Robustness of snakes

[Martin Whybrow]

I've just been fitting the supplied control snakes on my Uno Wot; this is the first time I've built a model using all plastic snakes, and I'm wondering how robust they are, especially in light of what happened earlier. In order to help me grip the inner part of the snake (clear plastic tube) when threading the 2mm metal ends into it, I wrapped masking tape around it; when I took the tape off, there was some residue, so I reached for the IPA and used that to wipe off the gunk. A couple of minutes later I looked at the inner and found the ends were white where I'd applied the IPA and the plastic had cracked and was now incredibly brittle. Given that methanol is considered more aggressive than IPA, I'm wondering how well these snakes are going to stand up to being smothered in glow fuel from the exhaust.

Anyone care to share their experiences? And any idea what plastic these inners are made of?

[David Ashby - Moderator]

IPA Martin?

 

Snakes are normally fine, a well used, tried and tested solution to control surface movement.

 

Obviously some solvents will affect the plastic but glow fuel shouldn't affect them and, being at the rear of the model (protrtuding from the fus) they shouldn't get coated in too much residue.

 

More important still is the need to properly brace and support snakes as they travel down the fuselage to prevent them bending within the fuselage when in use (blow back). That's obviously not good.

 

 

 

 

 

 

 

 

 

 

[Ben B]

Personally I drink IPA when building my planes :D Although I think we might be talking about different TLAs...

 

I really wouldn't worry. I recently retired my oil soaked trainer (Precedent Hiboy II circa 1991 no less) after a particularly bad take-off cartwheel took of the tailplane yet again. In that time I've got through countless props and spinners, lots of undercarriage saddle clamps and nose legs clamps, 4.8v battery packs, an engine(!) and eventually of course the oil penetrated the wood and it went a bit squishy- hence it not surviving my cartwheel take-off . The original snakes are still going strong- in fact as I put the plane away in the loft in the hope that the aeromodelling pixies will mend it I spied the snakes and thought "they'll be handy for my next model".

I imagine that the people who made the snakes made sure they were okay with methanol but didn't think of anyone using isopropyl alcohol on them....

[Biggles' Elder Brother - Moderator]

David - IPA - IsoPropylAlcohol.

 

Martin I'd been extremely careful using IPA or any similar solvent on any plastics. You should always test it of an off cut before use.

 

These days I don't use snakes that much personally - but I did go through a phase of using them on everything! I always found them ultra reliable and (excepting the bit that has been given the IPA treatment!) I would be very surprised if you had any problems from glow fuel on them.

 

BEB

Edited By Biggles' Elder Brother - Moderator on 18/05/2011 09:54:55

[Tim Mackey]

Interesting...I always use IPA on all sorts of plastic surfaces, including clear canopies etc, and have always assumed its the safe and less smelly alternative to using something like cellulose thinners. Cant say I have ever noticed any issues with it, and its great for cleaning off those bits of solarfilm colour bleeds that occur from time to time - but I will bow to your expert knowledge.

[Biggles' Elder Brother - Moderator]

Tim,

 

it will attack certain plastics - the problem is I can't remember which ones! Generally I agree its very useful stuff and I always have a spray can in the workshop, but I also always check before using it. It tends to be a softer flexible plastics like polypropelene that have problems.

 

Just a thought though. Martin did you use CA anywhere near the snake - because that also can cause a white crazing effect on some plastics.

 

BEB

[kc]

I would advise fitting new snakes if they seem brittle. Cut the ends off and use them on another plane that needs shorter snakes. You might test the damaged parts to destruction for everyones information.

Next time hold the snake inner by hand and use an old clevis with a bit of thread soldered in to drive the threaded adaptor into the snake. Open ou the old clevis 'arms' to get leverage. Make sure the threaded adaptors are the right ones for the snakes! There are slight differences between makes.  

If you need to grip anything delicate use a bit of soft leather such as the tongue of an old shoe to wrap around the part.  The inside rougher part of leather grips well and does not damage.

Edited By kc on 18/05/2011 10:41:07

[PatMc]

Posted by David Ashby - RCME Administrator on 18/05/2011 09:19:37:

 

More important still is the need to properly brace and support snakes as they travel down the fuselage to prevent them bending within the fuselage when in use (blow back). That's obviously not good.

 

I think you'll find that they don't actually need to be supported anywhere to prevent blow back so long as the ends are completely secure. However the exposed inners need to be as short as possible to minimise possible bending when they are pushing.

Snakes have two conflicting characteristics from our POV. They need to be as free as possible to reduce friction but the inner needs to be as close a fit as possible within the outer to minimise the the end play. I find that friction can be reduced by using a squirt of one of the furniture polishes (eg Mr Sheen) as a lubricant.

 

 

 

 

[kc]

Sorry I disagree, snakes need support about every 4 to 6 inches. Can be bound to 1/4 sq balsa with masking tape or balsa supports across fuselage.

Get a straight run by inserting a piece of piano wire whilst fixing.

[PatMc]

Posted by kc on 18/05/2011 10:44:28:

Sorry I disagree, snakes need support about every 4 to 6 inches. Can be bound to 1/4 sq balsa with masking tape or balsa supports across fuselage.

Get a straight run by inserting a piece of piano wire whilst fixing.

Why ?

[Ben B]

If the outers are fixed at both ends and they're never going to come lose then there's no real advantage to supporting them in between. Whilst both ends of the outer are fixed there is no problem.

Intermediate supports are, however, useful if (perhaps due to using heavyweight snakes and / or pulling some G) the outer snake becomes loose at one end. Then you're in trouble as the whole snake structure can flex and bend rather than actuate the control arm. If you've got intermediate supports not only will they help prevent the snakes from moving under G-force (ie come lose in the first place) but they'll also act as a replacement end fixing if the first one goes.

[kc]

The rotary servo arms tend to push the cable a bit sideways and the cables seems to bend instead of pushing. Also the end fixings cannot be completely immovable in balsa structures.

[Bob Cotsford]

Posted by kc on 18/05/2011 11:10:02:
Also the end fixings cannot be completely immovable in balsa structures.

now that I do not understand! I like to think that when I glue snake outers in a structure, balsa or otherwise, those snakes are not going anywhere! Roughen the outside of the tube, wrap in masking tape and glue in place with epoxy, PU or cyno.

I've never had a problem with the inner not flexing enough to accomodate the rotary servo output either, if they are bending I'd suggest you are exposing to much inner at the ends. I generally try to leave no more than an inch of unsupported inner exposed.

 

 

I do tend to tie snakes in at one or two points along their length, but only to stop them flopping about. In a fibreglass fuselage it's not often possible to fix them other than at the ends in any case.

Edited By Bob Cotsford on 18/05/2011 11:25:07

Edited By Bob Cotsford on 18/05/2011 11:27:13

[Colin Bernard]

I used to use the clevis tip above to screw the studding into the snake inner, but have since found it a lot easier to grip the studding in a pin vice and use that to drive the studding home.

 

I tend to follow that up by running a nut down the thread just to tidy them up where the vice has gripped, but in fact the vice has yet to deform the threads.

 

(By the way, when I went into my LMS to get some more last month there was a new lad in, obviously only just started, and not an aeromodeller. It was an interesting conversation when I asked him for some snakes!!)

[Martin Harris - Moderator]

I grip mine in my Dremel's chuck...

[Peter Beeney]

Patmac,

With the greatest respect, but you might like to conduct a little experiment. Using a full length snake, one end fixed to a servo, as normal, the other end fixed to a rudder on a 100 inch glider, again as simple standard practice. Both ends of the outer should be properly supported, but no other fixings; and the snake forming a shallow S shape to accommodate any slight surplus length. Operate the servo to full travel, keep it operated with the radio on, and then see what degree of pressure is required against the trailing edge of the rudder at it’s widest point to return the rudder to the neutral, or centre point. If this returns very easily, you might like to check as to where the lost movement is. This, of course, is when the snake inner is being pushed, not pulled.

Failure to do this led to one of my greatest disasters, many years ago, when I completely smashed a schoolboy’s brand new glider on my bungee. As a result checking the integrity of the complete control operating system has always been a requirement for me on beginners models. Even when built as per the instructions I’ve thought that sometimes they can be suspect, an area where I’m of the opinion that perhaps sometimes a little more attention could be given.

However, in very general terms, there appears to be few problems arising from this, so perhaps that’s yet another pointer indicating that the operating loads on the control surfaces are perhaps not always as heavy as we have thought previously.

Another situation can arise when sometimes the operating mechanism, what ever form it takes, ‘winds up’. On the elevator, say, it can blow back slightly, so the pilot has maybe full stick movement, then as the model slows up the elevator unwinds and goes to full travel. I’ve seen one or two classic examples of that!

PB

 

[PatMc]

Peter,

I have at least 7 models using snakes, all are only fixed at the extreme ends. Some have snakes on rudder only, some both rudder & elevator or AMT plus one V tail r/e mix. 5 of the models are between 2.5m & 3m span. There's very little lost movement on any of them but I am careful what snakes I use & how I install them.

In order to minimise the flapping about I pull the outer as taut as possible before fixing so that there is no "S". I usually use cyano & micro fibres or micro balloons as the fixing but occasionally epoxy.

Most of the snakes I use have steel inners but if they are plastic I insert 4" or 5" of thin piano wire at each end.

So long as both ends are secure it doesn't matter whether the snake is supported or not, lost movement will be the same. It's dependant on how close the inner fits in the outer. The loss is purely due to the side movement of the inner within the outer.

 

KC,

The end fixings have got to be immovable. The side movement caused by the rotary servos is taken up by the flexing of the exposed inner as is the side movement caused by the rotating action of the horn at the other end. But the amounts of these side movements are exactly the same for pushrods & pull-pull.

 

[Wiltshire Flyer]

I think that when you are dealing with the gubbins that control any model, belt and braces approach should be used, in models i have had in the past regardless of whether they tend to bow or not i have always at the very least secured them half way along their length.

 

Whilst checking their movement on the ground you may not notice any bowing, but in the air under tension when they need to push against the air resistence they may well bow.

 

Resulting in the controls not working as efficiently as they could, or to look at it another way, say you secure both ends, and after so many flights because the snake is bowing in the air the end nearest the rear of the plane does work its way loose, then you have a less than controlable plane going to end up who knows where?

 

So belt and braces every time i think.

[PatMc]

I don't understand why a snake should be bowing in flight. I've never had an end come loose yet & some of the models are - lets just say very well used.

IMO good installation practice, regular general inspection & pre flight checks are better than a simple belt & braces approach.

[Wiltshire Flyer]

Posted by PatMc on 19/05/2011 00:11:46:

IMO good installation practice, regular general inspection & pre flight checks are better than a simple belt & braces approach.

Good installation, regular inspection and pre-flight checks are all a part of the simple belt and braces approach.

But go the extra mile how could it hurt? and look at how many of us have advised to secure them at more than just eother end, we can't all be wrong

[Biggles' Elder Brother - Moderator]

Pat, I think what Peter and DF are refering to is this;

 

A snake operates two ways, in one direction it pulls - this is no problem, a snake under tension won't bow.

 

But in the other direction it pushes. Now suppose, hypothetically, the control surface couldn't move - but the servo was still pushing it through the snake. Something has to give - but what?

 

If the snake were perfectly rigid - like a very stiff pushrod - then the something that would give is the servo and the outcome would be that the servo will stall.

 

But if the snake can bend relatively easily (because it has a significant unsupported length) then it will be the something that gives and it will bend or bow along the unsupported section. This will happen because the servo end of the snake will move, but the control surface end cannot - the movement has to be taken up somewhere - the somewhere will be the bowing of the unsupported section of the snake.

 

Basically the system will give at its weakest point - either the servo or the snake. If the snake is well supported at regular intervals along its length then it will be effectively very stiff and it will not be the "weakest link".

 

How likely is this to be a significant problem? Well it depends upon how long the snake is, what the likely control surfaces forces will be and how much torque the servo has. If you mainly model sports type aircraft of a medium size and medium speed its not likely to particularly manifest itself I would have thought. But if your control runs are long, the model is fast with large control surfaces and you're using high torque servos I could see it being a potential problem.

 

BEB

[Craig Carr]

BEB, nice comprehensive answer, that was my understanding of the reason as well.

Like many others, Ive always support them along the run as well for the same reason, it just makes seems to make them more robust.

Craig

[Peter Beeney]

Pat,

Firstly I’m sorry I miss-spelt your handle in my recent post, I don’t know how I did that. Had I spotted it, I would have amended. Apologies, it wasn’t intended.

Re the snakes, I’m sure the way you install the snake control rod is more than adequate for virtually every occasion, but unfortunately most of the models I fly I don’t build, and there are sometimes little discrepancies in the building techniques. If I can hold the rudder servo arm stationary and then move the rudder sideways when pushing against the arm, thus bowing the snake, I would politely decline to fly it; but I’d certainly try and help the owner to improve it. When you have 30 feet of surgical tube bungee stretched out up to about 110 paces the launching forces are considerable. Some little snapshots of life you never forget and I’ll never forget how that young man’s glider went in! If the rudder servo moves, but the rudder is held in it’s central position by the massive slipstream, and the snake bows, it’s only ever going to end in tears. It’s quite possible, I’m sure, that a standard electric glider might never be subjected to these extreme conditions, so it might never be an issue; although maybe you don’t always really need high torque servos and long snakes; just the right conditions for the snake to give way first. I suspect that like many other aspects, when you get close to the outer limits of performance, the ability to go wrong starts to assume some sort of exponential curve, and disaster strikes before you even realise that it’s happening.
Like the gentleman from the West Country, Dorset flyer, I would also secure the outer run so that it was impossible to move, but I would personally call this good installation practice, not a belt and braces approach; and I don’t think this is always related to gliders, I’m convinced that some powered planes have suffered the same fate; for the very same reasons!

Looking at a similar case, but from a slightly different angle, I’ve flown for years with a friend who is well into small, very fast electric models. At one stage he was getting some erratic flying, which caused some head scratching. Then in an inspired moment he upgraded the servos to a more powerful variety. Instantly cracked it! We concluded that sometimes the servo itself must be ‘blown back’ because the pushrod linkage was secure, rigid and impossible to bend. Or perhaps was rather just simply refusing to operate against the slipstream again.

It might also be a consideration here that this could have been a bit murkier, too. If a stalled servo had caused the regulator, (BEC), to object in any way, and I’m sure there would have been every chance of that, and this had then led to the receiver coming out on strike in sympathy, the end result would again be a pile of rubble and a number of guesses as to what caused it; and what’s the betting that nobody would have got it right……
I’ve always thought that in general flight loads on control surfaces are generally light, and still do, but there are obviously some exceptions. Maybe it’s that old black magic exponential bend up to it’s tricks again, as the airspeed goes up to really fast, so the load increases to really heavy! And the bow in the control rod increases to really big!

We have just two different ideas on a common theme, I guess. I did a lot of thermal soaring with big gliders at one time. These usually had long snake controls, and they often seemed to go out of trim due to temperature changes. What we wanted was a fully temperature compensated servo, also manually adjustable, of course, to automatically keep in trim. Although as I remember, some of the Sprenbrook servos had a small screw adjacent to the arm where you could alter the servo centre slightly.

PB

[Stephen Grigg]

Ive built a Red Zephyr and it has a long fuselage.I thought I had supported the snakes ok but Im wrong.The Rudder is badly effected in the air it hardly lets the model turn right,.Its perfect on the ground and left in the air,but the snake is pushing when you turn right and the wind flow is obviously pushing the rudder back and flexing the snake.I may improve things if I can support the snake where it comes out of the fuselage.Its over 12 inches to the rudder and I believe this is where the problem lies.

[Malcolm Fisher]

I'm with Pat on this.

If the two ends are securely fixed, and the snake outer is as taught as it can be with only short lengths of inner exposed at each end, there is no way that it can bend unless the fuselage also bends.

I've used snakes that way for nearly thirty years on both gliders and power models with never any problems.

I even retrofitted snakes into a GFRP fuselage using home made clamps at each end. It worked well and was submitted and published as an article in Radio Modeller some years ago. No one suggested that there might be a problem with possible lack or loss of full control.

Malcolm