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Cable pulley tensioner/mount

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Malcolm C

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May 18, 2020
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My Flying flea plans call for the 90 degree bend in the cable run to pass thru a bent tube. Although it would be liberally greased I prefer a conventional pulley system. I have a hatred of those brass turnbuckle devices so I cobbled together this combined pulley mount and tensioner. The pulley is attached to a plate which slides in two 1" long slots in the mounting plate, when moved out to the correct cable tension the two 1/4" bolts are tightened. As an extra safety option the whole slide is locked by a 10/32" bolt of the correct length. Apart from the weight ( 12 oz ) which is offset by not having a brass turnbuckle it seems a simple answer to cable tensioning. I would get a one piece cable from the supplier, tension it and off I go. I would welcome any constructive criticism or improvement suggestions, thanks, Malcolm.
 

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Dan Thomas

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A turnbuckle and the cable terminals weigh a lot less than 12 ounces. With little airplanes, every ounce matters. I have seen airplanes where the builder wasn't concerned with a bit of extra weight here and there, and those airplanes are awfully heavy. There's an old story of a guy that built a Pietenpol, and he added a bit of dimension here and there in the structure to make it stronger. He also used oak instead of spruce. Its empty weight came out more than its gross weight, and so it never flew. What a waste.

Turnbuckles are the aviation standard for tensioning cables and rigging the control systems. They have proven to be strong and reliable and relatively light. Your method would work OK (though more cable safety pins would be better, to keep the cable from falling off the pulley if the cable goes slack) but it has a serious weight penalty.

I built my own gascolator out of 6061 aluminum bar stock. I machined off all the metal that didn't need to be there to save an ounce or two. My airplane was already overweight and I didn't want any more at all, since the useful load was already too small and the climb rate was nothing spectacular. It came out lighter than the ancient, leaky glass-bowled affair, and had more capacity.

Once one has an overweight airplane and suffers the consequences he gets a lot fussier about weight.

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Malcolm C

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I added a few more lightening holes and trimmed a bit here and there but couldn't get the weight down below 11 ozs I replaced the pulley mounting screw with a titanium one but that didn't help much. I had constructed the two prototypes out of 190" 7075 t6 I had no 125" at hand but might try making two more out of that and see how light I can get it. I will take a few pics of my weight saving efforts on other parts of the Flea and post them tomorrow.
 

BJC

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I’m sure that you can make it work. My preference would be to have a fixed pulley and devise an alternative to a turnbuckle in the cable. There is a recent thread on ways to do that.


BJC
 

pictsidhe

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I too would like adjustable pulleys instead of turnbuckles. Using a device mostly in tension to a hard point is the light way to do it. Here is a CBR600 motorcycle rear wheel adjuster for inspiration. It is good for many hundreds of pounds.

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TFF

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You will end up with possible unequal throw or a change in throw at the surface if re tensioned. It’s a flea so it might not really matter. It would drive a aerobatic or fast plane owner crazy.

The turnbuckle gets the lengths even. In a regular system with unequal cables you can either not get the throw you want, control not straight with surface. You might also feel an over center depending on how good the geometry is.
 

cvairwerks

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Something you didn't think of, is the cost for that cable....Very few shops anymore will loop splice cables that small. You are either going to have to do it yourself, or send if off and have it done. You can figure at least a C note per splice to have it done.....
 

Malcolm C

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Aircraft Spruce quoted for a 96" cable with two eyed ends for $ 34 each, I thought that was quite reasonable?
 

Dan Thomas

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Aircraft Spruce quoted for a 96" cable with two eyed ends for $ 34 each, I thought that was quite reasonable?
It is reasonable. And they might even be pull-testing the cable assemblies. Besides that, some EAA chapters own a Nicopress tool and the builder can borrow it and do his own and use the go/no-go gauge to make sure it's done right. If you can build an airplane you can build the cables. AC43.13 tells you how.

The OEM cables that have the cable swaged directy into the turnbuckle ends use some expensive and fancy machinery. You won't be doing that in your shop, but you don't need to. The loop and thimble and sleeve method is fine.
 

Malcolm C

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I have been careful about adding weight on the Flea . I have adapted mountain bike brakes for the tricycle gear operated from foot pedals to give my IMG_2208.JPG feet something to do lacking rudder pedals. Chain drilling two 063" stainless rotors was not nice at all.
 

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Malcolm C

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It would have to be a very special jigsaw to get through .063" stainless ! I don't hate turnbuckles I just wanted to find an alternate way of doing something. I have ordered the cables and will see how they work, I can always go back to using turnbuckles if they don't work.
 

cvairwerks

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You do realize that a complete turnbuckle assembly weighs less than 1/4 of what your slider contraption does? Using two bolted swage fittings makes the junction longer than the turnbuckle assembly too.
 

Aesquire

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Ugh. I hate drilling stainless.
Sometime in the last Century, late 1970's? I sat through training films titled "Stainless isn't Difficult, it's Just Different".

Then a few years later got a job at Lightnin Mixers and at one point was the Model lathe guy doing all the exotic metal stuff. Stainless was the cheap stuff there. Lots of Titanium, and other alloys used in the Chemical industry. ( you want to mix boiling Nitric Acid and Fluorine gas? Where are you and am I upwind? )

Carbide drills help, but most important is to slow the speed and get a feel for the "feed" aka how hard you are pushing the drill. Push too hard At too fast a speed and many Stainless steels work harden. Then you NEED carbide or other expensive tooling to cut it.

2 years ago at camp, ( SCA historical re-enactors ) they needed to drill 2 holes in a Stainless steel helmet. I offered to do it for them, but...no, they had it. 3 hours later I used a carbide concrete drill bit to tear out the hardened material and finish the first hole. Then I took a fresh drill bit ( They went through a Harbor Fright kit ) and drilled the second hole in 8 seconds, with a battery powered drill in front of my tent. Regular tool steel drill. The good news was it wasn't a hole that needed to be precisely sized, just to pass a leather string through. And the work hardened area was small and didn't matter to the application.

Yeah, it takes more time to drill stainless, just because you run a slower speed. It's all about the surface speed of the tool as it moves through the material.

Now, Hasteloy.... used in the afterburners of SR-71 Blackbirds, THAT stuff is nasty. look at it sideways and it work hardens and the drill sounds like popcorn as the cutting edge is eaten off. Then it's single point carbide or diamond to cut out the hard bit and.....
 

Dan Thomas

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The old Rapid-Tap made a big difference when drilling stainless. But it had 1,1,1-trichloroethane in it, a fluorocarbon, an ozone-destroyer, so that stuff got outlawed. The new Rapid-Tap is just an oil of some sort. Still works ok, but not nearly as well as the old stuff.
 

Malcolm C

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You do realize that a complete turnbuckle assembly weighs less than 1/4 of what your slider contraption does? Using two bolted swage fittings makes the junction longer than the turnbuckle that fills a particulat need on my aircraft too.
Yes, I was well aware of the different weights of the two devices but decided to try it anyway. And it is not a contraption it is a proof of concept device that fills a particular need on my aircraft. If we all followed the well trodden path we would all still be flying Wright flyers.
 
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