H5487

I see a lot of pull-pull set-ups for rudders but not for elevators? Is it due to the potential for flutter? I'm under the impression that pull-pull systems are tight and slop=free.

Many full sized aircraft (i.e. Cessna, Piper, etc) use pull-pull on the rudder because it's simple and light weight but use an aluminum tube pushrod on the elevator for safety. If a rudder cable breaks, you might have a wild ride during the landing flare but you'll likely survive. However, if an elevator cable were to snap, a controlled descent would be difficult, if not impossible. (Yes, I've read about how to use the trim tab to create resistance against the remaining cable but simulator tests have shown that the airplane will be totally out of control by the time the pilot figures out which cable has snapped and which way to trim.)

Harvey

DMcQuinn

My Feedback: (13)

The first plane I ever did pull-pull elevator was on a morris hobbies giant su-do-khoi. This is a 80 inch span profile fun-fly. It had three pull-pull systems (rudder, left elevator, right elevator). This puts all the servos near to the CG of the plane. I have done pull-pull on other elevators as well.

AA5BY

My guess is on a model it has more to do with the geometry. A rudder offers a symmetrical pull/pull cable installation where an elevator very often doesn't and elevators often require dual push rods and wouldn't that get more interesting with cables.

H5487

D McQuinn

Thanks for the info. Do you know why elevator pull-pull set-ups are rare compared to rudder set-ups?

Also, on your own aircraft with pull-pull elevators, do you run a complete set of cables to each elevator half or do you run them to one side only and connect the halves to each other?

Harvey

H5487

AA5BY,

Thanks for your reply. It looks like you responded to my question while I was responding to D McQuinn's reply. Does that make sense? I might have to lie down...

Harvey

allanflowers

It seems that most WW-I models I am familiar with have pull-pull on elevators as well as rudder. The geometry issues are not a problem if one is even slightly clever. The idea of a steel cable breaking on a model is absurd - certainly no more likely than any of the problems with a push rod.

Rodney

I have built at least 5 1/4 scale planes using pull-pull on both elevator and rudder. Much prefered over push rods and much cleaner and reliable to instal and use. Just be sure to have a bit of positive Ackerman built into the system. And,no, you do not need to have a direct path for the cables, it is easy to use nylon tubes as guides to go around obstacles or to change directions. Neither do the arms on the servos have to be the same length as the arms on the surfaces
If you can get a copy of the May 1993 issue of Model Airplane News (the article is also available in one of their How To books) I have a very detailed construction article on how to build your own pull-pull systems that is still very pertinent.

H5487

Rodney,

What is "positive Ackerman" and can you give me a link to the atricle that you mentioned?

Many thanks,

Harvey

Rodney

H5487, one of the better sites is http://homepages.ihug.co.nz/~atong Which is Allen's hobby shop, an excellent place for lots of advice. Just burrow down through the topics to Ackerman.

Rodney

http://www.rcuniverse.com/forum/m_72...tm.htm#7237017

Here is another good source with some excellent pictures to show you how one person did it.

allanflowers

Here are some pics of the pull-pulls on my Siemens Schuckert DIII, first in CAD then the actual model.
All the servos are in the front of the fuselage for weight balance reasons. The elevator servo drives four lines (30 lb Kevlar - McMastersCarr)) through Goldenrod tubes (I didn't use the supplied braided wires for radio reasons). Small brass plates soldered to Dubro rigging connectors make up a "splitter" so each elevator servo arm can drive two cables. There is no Ackerman on this system and it works perfectly.
The fuselage on this model is balsa skinned and non-accessible but the plastic tubes allow the cables to be slid in from the ends very easily. All adjustments are at the servo end so the visible hookups are scale accurate.

Rodney

That looks like a very neat setup you have there allanflowers. Yes, no Ackerman works but it is very hard to achieve, that is why I recommend that you put in a bit of positive Ackerman because the slightest error in construction can wind up giving you some negative Ackerman which can be catastrophic due to excessive current draw on the straining servos. I also always route my lines through tubes in case I ever want to restring one or change one out for some reason, it makes it real simple to do. IMHO the two most common mistakes people make when they first install a pull-pull system is to accidentally get a bit of negative Ackerman built in or are making the cables to taunt. You want them just barely tight enough so there is no sag in either cable at neutral.

Lnewqban

Harvey,

Some more info, schematics and references can be seen at this thread:

http://www.rcuniverse.com/forum/m_92...tm.htm#9233519

Regards!

H5487

Thanks, gentlemen, for your tries but the links to Ackerman is dead. Therefore, I have no idea what Ackerman is.

Harvey

khodges

My Feedback: (1)

"Ackerman" refers to the pivot point of the hinge relative to the position of the control horn's tip (where the control wire or rod is attached). Neutral would be with the tip exactly over the hinge point, negative and positive (I forget which is which) would be forward or behind the hinge point. Ackerman controls how much constant tension, or development of slack or increasing tension occurs in the control wires during control movement. You definitely do NOT want increasing tension, as it strains both the servos and mounting points of the control horns on the surfaces. Some slack on a slower flying plane with a large control surface is fine. The slack will develop on the "push" side of the control, as air pressure on the control surface will hold the "pull" wire in tension. A fast plane or one with a smaller control surface will develop flutter more quickly with increasing slack in the wires.

I've used a pull-pull setup once on an elevator system once, with great result. I did it primarily for scale detail, as it was a WWI plane (Bristol M-1). I used a single servo and bell-cranked it to a horizontal rod which had 2 arms, one for each pair of top and bottom wires. My elevator halves are tied together with a central rod, so if any one wire were to break, there is redundant control.

Like Rodney mentioned, I ran my wires through tubes to the exit points on the fuse. I used neutral Ackerman, and ensured all the tubes and pathways were symmetrical. this model was sort of a testbed for what I wanted to do on my later, larger scale WWI planes. The beauty of using the bellcrank and rod is that you can place the servo way up front if needed for balance, and run the rod back to the horizontal control, which allows you to shorten the runs of the control wires in the pull-pull setup. This can be for convenience of linkage location, and shorter wires are easier to maintain in adjustment for tension.

In the interior pics, you can see both the rudder pull-pull wires and the elevator pull-pull with the bellcrank arm to the servo, and the wires entering the tubes. I used coated leader line of about 25 pound test for my wires. This plane is 1/6 scale; I'll use at least 50 pound test for a 1/4 scale model.

da Rock

It's on your automobile. When you turn your steering wheel, your front tire on the inside of the turn angles more into the turn than your front outside tire angles. It's a good thing on automobiles.

How is is done? The same way it screws up pull-pull systems on airplanes.

BTW, all WWI airplanes had pull-pull on all their surfaces. Wires going everywhere. And they were so troublesome that almost every airplane had at least one rigger who did almost nothing but adjust those wires.

You know the rule that your pushrod should make a right angle from it's connect point to the hinge line. Well, that's so that pushrod doesn't wind up with Ackerman. I did a Google and got a number of hits on Ackerman. Bottom line is you need to have the connect points on your servo tiller equally as wide as the connect points on your surface's tiller, and the points on the tiller in line with the hinge line. Or the non-pulling line will either go slack or tighten up.

Pushrods and horns are so troublefree it's a wonder pull-pull is popular at all.

H5487

Khodges,

Thanks for the explanation. I also thank you for the pictures. Your attention to detail is outstanding!

Da rock, thanks for your explanation too.

In fact, thanks to all of you!

Harvey

Rodney

I still will insist that positive Ackerman is a good thing on all pull-pull. It will not cause flutter when the non pulled line goes slightly slack when you move the surface off neutral as the air pressure keeps the surface tight against the line being pulled. IMHO pull-pull is so much more reliable and trouble free than push rods that there is no comparison for large models. Even most small models such as the little electric powered jobs benefit as there is a great weight advantage over pushrods with a much larger choice of servo locations to adjust CG placement without adding dead weight. Positive Ackerman can be obtained by having the attach points of the movable surface being aft of the hinge point (away from the servo position) or by having the attach points at the servo (away from the surface being moved) ahead of the hinge point of the servo arm. It is pretty easy to demonstrate this to yourself with a few sketches using a compass and paper and pencil to scale out the geometry.

jaka

Hi!
Agree fully!
Wires is the only way to go if you want a light, exact connection between servo and flying surfaces.
It could be used all surfaces but is easiest to make on rudder and elevator.

Jetdesign

My Feedback: (8)

Ackerman on a pull-pull system to me sounds like a method to cover up mistakes. I've seen plenty of rudder pull-pull setups at the contests I've been to where both cables stay taught throughout the range of motion.

Take the time to lay everything out symmetrically, make sure your horns are square and the holes lined up with the pivot points, etc.

I do not see anything advantageous to slack in the control system of an airplane, again except to cover up a mistake that would cause excess tension in the opposite line.

allanflowers

I think you are correct that good geometry doesn't REQUIRE Ackerman but the gentleman is correct in stating that it is much preferable to binding & over taxing a servo.
By the way, making the horns "square" is not as simple as it seems. I did a Pup elevator where I wanted the cables to go to the sides of the cockpit so I used an angled horn, with lines perpendicular to them and pulleys to straighten every thing out rearward. The system works well and there is no binding and no Ackerman either.
daRock, the arm lengths (front to back) don't have to be equal either, as long as the line of action of the cables is perpendicular to the arms. My Siemens Schuckert has unequal arm lengths but the plastic guides bring the cables in at the right relationship and the tension does not vary.

Crash Campbell

Hi,

Along with the great info here already you may find this link of help. I too use pull/pull on rudder and elevator on some of my models, have't tried it on ailerons yet but may soon on a war bird.
http://www.mindspring.com/~rellis2/rcpattrn/ppull.htm

Cheers,

Colin

da Rock

Yeah, I know but cables eat plastic guides so I never chose to control routing that way. I'm just not a fan of pull pull anyway.

I usually don't go into all the ways to do the deed. Heck, getting people to understand the right angle concept is hard enough.

hook57

My Feedback: (21)

Interesting. I am familiar with Ackerman as it applies to an auto's steering (as applied to cornering geometry and such) but in over three decades never heard it in an aviation discussion (full size or modeling). However, I think I see your point in using it, say from a "toe in" point of view, because of the geometry of the cabling and the offset (movement) of the control surface (whether it is vertical or horizontal really.) It is not easy to calculat if one is not familiar, but the idea is not difficult to imagine or comprehend on a smaller scale. And sorryif Iintruded on the thread;but being familiar with pull-pull as old pattern flyer (and I saw it in use on elevators in the late seventies)I think bringing up Ackerman was a really good idea and a way to correlate the actions of the pull-pull set up. This pic is a pretty good description of it if you envision the rear axle as perpindicular to the fuselage center line looking aft . I hope the pic displays ok, if not my apologies guys.
Hook

(from a Bimmer site)