B-24D Liberator - Page 7 - RC Groups
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Jan 11, 2012, 06:24 PM
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That's a real fine Liberator.
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Feb 26, 2012, 08:12 PM
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Bruce, Say I did form light wooden pulleys. What would I use as a belt? Maybe rubber bands or elastic bands? As stated before a set up like that would maybe result in a large power loss due to resistance and friction. Though it would be expensive, I believe a small plastic chain and sprocket system would be best.

Feb 27, 2012, 02:25 PM
B for Bruce
BMatthews's Avatar
The drive cord would not do well if using elastics. You'd need something more like a cord. Otherwise the "belt" will stretch and jump out of the pulleys. So a low stretch "belt" or cord is needed. You also want a goodly amount of sliding friction against the V in the balsa pulleys so low friction plastics such as fishing lines are also to be avoided. And of course you need a way to form a knot which won't have a big bump in the line. All of this is why I suggested a profile test structure where all the drive line stuff is out where you can work on it and test options.

A successful drive line system for a rubber powered multi prop scale model depends equally on three factors. Weight, efficiency and reliability. And there's also a related but still very real factor of cost. Seems like your inquiries suggested a cost factor which made such a project iffy at best.

Then there's the weight. Plastics intended for small mechanism drives such as sold by outfits such as Small Parts are not geared towards the rubber powered hobby. This fact is also demonstrated by the shaft and bearing sizes that are intended to be used. As a result the overall weight of the drive line system for a 4 prop model is going to be rather heavy. Again I thought that you already came to that conclusion earlier in this thread.

Drive line efficiency is a big factor. We've had numerous threads here that go on about how rubber power is a marginal balance of power to weight even at the best of times. This implies that there is little in the way of excess which we can afford to see lost in any sort of power transmission. The question on this aspect is can we design and build a system of pulleys or shafts that run smoothly and with very little drag.

So where's that leave the idea of using balsa pulleys?

Well first off it would require measuring of the completed systems for drive line friction. It may well be that the one made from commercial parts would be better in this regard. But with a good design and good construction technique it's also entirely possible that a home made V belt and cord system could match the commercial parts just due to the added control you have over bearing design and shaft sizing which can be designed to minimize the friction through the use of smaller bearings that still supply adequite strength. I feel that this is possible and doable.

My only try at such a thing was many years ago when I built a Penni rubber powered helicopter. It used a big balsa pulley on the rotor shaft to connect to the tail rotor via a simple high strength sewing thread "belt". I was careful in how I made the pulleys and bearings but used only materials and techniques found in a typical modeler's home shop. A Dremel was used as a "wood lathe" to turn the pulleys. When done the system ran well with no perceptable friction that could be felt with my figners. I got many dozens of reliable flights from it until it was set aside or met it's demise through some means. Too many years ago to remember what became of it.

But it was my experience with this home made drive line that suggests to me that it is possible to build a home made system of similar pulleys and bearings and produce a servicable 4 prop from one motor drive system without TOO large a weight penalty or TOO much power loss.
Feb 27, 2012, 05:40 PM
Registered User
THAT makes sense. It's a project I have on the back burner for now.

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