B-24D Liberator - Page 4 - RC Groups
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Sep 20, 2011, 12:35 PM
B for Bruce
BMatthews's Avatar
Oh sure... get us all wired up on this and then go and take the "easy" way out....

Considering all the issues with trimming any multi prop rubber model I suspect you're going with the wiser option for now.
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Sep 20, 2011, 01:11 PM
Registered User
Right, Bruce but I didn't do it on purpose HaHa. I'm really considering the gear and pulley drive train system just to see if it can be done successfully. That's most likely what I'll do, my mind is pretty much made up about it and that was the original goal.

Sep 24, 2011, 05:21 PM
Registered User
Got the drive train belts in the mail today. They are plastic/rubber, round and clear. Pretty cool.

Oct 30, 2011, 09:56 PM
Registered User
Considering downthrust for four propellers mounted a little in front of the wing's LE...is it necessary? Or would a movable elevator provide the needed adjustments during trimming?

Oct 30, 2011, 10:44 PM
B for Bruce
BMatthews's Avatar
I would certainly build some downthrust into the nacelles. It's a guess at this point but perhaps go with 3 degrees.

Just thinking... if your drive connection to the prop blocks were to be done with a flexible wire or tubing then you could still have some degree of adjustable down and side thrust.

Will you make it so the props on each side turn in different directions?
Oct 31, 2011, 12:59 AM
Registered User
I've thought about using some kind of flexible prop shaft. It looks like at this point all the propellers will turn counter clockwise.
Oct 31, 2011, 03:00 PM
B for Bruce
BMatthews's Avatar
Have you given any thoughts to making up a "No-Cal" version to install and test the whole power transfer system yet? If you did such a thing it would soon give you a really good idea of how much thrust trimming is needed on an airframe which would be far and away easier to modify for good flying charactaristics. Then the angles could be replicated in the fully built up final model.

If you twist one side belt into a ( X ) figure "8" sort of crossover it'll reverse the rotation on one side. If the belt edges touch a small nylon roller on a vertical pin would provide a low friction bearing at the cross over. But I've seen such belts where they are twisted for the cross over and at the same time one side is flipped "inside out" and the twisting forms the belt such that the belts are vertical at the cross over and don't quite touch. But that may be a situation where the material needs to be very thin. The time I saw it in some equipment it was a thin plastic belt that was relatively wide. But try it and see if you can get it to work.

Keep in mind too that all your drum shaped pulleys for flat belts will want to be made with a slight barrel shape. The crown makes the belts become self centering. I know it seems counter intuitive that they would rise to the high point of the crown but that's physics for ya...
Nov 02, 2011, 05:34 PM
Registered User
I doubt I'll make a prototype. I'm pretty good at drafting plans which work on paper as well as built.
Nov 02, 2011, 07:16 PM
B for Bruce
BMatthews's Avatar
Well in that case I wish you all the best and hope you don't end up having to put sauce on those words before eating them... .
Nov 03, 2011, 07:11 AM
yes, its a flying lamb :)
draganbt's Avatar
Post pictures of the long anticipated build if you don't mind please
Nov 03, 2011, 01:13 PM
Registered User
Actually all I think it will come down to is finding the right rubber motor size, CG location and elevator/rudder adjustments; pretty much the same as my other planes.
Nov 05, 2011, 04:17 PM
Registered User
If I for go the gear system where the top gear turns the shaft which will turn the pulleys in the fuselage which have the belts which turn the propellers, and the bottom gear where the rubber motor hooks on to aft and in front, the winding hook at the nose area, and instead run the rubber motor from the fuselage pulleys to the rear, the motor length will be 18" approximately, and I will save weight and the drive train would be more direct. If I use the gear system the rubber motor would be about 23" but the weight would include the weight of two gears, four bearings and the drive shaft to the fuselage pulleys from the top gear, not to mention any drag from all that.
Nov 20, 2011, 01:09 PM
Registered User
I believe I have the pulley system figured out. In the fuselage and at the two inboard engines there will be two pulleys each. I'm going to glue them together and use two nylon thrust bearings as hubs with the shafts cut down to fit. I'll use music wire as the axle and two more thrust bearings to hold the rotating drive train at each pulley set up. The belts I have, have a 15% stretch capacity but I won't be stretching them to the maximum - too much stress on the structure. So I'm going to size the plan down so the distance between the pulleys are 1/4" less than planned. That will reduce the wing-span down to around 48". I'll be going to the printing store with the plan on CD and ask them to copy it so the distance from the center of the fuselage to the center of the inboard engine is 6 1/4". It'll be time for someone else to "Go figure".
Nov 21, 2011, 05:58 PM
Registered User
Here's how the plan is working out: On my computer screen it's a certain size so each of the three pages shows completely, separately of course. At 32% it's the exact right size to build from and to use to draft a "new" plan for free flight. At 100% it's real big. At 75% it's "real" size - the "inches" graph matches the real size of inches. I've got it on CD but I'm not sure what to tell the graphic designers at the printing company. Help!
Nov 21, 2011, 07:32 PM
Design is everything.

Suggested design


My 2 cents worth:

A long rubber motor inside the fuselage, wound by a shaft that runs through the nose.

The rubber is attached to to pulleys at one end, preferably sprocketed and belt driven.

Rough computer sketch attached.

You could probably built a sheet balsa model with the mechanism exposed to test the whole thing. Should be fun.

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