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Sky9pilot
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Re: Transmission idea for thick-winged twins & rubber
Reply #11 - Aug 30th, 2020 at 7:30pm
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Terry and Ian... some of us cheat on the nacelle placement and nudge them out 1/2 to 3/4" this can give you an inch longer prop and still clear the fuselage without distorting the look of the scale of the model.  Don't tell anyone we do this! Embarrassed Shocked Grin Wink
  

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pb_guy
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Re: Transmission idea for thick-winged twins & rubber
Reply #10 - Aug 30th, 2020 at 3:48pm
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Emmanuel Fillon used flexible cables in his peanut Gloster AS31 Survey (see: https://outerzone.co.uk/plan_details.asp?ID=1436 )

He does not, however, give any diagrams of his geared mechanism for transferring power.

ian
  

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terryman
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Re: Transmission idea for thick-winged twins & rubber
Reply #9 - Aug 30th, 2020 at 10:48am
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pb_guy wrote on Aug 30th, 2020 at 9:41am:
... One of the big problems for rubber powered twins is the clearance between the prop tips and the fuselage. I was considering the DH 104 Dove, but it would have to have a 30 inch wingspan to be able to turn a 4 inch prop!

ian


Yeah, small props, at least there would be two of them but nowhere near the swept area of a singe prop of say 9 inch diameter that would power a model that size.  Use high pitch on the small props?

Rubber belt drive seems most practical to me, even with frictional losses at bearings and pully sheaves.  At least all the dimensional control problems are relaxed a bit.  Counter rotation achieved by twisting a belt into a figure 8  and dealing with the belt cross-over point somehow.

Perhaps just pipe dreams once the impracticality of the of the various systems is considered. 

I also like the direct drive flexible cable (in conduit?) you mention.  Needs to be pretty small diameter and flexible.  Counter rotation needs some additional solution.

Terry

P.S. No, spinning cable in conduit doesn't work so well.  One still must somehow split the power from a single motor for routing to each prop.

4" props - Use  blades with large chord too?


« Last Edit: Aug 30th, 2020 at 12:41pm by terryman »  
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pb_guy
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Re: Transmission idea for thick-winged twins & rubber
Reply #8 - Aug 30th, 2020 at 9:41am
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Terry, the only way to deal with the sine/cosine problem is to even out the energy transfer by applying maximum power when the driven crank is in the 'slack' position. I looked for alternate methods of converting rotational to linear and then linear to rotational motion, but I couldn't find anything remotely practical. A geared direct-drive system through a flex drive cable would work, but would possibly add excessive weight if the gear box were in the tail, as I have seen proposed. A chain drive with a low-friction chain would transfer power evenly (if such a beast were available), but a belt drive would add slippage.

  One of the big problems for rubber powered twins is the clearance between the prop tips and the fuselage. I was considering the DH 104 Dove, but it would have to have a 30 inch wingspan to be able to turn a 4 inch prop!

ian
  
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terryman
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Re: Transmission idea for thick-winged twins & rubber
Reply #7 - Aug 30th, 2020 at 8:30am
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Ian,

Yeah,  I think the only thing I brought to the party is reducing the chance of sine (or is it cosine?) error, the length of connecting rod(s) changing if/when bent in the middle by the central crank.  As I said, back to the drawing board.

It's still an interesting problem to solve.  I salute you're effort to actually build a working model.   

Terry
  
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pb_guy
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Re: Transmission idea for thick-winged twins & rubber
Reply #6 - Aug 29th, 2020 at 11:28pm
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Terry, I am sorry, but I cannot see your idea working, but you are encouraged to build an example to test it for yourself. I did do a single conrod in early testing, as I built the rig. But if you try building a test platform you will see that it is 'slop' in the bearings that actually helps to create the 'binding'. What happens at times is that 'slop' will allow one crank to go in  a clockwise rotation while the other starts in a counter-clockwise rotation and the whole thing jams. That is why the single beam has the best chance of operatiing smoothly with any kind of slop in the bearings.

  Yes, some kind of adjustment 'turnbuckle' would certainly be helpful. My test rig has too much 'slop' in it as the wire 'cranks' are just a touch too small for the tubing.

ian
  
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terryman
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Re: Transmission idea for thick-winged twins & rubber
Reply #5 - Aug 29th, 2020 at 7:25pm
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Ian,


Intriguing model.  Not sure this entirely fixes the binding, my drawings omit a lot but show enough to give the idea.  The 2nd could provide counter rotation (I think) and I suspect its motion would look pretty wild in action.  Critical to prevent binding is the middle leg of all the wire Z bends must be equal (actually the radiuses of their rotation).  And as you already noted tie rods must be the correct length, probably best to build them with adjustable length, something like a built in turnbuckle or like the tie rod of a car.  I suppose small errors in these dimensions could be dealt with by adding slop in the bearings.  Also all bearing axes must be parallel.  Starting to sound like a pretty tall order in the flexing wings of an airplane but maybe possible.  Needs more thought and model builds to see where things break down.

Terry

P.S. Shucks, it looks like rotation reversal could still be a problem, the two drawings are the same machine.  The second one just chose to be in counter rotation.  Back to the drawing board.
« Last Edit: Aug 29th, 2020 at 10:54pm by terryman »  

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Re: Transmission idea for thick-winged twins & rubber
Reply #4 - Aug 28th, 2020 at 9:08pm
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Experiment 2 - Props rotating in the same direction.

I took the two separate rods and glued them together at the center crank. By the way, the conrods are made of bamboo coffee stir sticks from the dollar store with holes drilled with a pin drill. This setup is as designed originally by W Durant. It works 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 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 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 about a 10% loss in power due to drive/transmission friction (TANSTAAFL).

  The power is by a single rubber band taken from some veggies. It is the minimum needed to power this setup, and would not be sufficient for an actual flight.

Youtube videos
1. Top View: https://youtu.be/alvYuKWL6jI
2. Front View: https://youtu.be/l5WZWIJIdnc
Here are a couple of pics of the setup.
  

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pb_guy
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Re: Transmission idea for thick-winged twins & rubber
Reply #3 - Aug 28th, 2020 at 8:52pm
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Experiment 1 - counter-rotation - Is it possible?

I have been pondering this subject since I posted it originally. I was intrigued that no-one since 1934 had brought this subject up again. 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 2 weeks, I have been putting together a test device based on W. Durant's original article.

  I decided to make 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, the upshot of the first experiment is that you are not likely to be able to run separate conrods and to have props moving in opposite rotations.

  These are pics of the first setup. I used a jig to bend the crank wires to identical throws of 3/16" each. Tubing was 1/16" aluminum.

ian
  

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Re: Transmission idea for thick-winged twins & rubber
Reply #2 - Nov 18th, 2019 at 11:55am
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Seems like that would work and the friction would be very small.  I think it might need a counterweight at the center to reduce vibration, if you find that the plane starts to shake itself apart.
  
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Re: Transmission idea for thick-winged twins & rubber
Reply #1 - Oct 9th, 2019 at 4:28pm
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Interesting... Smiley
  

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Transmission idea for thick-winged twins & rubber
Oct 9th, 2019 at 2:27pm
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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 that would lend itself to thick-winged models like the Ford Trimotor. The model was the Boeing 247 - https://outerzone.co.uk/plan_details.asp?ID=11592
There is an article along with the plan. This is a shot of the transmission layout.

ian
  

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