meowy84

AJ and others have built a Cox reedie flat twin, which was covered in a few of the older posts. I've been considering doing the same but I would prefer to tie the two engines with a single r/c carb. Therefore I think that the TeeDee engines would be better for my needs since I wouldn't have to worry about adapting an intake system to the reed valves (which Wink has done with a single reedie and a single carb but not without a lot of trial and error to give good transition).
I figure that with the TD all I would have to play with is the length and diameter of the intake pipes (which I plan to make out of brass tubes).

Anyways, my question/concern: I have yet to buy my first TD and since my LHS doesn't carry them I haven't been able to inspect one in person and so I am concerned about whether the nose provides enough width (between the point where the plastic intake thingie ends and the tip of the crankcase where the thrust washer rides against) to be able to securely tie the two noses of the engines together. Obviously, tying the crankcases trogether at the back isn't a problem on account of the lugs.

Tim Wiltse-RCU

meowy,

Hey if you don't have a pair of TD's then go ahead an buy a pair of Norvels as they are cheaper. Just understand that you are building an engine that for the most part is a display piece. Good luck with your project.

Later

meowy84

I already have 2 Norvels (one r/c carbed and one needlevalve/venturi version) but prefer the look of the TDs. Plus, I'd also like to feed the 2 TDs off of a common carb. My Norvels seem to have the carb/venturi epoxied to the crankcase. Tim, why do you say that the twin would be mostly a display piece? Is it because of the too-high overall weight of the twin with the drive components (gears and bearings), as installed in a plane?

bsindel

Moewy,

Another problem with the N*rvels is that I don't think you can get a left hand crankshaft for them. Remember that with a geared twin one of the engines has to rotate in the opposite direction of the other. This is no problem with reedies as they will run in either direction.

Good Luck with your experiment.
Bill S. (the other Bill)

DICKEYBIRD

Hi Meowy, I got excited about doing a twin Tee Dee back when the AJ twin reedie thread was going but never did anything. The aluminum front housing retainer ring is about .275" wide which should provide enough meat for the gear housing to clamp onto. It is tapered slightly however which might tend to slide the housing off under the stress and vibes of the 2 engines running. Better to make new ones without the taper. Hafta ask AJ what the thread size & pitch is though!

The venturis overhang the front retainers considerably which reduces the amount of room you have to play with for your gear adaptor. (See pic.) I think you'd do better ditching the single carb idea and just use 2 coupled throttle sleeves with stock or reduced dia. venturis. Single Tee Dees seem to respond better to exhaust throttling than carbs and I 'spec it would get worse with 2 of 'em. No two of them run just alike so the likelihood of 2 them pulling the same through 1 carb is slim. Each piston/cyl. seems to have it's own unique power curve even if you buy 2 brand new ones.

Hi there udder Bill, I was thinking that 1 engine had to turn opposite-rotation as well but when you look at it, they both turn the same direction, just the gear & prop turn backward so a pusher prop will be needed....or 2 reverse-rotation cranks. When I built the little metal roll machine in the lathe thread, I had an epiphany when I realized that the 2 rollers were turning the same direction and only the idler gear turns opposite.

I hope you get it running and keep us posted!

Tim Wiltse-RCU

meowy,

The weight to power ratio is pretty bad with the little engines! I can't ever think of a person that is flying a geared Cox twin. The drag of the drive train sucks the life out of the set up. That is why I was saying that geared 1/2a twins are more for the "Neat" factor than anything else.

Later,
Tim

ajcoholic

My Feedback: (10)

One fellow that I sold a geared twin to DID fly it (in a lazy bee if memory serves me right). You can shave weight off enough to make the power to weight OK if it is in a floater type plane... think .15 size Cub or similar....

As weight goes, I used 1/8 to tie the engines together as a backplate, in practice that is not even necessary if you mount the engines directly to a firewall.

DRill lightening holes in the prop driver, etc.

AJC

combatpigg

My Feedback: (3)

I agree with TIM about this, the added weight and frictional losses make a project like this mostly practical from only a "coolness factor" stand point. I guess it depends on the results you are after. I think a lot of these projects have been tried with simple straight cut gears. The helical gear is probably a better choice for longevity. There must be some kind of power tool out there that has donor gears to try. The other thing to consider is how to arrange the firing sequence, simultaneous or 180, each has it's own advantages.

flyinrog

My Feedback: (1)

Well. I Learn something everyday here, see I thought the geared engines were done to swing a bigger prop, not for speed , but for power..an efficiency thing like electric motors...Rog

bsindel

Dickeybird,

Your right, I drew three circles to represent the gears, put little arrows on them to show the direction of rotation and bingo (or DUH) both engines have to turn backwards. Now, where can you get left hand crankshafts for Tee Dee's? I think I still have one NIP from years back, but I don't remember if it was from COX or not (coulda been aftermarket). Wonder what it would take to get AJ to gin me up a couple?????

Bill S. (the other Bill)

combatpigg

My Feedback: (3)

Gearing IS done to swing a bigger prop. The BYRON company used to sell a unit that made it possible for a .60 to swing a 16x6 well enough to fly a giant scale warbird. At the other extreme a speed guy could find himself going nowhere in a hurry with a 4 pitched prop turning 40K rpm. The only way to allow the engine to operate in the rpm range[let's say 40K] that it finds it's self happiest, and move the prop into a speed realm where it is most efficient[ let's say 6 pitch at 25K]we need a speed reduction gear. If the gain in prop efficiency offsets the frictional loss in the gear train, then there was a positive result. I think we need to be talking in extremes here before a geared twin becomes practical.

MikeSell

My Feedback: (2)

I have one of the Dubro belt reduction units for a .60 size engine to swing the big prop.The belts reduce the jolts and vibration transmitted by the gears. The unit weighs as much as the engine and does not need to maintain the same alignment and rigidity that gears do. The gears that are strong enough for extended use on the cox will add significant weight. The adjoining straps would have to be substantial to keep the gears from slight misalignment that would eat power.

Lets see if I have it right. You want an engine that weighs as much as two TeeDees, a set of gears, and a joining system and some manifold arrangement that will connect the two intakes so you can use one carb. You want no muffling or tank pressure. It has less power than two TeeDees and might turn a .10 size prop.

I am building a couple of flat twins. They fire as a single and thus need a built up crank. I decided to build them a little bigger before tackling the norvel radial I plan. I'm using K&B .65s with single crankcase, a mid rotor intake and no gearing. The same could be done with a cox or norvel.

ajcoholic

My Feedback: (10)

I dont have any reverse rotation cranks otherwise you would have them... sorry!

I agree, a geared twin 049 is not practical in many applications BUT in the right airframe it could be made to fly just fine.

Similarly to larger engine, ie, I have a 44cc opposed twin gasoline engine that a single 40cc can out turn, with less weight. But in a scale model the twin is OK.

The choice of model is limited but, for instance, a lazy bee with the 48" wing would easily carry the extra weight of the geared twin and fly. But it isnt going to be for everyone...

I allways encourage the "out of the ordinary" and experimentation is allways fun! And besides, if it doesnt fly well, just sellit on ebay and you can make a few $$'s

AJC

meowy84

Dickeybird, as for the venturi overhang problem I was planning on removing the venturis and using a single intake (a la the VW Beetle flat 4 method) so that both TDs would run off one carb. Removing the venturis would add some extra clearance. However, I'm discouraged about your comment about the TDs not responding well to carbs. What about a carb AND an exhaust throttle (like on the older Enya engines for example)? What about the Norvel .049-.061 engines; how do they respond being r/c carbed? I have a brand new r/c version of the Revlite .061 but haven't run it yet. The carb I was planning to use for the TDs is the carb that came with the older Aero/Ame .049 engines. Since I was planning to feed two .049 engines off of the single .049 carb I was planning on having them fire alternately and not simultaneously.

Out of curiosity though, what can I expect a single .049-.051 TD to be able to idle down to rpm-wise using just an exhaust throttle?

As for the weight factor, I agree with AJ. Since these little engines don't have stup-pulling torque you can get away with drilling many, many lightening holes. If I recall AJ used steel r/c car pinion gears for his reedie flat twin. However, aluminum pinions are also available at something like a 3rd of the weight of steel. And even those gears are overkill in strength I think. You can halve their weight by drilling lightening holes and even making their profile thinner. I think the average pinion is about 1/4 inches wide acroos the teeth. I think even if they were 1/8 inches wide they would have no problems handling the load of the 2 engines. Wat if you use hard plastic for some of the other components to further reduce weight? The r/c car manufacturers use plastic for everything from suspension arms, chassis to gear cases. So why not use plastic blocks to tie the cases together and for the central shaft/prop shaft carrier and even use plastic (or nylon) gears? This would no doubt shave considerable weight. The only metal parts would be the ball bearings, nuts and bolts and the central prop shaft. Of course the plastic gear wear might be an issue, although there are some lubricants available that claim very good adhesion to exposed surfaces like the gears would be.

MikeSell, your "You want an engine that weighs as much as two TeeDees, a set of gears, and a joining system and some manifold arrangement that will connect the two intakes so you can use one carb. You want no muffling or tank pressure. It has less power than two TeeDees and might turn a .10 size prop." assessment is essentially correct. Although I obviously see the problems associated with the extra weight and the frictional losses, I think it might be doable and furthermore practical for MY purposes since I'm not concerned with speed or performance but rather with relaxed, leisurely scale flight.

combatpigg

My Feedback: (3)

I considered a belt set up, but with a non bushing engine, I wonder if the belt tension would be a problem? Obviously much easier to set up, but if there was a main drawback, it would have to be the side load on the cranks. Too small of a pulley diameter at the engine will invite slippage also. I have played with the idea of making my own custom belts out of kevlar thread that has been "mummyfied" in that liquid rubber stuff. For a form I figure a deep ring could be cut in an aluminum plate, then sprayed with silicone for a mold release.

meowy84

Combat, didn't you mean to say with a bushinged engine the side load might be a problem more so than with the non-bushinged (i.e. ball bearinged) engine? I think with a ball bearing engine, the side loading would be less of a problem than with a bushinged crankshaft like with the reedie or TD. One solution to the side loading could be to use a tensioner on the toothed belt (like automobile timing belts) to provide just the right amount of snugness while minimizing side loading. Then again, if you tie 2 engines together with whatever method you pick, you will still get some degree of side loading....you get side loading even in a pinion geared twin since when the engines run their tendency is to try to push the gears apart.

ajcoholic

My Feedback: (10)

The gears are steel but are very lightweight. You can definitely save weight in a lot more areas - I have experimented with plastic and aluminum gears abd although they work they wear a lot faster.

Small engines are actually a lot fussier to work on than larger ones. It would be easier to work with a pair of 09's or even 15's than two 049's IMO.

Then again, I am toying with the idea of an true opposed twin using cox 010 piston/cylinders... however the way my life is going that wont get done untill about 2008 [X(]

Can anyone lend me a little of their free time?

AJC

MikeSell

My Feedback: (2)

Let me echo what AJ once told me. Even if it doesn't work better the cool factor makes it worthwhile. He said that the satisfaction of seeing your creation run is worth the effort. I am as previously stated building engines with all sorts of modifications. I built a .051 TeeDee with a home made carb with a norvel needle and spraybar (my avitar). It idles quite well for a small engine. I also added a header for the exhaust so I could muffle it.

A bushing journal is usually designed larger than the shaft using a ball bearing. The increased surface area will withstand more force and spread the wear. For very high loads bushings and wear plates are used rather than balls. The draw back in model engines is that many of the bushed units cannot be repaired as easily as replacing bearings.

The DuBro system uses a toothed belt so high tension isn't necessary. I think it may be hard to locate a belt and toothed pulleys the right size for this application

Remember that the intake timing will require the intake opening be in the same position that the venturi would be. Removing the front plastic will provide a good place to tie the engines together.

Plastics as you call them are not all the same. I've cut gears out of all different nylon formulas as well as nylotron GS. Some stand fair loads some don't. The Karbonite gears in the new HiTec servos are much stronger than the old resonoid gears in older units but not as strong as the brass gears. Aluminum is stronger than the karbonite while steel is stronger yet. I've had 3/16 wide aluminum gears wear out on electric setups with less time on them than break-in on a norvel. Now try to fit the pinions on the engines and keep them in place.

Of course what you want to do is possible. It will take some machining skill, some searching for parts, patience and trial and error. I hope your process is enjoyable and sucessful.

combatpigg

My Feedback: (3)

The shaft support in a COX is a hard surface that is plated over the aluminum case. I think it is like anodizing. If the film strength of the lube can't hold up to the pressure in this area, then the wear and tear probably reaches a point that is unmanageable, and instead of an assembly that lasts 20 years, who knows what you will get? I think a front and rear ball bearing works better in this application, plus they are easier to keep lubed from random oil migration. But who is going to do a project like this with a pair of ball bearinged 1/2A engines[X(]? The gears create a side load also, but I think tthe load is less than that of a belt. The area where the gears mesh gets hot, which tells me that work is getting wasted there. A belt system runs cooler, but just as much work is getting done. With a 1/2A project we all know that there isn't much power that can be wasted, so after weighing the [+]s and[-]s, the belts seems like a good choice. I wouldn't go with idlers, just go with an engine mounting block that allows the engines to be tilted away from the "mother pulley". I see no reason why the stock carbs can't be retained and like DB suggests go with dual throttle sleeves. A long runner[and flexible] manifold just adds to the confusion.

Silvaire

My Feedback: (50)

I must say that this thread stirs my immagination! I know the dissadvantages regarding weight, rapid wear, parts availability, etc., etc. of geared engines, but they are within my current abilities and tooling to build.

However, once upon a time G-Mark thought well enough of this concept to commercially produce a 5 cylinder geared radial with .061 cylinders. Dose anyone here have any pictures or details of one of these dissasssembled? How did THEY do it? I am curious, but am not yet willing to shell out the $300+ for one of them just to take a look!

By the way, two Cox TD .020's weigh less than a TD .049, and two TD .049's weigh only a little more than a TD .090. I have "several extra" TD .020's (some with broken carb bodies and/or plastic cancer on the tank outlet) and lots of ideas! Until I have some pictures of hardware to show though, I am too embarassed to discuss them here.

Arlen

meowy84

I realize that 1/2A engines are alot more fussy that larger engines but they're just so neat . I did however considered building a twin using .09 Cox engines and I might eventually try it perhaps if the TD twin doesn't turn out to be practical. For me practical is basically something that will stay in the air (lol). Since I'm more interested in the scale look rather than aerobatics, the max. speed, max. performance are secondary issues. In any case, like some of you mentioned I think the 'cool' factor of this type of an engine counts for a lot in my opinion.

Although I agree with MikeSell that for high loads bushings rather than bearings are used (i.e. with the exception of the more exotic machinery like the 50s Mercedes SLR and Porsche Carrera race cars using built-up cranks and bearings (needle I think) most real auto engines use con-rod, crank, main bushings instead of bearings). But on the lines of Combatpig I think that in most model engines the loads are tolerable enough so that ball bearings are actually better able to withstand side loads (with less friction and power loss) than a bushed crankshaft. I don't have any hard data on this but can only argue that most model engines have fairly beefy ball bearings that are fairly large in proportion to the rest of the engine, which I think gives them a certain margin of strength. This is of course academic though since the only commercially available 1/2A engine that I am aware of that has ball bearings is the small diesel engine made by a gentleman named Pfeffer (sp?) out of the Czech republic and like Combat said not many would wanna try to do a hack twin using a pair of these type of engines.

Keeping in mind what MikeSell and AJ mentioned about the gears it looks like steel (perhaps aluminum) is the way to go and weight would be better saved in other areas.

XJet

I've also been thinking about geared engines and figured that much of the lube/longevity problem could be solved by directing the engine exhaust onto the gears.

That constant supply of fresh/warm castor should reduce wear on the gears quite significantly.

You could either just have a manifold that dumped it onto the gears, or effectively build a muffler around the gears, this killing two birds with one stone.

mclintock

My Feedback: (3)

A possible source of toothed belts and gears to tinker with is inkjet printers. The platen or drum is turned on many models by a toothed belt, altho some may use a worm gear.
Speaking of worm gear, I've sometimes wondered if a worm gear system would work for a gear reduction drive using cox engine(s).

I once had a cox 'sandblaster' dune buggy that used a crude worm gear- the engine drove a plastic shaft with a helix of wire wrapped around it to form the worm, and that meshed with a moveable plastic gear on the car's axle. It worked fine- Pretty extreme ratio though, maybe more like the kind of thing to use in a helicopter. Hey there you go, look at the lite machines heli site and see what they are using.

Tim Wiltse-RCU

CP,

The only Cox crankcases that are anodized are the Golden Bee's, old Killer Bee's, Venom's, the Black Widows are Ceramkote I believe. The plain silver cases have no protection to them.

Later,
Tim

flyinrog

My Feedback: (1)

I had a Cox Dune buggy myself ,bought another on ebay last spring...I also have a Lite Machines heli and it is an 11 tooth gear on the metal clutch and about 100 teeth to the plastic rotor gear ,,the Norvels on these things have been no end of trouble , I spent a LOT of time breaking mine in,, of course I cant really fly it either, lets see at top speed the head is going (I think)2200 rpm(max) thats, what about 19,000 for the norvel .074...maybe thats alittle high, but its the maximum range for the heli ....Rog