How severe is pushrod temperature expansion?

[sparkysko]

I bought a 4 foot long sullivan gold-n-rod (The red and yellow one), but I keep reading posts about thermal expansion being a concern. I'd like to find out how much of an issue this is before I commit my build to the point of no return on replacing the pushrod. I've googled and searched for two days without any objective data.

[Klogg96]

I bought a 4 foot long sullivan gold-n-rod (The red and yellow one), but I keep reading posts about thermal expansion being a concern. I'd like to find out how much of an issue this is before I commit my build to the point of no return on replacing the pushrod. I've googled and searched for two days without any objective data.

I'd like to know the answer to that question ... I suppose it could be easy to measure. You could cut a 1 or 2 foot length at room temperature while making note of the temperature. Then put it in the oven set to like 150°. Re-Measure the length. I wonder if the expansion would be large enough to measure ... Then you could back out the expansion coeff (is it in inches / inch / degree)...

People have told me the same thing and I have been using the Sullivan braided steel pushrods instead. The problem is that they are heavy, especially in the end of the fuselage.

[Jack Hyde]

The thermal coefficient of expansion for steel is 8x10-6 in/in deg f. A steel pushrod 100 inches long will lengthen .08" for every 100 deg its temperature increases. Not much.
A rod 50" long heated 10 deg grows .004". Aluminum will grow about 50% more than steel.

[sparkysko]

Okay, and nylon expands 3x more than aluminum, sooo punching this into google:

((.004 * 1.5) * 3) * inches = 0.4572 millimeters

For 10 degrees @ 50 inches

[Tim Wolff]

The old yellow outer/white inner pushrods that came with the Cox/Airtronics kits would expand big time. Enough that the plane that flew just fine 2 hours ago was way out of trim in the afternoon. The Sullivan red/yellow are better. The pushrods with the grey/graphite inner rod have shown very little thremal expansion in the years I've had them in my Oly II.

[Garvey]

I have no quantitative data to offer. However, it is my experience that if you use a plastic snake for the elevator then the trim of the model will change every time you go out. I found that I needed to carry out a few handlaunches to establish the trim before hooking the glider onto the winch. Yes you can just winch launch the glider and trim it out at height, but I find it easier to trim the model when it is right in front of me.

[JINKSd]

SPARKYSKO

I have the Sullivan yellow inside blue outside push rod (It’s a little over 24” long) in a spirit 100 electric.

Early in the morning 85 degrees out the rudder is perfectly straight. Leaving the plane in the sun till 11 am the rudder is cocked a little over ¼” mind you the temperature on top the plane is over 105 degrees. Inside it’s even hotter.
The stab control is a Sullivan 1/16” cable. No problems with it stretching.

The picture is a BOT ARF.
The thermometer is not vary accurate but it does give me the difference.

P.S. I don’t think where your at the tempture gets this high.
JINKS D.

[Duke58]

It's not just the heat!!! take a plane out of a warm car and toss it into the cold coastal wind ,,,,SHRINKAGE HAPPENS :)

Earl

[sparkysko]

Is it unmanageable? The way I have my pushrods routed for zero flex, I would get binding galore if I later decided to swap these out with metal/CF.

[sekollera]

If it is a plastic pushrod in a plastic plane, the expansion should be roughly the same, no?

In any case, you have to consider the difference in material expansion coeffs between the fuse and the pushrod.

And around these parts you never know where a servo will center in 15 degrees F vs 70 degrees setup temperature. Sometimes it is waaay off.

So trim it and fly!

/Adam

[Ian Roach]

Thermal expansion/contraction of these rods is severe.

I have found that if you set everything up correctly on a cold day, then fly on a warm day, or vice-versa, you may not be able to satisfactorily retrim using the transmitter trim leavers alone. It could take two or three turns of the clevis on its thread to compensate for the expansion/contraction.

If you start the day with a correct trim setting the trim will most likely change as the day warms up or cools down.

The good news is that the Precision Rods (with black inner cable) from the same maker do not suffer this problem to any serious degree and they fit the same outer tubes. The multi-strand steel cables also work well.

Ian Roach

I bought a 4 foot long sullivan gold-n-rod (The red and yellow one), but I keep reading posts about thermal expansion being a concern. I'd like to find out how much of an issue this is before I commit my build to the point of no return on replacing the pushrod. I've googled and searched for two days without any objective data.

[GaryO]

Why aren't you guys using CF rods in teflon tubing? Aside from being lots lighter, they might be cheaper and not subject to the thermal expansion.

It could also be that what is seen as pushrod problems could well be thermal drift on the servos.

[lincoln]

I've got the Sullivan rods in an old Oly 2. If I go out when it's cold (like 40F), I need to retrim a bit, but nothing my radio can't handle. I haven't bothered to do anything about it in 15 years, so I guess I wouldn't call it severe. I understand one trick you can do is to put a bellcrank in the middle, but just use the two ends, so that when the first part of the pushrod is pulling, the other part is pushing. But of course carbon is going to be much better. The best match is probably the same material as the fuselage, and paint the fuselage white.

If you're really worried about this, it would probably be pretty easy to use an appropriately sized dowel for a pushrod, and even run it in the housing. Wood's thermal expansion coefficient is much lower than nylon's. At least along the grain.

[FrogChief]

Nothing a few clicks of trim can't fix. ;)

[satinet]

I had a couple of golden rods installed in a 60 inch slope plane. I set it up in my warm house.

when i took it out flying it was about 0C (32F), and the plane was running over a mm of down trim. If not more. The bad thing was that i didn't notice before I flew it.

I think severe cold can also effect the servos sometimes.

[jh2rc97]

A couple months back I measured the pushrod expansion on a Serenity at 78 degrees and at 100 degrees. The tailboom is carbon fiber so all it took was a 90 degree day to get the internal temp over 100 degrees. The pushrods are about 40" long. Going off a base measurement at 78 degrees and measuring the elevator position then measuring at 100 degrees there was 1/8" of elevator deflection. On this sailplane an 1/8" is alot of travel, about 30%. I ended up pulling the inner tubes out and replacing them with music wire that had one end threaded.

http://www.rcgroups.com/forums/showpost.php?p=12352233&postcount=138

Alan

[Andy W]

I have some in the basement. I will measure..
..a

[bjr_93tz]

Why aren't you guys using CF rods in teflon tubing? Aside from being lots lighter, they might be cheaper and not subject to the thermal expansion.

It could also be that what is seen as pushrod problems could well be thermal drift on the servos.

The CF rods in teflon I use on the outside of the carbon boom on the Bubble Dancer are rock solid. I've been out in zero celcius to 30+ celcius direct sunlight and never touch a trim lever on either rudder or the fully moving elevator, yeah it's a stable glider to start with but two clicks of rudder trim will see it drifting that way so the rudder is sensitive, any changes in rudder pushrod length (relative to the boom) would show up..

Not cheap though but very light and friction free as well....

[Ian Roach]

It could also be that what is seen as pushrod problems could well be thermal drift on the servos.

I cured the problem on several models by replacing the Gold-n-Rod inners with the black Precision Rod type running in the original outers. I left one model with the original Gold-n-Rods and it continued to suffer from temerature related trim changes for the rest of its service life. This shows that, in my case at least, the problem was not servo drift.

Ian Roach

[satinet]

I wouldn't use golden rods again. I found the thermal contraction to be really bad.

[glderguy]

Wow, golden rod/nyrod.....didnt even thing they made that stuff anymore.
I spent too much time battling temp related trim issues back in 70s during my first "career" flying gliders. Second time around, I now use CF.
Walter