Hi,

Any idea where one can get a round rudder servo arm (futaba) similar to the one in the picture, which Wolfgang Matt appears to use in his planes? The round arm should result in a more linear movement of the rudder. Any comments or suggestions would be appreciated.

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http://www.quickuk.eu/cat/general_pa...els.html?nav=2

Hangar 9 has them as well. You'll want the 45mm.

I've used them in all of my airplanes.

Ryan, do you have a link for a US supplier?

I googled around a bit and did not find them.

Sure thing, Bob.

For JR/Airtronics
http://www.horizonhobby.com/products...-large-HAN3512

For Futaba
http://www.horizonhobby.com/products...-large-HAN3515

http://www.horizonhobby.com/webapp/w...pe=productgrid

They also make a version for Futaba.

If you really want linear control, you need to use a wheel on both ends....I did this years ago on my EMCs...looks cool, zero change in cable tension through out the travel, and pretty much zero difference in control feel/response after rates/expo are dialed in. It does eliminate any side to side movement of the cables, so it is easier to make clean exits without the cable rubbing.

Regards,

Thank you, Ryan.

Don't know why I couldn't find them on the HH site...[]

Dave, are you using these now? Cables parallel (or thereabouts) or crossed?

I don't know either. I couldn't find it on the website and couldn't remember the part number, so I had to go into our internal sofware to find it! I'll see about getting that changed tomorrow so it's able to be found.

Todd has an elevator setup where the duel servos are at the center section of the fuse and two thin CF rods are making the direct connection.

Seems one can do the same for rudder? That is, no pull-pull but a long CF rod passing through several balsa "bushing" plates.

Not to argue that pull-pull is not ideal for pattern but like to bounce a potential different setup idea.

"Hangar 9 pull pull" in the search box produces -

http://www.horizonhobby.com/webapp/w...pe=productgrid


@ Bob - I stopped using the wheel on the servo arm (and on the rudder) since I really didn't find a benefit in terms of actual flight performance. The wheels can be a bit pesky with respect to the cables jumping out of the grooves.

I use crossed cables...with or without the wheels.

Regards,

In that case, it would be interesting to see the other end of Wolfgang Matt's "equation".

I suppose the only way to keep the tension the same in a pull-pull with a straight servo arm is if the connection points in the control surface horns were exactly the same distance apart as the servo arm connection points, and that the control horn connection points were lined up perfectly with the rudder hinge line. Which isn't very likely.

This sounds very similar to the Central Hobby DEPS set-up, but with 2 servos. I have used the single-servo DEPS and it works fine but was a little trickier to set up than I expected.

Now, I like the 2 mini's in the stabs.

I have seen Dan Landis' 2-servo set-up he (and some others) are using for the split rudder on the Passport. He uses 2 mini's flush-mounted in the fuse under the stabs.

With 2 servos on a conventional rudder with pushrods, it might take a bit of mixing to keep the servos from working against each other.

My experience has been that the most critical element for keeping equal cable tension throughout the throw is to have the connection point on the rudder exactly on the hinge line. For me, it doesn't seem to matter if the rudder connection points and the servo horn are the same distance apart (mine rarely are) as long as the connection point on the rudder is exactly on the hinge line. For example, the servo arm holes in play can be 2" apart while the rudder horn connection points are 1 - 7/8" apart. As long as the rudder horn holes are exactly centered on the hinge line, the cable tension will remain the same throughout the throw. Hope this helps.

Verne Koester

Hi,

On the rudder side Wolfgang uses a very traditional rudder horn that puts the connection point close to the rudder hinge line. A question to those that have used this setup (pull pull wheel on the rudder servo and normal rudder horn close to the rudder hinge line), do you get cables slacking and tensioning at the extremes of rudder reflection?

I have used the traditional approach (normal rudder servo arm) with the rudder horn on the rudder hinge line with good results. I agree that the servo and rudder horn lengths don't need to be the same. The pull pull wheel should however require smaller rudder wire exit holes in the fuze and less expo on the Tx. I however don't want to go the pull pull wheel route if it results in significant cable slacking/tensioning.

Regards
Clint

My question is, if you choose to use the 45mm pull-pull wheel on the servo, what would be a good width to use on the rudder control horn? Is there a ratio that would work consistently from 2m plane to 2m plane (I've always used a starting point like approx 1:1 and adjusted the length of the rudder control horn/clevis from the surface to get the desired feel)? I'm like Clint, I want to avoid any possibility of slack being created. BTW, Ryan, I called Horizon yesterday and was told by the phone rep that the above pull-pull wheel was only 35mm. Thanks, Everette

On my passport, both pairs of servos for elevator and rudder half are at the tail.

If the rudder servo (assuming one) is mounted at the center of the fuse, whether a push-rod setup is equally reasonable. The weight of a 2mm CF rod + some balsa plates probably is close to that of the pull-pull wires, and you definitely will not see any slacks.

Hi Clint,
It's been a few years since I used a pull-pull wheel but never had a problem when I did. The wheel, like a conventional servo arm is going to have equal throw in both directions (if the right side goes forward 1", the left side will go rearward 1"). The trick in all of this is to make sure the pivot point back at the rudder is exactly on the center of the rudder hinge line which is the pivot point of the rudder. If the connection points on the rudder horn are offset from the hinge line, you'll have unequal rotational movement of those points. That's when you get cables that tighten on one side while going slack on the other. You've got a rudder arm or wheel that's moving equally in both directions on the servo end and connection points on the rudder that aren't moving equally because they're offset from the pivot point of the rudder which is the hinge line of the rudder.

Finding horns that will put the connection point on the hinge line can be difficult. Most of the ones I've seen don't project the attachment point far enough forward of the mounting base of the horn. By the time you sand the bevel into the leading edge of the rudder, there's nothing left to screw the base of the horn to while still keeping the attachment point on the rudder hingeline. I designed some horns a few years ago that projects the attachment point 3/8" forward of the front of the base to take care of that. They were machined by Geoff Combs (former F3A pilot) at his rapid prototype shop in Ohio and are still available for sale. I use the shortest ones for the rudder, but all three sizes are designed to be interchangeable with the pivot points 3/8" forward of the front of the base. I'm not sponsored by Geoff and don't receive any royalties. I was happy just having a nice set of horns available that would take care of the problem. Here's his website with a direct link to the horns: http://www.aerosportproducts.com/rclinkage.htm

Verne Koester

I've found a simple way to do what Verne describes. I use a steel bolt as my horn and bend the bolt forward as necessary. Bending is also very simple if you can get a piece of aluminum stand off with the thread you intend to use (I use 6-32 which is about 4 mm).

The steel bolt is good quality but not hardened. The aluminum is screwed down on the bolt partway and is used as a handle to bend the bolt the correct amount by gently pushing with finger pressure. Then it is unscrewed a few threads at a time and bent some more, and so forth, until the correct bend is put in....

All thread is not recommended because it's usually made of punk steel and will fatigue and break easily in gas/glow applications. But it might work fine in electric set-ups

The concept of using CF rods with supports sleeves was around long before DEPS (which nicely packages all the bits together). I know myself and several others used DEPs style systems for the elevator in the mid/late 1980s, and single CF rods on the rudder. More recently, I used the Sullivan CF nyrods in DEPs configuration, and the CF nyrods are much easier to install as they are tolerant of some bends/curves. The problem with these type systems is that the thermal expansion rates are not the same for the pushrod material and the fuselage material....so there is a trim change with changing temperature (more of an issue with the CF nyrods than solid CF rods). My preference now for pattern planes is dual elevator servos - which when properly installed are easy to match throws across the range of travel with a little help from the balance function (on the JR 12X or 11X).

I used to be a fanatic about rudder cables maintaining the exact same tension throughout the rudder travel...and I tried all the various systems/combinations with varied degees of success. As mentioned by Verne, the biggest factor is having the control horn attachment points on the rudder hingeline, which, is not always possible, in which case it is very important to have symmetrical offsets on each side. With any scenario, there should be no offset in the servo arm. At this point, so long as the cables are tight at center, and center consistently, I don't worry too much about the slack cable, so long as the slack cable has the same behavior in both directions. Assymetries in the horns or servos do cause problems.

Regards,

Matt,
Very interesting. I had 4-40 and 6-32 bolts break on 2C 60 stuff back in the day. So I changed to 4-40 all thread, thinking it had lower temper, would be more flexible, and less likely to work harden and fatigue from vibration. I have a couple 60 2C planes with many flights without failures using 4-40 all thread, and I have several 2M glow planes with many flights without failures using 6-32 all thread.

What is the grade / composition of the steel bolt you recommend?

Regards,

Here's a good read on this subject.... http://www.qmfc.org/school/ackerman.htm

FWIW, I align the Rudder horns to the hinge line and measure the distance between the hole centres. This measurement is then used to make a CF servo horn which is screwed to a round servo wheel. Works well for me... In theory that should give me a nice rectangle.

Cheers,
Jason.

The thermal expansion rate of CF rods is very small. If the fuse is fiber glass, the long CF push-rod pull the control surface off the center in summer. This can be fixed easily through centering trimming from the radio. BtW, The thermal expansion rate of fiber glass available online is from 0.8% to 0.16%.

0.8% to 1.6%?

Not insignificant![X(]

See http://www.springerlink.com/content/m574015j3v6714r7/. The 0.8% expansion rate is at a high temperature.

The thermal expansion coefficient is 55x10-6 when the temperature is no more than 70 degree (C).