mm on the outside; I could either use that or I could just use the bushings I make for the above.
What a PITA -- and I've left my LHS owner sitting on some useless props.
For thought 3: Recommendations? I see that Cox International has props, but I assume that those aren't optimized for performance.
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What really irks me is that the ID ov the props is different for each size! I have a lathe, and can make whatever I need, but having to have a separate adapter for each engine/prop combo is the pits. Norvel 3mm, AP Wasp 4mm, Cox 5x40. What a pain! And why the big holes in the first place?
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What really irks me is that the ID ov the props is different for each size! I have a lathe, and can make whatever I need, but having to have a separate adapter for each engine/prop combo is the pits. Norvel 3mm, AP Wasp 4mm, Cox 5x40. What a pain! And why the big holes in the first place?
Who knows? The reason that I was thinking to ream out the holes in the props is to make them all the same size -- then I only have to have one bushing per engine.
I've already way overspent the modeling budget this month, so buying reamers is just going to have to wait until next month -- but I think I'm going to make some adapters with what I have, assuming I can modify the props without creating damage.
I take it you have no guidance for me on the issue of where to buy props that actually fit? I'd settle for props with 3mm holes that I need to ream out -- heck, I'd settle for props with no holes at all, and indeed another thought that I had was to fill the holes with JB weld and drill my own.
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Ha! Ha ha! Bwa ha ha ha ha!
"One bush to rule them all one bush to find them.
One bush to bring them all and in the darkness bind them!!!!!"
Item: Bushing.
Application: Fixing APC's screwups.
Material: Metal*
Tolerance: Yes**
Item to consist of a cylinder 0.230 inches long, with a hole reamed*** to fit match the mounting screw. Item to be turned to a diameter of 0.1875**** along 0.175 inches of it's length, with the remaining length forming a 0.242 inch diameter shoulder.
Sorry, no drawing -- they're for sissies.
This bushing works in all three props -- directly in the 6x2 and the 5.5x2.5, and in conjunction with the smaller of the two plastic bushings on the 5.7x3. Mount it from the back on the first two props, and from the front on the last one.
You may find that I've specified it to be a bit long -- mine is .235" long, which is .005" longer than the 5.7x3 prop is thick. It only works on the Cox engine because the center of the prop driver is relieved for the crank shaft. Subsequent ones will be built to the dimensions I'm giving above, or maybe a bit shorter yet.
* I.e., whatever you have lying around. Mine is made of aluminum, but I don't know if it's 6061, 2024, or some other hard, easy-to-machine aluminum. Brass would work just fine, and be even easier to machine. Don't use an old 1/4" bolt unless you're really good at putting a fine finish on crappy steel, and don't much care for your reamer.
** I.e., according to your pride. The propeller holes aren't all that precise, I aimed for 'snug' and was fortunate that I chose the prop whose hole was a hair bigger than the smallest-holed one, 'cause it'll push into that one and still be righty-tighty.
*** You really want to ream it, but if you're too cheap to send out an order to McMaster for reamers, you could get away with drilling it. Reaming it is just plain nicer -- drilling leaves it with noticeably more slop on the screw. In fact, I have a slightly undersized 1/8" reamer designed for sizing holes to press-fit pins; I think I'll see how my 5-40 screws fit in that.
**** Well, mine measures 0.186 and fits just fine, but I have cruddy calipers.
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Someone makes Cox spinners that have a nub to bush the prop.
Others have used a piece of fuel line or surgical tubing as a bushing. Make it long enough it gets good compression.
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I've been using 5.5X4 on a Testors 049 I use a piece of med fuel tubing as a spacer and it works fine but yeah I have no idea why the hole is so wide, boggles the mind.
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Tim, I contacted the guy who makes the aluminum cnc Tee Dee bodies on Ebay. I mentioned to him about the same problems you just mentioned. I was asking for a machined bushing. He wanted me to mic the parts, but I figured I would just mail him the props and the screw. I have this problem with APC props for the Norvel .15 in the 7x4 range. It was explained to me the reasoning for the different size holes is the way the prop is molded. The material enters the mold cavity through the prop hole. When smaller holes were used imperfections were created due to the material not being able to get in correctly. The fellow I contacted said he would make me anything I wanted and would even see if enough interest is being generated to make and sell them. You really can't beat the under $2 price tag on the 5.7x3, 5.5x2.5, 6x2 sizes. The others like 5x3 and 4.75x4 tend to add up in the $4 range especially when they break as easy as they do. Ken
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Just slip a short length of large fuel line on the shaft, then the prop. As you tighten it down the fuel line squashes and expands, self-centering the prop. Takes a little trial and error to get the right length but it's not critical. Works for me. For Cox & others with prop bolts, put the bolt thru the prop, then the chunk of fuel line, then thread it into the shaft.
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Tim, I contacted the guy who makes the aluminum cnc Tee Dee bodies on Ebay. I mentioned to him about the same problems you just mentioned. I was asking for a machined bushing. He wanted me to mic the parts, but I figured I would just mail him the props and the screw. I have this problem with APC props for the Norvel .15 in the 7x4 range. It was explained to me the reasoning for the different size holes is the way the prop is molded. The material enters the mold cavity through the prop hole. When smaller holes were used imperfections were created due to the material not being able to get in correctly. The fellow I contacted said he would make me anything I wanted and would even see if enough interest is being generated to make and sell them. You really can't beat the under $2 price tag on the 5.7x3, 5.5x2.5, 6x2 sizes. The others like 5x3 and 4.75x4 tend to add up in the $4 range especially when they break as easy as they do. Ken
Well, you're welcome to my "design". I don't know if it's what you'd want to use for bigger engines, but it certainly seems to be the ticket for the 049s that I have.
I'm going to whack out a few more for myself and my flying buddies, but it'd be nice to know that they're out there.
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APC props work great on my 1/2a fleet. I use the yellow fuel tubing for weedwackers and such for bushings available at your local hardware store cheap, it fits the id of the props perfect and will work on norvel and cox prop bolts. Also the cox 5x3 flexible props from the Cox store for 3 bucks apiece work great and last a long long time!
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APC does sell a set of metric adapter bushings. Part no. APCMPA1. They might help solve some of your issues.
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APC does sell a set of metric adapter bushings. Part no. APCMPA1. They might help solve some of your issues.
I couldn't find them on their web site.
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Yea, they only seem to list the props only. Just Google the part number. A bunch of on-line hobby shops carry them.
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Yea, they only seem to list the props only. Just Google the part number. A bunch of on-line hobby shops carry them.
Well, I've got the lathe all set up to churn them out, and I have four of them, and they're pretty (it is soooo easy to put a nice finish on aluminum), so I think I'll keep them. But I'll remember that it's out there.