ajcoholic

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OK guys, here it is finally after allmost a year. I have gotten around this weekend to putting two of my twins together. Hopefully this evening I will finish the sketching of the drawings and scan and post them here for you fellows. I will include all info you need to duplicate the engine, gears I use, bearings, etc.

Thanks for being patient and hopefully a number of you that expressed interest in this will get a chance to try one. They are quite neat!

More to come soon....

Andrew Coholic

Tim Wiltse-RCU

Cool stuff..look at those bad boys!!!!I'll be the first to step to the plate and get one together as soon as I finish my Cox DR .049(it's a disk rotor) project finished!


Later,
Tim

DICKEYBIRD

Cool! All it needs is a couple throttle sleeves!

michpittsman

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Hey guys-I remember the swap shop at Toledo in 1970(the year my eyes opened) where a gentleman had FOR SALE hand- built engines using Cox parts arranged in flat twin and v-twin configurations built on stock Cox crankcases! As I recall, the price on these jewels was out of my price range, being in the $25 to $30 range! A week's pay for a little motor? Outrageous! Anybody with a CNC machine should be able to knock these out on his lunch hour. JIM

jboy381-RCU

great deal aj...
looks like fun to give it a try...what do you think about a 5 engine setup???

john

ajcoholic

My Feedback: (10)

5 engines shouldnt bee any different, just arrange them on a larger backplate. But you will need to use the product backplates as the tanked backplates would not work too well!

OK here is what I did. I made some sketches of the parts and the required dimensions. As I post them I will also try and write whatever info is needed. But hopefully the scanned images are OK. If someone that wants them cant make them out here, I can also email you the JPG's but they are big files as full pages. Here i have them sized down to 40%.

Away we go!

ajcoholic

My Feedback: (10)

The main part is the central aluminum piece, that I call the prop shaft carrier. It serves a few functions, it holds the prop shaft bearings and in turn the prop shaft, it "clamps" onto the crankcase noses and rigidly holds the prop shaft (and gear) in respect to the other gears that are attached to the crankshaft of each engine. It is VERY important to get the spacing of the three holes correct (0.545" from center to center) in order to achieve the correct gear mesh with the Robonson Racing Product 48pitch gears (23 tooth on the engines and 28 tooth on the shaft).

The two holes that slide onto the engine crancases must be a light sliding fit (maybe 2 or 3 thou maximum clearance) and the two 5-40 screws are used to tighten the carrier to firmly clamp onto the cases. There is a little bit of fore-aft play that you can use to allign the gears so that they run in the same plane. Be carefull not to OVER tighten these screws, as they can cause the crankshafts to get snug and excess friction will result.

ajcoholic

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The prop shaft is made from 1/4 steel rod as shown on the plans. It is drilled/tapped on the end opposite the threads, to accept a 4-40 screw and washer, that holds the shaft from slipping through the bearings! The prop shaft gear is modified as shown (basically drilled through wityh a 1/4" bit) and slid onto the prop shaft, then silver brazed onto the shaft. Make sure to use as little brazing as necessary and not get any into the gear teeth. Clean up the shaft and slip the prop drive washer onto it, and tighten the setscrew to hold in place.

Its important to hold the gear in a lathe and drill the gear out DEAD CENTER as not to cause it to wobble. Also, when brazing, hold it with a jig to have the gear sit 100% perpendicular to the shaft, as not to wobble around when rotated.

The prop drive washer can be knurled on the face to grip the propeller better. If you can knurl then score the face with a file to make a cross hatch pattern.

ajcoholic

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The gears that go onto the engine crankshafts (the 23 teeth) are also modified. The heavy hub is turned down to leave only 1/16" left, and then the hole is opened up to the same diameter as the knurled part of the cranks. The drive washer on the stock engine is NOT used. I forget the #drill I used but pick the one that is the same diameter. Drill about 1/8" deep. Upon assembly, the gear will be pressed onto the crank slowly by placing a 5-40 screw into the shaft and tightening it up with the piston locked up.

ajcoholic

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There are a few other important things to consider...

If you are using engines with the plastic backplates, there is no need to do anything specail to mount onto the backplate. Just slip the carrier onto the crank noses, and lightly tighten. Then set the backplates onto the aluminum plate and mark the holes, drill and tap for 4-40 machine screws. I use only 2 per engine backplate (4 total).

BUT.... if you want to use babe bee, or black widow tanks, the engines are just a wee bit too "tight" for the gear spacing.

CAREFULLY file a small amount off the tankand backplate of each engine so they can fit slightly closer together. You should be able to put the engines onto a smooth surface and slide the carrier onto the crank noses without the tanks wanting to touch each other and flex outwards. Be carefull when removing material as not to got through the tanks!!

Finally assembly is as follows:

1- place the prop shaft (with driver and gear assembled) through the bearings and screw in the 4-40 screw and washer into the back end of the shaft. When tight, the bearings should NOT bind and there should be no fore-aft play. The fit can be adjusted by removing a little off the end of the shaft if it is too long. You want to lightly preload the bearings - LIGHTLY!!

2- slide the carrier onto the engines, and tighten the two clamp screws untill they cases are firmly held and no more, DONT OVER TIGHTEN!

3- place engine gears onto the shaft and take a 3/8 long, 5-40 cap screw and with the piston locked up, slowly and firmly tighten the screw, forcing the gear onto the knurled end of the crank untill the gear bottoms out. Repeat for the second side.

IMPORTANT: it is when fitting the second gear that you must decide when the engines will fire, ie, together, 180 degrees apart, etc. Once the gears are pressed onto the shaft the timing is FIXED.

4- place the engines onto the 1/8 mounting plate and fasten them with corresponding screws (4-40 for plastic backplates and 2-56 for metal tank backplates).

Fit a 9-4 or 9-6 prop and either use a 1/4-28 nut and washer or make a prop spinner nut.

To start up the engine, I use an electric starter and fire one cylinder first, with the opposite engine's glowhead out. Set the needle. Repeat with the opposite cylinder. Once both are set individually, then try a run with both engines.

Hope you like it and have fun

AJC

ajcoholic

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Jim,
I have just a basic lathe/mill. No CNC I can make one of these twins, including cleaning uo two old and dirty engines in about 2 1/2 to 3 hours. I made the two of these this weekend in 5 hours. Its not hard and an easy evening project. No CNC required!

bsindel

AJ

Where do the bearings come in to play?

Bill Sindel

OOPS I see, one would go in from each side of the prop shaft carrier into the
3/8's hole. Sorry.....[sm=spinnyeyes.gif]

ajcoholic

My Feedback: (10)

Yup. sorry I forgot that part. The flanged bearings go on either end of the carrier central hole. The use of a flanged bearing makes life a lot simpler, no blind bore hole from the back, etc. Also, to clarify, when the gear is brazed onto the prop shaft, and the shaft slipped through the bearings, the rear end (with threaded hole) of the shaft should be just about flush with the bearing race. That way when you put the #4 washer and 4-40 screw into the shaft, it can be tightened down very tightly without any excess force (preload) on the bearings. If the shaft is too long, just take a wee bit off the end. If it is too short, heat up the gear/shaft joint and tap it through a wee bit more.

AJC

Japanman

Thats a Beauty, Andy.
When i saw it first on the air tools cox site I was very impressed, Those cox engines are great for this kind of thing aren`t they!!

Genius!

Boy, a five cylinder version fully coweled in a 1930`s bipe gets my juices flowing!
Just think, a five cylinder radial for less than $200!! And how cool would it look???
I imagine it would be possible to run them from a common (R/C) carb...
Now THAT would be a good .25 sport plane!!

J.M

bsindel

Aj,

Would you give us a little more detail about the prop shaft rod and carrier.

1. What kind of steel did you use, and where is a good place to get it?

2. About the "Silver Brazing"; what kind of torch did you use, what kind of metal content is the brazing rod made up of, and where can it be obtained?

3. What kind of aluminum (hardness) will work best for the carrier?

The devil is in the details, eh.
Bill Sindel

ajcoholic

My Feedback: (10)

for the shaft, any steel will do really, I use 1/4 cold rolled or drill rod. The cold rolled thread a bit easier with a die or if you want to cut thethreads on the lathe it doesnt matter.

for the aluminum I use 6061-T6 as it is strong, and easy to get and cheap. any other common alloy will do, ie, 2011-T3, or anything in the 2000 series, but they cost more and the extra strength isnt needed. I imagine just pure aluminum would also work, but it is gummy to thread with a small tap like the 4-40 or 5-40.

the silver braze I use is just a high silver content wire, usually about 45 to 55% silver. Easyflow is one brand, I buy it from the shop where I get sawblades and they use it for brazing on the carbide teeth. normal brass brazing rod would also work but the silver is so sweet to use, and flows out like normal solder (allbeit at a higher temp) and is very strong.

a normal propane torch is fine to use, it is hot enough.

any more questions?

DICKEYBIRD

Just curious Andrew, which combination do you prefer? Is one way better than the other as far as the sound it makes or amount of vibration?

And thanks very much for your efforts; I've printed off the whole thing to go into my engines notebook. I can see one of these in my future. Lesse now, I have a Tee Dee reverse rotation crankshaft in stock. Hmmmm, if I can turn a couple of front housing retainers with parallel sides, I could have a "Famous AJCoholic/DickeyBird Twin!"

ajcoholic

My Feedback: (10)

Milton, I have built 12 of these so far, and allways set them to fire together. Its supposed to give a longer gear life, from what I have read about geared engines. However, fooling around with them, if you fire opposite it sounds way cooler!

This engine is more of a novelty IMO than a really practical powerplant. It can be used in a nice Cub-type plane - I was someday hopeing to try one in my Lazy Bee, but the weight is a concern for the average 1/2A plane...

Now a twin TD version....

Remember if the files sizes are too small to read here I can email you the full size file via email...

Andrew

ajcoholic

My Feedback: (10)

One other important thing...

The gear artio I chose (23 tooth driving a 28 tooth) was just something I chose arbitrarily. You can choose a larger prop gear for a more "geared down" ratio, and swing a huge prop slower. But, you would also have to modify the gear spacing on the carrier. If you went to a 30 tooth prop gear for instance, you would probably not have to file a bit off the tanks to get the engines to sit close enough. I havent tried any other gear ratios but feel free to do so!

AJC

jboy381-RCU

very nice work aj...i'll have to give it a try...

have you thought about 1/4 ply as a sub for the aluminum???will need to fuel proof it first...this might work well for me sence i have no metal working skills...thanks for your input...

john

ajcoholic

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I never thought of using wood as I have no need to replace the aluminum...

the only thing I would be worried about is flexing and adequate strength.

For the backplate, sure a 1/8th piece of ply would be fine but for the carrier I dont think anything (other than maybe a composite like G10, Bakelite, etc) will do any better than the aluminum.

Give it a try though, what can you lose?

Japanman

for a twin, how about a dense hardwood inplace of the aluminium, the bearings epoxied in with a good hard epoxy and two strips of sheet metal or even two big washers top and bottom to take the pinch bolt (and nut.)
I guess this would work, but would have a limited life span.
What do you think A.J?

J.M

ZAGNUT

My Feedback: (1)

if you can do an accurate enough job of drilling wood or ply you should be able to do the same with aluminium. worth trying.
with a well stocked junk box you might even find some gears that'll work without modification.
nice work AJ

dave

ajcoholic

My Feedback: (10)

Sure it would probably work fine.

Like I said, you make do with what you have, and they are many ways to tackle the same job. Whatever works for you go for it and have fun.

jboy381-RCU

thanks AJ

john