jeff4912

I recently built a Sig 4 Star 60 and really enjoy flying it, but have noticed on warm days, that the control rods to the elevator and rudder expand causing changes to the trim settings. The control rods are plastic.

I don't have the problem with the ailerons because they are steel rods.

My question is: can I replace the plastic rods with steel 2-56 rods without causing a interference problem with the airplanes receiver? I imagine that steel control rods would solve the trim change problems.

Thank you.

mgnostic

The heat doesn't impact the Aileron push rods as much because they are shorter. Although it may be to a lesser degree, the heat will also impact a long steel rod. As long as you don't have metal vibrating against metal, RF interference shouldn't be a big issue. This problem mostly dates to the days of AM radios. It is less an issue with FM radios and probably not at all with the latest 2.4 ghz stuff. I have had airplanes where I used the guide tubes for the plastic rods and steel rods that had bits of they yellow nyrod material pushed over the steel rod so that it didn't flop around in the tube. It worked about as well as the plastic rods. On the whole though, I think most of us just take off, adjust our trims and live with it.

sdstick

Go to Central Hobbies and have a look at their carbon fiber pushrods. Also go to Tower Hobbies and look at the Sullivan pushrods... you will be able to put together a great set of pushrod from the 2 sources I mentioned that will be far superior to what you have now... they will result in very little to no trim change problems.

Hope this helps!

Gray Beard

My Feedback: (-1)

This is a bit odd, I was using the 4*60 to teach building and the nyrod or clear plastic rods in the red tube is what we often used. I now live in Vegas and fly from a dry lake bed or asphalt runway. It goes from the 20s to about 115 here so you could say we have a temperature shift and the plastic rods don't create that much of a trim change. I'm from Tacoma and now what Washington is like and if your trim is shifting more then a click or a beep or two you have another problem.
If the outer tube isn't glued in tight all along it's length about every 6 or 8 inches it will allow the rod to shift and it will change the trim but your temperature doesn't get hot enough to be a big problem.
You can change out the nylon rods and use metal without any RF problem. I have never had any type of RF hit with a glow engine, even with AM or FM. I did with gas powered planes but trhat was before 2.4.

jeff4912

My outer tube is glued well in the airplane. As an experiment, I placed my Sig 4 Star out in my driveway in direct sun light for about ten minutes w/out the wings on. The rudder and elevator were centered when I placed it out there.

10 minutes later the rudder was bent to the left and the elevator was up. Neither by a lot but noticeable to the naked eye and enough to cause a trim change.

In my kit build instructions, Sig even warns about this and says to make sure the trims on the elevator and rudder are paid attention to because of the plastic rods.

I have a gas 15cc engine on it.

Can I just remove the inner plastic tube and replace it with 4-40 pushrods even though they are not as tight as the inner plastic tube was?

jeff4912

Gray Beard said:

"If the outer tube isn't glued in tight all along it's length about every 6 or 8 inches it will allow the rod to shift and it will change the trim "

Is that an in flight issue, because my trim changes are occurring on the ground prior to the flight when my Sig is sitting in the sun.

When I get my Sig home and inside the garage(out of the sun), the rudder and elevator go back to center.

Here is the paragraph from the Sig 4 Star 60 building manual:

Before flying, check and adjust all the pushrod linkages so that the control surfaces are in neutral position when the transmitter sticks and trim levers are centered. When you get to the field, don't be surprised if the elevator and rudder are slightly misaligned. Temperature and humidity changes can cause the nylon pushrod tubes to expand or contract slightly. Use the trim levers on the transmitter to return the control surfaces to neutral, and do the final trimming in the air.

sdstick

Sullivan makes or used to make a composite inner/outer pushrod called carbon rods or composite flex push rods. They are the best push rods I ever used in respect to minimal trim change from temp changes. I would search for some and set up a test.

Wish I could be of more help

MTK

Jeff
what you are experiencing is very common with most any push rod set-up. The nylon push rods that glide inside outer sheaths are more prone since they expand more as temp increases. But all materials (nylon,wood, carbon steel, ally) expand to one degree or another. Throw in the fact that the wood fuses also move with temp and you have a recipe for exactly what you are experiencing.

It's physics; every material has a COE or coefficient of thermal expansion. In Pattern and IMAC where precision set-ups are of paramount importance, we try to match the materials of the pushrods to those of the fuse. Many also will use servos as close to the surfaces as possible to minimize push-rod runs.

What I often do for elevators and rudders is to use pull-pull set-ups. There is no discernible difference in control cable lengths since the cables driving either side are made from the same stuff.

BTW- this is an age old situation, having been around since the start of model control.

Gray Beard

My Feedback: (-1)

The plane is an Up-Roar, the controls are nyrod with an 18 inch run. I haven't had it out of the shop in a week, before that it was in my van for two weeks. It's my plane I keep in the van in case I want to pull over someplace and fly.

jeff4912

Are the pictures showing me that the rudder has deflected because of the expanding of the nyrods?

Gray Beard

My Feedback: (-1)

I'm heading out to fly in about an hour so just for grins I turned on the plane and radio, the trims are still set at zero, the shop is at 95 degrees and it's humid. I took a shot of an ARF I assembled to sell that I installed the elevator and rudder aft to get the correct CG and used a 4-40 rod for the elevator and made up a pull/pull for the rudder. It never changes trim either.
I pulled out a piece of the red outer tube and slid in some 4-40 rod, a lot of room left inside the tube but it will work. Also going to change the CG with the added weight. Then I pulled out a piece of the Nyrod and slid a piece of 2-56 rod in it. Perfect fit and slides well.
When I was in Calif. and flying a plane with the nyrod the trim would maybe go out a click/beep or two but nothing worth changing out the controls for.
If it's more then a little trim change you have some other type of problem. Something is shifting?? A little is normal. What are you calling a lot??

jeff4912

Gray Beard..

At most, I will need to put in 8-10 clicks of trim on my Spektrum DX6 radio for the rudder and elevator. That equates to about 1/5 of the total trim available on the radio.

Gray Beard

My Feedback: (-1)

Sorry about two photos but my program only shows little thumbnails and I can't see them. The photo shows you the 4 anchor points on the guide tubes.
That really is way too much change and if you opened up the top or bottom of the fuse I would bet something else is wrong.
Removing the plastic rod is really easy and it would only take a few minutes to install a metal push rod to see if that will fix your problem.
It really is a fast thing to do.
Another thing I used to do a lot was make push rods from carbon fiber arrow shafts. You can eliminate all the braces and guides. A lot of the old kits gave you a dowel to make the control rods from.
You do have a problem but it's an easy fix!!

tailskid

My Feedback: (34)

The BEST pushrod IMHO is made out of the same material as the fuselage....so if you have a balsa fuse, use a (hard) balsa pushrod....fuse is fiberglass...use a fiberglass pushrod, etc....that is if it is possible

Gray Beard

My Feedback: (-1)

I just have to ask???????????????? I'm sure there is a good reason, I just don't know it but an inquiring mind wants to know. I only have one glass fuse plane and can't recall what was used as a push rod but I will look. I have never had a kit give me a balsa push rod but I have had them offer the hardwood dowel. I know my CG Extra has two dowels for the elevator halves. Not sure if I did that myself or not, that plane is a couple decades old.

jeff4912

Should I use 4-40 steel rods for this or 2-56 steel pushrods?

Does it matter if they are not perfectly snug(but slide freely of course) with the outer tube since they are rigid?

tailskid

My Feedback: (34)

If the material that makes up the fuselage expands, so will the pushrods if they are made of the same material.

049flyer

My Feedback: (18)

I recently had this very problem on my "go to" plane, changing of elevator trim. The difference is that my plane had wood rods with 2-56 rods.

Turns out that the Dubro metal clevice had worn the threads on the 2-56 wire just enough that it would jump a thread or two from time to time. Closer inspection revealed that the threads looked undamaged but when checked with calipers and a magnifying glass, they were indeed worn slightly.

In addition I discovered that the Dubro clevice was designed poorly when compared to Sullivan clevices. The Sullivan clevices lock at the seam, the Dubro clevices don't. I have now discaded all Dubro and similar clevices from my shop.

KaP2011

My Feedback: (17)

Use the 2-56 rods and here's a tip for you.
Use the plastic rod as a bushing, not the whole rod but 1/4" pieces of it spaced out along the metal rod and glued in place with ca. It will be as slop free as the plastic rod was. Be careful that you don't place a bushing in a position that will allow it to come all the way out of the outer tube as that can cause a control surface to lock in place.
I have been doing it this way for years and it has always worked well for me. You could use the entire inner plastic tube as a bushing but I've found that that usually causes a little binding due to the straightening effect of the metal rod.

jeff4912

I like your idea, thanks for sharing it.

Steve Percifield

My Feedback: (14)

I'm not going to read all the answers. I had the same problem. I replace the inner plastic rod with a 1/16" piece of music wire with clevises soldered to bothe ends, Cured the problem. The old Sig plastic pushrods were especially susceptible to the temp.

raptureboy

Holy Crap! Just change to the steel rods not a big deal and if needed add 3 or 4 small sections of the inner nyrod over the steel rod to act as guids. All arfs come with a plastic guide tube to run the steel rods through and it's not a problem.

Instructor

My Feedback: (16)

I believe someone mentioned the composite rods. They are called "Lazer" rods. They are made of a carbon and nylon composite and they won't expand or shrink do to temperature changes. I don't know if the inner rod from the Lazer rods will fit inside your existing sleeve......

Larry/Instructor

d_bodary

My Feedback: (85)

I used a metal 4-40 rod for my elevator. I just removed the inner nylon rod and put the rod inside the stock outer tube, I did not change the rudder pushrod, I just deal with the rudder trim change.