Pushrod Guides

Gang --

Reminds me of a story. Ten year old Virginia was watching her Mom preparing a ham for the oven. Mom cut 1/2" off the end of the ham before putting it in the pan.

Virginia: "Mom, why did you cut off that piece?"
Mom: "That is the way our family has always done it. I learned it from my Mom."

Mom was curious and asked her Mom why. Grandma said, "That is the way our family has always done it."

Grandma was curious, and found her Mom's old cookbook, and found the note, "Cut enough off the end to fit the blue pan."

Since everybody today is using carbon fiber pushrods, it is no longer an issue. But, even today, kits and plans still show a pushrod guide for pure music wire systems. If you built a non-fiber system today (.35 to .40 size profile with 5/64 wire), would you feel obligated to put in a pushrod guide?

I suspect the guides are not needed, and it is just that 1/2" of ham.

If the pushrod is "normal" length, you will, probably, need it. Try this: Give full up and hold the leadouts so they don't move. Now apply pressure to the elevator to see if you can push it back to neutral. If you can, the pushrod is flexing and you need a guide. Just a little flex will make a lot of difference. Take your pushrod and see how much you can flex it by hand before installing it. If it flexes, your plane will not corner (correctly).

(Oh, yes, and if you remember the note in the cookbook, it was necessary to cut the end off that ham in order to fit it into the blue pan. Sometimes there is a reason.)

Larry,

Do not ever use a wire push rod without a support.. I totaly agree with Leonard, however even if it passes the bend test when new, some day you might just adjust something and wonder why all of the sudden the plane won't turn inside.. Could be bad enough to not get you out of that wingover.. Been there done that plane wasn't repairable..

I've always used 3/32" (#40, .093") pushrods instead of 5/64"
(#56, .078"). They seem much stiffer. In addition, I make them 2-piece and use a coupler in the middle. This adds an additional point of stiffness. The resulf is a pushrod which is almost unbendable, but weighs a bit more.

I always wondered why we don't use a "pull-pull" system instead of a pushrod. Should be lighter, and the technology should be easy to come by from the R/C mob. I think "The Kiwi" referred to having used it many moons ago. My father said he tried using a pair of pulleys and connected the leadouts to the elevator horn. I gathered that the ratio was not well thought out (snafu), and it was a bit touchy. Late '40's or early '50's, I'd think. Steve

I've pondered the pull-pull idea myself. I have 4-40 sized pull-pull elevators & rudder on a large 60 sized RC funfly. They have been for dead reliable so far for several years & hundreds of flights. Easy to set up & adjust too. Maybe a regular pushrod from belcrank to flap horn, but use a special flap horn with a leg on the bottom as well as on top, then criss-cross the cables back to the elevators? You can also run pull-pulls through a plastic tube & route them anywhere you want. You could even set them inside a profile fuse, that would look good on a profile scale airplane, though it may make the controls draggy, I don't know. It works OK with servos, I do know that. One of these days I'm going to try this idea. I have a leftover set of pull-pulls in stock out in my shop, hmmm

You don't want to run anything through tubes or around pulleys, IMHO. If you HAD to, you might have another crank setup to make things line up correctly. We had a guy here tried a test model with "NyRods" when they first came out. Not a good idea! The needs of CL and R/C are quite different. Any friction in the controls must be eliminated. Servos don't care much. I believe the move to geometrically correct control systems will be a major step forward in CL flying. A sophisticated bellcrank like the "Expo-Crank" with less sensetivity around neutral would be another big improvement. I rather liked the BC design in the original "HUMBUG", but too much chance for wear and failure. Meanwhile, I found some small carbon tubing at a kite shop that will fit nicely over 3/32" piano wire or 4-40 rod, and it will stiffen the pushrod enough that you no longer need guides. This is about what Tom Morris' pushrods are. I have to figure out how to get some glue inside with the wire, tho. I think this would be a perfect use for Urethane glue. Steve

Steve, I wonder why you would want to put a carbon pushrod over a wire. It seems to be just adding unnecessary weight. A quarter inch carbon fiber pushrod is quite light--with ends it is just as light as the wire pushrod that needs the fairlead. But if you put a carbon fiber rod--even a smaller carbon fiber rod--over the wire pushrod, well, it might be a simple solution, but it would seem to end up with more weight...expecially when you are adding glue, etc.

Leonard, the carbon kite spars are .156" dia. x 29.5" long (about right to make two .35 sized pushrods flap to flap) and weigh 8.5 grams. In compression, these tubes are very stiff at full length, and when half length or a bit more to reach from flap to elevator, would be adequate by itself. However, with such a small diameter, the usual way to fasten the wire into the tube just won't work. About all you could do is bind short wires to the outside of the tube; really ugly. If the tube and glue add 5 grams to a 3/32" pushrod, I think it's well spent, and probably close to the weight of any guides you might use. I'm going to have to test CyA, Urethane and epoxy glues to see what works best, but I think this is a neat way to make a pushrod, especially for a profile,where the arrowshaft rods are too big and ugly. See Tom Morris' catalog! Steve

Yes, the C/F over the 3/32 wire is pretty cheap insurance that
an end is not going to come loose or split. Especially when young people are trying to learn the C/F tube ending system. I don't think it makes much difference for an experienced modeller where they take all precautions to ensure Rod ends don't split, loosen up, ect.

>I always wondered why we don't
>use a "pull-pull" system instead
>of a pushrod. Should be
>lighter, and the technology should
>be easy to come by
>from the R/C mob. I
>think "The Kiwi" referred to
>having used it many moons
>ago. My father said he
>tried using a pair of
>pulleys and connected the leadouts
>to the elevator horn. I
>gathered that the ratio was
>not well thought out (snafu),
>and it was a bit
>touchy. Late '40's or early
>'50's, I'd think. Steve

You don't need pulleys, you just need a cruciform bellcrank.

But, even
>today, kits and plans still
>show a pushrod guide for
>pure music wire systems. If
>you built a non-fiber system
>today (.35 to .40 size
>profile with 5/64 wire), would
>you feel obligated to put
>in a pushrod guide?
>
>I suspect the guides are not
>needed, and it is just
>that 1/2" of ham.

Leave it off, and you will quickly find why they are there, particularly with 5/64 pushrods! If you are using a bare wire pushrod, it's absolutely vital to use a fairlead of some sort! Heavier wire helps, but 3/32 MW pushrods are absolutely notorious for flexing on compression. I've seen a number of airplanes lost to this, and a whole bunch of others with "inexplicable" inside/outside turn differences. Using thicker wire helps a lot, but a fairlead and 3/32 wire is *much* lighter than 1/8 music music wire. 5/64 may end up bending anyway with a node right at the guide.

Use a graphite or fiberglass system, or at least 3/32 wire with a guide, on your typical 35-40 sized airplane. Beware of fiberglass pushrods growing after a while - no, I don't know why!

I haven't heard of the fiberglass pushrods growing, maybe it is because we have always used graphite once we started using composites. However, some "graphite" pushrods are really fiberglass in a different color. Are any of these what you are referring to?

On the subject of flexing, even the wire to the flap horn can flex if too light or set up wrong. I have two old Sterling Mustang stunters with moveable flaps. The one turned so tight I could fly at near normal level flight and do an outside half loop to inverted without hitting the ground. the second was built "exactly" like the first--even used the same paint scheme. But on insides it was mushy. It had a fairlead on the pushrod just like, and in the same place as the other plane. Still wouldn't turn. I put bigger elevators on it. Tried all sorts of things. Then one day I noticed that I could lock the leadouts and move the flap! The kit pushrod from the belcrank to the flap was flexing. I had to cut into the wing and replace that pushrod, but that solved the problem. It now turns as tightly as the original. (Both planes, by the way, were built in the early 50's)

Pushrods will bend under compression if not stiff enough or if they do not have fairleads to prevent that bending. Believe it now, or believe it later on when it comes time to fix Old Bowser.

Another reason all together.....

I use a system of guides that do barely "touch" so little to no drag, but it will capture the pushrod should the keeper let go. In essance, my systems could run without any keeper at all. You could call me paranoid, but from what I've seen, anything that CAN go wrong....WILL!
Eric

>I haven't heard of the fiberglass
>pushrods growing, maybe it is
>because we have always used
>graphite once we started using
>composites. However, some "graphite"
>pushrods are really fiberglass in
>a different color. Are
>any of these what you
>are referring to?
>

The yellow ones are the ones that grew. But I was aware that the black Dave Brown rods were in fact fiberglass and may have the same problem.

The graphite (picked out of the scrap bin at Aerospace Composites) doesn't seem to have this problem.