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Fuselage control run choice


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I have bought a used midel and the elevator and rudder control runs are absent although the model has clearly been flown. I cannot get into the fuz to glue in snakes so i guess the options are some sort of wood rods with threaded wire at either end or carbon rod with the wire. What are your preferences in this situation? 

Edited by martin collins 1
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One issue of pulling snakes through is that you may not be able to support them along their length. Standard dowel pushrods with threaded adaptors (bicycle spoke) is the way to go, remember to drill the dowel and bend the thread adaptor so it goes through the dowel before securing with thread and epoxy. I didn't do this once and the epoxy didn't hold the threaded rod in position on the elevator control, the end result wasn't pretty ?

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Yes, the reason i didn't go with snakes is that i cannot get doen the fuselage to glue supports to the wood. A friend had a model where this had not been done and on a hot day he wondered why he had virtually no control surface movement. This was due to the softened plastic of the snake bending inside the plane. There is plenty of room for dowls so i will probably go that route. 

Edited by martin collins 1
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My preference for pushrods is carbon tube, typically 5mm OD/3mm ID, with a threaded coupler glued in, then ball links bolted through the horn. Same both at servo and control surface. Precise, strong and rigid.

(sorry my pic won't upload for some reason)

Edited by Dale Bradly
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3 hours ago, martin collins 1 said:

Yes, the reason i didn't go with snakes is that i cannot get doen the fuselage to glue supports to the wood. A friend had a model where this had not been done and on a hot day he wondered why he had virtually no control surface movement.

 

You could use a dollop of expanding foam halfway down the fuselage?

 

Either way, if snakes are secured firmly & properly at both ends, the middle should not bend when the controls are moved. There was a recent thread on here picking over the bones of why that is.

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I've used snakes in gliders with glass fibre or plastic fuselages as well as built up gliders & sports models for decades & only ever fixed them at the two end points. This has always proved to be adequate.

Best to give the snake outers a turn or two of masking tape at the points that are to be secured & use epoxy or cyano with micro balloons as adhesive.

I would avoid using snakes altogether for any model that I wanted to use for precision aerobatics even if not for comps, carbon tubes as described by @Dale Bradley above would be my prefered choice

Edited by PatMc
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7 hours ago, Dale Bradly said:

My preference for pushrods is carbon tube, typically 5mm OD/3mm ID, with a threaded coupler glued in, then ball links bolted through the horn. Same both at servo and control surface. Precise, strong and rigid.

(sorry my pic won't upload for some reason)

 

I find 3mm carbon tubing with 2mm threaded rod glued in each end perfectly stiff enough, even when quite long, without any midway support.

 

Geoff

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Snakes need to be able to bow!

 

Fix the ends and they work on the Bowden cable principle.
 

Support them so that they are straight and/or over supported and unable to flex and they will lose their self-compensation for expansion and contraction in differing temperatures.  
 

As long as the ends of the outer are fixed properly and unsupported inners aren’t long enough to flex they work well with minimal slop. 

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The fact that a snake can flex to absorb changes in length due to temperature shows that there is some flex in the system. That said, I believe that Sullivan at least recommend supporting the outer every 25cm or so? So yes, whilst I agree that there is usually a minimal or at least, acceptable amount of slop in the system, a snake will, in my opinion, never be as accurate and consistent as a rigid, supported pushrod. 

 

But horses for courses in the end, we all have our own experiences and our own opinions and that's exactly how it should be ?

 

 

 

Edited by Matt Carlton
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Agree wholeheartedly that closed loop or a very short direct linkage are the best solutions but I think snakes can be quite acceptable in many situations as long as they're installed correctly.

 

That long bowed snake with two fixed end points can't have any more slop than the same snake run in a straight line and embedded in solid concrete!  It can only take up the clearance between the inner and outer. but it can expand and contract both inner and outer with changes in temperature to virtually eliminate trim changes.

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Now that I think about it, of course you are absolutely right. I was thinking of the system as it would be without the outer being fixed in position. I apologise for that, I was incorrect. Oddly, I have always paid pretty major attention to securing the ends of snake outers so I must've known somewhere in my head!

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  • 3 weeks later...

After saying I'd never experienced any temperature-related issues with snakes, that came back and bit me yesterday, but luckily it was spotted during preflight checks. I finally got my Dark Nights 2020 Mini Astro Hog refurb onto the field yesterday. Preflights the evening before had been fine -everything working properly. During the very warm day yesterday though, that wasn't the case. The elevator worked fine on low rates and mid rates, but selecting high rates for take off and working the controls, the elevator was very slow to respond and "sticky". At it's worst moving the stick resulted in no control surface movement at all. We checked that it wasn't at the external horn end -in terms of a loose connection, but it wasn't. Maiden flight postponed to do a proper workshop check.

 

Back home on the bench it quickly became obvious that the white nylon snake inner was exceedingly floppy in the unsupported inch and a half between the former and the servo. Working the control it simply deformed and bent, with no movement of the control surface. This had never been an issue on the bench before - nor when the model had flown in her IC incarnation many years ago. I replaced the floppy nylon snake with a bowden cable and that now works perfectly. I can't swear 100% that the issue was temperature related, but that was one very sloppy, bendy snake.927955016_MiniAstroHog21stJuly-13.thumb.jpg.6c8415a5d3a6945751213c3f404a1c9a.jpg

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Well there are snakes  and there are  snakes, as quality and materials vary enormously between different makes. I have never had issues with Sullivan types that use fluted hollow inners that slide easily in the outers. Slec also make a similar design that are very good and cope with bends well . Anthing else seems to stick if there are any bends. 

I had some white control snakes left over from years ago that had very thin hollow inners. Used as intended they always used to stick or bind and about as useful as a chocolate tea pot ! I use these inners as outer cases with thin piano wire running through them for inners on smaller models with excellent results . 

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I've had the same experience, no issues with Sullivan snakes and have successfully stiffened up some snakes by adding a wire insert. Unfortunately this model has the solid nylon snakes and having tried a number of alternatives -piano wire, carbon rods etc  to fit inside the snake outer - the Bowden cable ended up as the best replacement,

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