Is there a general rule of thumb for selecting servo arms?

I'm finaly completing the construction of my 15 year old Goldberg Eagle 2. The plans show servo wheels being used. The manual goes back and forth between servo wheels and servo arms. Which do I use in which instance, and why?

I have following wheel choices from my futaba radio kit.

- large servo wheel 35mm in diameter
- star shaped (6 arms) servo arm 32mm accross
- X shaped servo arm 39mm accross

Thanks,
Doug

use whatever does not interfere with other servos or equipment that gives you the correct amount of throws.i usually use wheels that are no larger in diameter than the servo case .and a single arm for throttle.

If you are attaching your pushrod to your servo with a z-bend, then you are usually OK with a round or any shape servo horn, as the pushrod rides above the horn. If you attach to the horn with a clevis, then you want to make sure that the extra arms are clipped off so they don't hit the clevis as it travels.

I hope that makes sense.

Bottom line is, do what aerowoof suggests, and try different combinations to see which fits each application the best. If you go with an X shape or a 6-legged horn, you should clip off the extra arms to avoid interference.

Good luck

I like the star arms.. I just trim off the ones I don't want or need.

I'm not a big fan of Z bends but they do work..
I prefer a soldered clevis on the servo end and threaded metal clevis on the horn end.
I put a small section of fuel tubing on each clevis to keep em closed. Just a little extra insurance.

I can take a picture of my Eagle setup when I get home if you want.

loved my eagle it died after 15 years.[sm=cry_smile.gif]

really though they are fine planes fly great and a good tough trainer great on floats also my instructor made me do a taildragger set up with no tail wheel...he said if you can handle this on the ground you can handle anything it took a little getting used to but he was right[sm=rolleyes.gif] this also taught me to use rudder and controlled thrust (throttle) to turn. especially handy on floats!

The smaller star horns or the small round ones are good for elevator, throttle and rudder. Use the 6 star horn on aileron in an "X" not cross config. with the rods conecting to the farthest horns or the horns closest to the LE. This set up should give some mechanical differential. As previously stated trim away any horns that obstruct. Use a saw not clippers to avoid cracking the hub.
(don't ask how I know this )
The reason to do this is it will cause the functional aileron (the one that goes down) to move fully but will cause the opposite aileron to move much less this way it won't cause as much drag therefore causing less adverse yaw. or in simple terms it will help the plane fly straighter.

I was very fortunate to have an instructer that told me the "WHYS" along with the cause I said so

Use an arm one inch or longer, if possible. That, coupled with an even longer control horn, gives you the best resolution and most torque.

Dr.1

The longer the servo arm the less force on the control surface. Thrust is the same for any length of servo arm as it is usually labeled in in/ounces so a 1/2 inch arm will have twice the thrust of a 1 inch arm. However you are better off to use as long an arm along with as long a control horn as you can as the slop in any connection will be minimized.

2 shots.. First one is the eagle.. I forgot that the clevises on the servo end were threaded.. I thought I'd soldered them.

the 2nd is my Sweet Stick.. I used DuBro single arms on the rudder and elevator servos.
Yeah the plane looks a little rough.. it's 20 years old.. give the old girl a break

Servo arm length?

Always shorter than the horn it's being asked to drive. (when possible)

It's easy to estimate. After the horn is attached, move the surface the recommended throw and watch the holes in the horn move. You get an idea which hole will wind up with the pushrod connection. And see how much fore/aft movement the servo arm is going to need to provide.

Until you build the experience to just look and guess....... a short ruler held beside the servo tells a lot. Work the servo with the TX stick and see how far each hole in the servo arm moves fore/aft. Do the same measurement at the horn, only you'll be moving the surface with your hand.

A little ruler is a very useful tool.

Great info. thanks. I'm redoing my servo arms and push rods to reduce weight. I'll have to post a pic when I'm done.

Yeah, with a lot of work you can save a few grams. Is it really worth the effort for the weight? Maybe for a more reliable installation.

I have to agree with Bruce here. The weight savings is going to minimal, and IMHO not worth the risk of having weaker servo arms. It would be a shame to lose a plane over a broken servo arm.

Ken

ditto

He's got another thread going about the pushrods.. He set it up with 4-40 push rods and it came out tail heavy.. like 8oz in the nose to balance heavy.. shaving 1/2oz on the tail should take 2oz of ballast off the nose

Yeah... I bored out the holes in the existing servo arms, to fit the 4-40s. I figured I would change arms because I didn't want the smaller diameter rods going into the bored out holes. It might have worked, but I didn't want even that little bit of extra slop in the controls. So, I'm going to try to make the servo wheels work instead of the arms I had orignaly used. It'll save me a few bucks and a run to the LHS.

So far, the wheel has worked perfectly on the elevator control. I'm going to see if I can get the wheel to work on the rudder/nose wheel control.

That does add another dimension to the story previously unmentioned. You MIGHT save that 2 ounces if you're lucky. Worth a shot there as 4-40 is overkill in a trainer.

starfire73,
Lighter pushrods are worth the effort. But then, anything lighter is worth any effort anyone is willing to put into the quest. Not willing? Don't do it. Like the challenge? Makes the hobby more fun for you. And this hobby is fun.

As for lighter pushrods.............
We want "lighter" and don't want to lose stiffness.
So I've been doing something special for the models that I think are worth the effort (and expense). Been using carbon fiber tubes. Epoxy shortened metal lengths of 2/56 size pushrod into the tubes. I found a tube with an I.D. that fits the metal rods. There were a couple of issues, but I think I got 'em solved.

First off, the CF tubes seem to have a plastic like finish. Not a problem for the outside, but it bothered me about the epoxy getting good purchase on the inside. So I lightly scrape inside before gluing. The metal goes in about 2 inches. And I use 30minute to insure good grip, but more about the epoxy in a minute.

Second, if CF tubes have a weakness, it's that they can be split. Not a lot of chance of that, but I'm covering new ground and don't like to take chances. So I originally wrapped the ends with epoxied thread to clinch the tube. The thread was wrapped over the metal ends to provide overkill for holding the rod and end together. It was labor intensive. I found a better way.

Turns out the rods are about as rigid as anything can be. And they're lighter than arrow shafts by a good margin.

And there are a number of plastic tubes that fit over the rods for support within the fuselage. Some are as slick as the CF rod, so there will be no wear. If the rods aren't bowed, there won't be wear anyway.

Recently, I've found a better way to do the ends. But overall, I've got what I want. Overkill stiffness. Overkill lightness. Easy to make. Installation looks clean and is clean.

Costs me about $4 for parts for each rod for my 40size. Worth it to me.

You might want to do something like this next time around.

Here are a couple of pictures of the CF pushrods.

What I've started doing for the ends is worth a description.
I wanted something to reinforce the ends in a couple of different ways. What I'm doing now seems to provide those ways. When I epoxy the short metal end into the CF rod, I also wipe epoxy along the outside of the CF and the metal. I've got a piece of heatshrink waiting on the CF rod and slip it over the glue area. It covers the mess AND......... When I hit it with a heatgun, I hit the ends of the heatshrink first. That sort of seals the glue in. And as the heat shrinks the tubing, it also liquifies the epoxy. It appears to squeeze epoxy into the connection to begin with, and then squeezes excess out the ends. The heatshrink actually absorbs a bit of the epoxy and becomes much better support. I've done a test build to have something right away to destructive test.

The result is shown.

BTW, the light gray tubing that you see sticking out of the fuselage tail is cut into short lengths and put over the finished rod as it's put into the airplane. A short length supports the rod at the opening in the picture. Another short length is halfway down the pushrod and goes through it's own balsa stick "bulkhead". Pushrods that're supported in a couple of different places stand up over the years better.

BTW, heating the ends to shrink the tubing heats the epoxy. It insures a stronger cure, and it gives a shorter cure. After I wipe off any excess that squeezes out, after just a few minutes, the rod can be handled and installed.

BTW, I usually use solder clevises on the insides. I only had threaded ones when I did this Tiger60. The inside ones have since been epoxied in place. And I guess I should mention that I do the soldering BEFORE I glue the piece into the CF rod.

I should also mention something in the 2nd picture that might cause some people to wonder.

The blue block of stuff down under the rods is 3/4" thick insulating foam. To get the CG back where I wanted it, the battery pack could be placed where that blue foam is. So I cut a few pieces of the stuff to make a battery box back there. It's quick and clean. It's light as a feather and very strong. The piece you see is the top of the box so to speak. All 3 pieces were glued in with Canopy Glue. Canopy Glue is pretty useful stuff. If I ever need to pull that battery, the top is an inteference fit with just a little glue to hold, so can be pulled out. It'll be ruined but so what. The stuff is plentiful and cheap. And cutting it to fit is so quick and easy, it's not worth worrying about.

And you can do things like this with it:

DaRock..

What size CF tube and what is the gray tubing used for support?

I'm planing on doing the same type of setup on my Citabria but I hadn't worked out the tubing size yet.

Good idea with the heat shrink tubing.. nice and clean.

Nice work da Rock. Where'd you get the CF tubing, and the heat shrink tubing?

Golly, I don't know the sizes. I do a lot of shopping for stuff that I would use to scratch build my next masterpiece. Only problem is I'm having so much fun with ARFs, I'm not getting to them. And I wind up with raw materials at hand when something comes up I want to do my way.

I found the CF tubing in the LHS in a big array of tubes holding all different sizes of CF tubes. It struck me that the I.D. looked about the size of the 2/56 threaded pushrod that also is sold individually. So I grabbed one of the pushrods and test fitted it. Next time I'm at that LHS, I'll look to see the size.

The gray tubing was another "opportunity" find. But I didn't select it for anything, as any that is somewhat vinyl like will work to protect the CF from abrasion. I've used the outer tubing from goldenrod, I'm pretty sure.

The heat shrink actually seems to absorb the epoxy which turns it into something of far more value than it looks. I wanted something on the outside of the rod/tube joint to backup the epoxy that's holding from the inside of the joint. And something to surround the CF tube to support from the outside. And after tearing into the test construction, it was good to see the shrink had seemed to absorb some epoxy. Insurance I didn't expect.