BULLSEYE

My Feedback: (3)

I am thinking about using a Pull-Pull System in one of my planes. I wanted to know if the control horn holes needed to be in line with the hinge point of the rudder. I know that this is usually the case with a conventional set-up (pushrods). I have heard that the control horns needed to be mounted differently for a Pull-Pull System. This going on a 40 scale plane.

Thanks,

Bullseye

jaka

Hi!
Same as on any other setup...holes inline with hinge line!
Use it where ever I can...on pylonracers, sailplanes and scale.

Regards!
Jan K
Sweden

Campy

My Feedback: (1)

Go to Alan's RC page http://homepages.ihug.co.nz/~atong/ If you scroll down a little there are 3 or 4 articles on installation of pull-pull systems including what is Ackermon and why you need it.

MinnFlyer

My Feedback: (4)

Actually, the whole ackerman thing is not that critical

It works great for cars, but it's really not needed on a pull-pull system.

But first, to answer your question... Put the control horn holes as close to in-line with the hinge as is possible, but don't go out of your way to get them perfect. 1/8" either way is fine.

Now let's tackle Ackerman...

Without going into a whole big explaination, the Ackerman principle states that if your pushrods connect forward of the hingeline, one cable (Opposite the one doing the pulling) will go slack. If the pushrods are connected BEHIND the hingeline, the cables will tighten slightly.

They say it is desireable to have one cable go slack, but I don't like that thinking. and since you have quite a bit of leeway as to how tight they should be, as long as you don't start off too tight, a little extra tightening won't really hurt.

To give you an idea of how tight I'm talking, if your control horns were 3" apart and 1/8" behind the hingeline, a 60 degree rotation (Which is a TON of rudder throw) would produce an excess pull of less than 1/8"

So try to get them as close as possible to the hingeline and you'll be ok

N1EDM

My Feedback: (2)

I don't know if it's mentioned in those other links or not, but I've read oneadditional 'principles'. The distance between the two wires on the control horns has to be the same distance on the Servo horn. That way, when you turn the servo horn, you are turning a parallelogram.

The rudder servo should be on the centerline, naturally.

Bob

MinnFlyer

My Feedback: (4)

Not true N1.

The servo horns DO NOT have to be the same length as the Control Horns

Deadeye

I put my horns slightly behind the hinge line. My horns rarley line up with the servo horn. If the rudder servo isn't centered, it will just have more throw one way than the other. I keep mine in the center, though.

jaka

Hi!
Not true!
The only thing you lose when you have a shorter servo arm than a rudder/elevator arm is traveling movement/angle at the rudder/elevator! And that is mostly a desirable feature! That way you get a very stiff setup.
The general rule in setting up any airplane is to have as short servo arm as possible and as long rudder arm as possible! The goal is to be able to push real hard on the elevator without it moving!

Regards!
Jan K
Sweden

BULLSEYE

My Feedback: (3)

Thanks guys,

I really appreciate all the great advice!!

N1EDM

My Feedback: (2)

It just doesn't seem right to not have equal spacing of the pull-pull wires... When you pull the rudder to one side, say to the left, then the right side will either slacken up or overtighten, depending on whether the spacing on the servo horn is narrower or wider than the spacing on the rudder horns...

You have to do the math.

Bob

JohnW

My Feedback: (6)

Diff size servo arm to control arm is fine. I do it all the time. Yes, it can change the slack in a line, but that isn't necessarily bad. As long as the non-working line is the slack line, you are fine. This works out, for example a rudder, it is being blown back putting tension on the working line. The non-working line does nothing, so if it is slack, or even missing, it won't matter... until later that is. I actually prefer this non-woking slack line setup. It reduces stress on servo bearing and helps keep a crisp neutral (slack line becomes tight as it approaches neutral.) The opposite I consider bad, i.e. non-working line tightens. If your lines at neutral are tight, then this only needlessly increases stress on the servo bearing at deflections other than neutral. Over time, I can almost guarantee your lines will loosen around neutral with that setup as everything is stretched out during use, which then causes a sloppy neutral, difficult to hold trim, etc. Cheers.

N1EDM

My Feedback: (2)

I know that people do it that way, but it still does not make good sense to me....

MinnFlyer

My Feedback: (4)

N1, it makes perfect sense. Regardless of different lengths, both sides of the servo arm move the same distance in opposite directions. It will pull the Control horn the same distance in opposite directions. If the Servo arm is smaller than the control horns, the servo will rotate more than the total rotation of the control horn (In degrees) but the distance traveled will be the same.

You can try a quick little experiment if you like...

Get 4 balsa sticks. mark one of them at center, and at 1" from center on each end (The length isn't critical as long as they're even) Mark another at center and at 1 1/2" from center. Pin both to your workbench through the center mark so they can pivot. Then join the ends with the two remaining sticks by putting a pin through the outer marks. Pivot the smaller stick, and the larger stick will follow.

N1EDM

My Feedback: (2)

I respectfully have to disagree with your analysis. To prove this point, I had to go and see for myself, so I made up two drawing in AutoCAD. I let AutoCAD do all the calculations.

First, I made up an uneven pull-pull system, with a 2" servo arm and a 3" rudder arm, spaced 9" apart. When the arms are at 90 degrees to the axis, the lines are even. When each arm is rotated on its pivot point, the lines become uneven lengths.

Then, I made another system, with the servo and rudder horns the same length. You can see that, even rotated, the lines are the same length. Sorry, but the numbers don't lie.

Whether you use AutoCAD or a slide rule (yeah, I learned how to use a slide rule, BC (Before Calculators), the math comes out the same.

Granted the numbers are a little extreme, but try it yourself - or give me some numbers and I'll generate a drawing for you to prove.... Give me any dimensions and angles, and I'll draw it out for you....

Bob

krossk

My Feedback: (27)

But I think that your mistake is in assuming the same angular displacement when the arms are of different lengths. The longer side in your first sketch will not actually stretch. THe length will remain the same and the longer arm rotate less, thus causing both sides to remain the same length. Certain geometries can allow slack in the side not pulling, but you won't stretch the side doing the work if you are using reliable materials.

N1EDM

My Feedback: (2)

Well, it looks like I have to eat a little bit of crow on this one...... but only a little.

I did another AutoCAD sketch based on krossk's email suggestion, and found out that I was technically right... but only 'technically'...

Different dimensions would yield different results, but the concept would be the same. As far as numbers go, I was correct - but only if you look purely at the numbers.

Let me explain the sketch below. It has three situations in it, with the rudder turned at 15, 30, and 40 degrees (note: this particular combination of numbers would not let the rudder turn to 45 degrees, as you might have in a 3D model). I had expected that the trapezoid with the 'rudder' at 40 degrees (the right-most diagram) would yield a really extreme difference between the two lines.... it didn't.

To review, the spacing between pivot points is 9". That makes the 'tension' cable of the pull-pull system 9.014". At its worst case for these dimensions (i.e., 40 degrees), the other line goes slack by a 'strong' 3/64", or about .045". That isn't a whole helluva lot... I have to admit.

Still, it goes to show that the closer the two pivot arms are to each other in length, the better off the system is. You keep tension on both cables which helps to reduce flutter, if that becomes an issue on your particular ride.

It also makes a case that if your arms are not of equal length, then the shorter dimension should be on the servo arm, not the rudder arms. If the situation were reversed, you could introduce binding into the system that could be hard on hinges - even a difference as little as .045".

Interesting discussion... Interesting learning experience...

Bob

krossk

My Feedback: (27)

And that is exactly why you don't tighten the control cables like guitar strings...

N1EDM

My Feedback: (2)

Well, my original premis still stands. Equal length on each arm.. It's just that the consequences of uneven arms aren't as bad as I thought they were going to be...

I was always under the impression that one of the features of a pull-pull system was that, with tension on both lines, you reduced the possibility of flutter.

We can each do it our own way...

Bob

JohnW

My Feedback: (6)

There is nothing wrong with unequal length arms on a pull-pull system. I do it all the time for the same reasons I don’t match servo arms to control arms on push/pull systems. I.E. reduction.

OK, technically I suppose one does reduce the possibility of flutter on a perfectly matched pull/pull system. However, getting perfect geometry is difficult. Not only must the arms be equal length, the attach point must be exactly in line with the hinge line, and this position must be maintained across temperature/humidity changes, flight conditions, etc. The scenario of having the non-working line go tight is really bad IMO and could introduce flutter. Because of the difficulty of getting perfect geometry, I feel a slight offset such that non-working line goes slack is justified as it ensures you won’t creep toward the bad geometry condition. I don’t mean super sloppy slack, just a little such that at full deflection the non-working line is no longer under any tension.

Think about flutter and how it is caused. If you get flutter when a rudder is under heavy deflection, the problem isn’t with your control system. Something else is messed up, such as hinge line leaks, turbulent flow caused by poor rudder/fin mating, etc. However, at neutral, the control system must be tight because the aero forces on the rudder are fairly equal left/right. At neutral, both lines “work.” This is why a tight center is so critical. If your center is loose, you could easily get flutter. Think about flutter on a push/pull system becasue of sloppy links (same basic idea as pull/pull.) It doesn't occur at full deflection. The sloppy link goes under tension or compression and firms ups. It is when the surface is at neutral that the sloppy link causes the problem.

I’ve set up a lot of planes this way (non-working line goes slack), including 200+ MPH ships and giant scale 3D/IMAC ships. I’ve never had flutter with the non-working line going slack setup. However, I did have a giant scale 3D/IMAC ship that had the “sloppy center” issue due to offset geometry in the “bad” direction. No matter how I tried to tighten the cables, I never could get a crisp center because my geometry was off just a little. I never had flutter, but the plane had tracking (wander) issues in yaw. In that plane, the arms actually were exactly the same length as I used a ball bearing bell crank and matching control horn. The issue was caused by the control horn not being exactly on the hinge line. It was only off by about 1/16” of an inch (on a 35% plane), but boy did it make a big diff. I eventually cut the arm out of the rudder and reattached slightly behind the hinge line so the non-working line went slack and the problem went away.

Cheers