Push Rod Size?
 

Is there a deciding factor when going from 2-56 rod to ,4-40 rod.? Some of the Arf's I have are 40-60 size and some are91 and up to giant scale.Some of the larger aircraft come with 4-40 and some with 2-56? Commen sense tells me to use 4-40 on planes with WS of over 60 IN..
Stress,speed and design of the plane seem to me, the deciding factor.Is a general rule on this?. I would like your input.</p>

RE: Push Rod Size?
 

When in doubt, use the bigger rod (4-40).

JMO,

Bob

RE: Push Rod Size?
 

I don't have experience with big models, but I can tell you that the limit for long push rods is the pushing force.
The reason is that when pushing, the rod acts like a slender column under load, and the possible failure is always due to buckling rather than breaking under stress.

Hence, a 2-56 threaded rod that is properly supported along its length is safer and keeps the control surface more firmly in position than a 4-40 threaded rod that is not.

That is the beauty of the pull-pull systems: all the elements work under tension (which is self-aligning); hence, they can be made smaller in diameter.

RE: Push Rod Size?
 

Stress, size, weight, type of plane. pushrod length/bracing are all factors. A Unlimited Class Fun Fly plane only has a 46" wingspan, a .32 engine, and weights less than 3 pounds, but requires 4-40 rods. My first 1/4 scale, 72", G23, 15 pound Fokker Dr1 flew nicely with 2-56.

The geometry of the pushrod run and the type of pushrod are more important than the size and will dictate what needs to be used.

RE: Push Rod Size?
 

The insertion of balsa or carbon rods for long spans tries to fight buckling and vibration.
Any bent rod is more prone to suffer buckling, just avoid bends and run rods as straight as possible.

Good reading about buckling:

http://en.wikipedia.org/wiki/Buckling

The difference in diameter is important (25%):

0.074" (1.80 mm) for a 2-56 threaded rod

0.095" (2.42 mm) for a 4-40 threaded rod

(Edited to correct mistake / See posts #6 and #8 below)

RE: Push Rod Size?
 

Diameter isn't what handles load, area is - and there's a big difference there.

Per your diameter figures:

2-56, .074D = .00429 sq.in.
4-40, .095D = .00708 sq.in.

Almost twice.

RE: Push Rod Size?
 

So I guess, use 4-40 with proper support on all 60+ size models? I am asking this because, I have just purchased a GP Extra 300SP 46-80. In the RTF forum, I noticed a lot of posts stating that it is prone to control surface flutter and that there are a lot of comments regarding the hardware. I am planning to put either a 55AX or a OS-72A on it. Wrote a few PMs to the people who posted in the thread but did not get a response.

Also, when using carbon rods, arent they likely to shatter rather than bend? I have a Phoenix Yak54 60" that has carbon rods but it is still in the box and will continue to be in the perceivable future.

Another thing, which type of linkage is more desirable: clevis-horn type or ball link type or any other? Within that, whether to use nylon clevices or metal ones? All the models I have have clevices (Including the ones above) but I am aware of a few high-performance once that have other types. Also my Phoenix tiger has metal clevices on nylon horns while all the others have plastic ones

Ameyam

RE: Push Rod Size?
 

That is correct for tension and compression strength, which are proportional to the sectional area.

For avoiding buckling, the diameter is more important than the area, since rigidity depends on the section's inertia module, or how far from the center the material is.
That is why a tube is more resistant to buckling than a rod of similar sectional area.

Since that section's inertia module is proportional to radius^4, for similar ideal conditions, the 2-56 threaded rod will fail due to buckling at only 30% of the compression force that would make a 4-40 threaded rod fail by buckling.

Thanks for the correction, Dr1Driver; the small difference in diameter does make a big difference for practical use.
I have corrected my previous post.

ameyam,

Good reading here:

http://www.airfieldmodels.com/inform...ware/index.htm

RE: Push Rod Size?
 

I've used Sullivan Carbon Rods with tubes. The tubes are supported at each of the formers and at the exit point with epoxy.

I have a method that I use on them all to secure the steel rod at the end of the carbon rod... I mark the steel rod for depth of insertion into the carbon rod, making sure that the depth is well within the tube and within the fuselage, then I secure the steel rod with the chuck of a varriable speed drill, I then coat the steel rod with CA and immediately push it in the carbon rod while the drill is quickly turning, and don't stop until I reach the mark where it is supposed to go and stop. Once I stop, though, the CA is set and it is NOT coming apart and makes a real light-weight and strong push rod.

I have this set up on my Venus II as well as my Tiger 120. My Excelleron 90 has pull-pull for both the elevator AND rudder. Nice, but there is quite a bit of cable going back there... :eek: ;)

CGr

RE: Push Rod Size?
 

I second that. All beginners should read through that, especially regarding the hinges part as no one usually guides you on that. Thanks a lot

Ameyam

RE: Push Rod Size?
 

So true.

You prevent flex and buckling this way, which in turn reduces flutter and helps centering.

A thicker control rod does not necessarily get rid of lateral rod movement... but supports WILL.