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Old Sep 07, 2011, 09:26 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
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Quote:
Originally Posted by Gluehand View Post
I recall someone using cranks for transferring the power from the rubber main shaft to the props, thus minimizing the drag from gears or pulleys.. (was it a B-17..?)

Does anyone remember...?...who was it, and did this work...?
While cranks will work they will induce a lot of vibration. Far better to go with pulleys and belts.

Kevin, even 1/4 wide Gilmer belts and plastic pulleys are heavier than fabric store ribbon and turned balsa pulleys. By now you should be in tune with the need for the lightest possible weight in any rubber model. This primary dictum is only that much more important with something like a scale B-24 where you'll have an extensive drive system for power transmission. Shaving even a few grams here and there in the drive system is very important.

To get an idea of a valid target weight for your 48 inch model ready to fly I looked up a 3 view drawing and did a quick and dirty CAD sketch of the outline of the wing. At 48 inch span this thing only has about 206 square inches of area. To fly decently as a rubber model you're going to need to keep the overall flying weight down to around 8 to 10 oz. And 10 is pretty much a top end cap. A model framework of this size is going to come in at around 4 to 5 oz covered and painted without the drive train and only if you're very careful with the design and wood selection. To that you're looking at using about 2.5 to 3 oz of rubber for even a reasonable flight duration of about 50 to 70 seconds in dead air. That only leaves ideally 1 to 1.5 oz for the drive train. So there's really not a lot of allowance for relatively heavy fabric and rubber timing belts and fairly stout plastic pulleys.
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Old Sep 07, 2011, 10:35 PM
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Okay, Thanks Bruce. Another option is to only power the inboard engines. That way there would be less mechanism to have to use so that would save weight. I would then have two 7" propellers to power the reduced weight of the plane but I don't know if that would be enough thrust. With just using the inboard engines there would be just a tad more room for a larger pulley. I don't want to use a whole lot of rubber one, because I don't want a whole lot of stress on the fuselage, I'm thinking two loops of 1/4" for the maximum, and two, I don't want the motor run to be too short. I'll search for wooden pulleys with flanges. I'm sure I can find the larger rubber bands to use.

Kev
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Old Sep 08, 2011, 11:31 AM
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I bought some assorted things this morning - tiny paper thread spools, little wooden spools, small round aluminum candle wick sustainers, small flat wooden circles, wooden furniture plugs and more CA glue. I'll see what I can do. I'm thinking of making hubs from aluminum tubes. If I use the wooden spools I'll have to make better flanges and cut them down and glue them back together to make them a little larger than 1/4" in width. I'm thinking if I use the candle wick sustainers as hubs the pulleys will be very light especially if the paper spools will work.

Kev
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Old Sep 08, 2011, 12:09 PM
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The paper spool is what looks like will work. The outside diameter is 3/4". The width is 3/8", the hub diameter is 5/16" outside and inside diameter is 3/16". The hub outside diameter with some thread left on is 1/2". The wing airfoil height at the outside engines is 1".

Kev
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Old Sep 08, 2011, 12:42 PM
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The distance from the center of the fuselage to the center of the inboard engines is 5". From the inboard engines to the outboard engines is approx. 6 1/2" I might go with plastic gears for the nose transfer system which transfers the rubber motor to the prop shaft mechanism. They can be 1" in diameter.

Kev
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Old Sep 08, 2011, 12:52 PM
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I believe the wick sustainers are tin. Their diameter is just slightly smaller than the paper spool diameter. The wick sustainers have a hub, too, which is 1/8" in diameter. I can easily glue the wick sustainers to the spools for a little strength. It looks like all the spools will be made with extended wire axles inside aluminum tubes inside the spools which reach to the propellers. Of course there will have to be a way for the spools to rotate freely and turn the propeller shafts...more aluminum tubes with the whole mechanism mounted.

Kev
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Old Sep 08, 2011, 05:54 PM
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I'm going with an all paper pulley spool with white glue added to the hub to strengthen. The hub is 1/4" in diameter. I made a pulley with some thread still on the hub and glued on two wick sustainers and it's heavy.

Bruce, I won't be painting the model so I'll save a little weight there.

Kev
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Old Sep 08, 2011, 11:26 PM
B for Bruce
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Kevin, do you own a Dremel Moto Tool? If so you have the ability to use it as a small wood lathe to allow making all your own pulleys for a project of this sort. It's really not that hard to set up and do.

And that way you can make the pulleys a little bit barrel shaped so that the rubber bands or ribbon belts tend to self center themselves. It's worth doing even with the spools or you may find out just how little retention you get from the flanges at the edges.

Also you'd better make allowance in the fuselage for the motor to be about 6 to 8 strands of 1/4 to make the motor heavy enough to have the power needed for a decent flight time. And in truth you may even end up needing 10 strands of 1/4.

To get the model to achieve a reasonable climb and not run down just at touchdown you need to make the motor weigh about roughly 30% of the total model weight with motor. You won't get a climb from just 4 strands of 1/4. At most you'll get a cruise climb up to a few feet above the launch height and then a powered glide to touchdown. Meanwhile contest models have been shown to fly their best balance of power to glide when the motor weighs 50% of the total model weight.
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Old Sep 09, 2011, 05:25 AM
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Bruce, Do you mean a barrel shape where the spool hubs are concave or convex? I've been considering elastic fabric bands instead of rubber bands but I'm leaning toward rubber bands. At any rate I'll need to get elastic bands and see what their properties are. I don't own a Dremel tool. I want to go with the paper spools since they weigh next to nothing and they are the right size. Thanks for the rubber motor info.

Kev
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Old Sep 09, 2011, 08:50 AM
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I'm going to bolt the pulley mounts in place with tiny bolts if I can find them. that way I can remove them for maintenance. They will need to be mounted for three degrees of down thrust. I'll glue the propeller shafts in place at their mounts and at the propellers, so they won't be able to free wheel. I'll have to drill out the propellers to take the music wire shafts if I can't use the smaller music wire shafts...that depends on if I can get aluminum tubes which will fit the thin shafts in tightly. The 1/4" x 1" plastic/metal gears for the nose transfer system cost a small fortune - around $25. each. At that size they have 42 teeth.

Kev
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Old Sep 10, 2011, 01:43 PM
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I'm making my two nose gears from balsa. I've laminated two hard balsa 3/32" sheets together with a 1/4" soft balsa core cross grained. They are 1" in outside diameter and 3/4" inside diameter. I haven't decided on hub size yet but I'll start with a 1/8" hole. They will have 12 teeth each. After formed I will coat the teeth with several coats of white glue. I've drafted the two cross sections on tracing paper and will glue onto the block so the drilled hole matches the center of the circles.

Kev
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Old Sep 11, 2011, 08:25 AM
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I reckon I'm a stick in the mud, but I remain convinced that a traditional direct drive setup is preferable. There are some folks experimenting with chain/belt/gear drive, but direct is more efficient and lighter. Only real disadvantage is the time required to wind them all up.

8 rubber motors takes forever to wind, but sooooooooooo worth it:
Spruce Goose Flight 2 (0 min 47 sec)
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Old Sep 11, 2011, 09:22 AM
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Great vid jwfinn. The dialog sounds as if it were from November 2, 1947!
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Old Sep 11, 2011, 10:50 AM
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Great flight, Joshua, Thanks for the video. I've seen the pictures of your great model and you did a good job. Are you suggesting I add a motor stick which extends beyond the wing's TE for each of the four propellers so there will be a longer motor?

Kev
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Old Sep 11, 2011, 05:09 PM
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I picked up some hardware that might work for the drive train - a couple of screen door plastic rollers and some sanding bands. I discovered a rubber band will slip on just paper hubs so I'm gluing on the sanding bands to the paper thread spool hubs for friction. The plastic screen door rollers are 1" in diameter and are shaped like a pulley which can fit in an 1/8" rubber band. They are too big for the wing drive train but just the right size for the nose transfer system. I'm thinking I can glue two of them together for a double short rubber band pulley instead of using gears, or maybe three wide. I'm just about done making the two balsa gears for the nose system. I'm coating them with white glue now.they might work. I'll post pictures of them tonight. At the hardware store I picked up some small nylon bolts, nuts and washers, too.

Kev
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