It's not night yet but I've learned how to use this camera. Here's the gears.

Kev

...a second view.

Kev

I can cut the outside diameter of the paper spools/pulleys down a little. I've drawn up rough sketches of the drive train, gear and pulley "boxes", 3- views.

Kev

I picked up aluminum tubes in a variety of sizes. The paper spool hub inside diameter is too small for the nearest size tube and to big for the nearest smaller size. I found out that plastic drinking straws fit perfectly over the smaller tube and inside the paper spool hub snugly. I also picked up some small nylon cable/wire retainers and the tubes I'm using for the mount hubs fit perfectly inside them. I'm going to try to use the retainers so I can get the pulleys off of their mounts so I can change rubber bands, they use a small wood screw through two holes. I'm still working on the balsa gears - coating them with Elmer's white glue. I've put on around seven coats. This makes them hard and smooth. The hubs for the gears will make the gears mesh right at 5 1/5 PICAS from center to center. The gear teeth need to be a little loose to turn freely. I'm having trouble finding the right rubber bands to turn the pulleys. I have 1/4" wide bands but stretching to 6 1/2" will put too much stress on the frames. I could use a 1/8" band but like B Matthews said, I need to make the pulley hubs concave to keep the rubber bands in the center.

Kev

I'm reinforcing the paper spools with card stock like paper. A paper hole puncher makes holes just the right size. The wire/cable retainers are 3/8" wide. I'm not sure if I want to make the pulley mounts that large. I'm considering making the mount hubs with the plastic straws. That would save weight.

Kev

I hate to say this but balsa for the gears is not the ideal material. Despite what appears to be pretty darn good craftsmanship there's just no way for balsa to hold the degree of accuracy of shape for efficiently transferring the power. You're going to find that they run lumpy and with more drag than you can afford. Gears need to be precision cut and evenly and smoothly sided for them to work well. If you look at how they work the sides need to closely follow a circular path shape so the teeth engage smoothly and hand off the load to the next tooth in line in a smooth manner. And in light of that your home made gears are just not going to do the job I'm afraid.

Looking at the Small Parts online catalog here's what I would suggest you need to make the front end transfer box. In this case I'm showing a 1:1 ratio. But there's much to be said for making it a slight step up ratio.

For a 1:1 ratio you would want to get two of these gears or similar;
http://www.smallparts.com/acetal-deg...rowse=16414721

For a build up ratio maybe get one of the above then one of these;
http://www.smallparts.com/acetal-deg...rowse=16414721

These both require that you use 1/4 shaft material. Get some aluminium rod for this or thick wall ALLOY (not K&S, it's way too soft) tubing for the shafts. Small Parts has this 6061-T6 alloy which would work perfectly.
http://www.smallparts.com/dp/B000FMZ...sp_dp_g2c_asin

Then you also order 4 of these bearings to use for supporting the two shafts;
http://www.smallparts.com/kilian-dia...ed=0.25+inches


All this may seem counter to what I said before about having to keep things light. But this is just about as light as you can manage and still not use up much of the rubber's power in just trying to connect the output of the motor up to the props. In this case efficiency trumps saving a few grams. The use of the delrin/acetal plastic gears and the aluminium shafting is a serious nod to keeping the weight down. The use of the flanged ball bearings is a nod at tolerating the weight to achieve as low a gear box friction loss as practical.

What I foresee the drive train looking like is a single motor connected to the gears in the nose which hide just behind the nose bubble. The motor sitting about in the middle or slightly low in the fuselage. The output from the gear box will connect to a drive shaft (think carbon tubing here) via universal joints at each end to the pulley which drives belts that extend out into the wings to drive the props. The center pulley cluster as well as the nacelle pulleys can use small diameter shafts and get away with plain brass bearings as a result. Just provide a way to get in and flush them clean then re-oil them from time to time.

Thank you very much, Bruce. The largest gears I can fit into the fuse is around 1" each. Two 1.5's would require 3" of space inside the fuse. There is that much room but I have to take into consideration the box also and mounting it. As far as different gear sizes for a ration other than 1 to 1, I thought that was against the regulations, no?

Kev

Just checked the plan again, I could fit in a 1.5 diameter with a 1" diameter combination. Which gear would be directly connected to the rubber and which to the prop shaft, or does it matter?

figured it out - the larger gear to the rubber motor.

I'm having reservations with the paper spools with hubs wrapped in course sand paper. They would be light, though. The screen door wheels would be perfect if they were only smaller. They are 1" in diameter with a concave groove that fits a 1/8" rubber band perfectly and a hole in the center for an axle. So I've thought I could run the bottom band under the wing while the top band would be inside the wing. This seems to be the better solution.

Kev

or I could scale it up to 60", that would give me close to 1 1/4" to work with inside the wing.

Hobby Lobby carries plastic spools just like the paper spools I have. Having slept on it, I'm going to try them instead and see. They will be much lighter than the screen door rollers and smaller. I won't think any more about running the bottom of the rubber band pulley belts under the wing, that would cause major problems with drag having two open bottom sections in the wing. I can sand the plastic spools down to get them to around 1/2" while still having a 6/16" hub. I'm back to building a 48" plane.

Kev

For the pulleys forget the spools. You need to make your own so they have a slight barrel shape to them. Having a slight crown in the middle will make the bands self center. It's the same reason why the drums and wheels on belt sanders and bandsaws are crowned in the middle. Using spools with angled sides will actually cause the bands to walk up the flanges and off the pulleys if they run over to the sides. I know it sounds counter to what you'd think but that's the way the physics of pulleys works. With flanged end spools your alignment would be critical. With non flanged "barrel" shaped pulleys you don't even need end flanges as long as the alignment is somewhere close.

And the best way to make such pulleys is with a Dremel moto tool used as a quick and dirtly wood lathe. The laminated balsa pulleys you can make with such a setup will be light and very effective.

As for the return run I'd use idler pulleys to "bend" the return run back up into the wing's thickness so the return run is up within about a 1/4 to 3/8 inch away from the tension side.

Speaking of wing thickness. No flat bottom skinny airfoils for THIS one. To produce the strength and rigidity (they are not the same thing) needed to adequitely support the prop drive nacelles and give you the room you need in the wing for all the drive stuff you're going to need to use a thicker airfoil. But if you make a thick airfoil with a flat bottom it'll have too much camber and not fly well. So you're stuck with using a "semi symetrical" airfoil so you get the right amount of airfoil camber and still have the thickness you want to install the drive stuff in the wing. But if things just get too crowded in there then running the return path along underneath the wing is still an option. Not the ideal one for sure. But better than trying to build the wing TOO thick.

You know..... a B17 with the wider and thicker "old school" wing would be a lot easier to do this. But if you've got a thing for the B24 then have at it.

The wing is nowhere near flat bottomed.

Kev

Well, that'll help a lot. I only mentioned it because so many scale rubber modelers automatically use a flat bottom FF style airfoil instead of considering the use of other options.

It seems as though you're well along with a lot of the concepts for this. All the best on it. And as others have suggested a full blown mockup of the power transmission system would not be out of the question before commiting to all that scale structure. Perhaps an all profile fan fold foam or depron test bed to even see the power system in flight would not be a bad idea. You could load it up and play with props and get a far better idea of allowable overall flying weight and other aspects and also spot and correct any weak points where it's easy to work with instead of being buried in a bunch of sticks and tissue.

Hell, even the profile version would be amazing to see in flight. Then building up the framed and covered version would just be the icing on the cake and raise the "WOW!" factor off the scale...

After searching every store which would carry rubber bands and not finding what I need, I found a great company - DURA-BELT. They can make belts for me that are just right - 1/16" diameter round belts with a 12% stretch factor to fit in the 1" diameter x 1/8" plastic pulleys I have that have a 1/8" debth concave groove ( screen door roller wheels ).

Kev

I'm going with 3/32" diameter round belts. If you want belts like this contact EBELTING on the web of I can give you the email address. I talked to the owner and he helped me calculate the specs. YIPPIE!

Kev

I'm thinking of using four propellers like this balsa hand made example. I'm looking into using motor sticks which extend beyond the TE instead. The FAC rules appear to allow this, the motor stick clause for multi-propeller planes isn't specific about it.

Kev