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Old Jun 29, 2014, 12:37 AM
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Question
Thermal expansion of Plastic Pushrods

Anyone know the coefficient of thermal expansion of common plastic pushrods, e.g. Goldenrods etc? Methinks it is negligible but not sure.
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Old Jun 29, 2014, 08:17 AM
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You are correct, in most cases it is negligable. Very often, the dimensional change in the structure itself is just as valid a concern which is usually none.
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Old Jun 29, 2014, 12:26 PM
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Thanks, Rodney. Do you have any ball park numbers for a coefficient?
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Old Jun 29, 2014, 01:23 PM
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It is related to the control horn length and degree of model sensitivity to pitch.

On my sailplanes I NEVER use plastic in plastic rods. The models are set up with the CG well back and so pitch trim becomes about "5 click critical" to go from a mushy almost stalling flight to a speedy penetrating glide that isn't quite a shallow dive. With things this critical the thermal changes from Golden Rods or other plastic in plastic rods plays havoc from one flight to the next on days when the temperature varies from early morning flights to warm mid day flights.

Similarly on RES gliders that use a lot of rudder deflection the control horn is so short at the rudder that even the small thermal change produces a good 3/16 to 1/4 inch of trim change from cold mornings to hot sunny afternoons all in the course of a single day.

On sport flying models which don't use extreme trim setups or extreme throw control surfaces the change is still there but it's so minor that most models don't react to it.

I've never bothered to measure the coefficient for this. But it would be easy enough to do it though. Just set up a couple of feet of the rod so it's fixed at one end then put it into the fridge and mark the other free end. on the mounting board. Then stick it somewhere quite warm and measure the thermal growth for that much temperature change. From that you can determine the percentage change per degree easily enough.
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Old Jun 29, 2014, 01:57 PM
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Quote:
Originally Posted by BMatthews View Post
It is related to the control horn length and degree of model sensitivity to pitch.

On my sailplanes I NEVER use plastic in plastic rods. The models are set up with the CG well back and so pitch trim becomes about "5 click critical" to go from a mushy almost stalling flight to a speedy penetrating glide that isn't quite a shallow dive. With things this critical the thermal changes from Golden Rods or other plastic in plastic rods plays havoc from one flight to the next on days when the temperature varies from early morning flights to warm mid day flights.

Similarly on RES gliders that use a lot of rudder deflection the control horn is so short at the rudder that even the small thermal change produces a good 3/16 to 1/4 inch of trim change from cold mornings to hot sunny afternoons all in the course of a single day.

On sport flying models which don't use extreme trim setups or extreme throw control surfaces the change is still there but it's so minor that most models don't react to it.

I've never bothered to measure the coefficient for this. But it would be easy enough to do it though. Just set up a couple of feet of the rod so it's fixed at one end then put it into the fridge and mark the other free end. on the mounting board. Then stick it somewhere quite warm and measure the thermal growth for that much temperature change. From that you can determine the percentage change per degree easily enough.
This will only be accurate if you also figure in the expansion of the material you mount it on; if you are not careful, the mount can change more than the item you are trying to measure.
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Old Jun 29, 2014, 02:30 PM
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Gold-N-Rods/Nyrods/etc. were very popular in the early 1980s as they vastly outperformed what was previously the state of the art -- a 1/4" square balsa stick with a dog-leg bent steel rod glued to it with yellow glue and sewing thread. But no, the thermal expansion is not negligible at all and is the reason these pushrods have been obsolete since the introduction of carbon fiber in the 1980s.

CTE of a typical nylon pushrod is around 45e-6 in/in F. So for example a temperature change of 12F on a 24" pushrod is 0.000045 x 24 x 12 = .013 inches. If your servo arm is a typical 12mm (1/2") this equates to 1.6deg of servo movement or about 3 clicks of trim.

Or for a 1 meter pushrod over a 30C temperature change it is .000081 x 30 x 1 = 2.4mm which equates to 11.5 deg of servo movement or 23 clicks of trim.

Steel has about 1/5 the expansion rate so it is a much better choice but carbon is often preferred since it has basically zero expansion. Of course everything is relative -- you must consider the fuselage expansion as well! A wood or even fiberglass fuselage will expand similarly to steel so you might find that steel is more stable than carbon in these cases though the difference is small enough to be negligible. And of course a styrofoam or plastic fuselage will be most stable with a nylon pushrod.
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Old Jun 29, 2014, 02:59 PM
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Thanks Vespa. You're quite right that the CTE of the substrate to which the pushrod is mounted is just as important as the CTE of the pushrod. Thus it's likely that a nylon Pushrod would be a superior application for something like a Radian.
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Old Jul 01, 2014, 12:50 PM
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Oops, I must admit that I hadn't thought of the CTE of the mount. So substitute measuring with a good quality ruler and the mount is simply to hold the plastic straight and to slow down the heat transfer when removed from the cool and hot environments.

After playing around with such things I simply went back to using pushrods made from the same material as the fuselage for my more demanding projects. And in one notable case I perhaps just got lucky. I now only use plastic in plastic or wire in plastic pushrods on special cases where using solid rods is simply too awkward or I don't need the highest precision and stability.

So for me this means I use built up wood rods with wire ends by choice for the most part since I seldom build with plastics.... VERY seldom.

That one notable exception was a pod and boom electric fuselage I built up for an existing 2meter wing and stabilizer. I used wood rods side by side in a carbon tube for a boom. The shaft is the guide and the wood rods are shaped to mirrored "D" sections to fit side by side in the tube closely but with no friction. This worked out amazingly well. But I got lucky in that the carbon and CTE's seem to be very close. Mind you the fuselage is also painted stark white to fight sunlight absorption.
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Old Jul 04, 2014, 10:41 AM
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For something like Goldenrod, the outer and inner will expand and contract roughly similar amounts, so the net change may be small. Of course the structure also expands and contracts, balsa being not far off plastics, so the net effect would have to be calculated.

Balsa: Thermal expansion 40 55 e-6/K Idemat 2003

http://www.matbase.com/material-cate...tml#properties

The bigger changes may occur with a carbon fibre structure such as a CF tailboom, and typical pushrod materials, or CF pushrods and balsa or kevlar or glass composite structures. CF/epoxy composites may have a negative or very small positive CTE, depending on the lay-up.

http://www.engineeringtoolbox.com/li...ents-d_95.html

A nylon pushrod (CTE = 80 * 10^-6m/mK) would expand or contract about 1.6mm over a 1m length for a 20C temperature change. If a CF tailboom had a zero CTE, then that combination could introduce considerable trim change, depending on the mechanical control set-up.

A steel pushrod (13.0 10^-6m/mK) would expand or contract about 0.26mm over a 1m length for a 20C temperature change. A balsa fuse (50 10^-6m/mK) would change about 1mm in length over a m for 20C delta, so there could be a 0.75mm difference.

You really have to calculate the differences in material CTE, and then take into account if the outer sheath on a flexible pushrod is also changing.

Kevin
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Old Jul 07, 2014, 09:16 PM
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Kevin, I know that SHOULD be the case but it simply didn`t work. Once you lock the ends of the guide rod to the bulkhead and side of the fuselage you produce a system where if the rods and supporting structure do not match for CTE the inner rod will buckle or stretch due to the limits imposed by the fuselage. And meanwhile the inner rod flexes with the outer tube and the tail surface shifts in proportion.

But as you say it`s the difference that matters. If the plastic rods grow by, say, 1.5mm while the fuselage only grows by .5mm and the servo stays the same then the 1mm difference shows up at the control horn which moves the rudder over..... which is exactly what happened to me and why I simply do not use plastic in plastic rods on anything that really matters.
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Old Jul 14, 2014, 08:57 PM
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Quote:
Originally Posted by BMatthews View Post
It is related to the control horn length and degree of model sensitivity to pitch.

On my sailplanes I NEVER use plastic in plastic rods. The models are set up with the CG well back and so pitch trim becomes about "5 click critical" to go from a mushy almost stalling flight to a speedy penetrating glide that isn't quite a shallow dive. With things this critical the thermal changes from Golden Rods or other plastic in plastic rods plays havoc from one flight to the next on days when the temperature varies from early morning flights to warm mid day flights.

Similarly on RES gliders that use a lot of rudder deflection the control horn is so short at the rudder that even the small thermal change produces a good 3/16 to 1/4 inch of trim change from cold mornings to hot sunny afternoons all in the course of a single day.

On sport flying models which don't use extreme trim setups or extreme throw control surfaces the change is still there but it's so minor that most models don't react to it.

I've never bothered to measure the coefficient for this. But it would be easy enough to do it though. Just set up a couple of feet of the rod so it's fixed at one end then put it into the fridge and mark the other free end. on the mounting board. Then stick it somewhere quite warm and measure the thermal growth for that much temperature change. From that you can determine the percentage change per degree easily enough.
My experience exactly. I ended up putting the fuselage in a hot car and set the pushrod end mechanically for the long end of the trim. Then when it cooled it went toward neutral.....
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Old Jul 15, 2014, 09:39 AM
Marion
USA, NC, Hillsborough
Joined Oct 2003
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I believe the balsa expansion and contraction DUE TO MOISTURE is also a big factor. I have run test of that factor on the tightening and loosening of covering material. Leaving the model in a high humidity environment for a few days (thus expanding the wood) will pull out covering wrinkles. And vise versa, storing the model in a dry enviromment will reproduce those wrinkles. At least that is my experience. How much does that effect the model trim, I can't prove to anyone other than myself :-)
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Old Jul 15, 2014, 12:15 PM
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Test?
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Old Jul 15, 2014, 12:58 PM
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The entire conversation is quite interesting, though I find odd that these effects should have such a massive influence. I can think of a couple of solutions that might be useful on larger planes, though. Set up all controls as push/pull pushrod pairs and allow the servo to slide slightly to accomodate for the expansion of the rods. Devise a material that contracts with heat (it can be done using a cellular layout for example) and mix and match materials so that the thermal expansion matches the wanted for the entire length of the pushrod. Build pushrod out of slivers cut out of a modern train rail (those things can be hundred of meters long with minimal gaps at junction points)
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Old Jul 15, 2014, 08:56 PM
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Brandano, you ever looked at the joint gaps in rails in winter when it's cold? They are opened up to something like 3/8 inch. And in the heat of summer the joints are all but closed.

The pushrod effect is minimal until you're using a really short control horn on the surface. Or when something like the pitch stability is set to a very small value by moving the CG well back and near to the Neutral point. Such as with RES sailplanes where the rudder throw is very large. Then the temperature changes make a HUGE difference.

These days with some servos that come with overlong output arms we could get the throw we want with a longer control horn. WIth the longer arms at both ends the effect of any expansion and contraction would be minimized. But with more normal sized output arms used with short control horns it's still an issue.
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