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Old Aug 10, 2012, 06:44 PM
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carbondale il
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B 24-D Liberator

I'm going to build the Liberator. I bought the plan from War-Bird-Plans on Ebay. After downloading I put the 3 page plan on disk. I took the disk to a printing company and had it printed on three - 12" x 18" sheets of card stock. I copied them in sections on a standard copy machine at 182% and pieced them together with tape on a make shift light table - my glass top dining table with a lamp underneath. The wing-span is 50". The fuse is 30". The root wing chord is 6". It will take four 6" props. Some of us have talked about this and I'm going with a micro sprocket / chain drive with a single heavy rubber motor running the length of nearly the entire fuse and inside. The fuse will be built box and former. I'm thinking of weight and I'm wondering if four -1/8" hard longerons with 1/16" x 1/8" cross members and diagonal braces for 2" fuse bays will be enough. The wing dihedral will be 3 1/2 degrees.

Kev
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Old Aug 10, 2012, 09:50 PM
B for Bruce
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The 'Wack, BC, Canada
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That sort of fuselage will hold up against 10 to 12 strands of fully wound 1/4 rubber. So I think you'll be fine. Mind you if it blows while in the fuselage then goodbye model. If I were you I'd look at having the fuselage done as a "pass through" style where the nose and tail both are removable.

The tail plug/rear turret would be the part that anchors into the winding stooge. Then the winder equipped with a LONG extension tube with hooks at each end is attached to the front end of the motor after removing the nose block. The model is then drawn forward onto the winding tube where it rides during winding the motor. At the end with the motor wound you or a helper moves the model back over the motor to the tail plug and then suitable cross handles and tricks are used to transfer the front hook of the fully wound motor from the winder extension rod to the nose block.

The nose block would have a set of transfer gears that connect the motor to the internal drive links that run along the top of the fuselage over the motor. The nose block gearing would have an easily replaced "plug and socket" sort of driver to extend the power. Something like a hex ball driver tip into a hex cap screw.

It sounds like a lot of trouble. But after all the work you'll need to do for this model you REALLY DO NOT want a blown motor to shred the fuselage. And 10 to 12 strands of 1/4 or the equivalent in smaller width rubber will certainly do that.
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Old Aug 10, 2012, 11:08 PM
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Sounds like a plan, Bruce. The rubber motor will run just underneath the wing. The wing has two 3/16" aluminum tubes which run through the fuse and through the wing ribs to the outside of the inside nacelles. Do you think 3 1/2 degrees will be enough dihedral? I'm thinking of using every other rib instead of the 1" apart plan ribs. Also, for the two nose transfer gears - one gear of which would turn the props and the other gear turned by the rubber - should they have "a lot" of teeth or fewer larger teeth? Thanks A LOT for your interest!

Kev
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Old Aug 11, 2012, 02:53 AM
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Hi Kev, glad you're going for it on this project - it's very exciting Please keep us up to date with your progress and lots of pics!

I was looking at doing a Fillon Twin Drive recently, different to your idea but similar challenges. I was recomended this chain and sprocket supplier by someone in the know. Apparently the advantage of chain and sprocket is very low friction and it doesn't require quite the same standard of accuracy to build.

My one concern is that it is not the very lightest, especially with 4 motors so you willl need to keep a close eye on the weight. But at that large model size you will have some wiggle room. Here's my thread on HPA which has some weights given for the stuff above. I was also interested in the thrust and RPM of the smaller props. The same principles apply: the bigger the prop the better for rubber power. Do you know what size props you can swing yet?

If I can help at all with any of the calculations (weight target from wingloading, prop sizing and design, gear ratios etc) I'd be glad to be useful: it's going be an epic project Regarding the dihedral: a lot of it is about what you can/want to cope with in the trimming, more is easier of course, but 3.5 degrees should give you something to work with and the high wing helps. It's not as if you will have massive torque from a single prop

Jon
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Old Aug 11, 2012, 07:36 AM
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United States, FL, Perry
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How about using a beefy motor stick that can be wound outside the model, it will take the torque and the fuselage could be built lighter. As for gearing I personally would stick to 1:1 but only to keep it all simple.
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Old Aug 11, 2012, 09:11 AM
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Yak, thanks for your offer to help. The largest size prop which will fit is 6". I have been thinking of making the props 4 bladed by notching two plastic props and CA gluing them together. And thanks for the chain and sprocket link, I'll check it out. FLYBOB, a removable motor stick is an idea but I'm thinking with all the mechanics in the plane to drive the props, I don't think, at least right now, it would be better. It would have to be one heck of a stick to hold 5 loops of 1/4" rubber. Picks of everything are on the way. The gearing ratio has to be 1 to 1 for competition.

Kev
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Old Aug 11, 2012, 10:31 AM
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I was thinking a box stick made from 1/16 balsa sheet, my mind is still working on how it would connect to the gearing system.
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Old Aug 11, 2012, 11:30 AM
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FLYBOB, I have a good idea of how I will design the drive train mechanics: Use plastic gears at the nose inside the fuse, one on top of the other. They would be secured to the fuse. The top gear hub will have a wire which will extend back and turn a sprocket and chain system which will turn the props, at the fuse wing area. The bottom gear will have a similar wire in the hub which will have a diamond shaped hook to hold the rubber motor. I'll probably buy plastic gears with either a 1/8" or 1/4" hub hole. I'll CA glue into the hub holes a nylon thrust bearing. I'll CA glue a wire inside the thrust bearings which will also serve as a hub shaft and will be held in place with two more nylon thrust bearings secured inside the fuse. The box you mentioned could "float" inside the fuse and wouldn't need to be a part of the drive train. Bruce, I need to see a diagram of your solution to winding, I can't picture it all in my mind.

Kev
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Old Aug 11, 2012, 09:42 PM
B for Bruce
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The 'Wack, BC, Canada
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An external motor stick isn't a bad idea other than the issues with connecting it to the gearing. Although if the motor unit itself had the transfer box on the front of it or even the whole outer nose turret and hooked into a prop drive similar to what I was suggesting that could work out nicely as well.

Kevin, to give yourself room for the power transfer shaft from the nose to the wing center section as well as give the rubber some room to thrash around I'd suggest that the motor will be roughly in the lower half of the model. If you split the height at the wing into thirds the top of the lower third would be where I see the motor axis being located. That'll give you good room to put in the power transfer stuff and give the motor enough room to thrash about as it unwinds.

If you have not worked with motors of up around 10 strands of 1/4 or the equivalent this is an important factor. Don't skimp on the room for the motor to move around.

If you insist on 6 inch props then I agree with the idea of gearing up the prop RPM. 6 inch props, even four of them, isn't a lot of prop disc area for the amount of rubber you're going to need. If you can get the props up to around 8 to 9 then you could get away with direct 1:1. But much less and I think you'll find that your motor winds spin out too fast with too little effect. Or you may get a wild power burst and then nothing. And you DO want it to fly like a bomber and not one of the escort fighters....

All of this is still why for a project of this nature where there is little information to go by that it's a very good idea to make a mock up of the drive system or even to build the system up onto a simplified "stand way back and squint scale" test platform. You're going to build all this stuff into a massively time consumning project and if it needs any tweaking you'll end up having to destroy a quarter or more of the model to make any changes. I know you dismissed that option before. But it would be well worth the time to re-consider it.

Mind you that's one advantage of the separate transfer gearing up in the nose block. THAT gear box should be adaptable to altering the ratio. If you keep that option open along with the ability to swap to different props as needed then the internal system could get away with no attention with any luck.

I know if it were me I'd be building it with hatches to access the major points even if it were simply to allow cleaning and lubricating.
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Old Aug 11, 2012, 09:50 PM
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I checked out the link supplied by Yak. Pretty good products and it appears they have what I've been looking for. The site is Servo City and they have plastic micro chains with snap together links and plastic spokked sprockets. It looks like everything for the drive train will be lighter and much cheaper than anticipated. This is looking like a long term project. I'll keep everyone posted on every little development and through each phase of the building. So far I have the basic fuselage plan drafted along with the formers, top and side. Trying to find the best way to make the drive train. It looks like with 3/4" diameter sprockets, I'll be able to run the top of the drive chain inside the wing. The bottom, however, will have to run just underneath / outside the wing. I'm really concerned about only 3 1/2 degrees of wing dihedral. It doesn't look like much when I look at the plan. I'm wondering if I make the wing polyhedral and raise the wing tips, would that disqualify me from competition? It certainly wouldn't be scale.

Kev

Kev
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Old Aug 11, 2012, 10:00 PM
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Exactly, Bruce...The rubber motor is right where you have suggested in the plan. The big problem is there is no room for any larger than 6" props. The distance from the inside nacelles to the outer edge of the fuse is just over 3" and the distance between nacelles is just over 6". The inside nacelle LE extends beyond the outside nacelle but not enough to allow for overlapping props. How about if I make 4 bladed props? Or I could move the nacelles a little further down toward the wing tips but that somehow seems like a bad idea to me.
A major goal is to compete and win and so any other gear ratio than 1 - 1 is a no no.

Kev
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Old Aug 11, 2012, 11:18 PM
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I would not move the nacelles at all.

The original had 3 bladed props. I'd suggest using 3 blades and go with a fairly coarse pitch where the angle is 45 degrees at around the 2 inch out from the center. If you're stuck with 1:1 then that's about all you can do. I would make it so the blades are made separately and have dowel stubs that clamp into the hubs. That way you can play with different blade widths, alter the pitch to see which works out best and even swap them out due to any damage more easily.

The more or less normal prop for up to around 10 strands of 1/4 rubber is a 16 to 18 inch two blade. Let's split the difference and call it 17. That's 227 sq inches of swept area for the prop disc. Your 4x6 option will have a measely 113 sq inches total. So you can see why I'm a bit concerned over the 1:1 situation. You simply do not have the prop disc area needed to soak up the power from the sort of motor that will be needed for a model of this size and weight.

One option would be to use reversing gears on the tail and run TWO motors in tandem so that you get half the strands and twice the length. Or go with the single motor and keep the overall model as light as possible, good luck with THAT on a 4 prop geared drivetrain model, and run the minimum rubber needed to achieve about a 25 to 30 second climbing cruise with a cruise and powered descent of around another 25 to 30 seconds. Then hope like hell it still has some altitude to glide for another few seconds.

You can certainly get enough power into this thing to fly. But the problem will be that it'll want to spin out in much too short a time while producing a P51 like climb. And if it climbs too steeply you'll run into additional trim problems from the scale dihedral.

Hey, as long as we're living on the edge I wonder about the idea of a torque brake? It would only come into play when there's enough tension in the motor to apply the brake for the first 20% or so of the winds. It would hold down the power burst and spread it out over a longer time. This could work because rubber motors are not like conventional engines and the like. Turns held back are still turns in the "power bank" so a brake doesn't produce the same sort of loss you would get if it were an engine or electic motor.
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Old Aug 12, 2012, 12:00 AM
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Well, Living on the edge is great, huh? You are talking about a real high tech propeller construction with removable blades. I wonder if RC props would do the trick. I know they come in three blades and differing pitches. All I would have to worry about is making the hub fit in a small prop shaft. I imagine they *could* be made of wood but that seems to be a difficult project with the limited tools I have. If I ran a tandem rubber motor, how would it be wound? And how would a torque break be made? Like the old torque converters used in cars back in the day? I have no idea. So, it looks like right now 4 x 4 bladed props will help.

Kev
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Old Aug 12, 2012, 02:05 AM
B for Bruce
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You're making it sound harder than it is. If it comes down to it since the props are only 6 inches in diameter you could cut the blades from large yogurt containers and glue them onto 1/8 to 3/16 dowel stubs. By cutting the blade shape from the tub on the right angle you get the sort of twist you want in the blade.

You'll likely want to stick to 3 blades since all the pictures I'm finding of B24's seem to show 3 blades instead of 4.
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Old Aug 12, 2012, 06:17 AM
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Good thing about home made props is they will be up away from the ground on the 24 so they won't shed blades on landing.
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