Hi Folks
I am installing the gear in my MK Skystar and I have a dual elevator coupler at the back and a CF pushrod to connect it too the servo.
Do I need to support the CF pushrod at one or two point to cut down slop?and vibration. if so what is the best way to do it.
I would estimate the distance between the servo and the coupler as just over 24 inchs.

Mike

I have a 22" pushrod to a MK bellcrank in my MATRIX with no support and
it works fine. I used a 1/4" rod from the servo to the bellcrank and 3/16" rods
from the bellcrank to the elevators.
I am scratch building a new plane which will require a 29" push rod and I boxed in
a 1/4" CF pushrod with 1/16" balsa. Just CA'd the balsa strips around the push rod
the full length almost to the threaded inserts and it stiffened it up considerately.
Didn't add hardly any weight and it just looks like a square balsa pushrod now.

tommy s

Depending on the length of the rod you may want to use support. The reason why the rods break is the vibration gets the rod to move to a certain harmonic pitch and then snap! What I did and learned this from many pattern flyers is that I have a piece of foam about midway between my bellcrank and where the servo is mounted. the foam acts as a dampener for the push rod, and will not allow it to get the harmonic pitch that would cause failure. I use silicone to attach it to the inside of the fuse. (of course this is just my 2 cents)

I also use a piece of foam block (similar to the ones in the JR & Futaba aluminum case) in the middle also. Just run a hot piano wire slightly smaller than the pushrod diameter. Attach the bellcrank side first. Then, insert the foam from servo side. I used another long carbon rod to push it in. The foam is slightly wider than the location where it goes, so the fuse sides are pressing the foam a little. It never moved for the last two years.

The foam piece by itself will be enough to prevent the harmonic. You don't have to attact it to anything do you?

We modify our MK devices and make them pull pull. It's the best way I've seen to do it so far. It's almost no weight also. Mike

Hi Folks
I take it there is very little binding where it runs throught the foam so it wont effect the linkage performance?

Mike

I have a carbon pushrod going to the splitter in my Alliance and have had no problems either with vibration of the rod or fatigue of the connectors in the 2 1/2 years it has been in there. This must be a run of a good 2 1/2 - 3 feet and I was using M3 connectors at either end. Up to you but you may want to try it without any support first and once you have had the engine running you will be able to see how it is responding. If there are no resonances set up at any rpm I wouldn't bother fitting anything. This is the simplest and lightest way to fit the pushrod! and if there is some vibration you can always fit a support afterwards.

The pull-pull system sounds good too, I have seen them done and they are very precise and light, although I would still probably use a carbon rod in my next plane simply because I am familiar with that set-up.

James

[quote] James,

I really must disagree with the above statements. I lost a plane due to the push rod end fatiguing due to resonances and I never had a clue that there were any resonance issues. My feeling is it's better to be safe than sorry and to just add a support and be done with it.

Sam

Sam

What kind of ends did you have on the CF pushrods. I use the titanium
ones from Central Hobbies.

tommy s

Tommy S.,

This was years ago in the mid 1980's. I seriously doubt that the Central Hobbies ends will flex enough to fatigue. (When I read that statement about not supporting the rod I instantly went back almost 20 years and saw that plane going in. I honestly didn't think about the stout stuff that Central Hobbies has.) Saying all of that in the ( ) if I were making a push rod much over 15 to 18 inches I would still support it somewhere close to the center. If you do have a harmonic it will raise hell with everything in the assembly and cause premature ware. We use those push rods because they are stiff and transmit exactly what we want without flax. If we then shake hell out of everything what's the point? We defeat our purpose.

Sam

Honestly the only way these pushrods can vibrate harmonically is if they have
some flex to begin with and at 22" there is absolutely no movement in these
carbon fiber rods at all. The only movement at all is the rubber mounting of the
servos and that is almost nothing. If you haven't used the CF rods with the
titanium ends you cannot believe how stiff they are and if you use good ball links
at the control ends there is no slop at all.

tommy s

I used a solid 1/4" carbon rod in my Goldberg ultimate, after the guy I bought it off used, well pretty much BROOMSTICK!

yerk, the pushrod alone weighed 4 ounces! I replaced it with my out carbon version with 4-40 rods in a Y configuration at the tail and WOW it's soo stiff and it's unsupported, but I too feel that there should be some damper if not only to stop the noise of the pushrod humming.

Two great issues here
1. supporting a carbon push rod -- I don't think the pushrod will bend, but there's a stress point wherever the assembly goes from carbon to the pushrod ends. I found (the hard way!) that 2-56 hardware ain't strong enough to deal w/ the stresses, whether caused by harmonic vibes or whatever... flight #5 on a new Focus ended ...poorly... Same pushrod w/ 4-40 ends (same plane too after some major repairs) now has 200 or so flights on it. I do use a bearing at the tail of the airplane to support the rod aft of where it Y's out to the elevator halves.

2. The other issue -- stress induced by harmonic vibration - an excellent one. Any foam or balsa that constrains the pushrod's lateral movement will work I think - I've used both. One thing though - I DON'T think it's a good idea to put the vibration dampener at or near the 1/2 or 1/4 point - if the rod vibrates, chances are the 1/2 and 1/4 length points will be stationary nodes, and there'll be no movement there... just on either side of it.
This is one place where it's probably better NOT to measure precisely - I just put the damper somewhere near an end, NOT at the middle - and so far it's been a good solution.

-Rick

2nd and 3rd harmonics huh???

good point.

I don't use 2-56 on anything, even on my 40 size Venus and Matrix I have
4-40 and 3mm titanium ends. Maybe overkill but makes me feel better.

tommy s

the best way to kill the vibration, if I understand this problem correctly, is to put some foam rubber, similar to what the radios come packed in, inside the carbon fober rod (assuming that it isn't solid). This is what is done with golf clubs to reduce the transmition of vibration. It reduced vibes in steel about 98%, Sensicore if any of you play golf, and I'm sure it will do the same for carbon fiber or graphite.

The foam in Sensicore golf shafts does nothing but support a plastic device
that looks like a soda straw which actually does the dampening by vibrating
itself at a similar frequency and canceling out the shafts vibration.
The problem with Sensicore, and why some golfers don't like it , is it adds
so much weight to the golf shaft. Probably same problem for us if we used
it in airplanes.

(My brother runs a golf store)

tommy s

the foam does the vibration dampening, and the shafts with sensicore actually weigh less than normal steel shafts, but arguing golf isn't my point, all you would need in a push rod is about 1" or 2" of the foam and the weight of that is negligible,

True Temper Dynamic Gold Lite Iron Shafts - 110 grams / with sensicore 114 grams
True Temper Dynalite Gold Iron shafts - 118 grams / with sensicore 122 grams - etc,etc,etc.
Out of True Temper Catalog.
My brother is on Ping's and Callaways and several others staffs and has tried every
imaginal shaft combination you can think of. I can tell you without any doubt the foam
inside the shaft does nothing by itself. Some companies use a lighter weight shaft if it
is to be used with Sensicore as adding Sensicore to a shaft definitely adds weight.
If I were to use foam to dampen a carbon fiber pushrod I would put it around the outside
of the rod as discussed earlier in this thread.

tommy s

like I said I don't want to argue golf stuff, but the foam inside the carbon fiber rod will dampen the vibrations

If you think it will, then stuff it in there. The hole through a 3/16" carbon fiber
pushrod is probably smaller than a matchstick and that little foam in that small
a space will have no effect at all in my opinion.

tommy s

if it is that small of a hole then it will have a minimal affect, you're right