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Author Topic: Drive Transmission for Rubber-powered Twins  (Read 428 times)
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PB_guy
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« on: August 28, 2020, 11:41:10 PM »

Usually, twins have nacelles that are too short for using a direct drive with rubber. However, there is an article on Outerzone with a model that used a unique crankshaft transmission. The model Boeing 247 was published by W. Durant in 1934 in MAN: https://outerzone.co.uk/plan_details.asp?ID=11592
There is an article along with the plan. Below is a pic of the transmission layout.

  It does have the disadvantage of a visible conrod (connecting rod) between the fuselage and the motor. However it would allow a twin with small nacelles to be powered with a rubber motor buried in the fuselage. I have two scale models in mind for this possibility. So for the last couple of weeks, I have been putting together a test device based on W. Durant's original article; a mock-up of the transmission idea to test out any problems or pitfalls.

   Early on it occurred to me that it might be possible for a central drive to swing props in opposite directions by placing one prop crank at the top and the other at the bottom of the cycle. So, I started by having two separate conrods; left and right. Because of the nature of converting a rotating motion to linear motion and back again, there is a cycle of 'energy transfer' from peak energy when the crank is vertical, to minimum energy when the crank is horizontal. When the linear motion reverses, there is the potential of 'binding' if the rod is either too short, or too long. I made the conrods in two pieces so that I could cut them apart, readjust and re-glue them to correct for errors (and yes, I adjusted them several times). With the two rods, I was counting on inertia and centripetal forces to carry the crank past the 'binding' point when the cranks were horizontal. Unfortunately, this did not work well, and I got the 'washing machine' effect with the props reversing direction on most rotations. So, you are not likely to be able to run separate conrods and to have props moving in opposite rotations.

  After the initial failure, I took the two separate conrods and glued them together at the center crank.  This setup is essentially the same as designed originally by W Durant. It works fairly well, and you don't have the problem of rotation reversal since the three points of support (crank attachment points) cannot deviate from a straight line. I was concerned about possible vibration, but it turned out that vibration is quite minimal and almost entirely unnoticeable. Nevertheless, you still have to be very careful about not allowing the possibility of 'binding' when the cranks are horizontal. I had to separate and re-glue the conrod joints when this occurred. In order to drive two props, you have to use the same amount of rubber that you would use on two motors, and then consider that you are likely to be incurring at least a 10 to 15% loss in power due to drive/transmission friction (TANSTAAFL).

  These are pics of the test rig. I used a simple jig of nails driven into a chunk of wood to bend the crank wires to identical throws of 3/16" each. Tubing was 1/16" aluminum. Prop blades were cardboard glued to bamboo toothpicks, using bamboo splinters for the prop drive pins. The conrods are made of bamboo coffee stir sticks from the dollar store with holes drilled with a pin drill. they were glued together on the center crank after my initial failure with the twin conrods. The frames were from scrap balsa from old kits. The power is by a single rubber band taken from some veggies. It is the minimum needed to power this setup, and would not provide sufficient power for an actual flight.

Youtube videos
 1. Top View: https://youtu.be/alvYuKWL6jI
 2. Front View: https://youtu.be/l5WZWIJIdnc
Attached files Thumbnail(s):
Drive Transmission for Rubber-powered Twins
Drive Transmission for Rubber-powered Twins
Drive Transmission for Rubber-powered Twins
Drive Transmission for Rubber-powered Twins
Drive Transmission for Rubber-powered Twins
Drive Transmission for Rubber-powered Twins
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TheLurker
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« Reply #1 on: August 29, 2020, 03:17:11 AM »

Interesting.  Have you discounted the possibility of a Moore Drive?

There's an example here https://outerzone.co.uk/plan_details.asp?ID=9352
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dputt7
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« Reply #2 on: August 29, 2020, 03:40:36 AM »

 Excellent stuff Ian, I wish you all the best for your future experiments.
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OZPAF
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« Reply #3 on: August 29, 2020, 04:07:17 AM »

Interesting stuff Ian. Clearance for the vertical movement of the rods in the wing might be a problem.

Good luck with it.

John
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DavidJP
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« Reply #4 on: August 29, 2020, 04:34:46 AM »

Yes - interesting but at first sight it looks a little energy consuming.  Moore’s gizmos have always fascinated me but you don’t hear much about them but thank you for reminding us Lurk. The Scion could be an challenging topic.  I do recall some boffins saying his gear box seemed to absorb more “power” than a conventional drive so was not in many cases worth the effort. 

With all the technology etc. we have today I feel that the need to invent things has diminished and we don’t see things like the “drives mentioned above.  Quite possibly of course I am not in touch enough.  Certainly we have sufficiently talented people among us still to “invent” things in spite of the trend to buy a ready to go device. 
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billdennis747
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« Reply #5 on: August 29, 2020, 04:36:32 AM »

Wow. I think experience has shown that contra-props are unnecessary. My concerns would be a) the CG and b) it all flying apart with more power. I know that people have succeeded with even short nacelles as you can pre-tension modern rubber motors to remarkably short lengths.
Good luck!
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vintagemike
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« Reply #6 on: August 29, 2020, 04:45:59 AM »

What about the drive used by Towner on the Astral twins? angled drive using thin wire inside curtain wire in the manner of a Bowden cable to provide the angle.
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billdennis747
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« Reply #7 on: August 29, 2020, 06:48:02 AM »

I know that people have succeeded with even short nacelles as you can pre-tension modern rubber motors to remarkably short lengths.
Good luck!
What I meant to say was short nacelles with straight short motors.
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DavidJP
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« Reply #8 on: August 29, 2020, 07:12:22 AM »

What about the drive used by Towner on the Astral twins? angled drive using thin wire inside curtain wire in the manner of a Bowden cable to provide the angle.

Do you think that that Bowden cable style used in R/C models might work as well and be lighter - the outer is a kind of nylon and almost friction free?

I am tempted to download that Scion plan.......

I have not had much success with pre tensioned motors so far - probably not doing it right!
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danmellor
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« Reply #9 on: August 29, 2020, 08:56:11 AM »

I have a Fillon plan that shows his twin system. That's flex cables, I think. It's a while since I looked at it and thought "Nah!" For a peanut Short something or other.

Cheers,

Dan.
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danmellor
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« Reply #10 on: August 29, 2020, 08:59:11 AM »

The Fillon Scion plan is up in the Plan Gallery, but not the sheet showing the drive system. I'll see if I can find my copy out.

Dan.
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flydean1
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« Reply #11 on: August 29, 2020, 10:58:39 AM »

Fulton Hungerford, the silk-spoke wheel man. did a Ford Tri-Motor with 3 functioning props. 

Central was with a conventional motor in the fuselage.

The wings, which were built scale with open ribs had the motor running from the wing tips inboard to just short of the nacelles.  Drive to the rear of the nacelles was with a flex shaft drive made from winding spring steel around a 1/16 mandrel.

I actually held this model in my hand as he was packing it to ship to a model museum in the Northeast.  I think the museum folded a few years later.  Don't know what happened to the exhibits.
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PB_guy
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« Reply #12 on: August 29, 2020, 01:37:16 PM »

I had previously evaluated the concept of the Moore Drive, and considered that it would consume too much power through friction. I have also favourably considered the 'cable' drive, but it requires some gearing and I am not familiar enough with it to acquire the right materials to attempt a set-up test rig. But I may give it a shot in the future. Because of the 3/16" throw, the diameter of the moving parts of Durant's drive is 3/8" plus an additional amount for the thickness of the conrod, so you are looking at 1/2" minimum in a small (18 to 24") W/S model. That requires a pretty thick wing and perhaps it might work in a Ford Trimotor, but otherwise, the non-scale exposed conrods as used in Durant's Boeing 247 model are much more practical, albeit a bit unsightly. Of course, embedded electrics would work today, but I am intrigued with the thought of a rubber-powered twin to see how it would work.

ian
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fred
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« Reply #13 on: September 01, 2020, 02:56:28 PM »

That crank drive is Klever.
 Remembering that early RC escapements were also half Rube Goldberg and half Genius.
Frankly I would seriously consider a pair of 8520 Gear drives and a 10 F capacitor on each.
Lighter with absolutely reliable /replicable performance.     Just a thought.
Can still be freeflight even though it doesn't have a Gummy band .
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deltadart48
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« Reply #14 on: October 02, 2020, 04:54:42 PM »

Howdy all,

Take a look at the plans for the DeHavilland Mosquito D-V uploaded here by Gravitywell. They have an interesting belt drive (ok, rubber bands) and spool arrangement for a single rubber motor and twin props. This might even work for four engine aircraft as well.

Ed
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