The crankcases to the manifold block would be a bit odd. I'm sort of thinking that the the crankcases would have 2-56 studs in the holes that match with locating holes in the manifold plate. From there long 4-40 rods are then used in the holes of the front and rear plates to pull the whole mess together. Not much else that would work since the holes are all in alignment so there's no way to put the screws into the last one. The other option that may work is that if the center manifold plate may be thick enough that notches could be cut into the edges to allow the threaded studs to protrude into the center line cutouts to allow 2-56 nuts to slip in there and go onto the ends of the studs. It would be a tricky bit of assembly but it could be done with only a bit more room than what is needed for the double nuts alone.
Patrick, granted that this would not be super powerful but with care in the building it would easily produce as much power as a typical low power loop scavenged .20 at only about twice the weight... But pure performance is not what this sort of thing is all about anyway. But mounted in a lightly built scale homebuilt design it would be TOTALLY KEWL!
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You know Bruce, in the past some people have said the twins are nice to look at, but not really usable in an airplane. I think your right. Not only would it fly something, it would be excessively cool doing it. Thus the "flyable power" rating. If there was so much power loss that it woulldn't fly anything, it wouldn't be worth doing.
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I think the "Cool Factor" alone would fly it!!! Seriously though, I think it would be powerful enough to fly something cool. Not a rocket, not a prop hanging 3D thing, but a plane which would take off in a normal fashion, do some circuits around the field, not to mention some low, slow flybys (it would be throttled wouldn't it?) and a touchNgo for good measure then land like an airplane lands with a few exceptions.
Exception #1 I am sure that most if not all heads would turn following every move and manuver.
Exception #2 Most if not all would enjoy the sound emitted by such a creation especially when each cylinder note was add to the choir. Exception#3 Most if not all would jump at the chance to help restart the wonderful creation the next time around. Exception #4 Most if not all would wish their engines would sound so cool!
There was a fellow at our field a few years back that had one of the rare Gmark 5 cylinder radials. He set it up to run and I was right there in anticipation of the music to follow. However I was actually somewhat dissapointed. The gnashing of the gears combined with the sound of 5 exhaust pulses per rev conspired to make it sound nasty by anyone's definition. However I've also heard a geared twin before PBJ's movie and it sounded nice, Mostly due to the matched firings I suspect.
Love the twin! I have seen plans for these on ebay, and figured it would be a fun project.
Here's another idea - How 'bout making a Sure-Start version of this: http://raysrocketry.websanon.com/index.php?itemid=32 Cant see why it wouldnt work, apart from it being hard to start (rear prop first i guess) !!
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Indeed Bruce I was basing my anticipation on the sounds I heard on the twin-bager video. I could be wrong. I bet a belt drive would make some difference. Though I haven't seen very much done with belts. I know there are some but I don't know anyone who personally has used such a system. Never have I seen one used for something like a multiple Cox set-up. Not saying it hasn't been done, just that I haven't seen one.
All I can say for sure is that I just loved the sound of PBJs creation and wish one day to have one myself!!
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Wow... I really didn't expect to stir up this much interest in the twin, (or a flat four for that matter). Thanks for all the feedback!
PT, the attached rendering should give you a better idea of what I had in mind. Alternatively, I suppose I could omit the full length studs, have threaded "ears" or "bumps" on the manifold and run screws from the outer bearing blocks into the mainifold, effectively sandwiching the engines together...
Bruce, 2-56 studs, notches in the mainifold and (16) little #2 nuts? Yikes! No thanks!
I'd have to look closer, but if a person would drill the backplate mounting holes all the way through to the front of the crankcase, would that create a leak? That may be another way to go....
indoorff, plans for the twin on e-bay? Don't bother, Andrew Coholic posted free plans here on RCU a coupla years ago (and that's where I got the information to builld mine). If you're interested, here's link to the original thread. I meant to include that information in my first post...
Well, here's where this impromptu "project" stands... (D*mn you Bruce! )
WOW! These 3D shots are looking real fine. It looks like I should be able to reach in through the screen and fondle them....
Joe, the last pic is the perfect place to show the location studs I reffered to in a previous post. The threads in the case are 2-56 of course so what "we" need is some short threaded studs of 2-56 thread to a straight sized pin to extend about 1/8 inch from the case. The plastic manifold would have matching holes to recieve the studs. This would prevent the engines from rotating within the mount until the front and rear clamps are tightened. You wouldn't need all four studs by any means. Perhaps just one or two. They are only there to key the Sure Starts into place until it's all snugged down.
As for drilling through to the front I'm not sure that will work with the upper ones. Certainly the lower two are safe but I believe the threads would be exposed a little through the cylinder area on the upper ones. Whether or not this would foul the piston or rod in their travels I don't know. But if it didn't then a simple fiber washer gasket on the "upper" bolts would seal things just fine.
This is really shaping up to be a viable project from the looks of it...... As if you need any more encouragment...
< Message edited by BMatthews -- 2/4/2006 7:05:58 AM >
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OK, here's where the project stands:
1.) I've incorporated mounting holes in the bottom of the front and rear bearing/engine mounts (for beam style mounting).
2.) Added locating holes in the manifold and pins on the engines per Bruce's suggestion.
3.) Ditched the screw-in carb w/ locknut in favor of the more common setscrew fastening method. (Hey, I already had that carb in CAD)
4.) Went ahead and added bearings in the manifold block. These will be of the sealed variety because when I redesigned the manifold block in an effort to make things more compact, the prop shaft now passes through the intake passages. Will have to weight the pros and cons of this later...
5.) The front and rear mounting plates are now thicker to provide a better grip on the engines with a marginal weight increase.
6. Model now shows the proper propshaft gear with a 1.5:1 gear ratio. This ratio was chosen because this was the largest "over the counter" RC car pinion gear available. A smaller gear on the engine would allow higher ratios, so some experimentation may be in order...
Now, some concerns...
1.) Need to come up with a convenient way to mount the rear propshaft gear so engine can be disassembled. (The front gear is silver soldered to the shaft.) Not a biggie, just isn't done yet.
2.) It would probably be just as easy to machine new reed holders as it's kind of a PITA to hold the new style backplates (SureStarts) in the lathe for turning. Again, no big deal, and it actually allows a bit of customizing and design freedom - it just adds another level of complexity to the project.
3.) A radial mounting option may be nice.
4.) Gear covers?
5.) Exhausts? Even if it meant collectors around the cylinders and four exhaust stubs...
6.) As it stands, the engine weighs (according to CAD) about 10 ounces...
Thanks for all the encouragement guys, but don't expect to see one running soon.
Now with that being said, if anyone is interested is tinkering with the design, I have no problem sharing the 3D models (no plans are available at this time). All that I ask is that you don't claim it as "your own" and try to profit from my generosity. A lot to ask in today's society I know, but I think for the most part we're all in the same boat here and just want to see one of these "come to life".
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10oz ain't too bad! A .25la weighs 9.5oz and has all the power of four good reedies. Shoot, if I had any clue how to use 3d software... Now, forget the exhaust and gear covers and get back to work!
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All Forums >> RC Airplanes >> "1/2 A" & "1/8 A" airplanes >> RE: Geared Cox Twin Video 
RE: Geared Cox Twin Video - 
But pure performance is not what this sort of thing is all about anyway. But mounted in a lightly built scale homebuilt design it would be TOTALLY KEWL! 
Thus the "flyable power" rating. 
If there was so much power loss that it woulldn't fly anything, it wouldn't be worth doing.
