flyerdave

My Feedback: (212)

50cc Sbach with pull pull rudder.Cables are tight at neutral, but when I turn either way, the cables go slack. I have attached a picture of the servo arm. Is there anyway to correct this problem or do I need to go to another type servo arm? Thanks for the help.

sensei

You might want to read this, Pull-pull setup

Personally I like push pull setups, but that is just me.
Bob

speedracerntrixie

My Feedback: (29)

Can you post a picture of the rudder horns as well? From what I see you have an offset aft of the servo pivot. For starters you would need to have the same amount of offset aft of the hinge line on the rudder. Second is that your servo arm is way longer then what it needs to be. You are loosing a ton of power and resolution with that setup. Best case scenario is that your servo arm is the same length as the control horns, neither have any offset and the cables are not crossed.

flyerdave

My Feedback: (212)

I will post a pic later today of the rudder horns. I will also measure the length of the horn. If I understand right, this length should be as close as possible to the same distance as the servo arm. Am I understanding the above post correctly that the best case scenario is NOT to cross the cables?. I was thinking that crossed cables were the best scenario.I do appreciate the help, been flying RC for 35 years and I am surprised sometimes at how much I still have to learn. Thanks

Rodney

You have a slight it of positive Ackerman on the servo arm (the attach points of the cable is slightly aft of the center line on the servo). Nothing wrong with this, in fact it is preferable for the non pulled line to go very slightly slack as you move off neutral position. I've been using pull-pull on all my large models for that last 20+ years and ALWAYS make sure I have a bit of positive Ackerman as it is much easier on the servos. Some argue against this but, there is never any problem caused by having that non pulling line go a bit slack as you move off neutral as long as there is no slack at neutral. Another error many make when using pull-pull is to have the lines to tight; you want them just taunt enough so there is no slack at neutral.

sensei

I agree with Rodney, as long as the cables are snug at center it just doesn't matter that the non loaded side goes a little slack when the rudder is deflected because it adds nothing in holding power to the loaded side during deflection.

Bob

Jennifer Curtis

Crossed cables are not better or worse.

Crossing them makes them exit the fuselage closer
to the tail so there is less outside the fuselage.

They should not rub the fuselage or each other
no matter how they are routed. It is ok to have
the rudder horns slightly higher on one side and
lower on the other to keep the crossed cables
apart.

I wouldn't consider the arm length a problem
as long as most of the servo travel is used
going from full left to full right. If you have
to set the servo endpoints at 50% or less
consider shorter servo arms or longer
rudder horns.

The cables should slacken only VERY
slightly as the rudder goes off center.
This is caused by the clevis pins being
out of line with the hinge pin on the
rudder and/or the center of the servo
shaft.

If there is a lot of slack because both
ends are off, you can try using the
inline holes on the servo arm. If the
clevises bind on the arm, you can
flip the arm around so the offset
holes are forward.

It is possible to use one offset hole
and one inline hole, but you need
to be sure that when the rudder is
centered, the servo arm is slightly
cocked to make the length of each
cable equal.

Jenny

flyerdave

My Feedback: (212)

Thanks for the help. I think one problem is the difference in distance at servo arm and rudder horn. Rudder horn length is 1.25 inches and servo arm length is 2.00 inches from servo center.I'm going with a shorter servo arm or at least move them to match the length of the rudder horn.

Rodney

As long as there is no binding at the extreme activation, the difference in lengths should not cause any problems. If the servo arm is longer than the rudder arm of coarse you will get more rudder throw in degrees moved than the servo moves and some loss of force. You can make the rudder arms as much longer than the servo arms as you want, you will just get less throw but more force. The problem is when the rudder arms arms are so short that the rudder throw results in a binding at the hinge line taking place at extreme throw.

flyerdave

My Feedback: (212)

Tried moving the connections in on the servo arm and still no better. The servo arm that was being used by the guy who had it before me is in the picture. I'm using a different type servo so I'll need to change the servo wheel that it's attached to. I'm going to try this one and see if it helps. thanks

Lone Star Charles

flyerdave

If you will give me the measurements of your Sbach installation, I will do a drawing that will give you a pretty good idea where to put your servo arms and control horns. I'll need the dimensions from the servo output shaft to the rudder hinge line as well as the arm or horn length as well as any forward or aft offsets. Usually, I leave the rudder with no offsets and compensate with offset in the servo arms, but can go any way you want.

Charles

acerc

I make servo arms out of flat brass on my scale builds. You have to match the center of rotation (the servo screw and rudder hinge center) and the linkage attachment points. Once you have drawn them out for a servo horn draw a line center of the screw and the linkage mark, the linkage can attach anywhere along this line and keep linearity in movement. I measure how much movement I want in the rudder horn and drill the servo horn for maximum travel. Pic 138 is where I drew it out, the black circles are a match to the rudder but the red are where it needed to be for maximum travel.

Rodney

Your picture shows a large amount of positive Akerman at the servo which may have been compensated for at the rudder (can't tell from the picture). Nothing wrong with having positive Ackerman, just commenting on it. Now negative could be a problem.

acerc

Like I stated the servo matches the rudder so there is no slack in movement. I suppose it is called Ackerman and yes negative would be damaging. What is important is to match the movement on both ends, when doing rolling circles slack in the cable is not a good thing.

BarracudaHockey

My Feedback: (11)

About the only way to get the correct geometry on that plane is to use the arm that came with the kit (that posted a picture of)

Or
Not worry about the slack because the air load will keep it tight.

Rodney

Slack will not bother you as long as there is none at neutral.

flyerdave

My Feedback: (212)

What I really needed was to use the arm that came with the plane(pictured).I was using a different brand servo than the previous owner so the servo wheel that was attached to the kit supplied arm wouldn't fit. Last night I changed the servo wheel to a savox and hooked everything up, PERFECT. Thanks again for all the help and Barracuda you knew it would work didn't you.

BarracudaHockey

My Feedback: (11)

Been there done that.

Now all you have to do is deal with those hideous markings

flyerdave

My Feedback: (212)

Oh I've decided to leave the name on there so the plane will fly better.See I've done my research and found out that the original owner of this plane was one great pilot.

BarracudaHockey

My Feedback: (11)

Oh LMAO

Is that my old SBach?

EDIT: Yep I just noticed the scale N number on the tail.

Small world eh

flyerdave

My Feedback: (212)

I'm getting ready to fly her for the first time this weekend. I'm at home today because of a pretty big ice storm here last night, so I'm about to go put a little run time on the DA 50 that I put on the front of your old SBach. I am really looking forward to flying this plane.

BarracudaHockey

My Feedback: (11)

I loved that plane. Only reason I sold it was to make room for my Corvus that I got for review and didn't have room for all the 50cc planes.

I take back what I said about the markings, you should see the crappy stock graphics.

And a plane with "Pilot: Andy Griffith" on the canopy belongs in NC!

daytonarc

My Feedback: (5)

lol

flyerdave

My Feedback: (212)

Well, that SBach is out in the back yard right now with a sweet sounding DA50 purring along while its 28 degrees and my daughter just came over and told her Mom that only a crazy man would be out there in this weather playing with a plane. I guess I'm crazy!

jester_s1

I'm glad you figured it out. For future reference, it's hard to make things any easier on yourself than to use the DuBro threaded control horns. I have the servo arm like your black one and was able to match the length perfectly at the rudder to it. I have no Ackerman in either direction with the cables making a low bass note when plucked. It took all of maybe an hour to set it up once I had the cables made. And the really great thing about the threaded control horn is that if you make a small mistake in the geometry, the adjustability lets you fix it.