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Dec 28, 2006, 07:53 PM
Registered User
ufoDziner's Avatar
Quote:
Originally Posted by soholingo
I didn't put the pull/pull system down, and it IS more complicated to set up than a push pull linkage. Last I checked you have to setup two linkages in a pull/pull setup everytime. Not just during the first installation.

Your comment about it getting easier is moot. Pushpull gets easier the second time too, so does mounting motors, installing servos, etc...

Don't take my comments out of context, I just wanted to know the benefits of pull/pull, and the only viable one is weight savings.
I don't think I'd say it's more complicated. It is more work. Since you have to connect twice as many items to control horns and the servo. But, it's still basically the same types of connection.

Wes
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Dec 28, 2006, 09:20 PM
BANNED!!!
soholingo's Avatar
Quote:
Originally Posted by ufoDziner
I don't think I'd say it's more complicated. It is more work. Since you have to connect twice as many items to control horns and the servo. But, it's still basically the same types of connection.

Wes
No argument there...

Jay
Dec 28, 2006, 11:37 PM
Registered User
rcav8r2's Avatar
I've used both and for me it really depends on the situation, and what I have on hand. I've used PP in some places that a conventional push rod, or is that a pull rod? , would have been a night mare, and vice versa. It's nice to have a lot of tools in your bag of tricks
BTW, I make my own PP "kits". I went to Gander Mountain and got 2 sizes of nylon coated Stainless Steel fishing leader. More than enough for a life time of models for about $4.00 a roll. I then cut small aluminum or brass tubing as ferules. One the servo side I just thread the wire through the appropriate hole. One the control side I use Dubro rigging couplers w/ a standard clevis. In a pinch I'll bend an eye in a short length of push rod material and use this with a clevis for adjustment.
Dec 29, 2006, 09:14 AM
the-plumber
Quote:
Originally Posted by soholingo
No offense but isn't all of this avoided with a properly configured and secured control rod? If there is no slop in the connection then there is no advantage to pull pull correct?
Exactamundo !

Unless you consider the miniscule weight savings of lightweight aramid fiber pull-pull cables versus the "weight" of a push-rod and bracing.

OTOH, some scale models would seem to dictate pull-pull where that was the configuration in the prototype, e.g. the rudder on a J-3/L-4 or an Extra 230, or other pull-pull flight surfaces.

Push-rods are simpler to implement but more difficult to make sufficiently robust for highly loaded flight control surfaces, i.e. aerobatic models or large models (the tail feathers on a 140" L-4 have a -LOT- of surface area !!!).

EDIT : There was a small booklet called "Control Systems" which was published by Air Age (Model Airplane News), but it is long since out of print. If you could chase one down somewhere it's a worthwhile addition to your library. I'm thinking about adding a control section to my web site, one of these days when I have nothing else to do . . .

Lots of would-be builders have no real foundation from which to work with respect to correctly implemented control systems and the salient hardware. Can't tell you how many times I've seen a clevis installed where it must flex in the plane of the pin.
Last edited by the-plumber; Dec 29, 2006 at 09:24 AM.
Dec 29, 2006, 10:12 AM
Ascended Master
Sparky Paul's Avatar
Quote:
Originally Posted by rcav8r2
I've used both and for me it really depends on the situation, and what I have on hand. I've used PP in some places that a conventional push rod, or is that a pull rod? , would have been a night mare, and vice versa. It's nice to have a lot of tools in your bag of tricks
..
.
Pull-pulls set up so easily some planes cry for them! Foamies, where long control runs mean extra bracing for the necessarily thin push rods for one.
Gliders use them quite well on rudders.
Having pulls for both directions -is- better than pushing in one, with the bracing usually called for to prevent flex, and vibration created whipping.
And, no one HAS to use pull-pull.. It's just something the attuned person will see as right and proper for some installations.
Dec 29, 2006, 06:19 PM
BANNED!!!
soholingo's Avatar
Quote:
Originally Posted by Sparky Paul
.
And, no one HAS to use pull-pull.. It's just something the attuned person will see as right and proper for some installations.
Agreed...
Dec 29, 2006, 11:43 PM
Registered User
Bill Mixon's Avatar
Pull-Pull

I'll add my 2 cents on this subject.

One problem many people have with pull-pull is getting it setup properly. Many simply put dual horns on the surface, and run a set of lines/cables without regard to any of the geometrical alignment issues involved with pull-pull setups. Even though severely screwed up, most bad pull-pull systems will still work, however they certainly are not right or optimum.
here are some pull-pull setup guidlines.

To be geometrically correct the servo arm or bellcrank geometry must match the control horns geometry on the surface. So if you have a straight double servo arm with the cables attached at 1" from center, then your control horns should also be set at the same 1" distance from center, and with the connection points 180 degrees apart relative the hinge line.
If you have the horns mounted forward or aft of the hinge line and/or the lines are not connected at the same distance then your setup is off.

Sometimes you may find that you need more throw, and the typical fix is to move the linkage connection point closer in on the control surface (3/4" for example). While this may work just fine for a given application it isn't right, and the more off this is altered the worse the setup will be (typical result is slack in one side when deflected).
Also many people don't take the time to mount the control horns in the proper place. It is very common to see the horns mouted with the holes 1/8" or more aft of the hinge line. This happens a lot because the bevel on the control surface may not allow the horns to be easily mounted far enough forward on the surface.
This is a big problem especially on really bad setups where the horns are like 1/2" aft of the hinge line for example. If you have to have your control horns aft of the hinge line, then you need a special made servo arm that will match the geometry of the control horn/hinge setup. (keep in mind though that this isn't optimal, the connection points relative the pivot need to be at 180 degrees)
The result of an aft control horn placement will be properly tensioned lines at neutral, and LOTS of slack when the surface is deflected.
The exact opposite is true if the horns are ahead of the hinge line. In this situation the setup will be properly tentioned at center until the surface is deflected. When deflected the geometry is off and you will not have enough cable or line, so it either binds up (line tension will actually try to pull the setup back to center) or the setup may be stretched and you now have slack in the lines at neutral. This is really bad because it can induce flutter.
Control horn placement is very important.

Another typical improper setup is to cross the lines/cables. Even on setups that are geometrically perfect otherwise, simply crossing the lines can cause problems. I know, I know, many companies and airplane designers recommend a crossed line setup, which works great for making a better exit route through the fuselage. In most cases this isn't a big issue, however it does lead to a slightly less than optimum setup with slack in the lines when deflected.

Next is the plane of rotation for the arm, and the surface, and alignment relationship between the two. This could be a bit difficult to explain without some drawings.
A good example would be a four line elevator setup. Here you want the servo laying down in the fuselage so that the rotation planes are the same. Again this isn't really a major problem, but in extreme cases having the servo mounted upright with the planes of rotation 90 degrees off can cause ineffeciencies in the setup. This is simply because of the direction in which the lines have to be routed. On one side of servo you will have your "up lines", and on the other side the "down lines". Now if you look at the two lines on each side, note that one runs nearly straight back to the horn and the other line is much longer and has to run at an angle across the fuselage to connect to the other surface. Again the geometry is off with such a setup, and can cause problems.
Another related problem is having the servo arm or bellcrank mounted off from the center line of your control surface. An extreme example of this would be a T-tail configuration where the servo may be mouted way under the control surface which requires that the pull pull lines are run at steep downward angle from the control surface. The 180 degree hinge to horn alignment needs to be perpendicular to the route of your pull pull system not the control surface. In other words this setup would require that special horns be used so that the top or up horn is leaning forward over the surface, and the lower horn is leaning aft by the same amount.
Again this is a extreme example where proper alignment is obvious.
A less obvious setup would be where your elevator servo was 24" ahead, and 7" below the ceterline of the horizontal stab.
In this case if you simply used 90 degree oriented horns on the surface your geometry would be off. And as a result up and down deflections would not be the same, and down elevator possitions would be less precise. This geometry problem would also apply to a push pull system as well, but with a pull pull setup it could be more problematic.

I use pull pull and push pull depending on the application. Whenever possible I'll use pull pull on my 3D planes and sailplanes to save weight and have a good solid control system.
Setup properly pull pull works great, setup wrong it can be a nightmare.

regards
Dec 30, 2006, 12:31 AM
Suspended Account
wow...

i knew i should have paid more attention in math class.

nice write up!!
Dec 30, 2006, 06:33 AM
Registered User
dspecgsx's Avatar
Quote:
Originally Posted by Bill Mixon
A good example would be a four line elevator setup. Here you want the servo laying down in the fuselage so that the rotation planes are the same.
Like this.....
Dec 30, 2006, 12:37 PM
Registered User
Bill Mixon's Avatar
Exactly,
That's a perfect example shown on your pattern plane. Having the servo laying down is key to a precise setup. Many people dont take the time to do it right because of the servo mounting issues, and simply mount the servo "upright" so it's easier to install/service.
Dec 30, 2006, 01:26 PM
Ascended Master
Sparky Paul's Avatar
In the cabin gets rid of that slight geometry at the end points.
With side-mounted servos at the back end, and one of them electrically reversed, the geometry remains the same.
Then it's getting the end points the same that matters with either configuration.
Jan 01, 2007, 12:05 AM
Promoting Model Aviation...
Murocflyer's Avatar
Wow, this has turned into a very informative thread on the Pull-Pull System. I for one appreciate it very much.

Frank
Jan 01, 2007, 01:09 AM
Deletedfor proving Nauga wrong
good info on the way to deal with dual servos and pushrods too....
Jan 01, 2007, 07:33 PM
Promoting Model Aviation...
Murocflyer's Avatar
Well I tried out my very first Pull-Pull system on my scratchbuilt plane. I used it on the elevator.

This is the one I used.

http://www2.towerhobbies.com/cgi-bin...0&I=LXD858&P=K

It was actually pretty easy to set up. I think next time I'll just use some spiderwire. Maybe I got lucky, but I didn't have to do any adjusting and it seems all set up correctly.

Thanks again for the very informative thread. I hope others will benefit from it also.

Frank
Jan 02, 2007, 10:43 AM
Registered User
Hi all, I've never done a pull/pull system before and about a month ago I purchased the Dubro Micro http://www2.towerhobbies.com/cgi-bin...?&I=LXAPC6&P=7 for a corsair I'm building. I should have read the forums first but I didn't. I have the corsair rigged already but I have the elevator and rudder wires crossed so it runs easier in the fuse. They do touch each other so I'm sure they will rub when moving, is this bad? If so, if I get two pieces of tubing and glue them in an X formation so the wire's don't touch each other, would this be ok?

Thanks in advance.
Ralph


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