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Old Aug 16, 2012, 10:25 AM
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carbondale il
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Since the wing ribs are a tapered smaller thickness from the root toward the tips, The drive train chain is clearly below the wing at the outside nacelles. Toward the roots, the chain is more closer to the bottom, until from the inside nacelles to the fuse, I had to make a 1/8" deep and 1/4" wide slot in the bottom of the wing to accommodate. In light of this, would it be better if the chains were inside the nacelles in front of the LE after all? Two 10" props will give me 157" swept area.

Kev
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Old Aug 16, 2012, 11:38 AM
Balsa Flies Better!
Stamford, CT
Joined Oct 2000
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Kev

There is no way you are going to be able to come up with the structure to deal with the torque loads of even 9" props on this airplane if you're going lightweight. A commercial 6" plastic prop weighs about 3 grams. A 9" prop weighs 8 grams.

I don't know many airplanes that can use 1/16" wood to handle the loads of a 9" prop. Typically you've got to 3/32".

You need to figure out what type of building style you're going to use. As an example, I have an Easy Built 50" P-40 lying around that's 200 grams with no nose block or rubber- figure it'll be an easy 300 plus grams all up- maybe more. I flew the airplane for a bit on a 14" prop with probably 4 loops of 1/4"- maybe more. But I suspect that a 50" Liberator should actually be a smaller airplane- it's got a long skinny wing- hence my comment that it should be closer to 200 grams. At 200 grams though- 4 6 inch props should work- and they will need a lot less structure than a 9" prop.

I'm guessing that the Liberator at 50" is actually pretty close tot the same scale as my B-26 at 30"- the long skinny wings don't have a lot more area. If my B-26 flies on a pair of 6" prop- then your Lib should fly on 4 of them.

Do beef up the wings- especially out to the nacelles. After that, things can get lighter, but these airplane put a lot of torsional loads on the wings in between nacelle and the fuse.

Sam
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Old Aug 16, 2012, 03:20 PM
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Thanks Sam, that's a relief. 6" props will give it a scale look and weigh less. I want to try the Superior 6" 3 bladed balsa props if they get them back in stock. The Czech props are billed as the best performing props available. They are plastic 2 bladed. I don't think I can make them free wheeling but I'll give it some thought. Actually, free wheeling props add lots of drag after the motor runs out.

Kev
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Old Aug 16, 2012, 06:20 PM
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Worked on the plan some more today. I moved the drive train just in front of the wing. I'm happier with it like this. Bought the nose transfer gears. They should arrive tomorrow - two 1" x 3/16" with a 1/4" hub opening, 32 teeth. They will fit in the first bay between the first and second formers. Going to beef up the wing a little up to the out board nacelles. Designed a simple and easy way to build the sprocket unit mounts. The clear plastic parts including the canopy should arrive tomorrow, also. Designed a way to get another 2 1/2" of "motor stick" length.

Kev
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Old Aug 16, 2012, 07:04 PM
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The Liberator airfoil is an RAF 34.

Kev
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Old Aug 16, 2012, 07:37 PM
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Does anyone know how to convert fractions into decimals? I need 1 1/8" and 13/16" converted. It's not a lot but I'm considering changing the gearing ratio and forgetting about competing.

Kev
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Old Aug 16, 2012, 09:18 PM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
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Use the calculator in your computer and divide "1" by "8" then add the whole number "1" to the decimal amount.

The "/" is what denotes a fraction but it also indicates that you can divide the top number by the bottom to produce the answer you're after.
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Old Aug 16, 2012, 10:17 PM
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Thanks, I didn't do well in Jr. High math but that's easy enough.

Kev
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Old Aug 17, 2012, 10:35 AM
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The distance between the nacelles sprocket unit and the prop itself is about 1 1/2". I've been planning on running a prop shaft the distance at 3 degrees down thrust, which would not be able to be adjusted. What if I spanned the distance with a rubber band? That way I would have a bendable prop shaft and could change the down thrust. It would have to be heavy, like an office band looped several times. It also would serve as a kind of torque converted.

Kev
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Old Aug 17, 2012, 11:08 AM
B for Bruce
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All of the ideas you're dreaming up in the last dozen or so posts is why it's a good idea to make up some sort of test bed model of the B24.

With the test bed model you can find out what sort of thrust line adjustments are needed, determine the maximum weight for a given motor power that will allow the model to fly "long enough" and even mess about with gearing ratios and props to find out which works and which do not. With a simple rectangular box fuselage you can even work on the practicality of the motor winding scheme. And you can do it all in a platform which won't bring sad tears to your eyes if you have to hack n' patch to make some of the adjustments.

With this project you're traveling a path few free flighters have taken. Especially since you're using commercial gearing parts that I've never seen used in any model in all the magazines or forums I've read or visited. So you are bound to take a few wrong branchings along the way. A test bed model simply makes it a lot less of a loss and faster to recover from the wrongly taken side paths. Why you're so dead set against a test bed model is beyond me. If I were to tackle a 4 prop/1 motor geared project it would be my very FIRST step of choice.

As the old saying goes "Rome was not built in a day".......
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Old Aug 17, 2012, 12:04 PM
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I'm not against a test bed model. It's that I need to work out certain design problems for the finished model first. I'm glad you have advocated for it. Once I get down on paper what seems to be the best solutions then I can test them. If I run into problems which can't be solved design wise for the finished model, then there is no point in going on. I'm putting the horse before the cart.

Kev
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Old Aug 17, 2012, 12:16 PM
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With the 1 to 1 gear ratio, where both gears are 1", the fuse sprocket unit is too low and the motor is too close to it. If I raise the fuse sprocket unit and change the gear ratio so the top gear is 3/4" and the bottom gear is 1 1/2", the fuse sprocket unit will be high enough to be away from the motor.

Kev
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Old Aug 17, 2012, 12:36 PM
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Quote:
Originally Posted by kevin matthews View Post
With the 1 to 1 gear ratio, where both gears are 1", the fuse sprocket unit is too low and the motor is too close to it. If I raise the fuse sprocket unit and change the gear ratio so the top gear is 3/4" and the bottom gear is 1 1/2", the fuse sprocket unit will be high enough to be away from the motor.

Kev
Or simply select 1:1 gears that are a larger diameter. Two 1.5 inch gears will still provide a 1:1 but now your input to output axis distance is 1.5 inches instead of 1 inch. Or if the model isn't wide enough to accomadate gears of that diameter within the fuselage then use 3 one inch gears where the middle gear is an idler for a 2 inch distance between the input and output axis.

I suppose it's your relatively low time in the hobby but a lot of the stuff you've been worried about, such as prop selection, is really only something which can be dealt with on an actual flying model. There's simply no way to know if 1:1 with 2 or 3 blade 6 inch props will fly well and provide the prop resistance needed to achieve the sort of power plus motor run duration you need for a project of this sort. And props are by far the easiest issue to solve since you can always make your own.

The sort of things I'd be worried about at this conceptual point in the planning are what the total weight of the model and drive train will be and can I stick enough rubber into the model to fly it and if the wing loading will be within acceptable limits for rubber scale. And if not do I need to consider getting the plans blown up by 5 to 15% to get the wing area I need to make it work. Or will even THAT be enough? The rest is either a case of coming up with a suitably elegant design solution to any problems or stuff that can only be determined by at least assembling a test jig for the drive and then a test bed model to see if it'll all fly.

Heck, in the end assembling all the drive component gears, bearings, shafts and chains may show that it has enough friction that the whole mess is a non starter before you even get any further. But you simply can't know this until you assemble and try the drive system at least on an open format test jig which replicates how it would be set up in the model. And last I read you had not yet ordered the drive components.
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Old Aug 18, 2012, 11:15 AM
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I've designed the wing with a center section. I'll be able to build the semi-spans and then attach the center section so the wing will be one piece before it is mounted at the fuse with 5 1/2 degrees dihedral. The wing will rest on wing saddles built on the sides of the fuse. After mounting the wing I'll fill in the fuse gap on top and place the area formers and top stringers. There will be two wing spars which run inside the fuse. They will have an arched bottom so there will be enough clearance for the rubber motor to run underneath. The wing TE is sharp in the plan I'm using to go by. I'm making them squarish with a 1/16" edge x 1/4" shaped to the airfoil. I'm still debating the prop size. It's either going to have four 6" props or two 9 1/2". Bruce, I have the sprockets and the chain.

Kev
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Old Aug 19, 2012, 01:13 AM
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I've been selecting the wood ( on paper ). I should finish drafting the plan tomorrow except for the last details of the drive train. I'm waiting for teflon washers to arrive. Then I'll know exactly the distance between the hub bearings and the hubs. I have a pretty "tight" draft of the drive train done, and I still need to buy the nose gears and draft the transfer unit I'll use. I have good drawings of that done with different gear sizes. Bruce, your suggestion to use a third middle gear so a 1 to 1 ratio can be used with the placement I'll need is necessary if I want to compete. There is no other way.

Kev
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