B-24D Liberator - RC Groups
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Sep 05, 2011, 04:37 AM
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B-24D Liberator

This is a combination of an idea and a build log. I thought I'd get started. I've been thinking of building a Liberator for a few days. At first I was going to build the 2000 series Guillow's kit and modify it for four propeller rubber power. But I found a really great plan for a 60 +" RC Liberator. I bought it, downloaded it and burned it to CD...Yea me! I can do this. The plan is super detailed and absolutely covers everything. I was going to buy the Guillow's plan for $17. including shipping, what a mistake that would have been. I would have done that and drafted a plan based on it. I can convert the plan I got without a lot of trouble but it will be a major job. I'm going to build the Liberator rubber powered and rubber drive four 6" propellers. I found this great company which sells micro sprockets, belts and chains. I've already figured out how to make the power system mechanism. The wing span will be 48", at that size I can buy the Guillow's clear plastic parts sheet from them for the canopy and front and aft windows. Guillow's has replacement parts available so I won't have to buy the whole kit. I can't tell you how excited I am about this project. I've been wanting to build a large plane for some time and also I've been wanting to build a plane that will earn extra contest points. I'll finish up with the Warhawk in about a month. I have to wait till the dope drys well before applying the dark green camouflage tissue and I have to buy a Gizmo thrust button. But back to the subject: The Liberator engine's are below the center line of the wing so running a long prop shaft from the sprockets for 3 degrees of down thrust is no problem. In fact, it works out nearly perfectly. The large camber ( if I have my term right ) of the airfoil makes it perfect for housing the drive mechanism. I've figured out that I can use a 3/4" double sprocket chained to four 1/4" sprockets for a 3 to 1 ratio. the inboard engines will have double sprockets, also. I'm going to build a way to use a winder at the nose to wind two lengths of rubber with the fuselage double sprockets in the middle of them. After I have the plan printed and sized I'll begin drafting a plan for free flight. I'm going to design the power system at a 100% size and send it to this company and just request they fit their product to match what I need. And of course I'll use contest balsa and Esaki Jap tissue.

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Sep 05, 2011, 05:59 AM
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Gluehand's Avatar
Originally Posted by kevin matthews
I found this great company which sells micro sprockets, belts and chains. I've already figured out how to make the power system mechanism.
Wow, what an exiting project, Kev...please share your progress here....

As this is "unbroken ground", a good idea would be to make a test rig of the complete drive system, trying out prop sizes etc, before fitting anything in the plane....

Sep 05, 2011, 07:18 AM
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Gluehand, I certainly will keep everyone posted with this build log. It will be awhile before I start building but maybe I'll post the plan as I progress with it. Building a test rig is a good idea, I'll have to get two of everything in the mechanism Ha.

Sep 05, 2011, 12:42 PM
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I've run into a problem. If I make a rubber motor forward of the wing and another aft of the wing then the front motor would be shorter. So it would wind up to the max first with a lot of turns left possible for the aft motor. Another problem, even if I make both motors the same length, winding them only from the nose would wind the forward motor first...the drive rig would be in the wing so both motors would end there. IDK, maybe after the forward motor is wound pretty much to the max - it wouldn't break just yet because the aft motor would be winding taking some stress...then the aft motor would wind closer to it's max. If I use only an aft motor it would be 15 1/2" in length.

Sep 05, 2011, 07:54 PM
yes, its a flying lamb :)
draganbt's Avatar
This should be a very interesting project, Kev.

Oh, and get used to problems
Sep 05, 2011, 10:32 PM
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TLyttle's Avatar
I would look at having the front motor drive the outer props, and an equal-length motor driver the inners. That way both motors would run out together, and still give good endurance. Lots of options for actual drive, lots of luck with your choice! Also look at profile construction for your "mule", saves a LOT of time...
Sep 06, 2011, 04:01 AM
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TLyttle, I think I've come up with a solution. I can run the motor length under the wing from the nose to as far aft as I want and set up a sprocket rig in the nose which is connected and drives the sprocket rig at the wing which in turn drives the propellers. You will have to tell me what profile construction is.

Sep 06, 2011, 08:53 AM
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I talked to an engineer at Stock Drive Products. He told me a belt drive would be better than a small chain. The smallest chain size they have is 1/8" and they have no 1/4" diameter sprockets to match. He was very helpful. I can send a drawing and they will match the best system to the purpose. I don't know the technical term but I'm needing to know how much torque a 22" or so rubber motor which is two loops of 1/4" unwinds at. Anyone?

Sep 06, 2011, 11:35 AM
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Another "drive" system to try could be a through shaft with mitre or bevel gears to couple to prop shafts. Note, there is a differance between mitre (1:1) and bevel gears. Possible use of C/F rods for drive shafts?

Some of the "Old" multi drives were wire and stirrup.

Regards Ian.
Sep 06, 2011, 06:12 PM
Registered User
Kevin, my best wishes for some successful flying. I tried what you are planning with a 48" span B-35 many years ago and let me pass on some hard learned lessons. I tried to turn all the corners with some neat bevel gears used in the model RR world (a good source of gadgets and gizmos). To make a long story short, they stripped with a scream at the first winding. The torque of a rubber motor was a LOT more than I had anticipated. In talking with Walt Mooney who was in our club, he said my failure was a blessing as bevel gears are very inefficient at transferring power---I think it was about a 75% efficiency. That meant that the power was reduced by 75% as it went through each gear. I would have had about 2 Butterfly Power at the props had it worked.

He also said a chain and sprocket affair would be much more efficient IF the chain has very little friction between the links. Using a belt and pully arrangement, the belt drive would have to "grab" the pulleys very tightly or the torque will cause them to slip and you then will have the loss of torque through each pulley as with the bevel gears.

As far as the torque goes, according to a chart I have that the competition guys say is pretty accurate, the breaking torque for 8 strands of 1/8th inch Tan Super Sport is 32 in-oz (which is the same as 2 loops of 1/4th).

Since whatever route you take, it is going to cost you some money you may want to reconsider the 3:1 gearing of the props. You mentioned "contest" in the first post and FAC rules disallow anything other than 1:1 gearing. It would be a bummer to get knocked out of a contest because of that. Actually, I think the 1:1 ratio will work just fine----look at Chris Starleaf's B-24 that flew 54 seconds at WestFAC III with just common direct drive rubber motors.

Good luck in any case---it will be a magnificent sight in the sky. (My B-35 uncompleted airframe ended its days as a wall hanging in my shop. sigh.)
Sep 06, 2011, 07:17 PM
Registered User
As mentioned by Tom Arnold above, rubber powered B24's have been done before but with the more conventional method of transferring power to the propellors, here is a video of one performing rather 'hairy' flight at Geneseo in 1990:

Rubber powered B-24 Liberator, Geneseo 1990 (1 min 51 sec)

Pictures of another, Chris Starleaf's 60" example, can be found if you scroll about a third of the way down this link:


Good luck with your ambitious project, don't forget to post pictures!
Sep 07, 2011, 12:47 AM
B for Bruce
BMatthews's Avatar
Kevin, what span are you looking to make the model? If you're not sure yet I'd suggest something up around 40 to 45 inches. It's large enough to more easily carry the unavoidable extra weight of the drive train to the props yet at the same time given the higher aspect ratio of the B-24 it'll produce a "small" enough airframe to be reasonably rugged without undue engineering.

I know the guy at Small Parts means well but he won't have any possible idea of the need for keeping things stupidly light. By all means get some smaller gears. But from there rely on fairly thin music wire or carbon rod and lots of fairleads for contecting drive shafts.

Bevel gears are very draggy so are best avoided. I suspect you'll find that some sort of pulleys and simple belting will work just fine I'll bet 1/4 wide ribbon sewn neatly to form a belt along with a tensioner will provide the drive system through the wing. I once restored an old belt drive turntable to operation using fabric store ribbon to make a belt in just this way. The belt lasted for some years of regular use.
Sep 07, 2011, 05:44 AM
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Gluehand's Avatar
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...?
Sep 07, 2011, 06:00 AM
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Tom, Bruce, Pete, Thanks, I'm going to make it 48". It looks like I'll be using 1/4" wide timing belts.

Sep 07, 2011, 08:01 AM
Registered User
Ron Moulsons "Flying Scale Models" has a section describing rubber drives. One system used was the "Moore" drive which shows wire cranks and stirrups and the other, not shown, is to use the twisted "Curtain wire" flexible drive (Like the Prop shafts) taken back to a gearbox in the fusegulge connected to the rubber motor.

Regards Ian.

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