CoosBayLumber

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Way, back in May 1966 issue of American Modeler magazine was a three page article on how to make a twin cylinder engine using two Cox Baby Bees bolted and geared together. The article pointed out the use of the twin in a Sterling Cub, but I have thoughts of using such in an Aeronca C3 kit which I have. Some fellow was trying to sell copies of the article on eBay for more than an original magazine was worth also.

I have a number of questions for anyone familiar with the design.

Has anyone ever built one, or saw one in operation?

After all the effort, do you get any better runs than the standard three minute ones from a standard Baby Bee?

One of the gears noted within the article is not listed as available in current catalogues, is there a replacement or set of substitutes now available?

Perhaps is there anyone who has carved up the steel and aluminum parts and have these commercially available?

I used to see these twins at swap meets for years, but all looked as though they touched down on a busy freeway and am now considering to build one myself..


Wm.

BMatthews

I'll have to check but I may have that article still. Actually I DO have the article but I'd have to find it in the files. No small feat let me assure you....

It used a 1:1 gear ratio to turn a 7x4 prop. If I was doing this I'd set up the ratio for 1:2 or so to let it swing at least an 8 inch and preferrably a 9 or 10 inch. That would be a lot nicer in something like a scale'ish model. It would make starting super easy also.

Using Black Widows would give a longer run. Or you could go for the plastic backplate "product" engines and use a single external tank and a T fitting. At least that way the engines would run out of fuel together.

I'll see if I can find that article later this weekend.

ajcoholic

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Boy, oh boy! I have the articel andmag right here! I have made a dozen at least of these twins, and I have changed the design to better suit my needs. I use a dual ball bearing supported main prop shaft, and instead of having the shaft assembly attached to the back mounting plate, I machine a carrier that attatches to the crankcases. For gears I use the robinson racing (rc car gears) ultimate pinion, 32 pitch gears in 23 and 28 teeth.

I am at work, and have no pics here. But when I get home I will post some pics.

I used the small and large cox tanks, and also the backplates that allow a separate tank.

Will get the pics soon, I am here for a few more hours...

Also, way back when, on a thread for the 5 cylinder radial "tanzanos" posted pics of I had posted some pics of the engines I am sure. Try a search...

Andrew Coholic

ajcoholic

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Here is the thread:

http://www.rcuniverse.com/showthread...1&pagenumber=2

I promissed 5 guys (I still have there emails) that when I built another and did a drawing of my mods I would send out plans, I am still working on it! I didnt get a chance yet to build another and make drawings, it has been a CRAZY winter for me. Will do in the future... I am good for my word!

Andreww

CoosBayLumber

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ANDREW:


That REALLY looks impressive. I have a friend who has given me time on weekends to use his CNC mill. Had set up the program to cut several back plates and front mounts as per the article. However, I then began investigating the gears, and altough many places had them in their catalogues, there was no stock. I never thought of the RC Car racing crowd. You gave me a good thought therein.

Big question is how long does the set up run timewise? I see that you installed a separate tank in one view, and I was more along thinking if possible to do this.

You gave me a project now for the Aeronca.

Wm.

BMatthews

And don't forget to figure in a reduction gearing.

With a 10 sized twin turning a 10x 8 or so prop you'll be needing a 50 inch or even larger lightly built Aeronca to keep the speed down. Does that sound about right?

Do I see a lot of balsa sticks in your future?

The original article used a bearing in the rear plate. But I like ajchoholic's idea of keeping it all in the forward unit. With the beefier noses on today's crankcases they could be machined to center on the crankshaft and have lots of strength to withstand the structural loads with no problem.

I'm kind of wondering about the gear mesh. Is it worth making the meshing adjustable somehow?

ajcoholic

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I found the 23:28 gear ratio great for turning up to a 9-6 prop, but Robinson makes a good selection of gears so you could use a larger prop shaft gear or smaller engine gears. The hardened steel teeth of the "ultimate" series seem to take the beating. I did not do adjustable mesh, but figured my gear spacing and milled the holes accurately as to have the right mesh. Only good for one gear size however.

Since the engines are running at the max rpm, you are getting the same duration, ie about 3 minutes for a babe bee - I'd go with an external tank. A 2 ounce would do nicely.

Andrew

CoosBayLumber

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I forgot to ask, but did you put a keyway into the crankshaft and gear as mentioned via the American Modeler? It looks tiny, and was wondering if prop screw tightness was sufficient to hold everything from slipping.

Wm.

ajcoholic

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No keyway, the gear is drilled for 1/8 shaft which ius the exact diam of the 5-40 prop screw. Just used some locktite and things hold up well without slipping. I silver brazed the prop gear onto the 1/4 inch diam steel shaft.

Andrew