Struggling with first Pull-Pull
 

My instructions don't talk much about tension, and I think that's where my problem is but I'm unsure. How do you set the tension?

When the rudder is neutral, cables are snug but not overly tight. When I input left rudder, both cables end up loose, and the return to neutral is very jerky. When I input right rudder, both cables remain tight throughout the range of motion, and it is very smooth and accurate. There is enough tension however that the surface the servo, battery, and receiver are mounted to flexes with right rudder, though not too bad.

What do I do? Here are some pics of neutral, left input, and right input, respectively. You can see that the servo horn has a slight amount of Ackerman as the connection points are aft of the center screw. The lines cross inside the plane.

Please help!! Thanks.

RE: Struggling with first Pull-Pull
 

Can you shoot some pics of the rudder end?

RE: Struggling with first Pull-Pull
 

How's this? I tried adjusting one of the ball links before which is why you see the rudder not centered perfectly. A click or two of trim centered it again, same operation issues.

RE: Struggling with first Pull-Pull
 

You must have the pivot points (if establishing positive Ackerman at the servo end) ahead of the pivot point of the servo arm. If establishing it at the control horn on the moveable surface, then the attach points must be aft of the hinge line of the moveable surface.. You want the lines to be barely snug at neutral with the line not being pulled going slightly slack as the surface is moved off neutral. If you get negative Ackerman by doing the opposite, you get in trouble fast as the line become to tight and can cause binding or cause excessive servo current drain.

RE: Struggling with first Pull-Pull
 

The center of the attach points on the rudder horns are even with the leading edge of the rudder, so either at or just barely behind the pivot point.

RE: Struggling with first Pull-Pull
 

OUCH! I found an issue. The rudder horn on the left side is longer than the one on the right side. Regardless of where the pivot point is on the servo, left rudder always ends up in more slack.

The hole in the rudder horn on the left side of the plane is 1" from the rudder, while the hole on the right side is 3/4" from the rudder. The rudder horn is one piece that slides through the rudder and is epoxied in place.

What I see my options as being are:
1) Drill a new hole at the 3/4" mark on the left side, trying to keep the hole the same distance fore or aft of the hinge line. My question here is about uncertainty of PC board strenght, drilling, and plugging the old hole.

2) Cut off the existing horn and try a new style. Issue here is the existing cuts and parts epoxied in the rudder either being in the way or weakening the attachment point.

3) Somehow modifying either the servo horn or adding some kind of tensioning device.

I like option 1 but would like to hear some comments (especially on drilling and plugging holes in PC board horns 1/4" apart).

RE: Struggling with first Pull-Pull
 

Well Joe, that really sucks but it's one of those little Nitroplanes things I mentioned. Other then that those look like pretty good little items. On most ARFs I tend to use after market hardware unless it's one of the higher end planes but then you are paying for it and the ARF will cost you a lot more. Wild Hare gets about $75.00 for there hardware package for there big planes and look at the price for a Horizon ARF, they usually have very good hardware but you pay for it. Time for you to start stocking a building box of your own. The photos show one of the items I use for my pull/pull systems, I think it is Dubro? I keep several sets of different sizes on hand. I make up my own pull/pull systems too using stainless steel plastic coated fishing leader of different sizes too. I would just remove the horns you have now and install something like these. I also make these up myself too sometimes using 4-40 all thread, not very hard to do. If you have some old plastic control horns around you can use those too and just screw them on from side to side. Unless you can get the wires even you will never get the system to work correctly.[&o]

RE: Struggling with first Pull-Pull
 

Thanks Gray Beard, you rock!

Now the hard part is figuring out how to get all that epoxied PC board crap out of the way:eek:

RE: Struggling with first Pull-Pull
 

So if I'm going to get new hardware, including rudder horns and maybe servo horn, do I install the new rudder horns behind the hinge line, or do they need to be bumped forward? For either option, how do I account for that on the servo side?

I understand the concept of Ackerman, but I guess I don't have a full grasp on it. I've got to go and study heat transfer so can't put time into learning about Ackerman right now.

Thanks.

RE: Struggling with first Pull-Pull
 

You may not be able to get it off clean but if you have a heat gun you can get the epoxy real hot and it will peel off sometimes. See, heat transfer!!:) Try not to over think this. On a pull/pull set up you want the servo arm to be even and you just go into sub trim and move it until the arm is centered. If your servo arm is one inch long you want the wire at the rudder to be at one inch long. If you have a nice straight shot for the wires then you want to connect them so the wires are right side on right side and left on left. If not then you can cross the wires. Sometimes you will just have to live with one wire going slack while the other is tight but if done correctly they are both tight when the rudder is deflected. That's in a perfect world but with an ARF sometimes you just have to live with how they have things set up and it's out of your hands. When I build a plane the set up is always on my mind and I build with that thought in the back of my head so things go smoother for me in the long run. Now if I could only remember to install the servo wire tubes in my wings before the wing is assembled. :eek:

RE: Struggling with first Pull-Pull
 

I see a couple of misconceptions in some of the above comments. First off, you are better off with some positive Ackerman in any pull-pull system and that means that the line not being pulled will slacken off a slight bit as a control surface is moved away from neutral position. The second one is that the length from the hinge line to the attach points on the horns must be the same as that on the servo arms. In fact, you can have the horn lengths as long as you want with no problems other than reduced sensitivity. There is a limit to how short they can be though.
as long as when the servo is at full deflection, the moveable surface is not moved beyong the limits of the hinge line, you are perfectly safe. Another popular misconception is that the run from the servo to the horns must be perfectly straight. Not at all true, if you use the proper guides you can turn them 90 degrees, route them around obstructions or most any other shape. Real planes have been doing this for ages with no problems and I have at least 10 large scale planes that I route cables around structure with no problems. Once you do a few pull-pull systems, you will never use push rods for the bigger planes or where minimum control slop is allowed. Probably the biggest mistake I see in most pull-pull systems is to tight a tension on the lines. You want just enough to prevent any slack in the neutral position.

RE: Struggling with first Pull-Pull
 

Well before starting to cut things up (and of course studying my Heat Transfer...after this, I swear!) I figured I'd try re-drilling the rudder horn. Now the two attachments are within 1/32" of each other, laterally and fore/aft.

Things are looking a little better. I still get more slack in the system when I use left rudder input than when I use right. I tried moving the points on the servo horn in and out, and fore and aft. Not much difference, although enough to decide I don't want the servo horn in front of the servo pivot.

How much slack in the system should I be getting when turning? I'm at a bit of a disadvantage having never really seen a pull-pull before.

Maybe a new set of horns is still the way to go for in increase in adjustability?

RE: Struggling with first Pull-Pull
 

Most systems will have a little slack on one side during full deflection. Unless your geometry is perfect this will happen. Most times it's not a big deal. When the plane is flying the wind will keep tension on the deflected side. Have you tried crossing the wires? This will help under some circumstances. As far as tension when centered. To much is worse then not enough. To much will wear out the servo and could cause a drain on the battery. Wearing out the servo is the biggest issue.

David

RE: Struggling with first Pull-Pull
 

Ok, thanks. There may be too much tension at neutral, and going from deflected/slack to neutral the servo starts fighting itself? It's really 'jumpy' as I let go of the control, and the tail shakes as the cables bounce around.

RE: Struggling with first Pull-Pull
 

Joe, if you do a search just by putting in the name Ackermon you will find a thread devoted to it. Go look it up. Most modelers have been setting up pull/pull without ever hearing the name and it has worked out great without any problems. A couple months ago there was an article in RC Report on how to install the pull/pull system with photos and some light math, very good article and not a lot of thinking to it. Go take a look at the thread here on RCU. No reason to go through it all again.:eek:

RE: Struggling with first Pull-Pull
 

gaRCfield

Go here it will answer all your questions, http://www.mindspring.com/~rellis2/rcpattrn/ppull.htm

RE: Struggling with first Pull-Pull
 

One thing, I don't know if it's a trick, that I do is to set the rudder control arms to the same width as the arms on the servo horns. This seems to make the tensioning more consistant. I also set the pivot points on the rudder end about 1/16" behind the hinge line, though it does cause the slack line to be a bit sloppier than some would like. Since I seldom fly high alpha there is always wind resistence on the rudder side being pulled inward, so slack on the off side is irrelevent. But it is forgiving of a not-perfectly-centered hinge line and it works well enough in practice. A little slack is worth the price as insurance since an increase in tension on both lines as the rudder is deflected is a bad thing.

Another system I have seen that seems like a great idea is to use a round sheave for the servo "arm" that is grooved for the cables. The cable is anchored on the rim. This gives an almost linear push-pull (it's like the steering system used on some sailboats) as it always exits the servo radius from the same point. I'm thinking on working this into my next model and I may build a mock-up just to toy with the concept first.

RE: Struggling with first Pull-Pull
 

I have seen the round wheel with the groove as well.. but have found from others that the groove / tightening mechanism isn't the best, comes loose and is expensive. Rather i was told by many that what is better is to get the standard wheel that comes with your servo.. Drill holes round the perimieter (5 or 6) with starting and ending holes 180 through the diameter and thread your line through them. Same affect and does not come loose ever. Adjustments are made at the control surface end.

RE: Struggling with first Pull-Pull
 

Thanks for everything guys. I have enough information to tackle the rest of this project. For some reason I wasn't expecting this to be such an exact science, but I guess that's what happens when you're a bit of a perfectionist.

RE: Struggling with first Pull-Pull
 

That's why I get a little poopsy when friends think of what we fly as "toys". I guess it's all how you look at exact science wise... with 6lbs or so going 50+ mph you better be sure the kit you built or ARF you put together and corresponding linkages are sound!!

RE: Struggling with first Pull-Pull
 

GA, I can't believe that it didn't arrive yet, But there is a full pull pull setup on it's way with a Hi-tec horn on it.

RE: Struggling with first Pull-Pull
 

Actually I just got it, thanks! I have to sift through and figure out what I can use. Unfortunately the control horns are all different sizes and are already epoxied in my plane so I have to figure out what to use, what to cut out, etc. The real problem lies in the rudder horn, which for some reason is asymmetrical and I think a large part of the problem. Cutting that out though an installing more pc board horns in the same area would be a bit nasty I think.

RE: Struggling with first Pull-Pull
 

Joe,
I had a similar incedent with my Yak, I would reccomend drilling a new hole rather than trying to remove the old horns. If you are doing any 3D with this plane
you will need all the strength possible. the only other option IMO would be to cut the horn flush with the rudder and drill a hole clean through rudder, dill a 4-40 hole through
a 3/8 piece of dowel and epoxy that in the hole. this will create a hardpoint (the rudder design is kind of weak in the NP yak) and use a setup like Graybeard posted

RE: Struggling with first Pull-Pull
 

I can recommend this horn type for 1/5 & 1/4 scale. It is easy to adjust to even width and robust in use. As Overbored77 recommended - I either make thin plywood cheeks around the base before covering or set it in a 1/2" dowel epoxied in the rudder for ARFs.

http://www3.towerhobbies.com/cgi-bin...?&I=LXBWL4&P=7

http://www2.gpmd.com/image/s/sulq3268.jpg

RE: Struggling with first Pull-Pull
 

Thanks, that's what I've been looking at most recently.

I've got to return some stuff to Tower and figure out what I need for exchanges, and then it's time for a break from this plane; it's starting to not be fun anymore. For a plane the same size and weight and power requirements of the Venus, this one is so much more complicated and expensive.