Thread: Idea B-24D Liberator
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Old Sep 07, 2011, 09:26 PM
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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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