Let me axe you,

If the rule is to have the control horn the same length as the servo arm, how do I get more throw?
If the servo moves 30 degrees, the surface is gonna move 30 degrees, as long as both are the same distance from center, correct?

Do I have to go back to heavy pushrods to get 45 degrees?

Thanks in advance,

Joe Petro

Joe,
With a pull/pull system is is necessary to keep the arrangement equal. If you don't it will either bind or get loose as the servo moves. To get more throw you will need to adjust your radio to do that. Most are capable of changing the throws in a function called ATV or ?? It depends on the brand of servo / radio. Some servos are programmable for this function. If you have a basic radio (non computer) it will be a difficult task to change in some cases. Check your radio manual to see if the throws can be increased. Bill

In pull-pull you don't have to keep the servo horn and control horn lengths equal. You want to maximize the servo throw using your transmitter, then, if you need more throw, more length on the servo horn relative to the control surface horn, like you thought.

This does change the geometry but as long as you are not planning on doing it to the extreme (like 4" off the servo center and .25" off the hinge line ) then doing this wont cause any problem. A little slack on the inactive cable does not cause any problems other than looking a little stupid, but who cares about that right ?

I've had this question a while too. I think I read somewhere that some servo arms are offset to compensate for the arm/control horn dimentions being different. By offset I mean (I think) that if you drew a line from one hole on the servo arm to the other hole, the line wouldn't pass through the servo output shaft. (see the picture below) Somehow this keeps both lines tight even though the servo arm is bigger or smaller than the control horn. I'm not really sure about all this though, so someone who really knows... please enlighten us!

Actually the servo arms are offset to compensate for the fact that you get varying line tension if the servo arm is straight and it's connected to the control surface that is NOT straight (See the attached image, which WOULD give undesirable varying cable tension).

If you could set up the control surface so that it too is straight, then unequal length arms between the servo and the control surface would not matter, because the ratio or distance one side moves is exactly matched by the ratio/distance the other side moves, and cable tension stays constant.

To minimize this varying line tension, you can (1) use an offset servo arm to match the amount of offset at the control surface between the hinge/pivot line and a line drawn between both clevis holes or (2) offset the control surface arms to so the clevis holes are at or near the hinge/pivot point, thereby matching the straight servo arm.

Actually, it isn't absolutely critical that both lines stay absolutely tight. What IS critical is to avoid the cables becoming tighter when the surface is moved from neutral to extreme travel. It's fine (actually a bit desirable) for the cable to loosen slightly at extreme travel. Air pressure from flight loads will keep the loaded (pulling) side tight, and since one side of the surface is under a load, flutter isn't an issue.

Is to use a bellcrank. Hook the cables to the surface and the bellcrank with equal distances. Then use a pushrod from servo to bellcrank. Now you can increase throw to the bellcrank by moving the pushrod out on the servo horn. You can get way extreme throws this way.

If you would like more travel/deflection with a pull-pull system and its unavailble due to other restrictions add a Tiller-bar to your rudder linkage. Its a simple bellcrank activated by a couple of short pushrods coupled to the rudder servo output arm.

I would make it a point to keep the rudder control horns pivot attaching point and tiller-bar or servo output connecting points as close as possible. This will create user freindly system that will not have any adverse effects of tighteneing and or slack in the cable system.

If you use a Tiller-bar system insure that your pivot points are the same at the servo output arm and the Tiller-bar, for the reasons mentioned above.

One of the advantages of using a Tiller-bar is the application of the offset unit as mentioned above. This can be applied to the servo output arm as well. Neslon Hobby makes some nice hardware for our applications. Remeber the offset arm allows for the offset in the hinge pivot line.

http://www.nelsonhobby.com/

The other method used to offset the rudder hinge point pivot line is adapters which are attached to the rudder post/stud that allow the correct offset forward, toward the pivot line and provide for the cable attachment point to be in line with the rudders hinge pivot line...

Michael, I don't see how the added bellcrank in your picture does anything to change the effective gear ratio. It seems like the added linkage would only contribute to overall slop (ouch).

The suggestion to use a bellcrank with a single linkage to the rudder defeats the purpose of using pull-pull. You might as well have one pushrod going back to the rudder. I will agree it provides a simple way to change the max deflection.

The best answer is from Tim. Control and servo horns can be different lengths. If the horns are in line with the pivot point, there will not be any slack, even at max deflection.

Tom

The tiller bar is wider than the rudder pushrod pivot attaching points. By using the holes furthest from center on the bar to attach the cable ends you change the ratio, thus more travel is realized. You cannot see it my picture the cables ends are attached on the bottom side.

Using a single pushrod as Chuck suggested does in fact work and maintains the pull-pull feature when coupled with a bellcrank. However it also loads one side of the servo rather than both sides with a dual pushrod system.


Quote from greenboot
The best answer is from Tim. Control and servo horns can be different lengths. If the horns are in line with the pivot point, there will not be any slack, even at max deflection.

I disagree here without exception... If the arms/pivot attaching points are not the same width/spacing you will have cable slack. Try it and see. It all comes back to something known as Ackermans Theory

Tim's right about the geometry. What you are all talking about is a principal called ackerman. There is extensive information about it (can't find my links) on the net. Do a google search for ackerman and pull pull rudder for more than you wanted to know.

As for the tiller bar/bell crank, the real advantage for this item in this application is not for adjusting throw but for saving the servo bearings. With the rudder linkage the entire weight of the aircraft plus some will be put on the pivot point of the pull pull in a knife edge loop. The tiller bar and it's bearing support that load not the servo.

L-

Tim is right only if you mount the cable ends to the same location/spacing as the rudder servo pushrods. This is not how I did it as previuosly mentioned.

Ackermans Theory is directly resposible for the slack realized when using unequal length pivot points on the tiller-bar and or control horns/rudder stud. Combined with the rudder hinge pivot offset or lack thereof, you will see cable slack.

Absolute, a significant advantage to the bellcrank/tiller-bar system is the cantilever load created by the pull-pull system is not transferred to the servo output shaft. Secondary is the ability to compensate for the offset hinge pivot line with an offset bellcrank/tiller-bar.

Thanks for the great input, everyone!

Just to double-check, if I understand correctly, the servo-arm/control-horn size ratio gives you more (or less, depending) rudder throw than servo throw AND causes the non-tentioned cable to go have slack, and matching the offsets on each end (match servo-output-shaft/servo-clevis offset to hinge-line/rudder-clevis offset) just causes the servo-arm/control-horn size ratio to be proportional (i.e. rudder throw is always X times servo throw, for any given deflection)?? I have trouble explaining what I mean, please see the picture. If you wanted both cables to be at the same tention throughout servo/surface throw, the servo-arm/control-horn would have to be the same size and the offsets would have to be the same.

I either finally understand or am really confused - I hope it's not the latter!

Michael, your pull-pull setup shows exactly what we're talking about. Your rudder doesn't know or care what the spacing is on your servo arms, it only sees the spacing on the WIDE TILLER bar that you use. Like my example, your tiller bar is "wider than the rudder pushrod pivot attaching points", and this gives the increased travel. The rudder knows that it's being moved by something that's longer than its own horns, but neither it nor the cables would realize a difference if it were a 3" tiller bar, or a 3" servo control arm attached directly to the servo. The ratio is the same, and if the servo arm's holes lined up straight like the tiller bar's holes do, any variance in slack (or lack thereof) would be identical. I'd use a tiller bar if I were concerned about the loads that the servo bearing see. Since this is a relatively small load only requiring a single servo, I'd feel the servo's bearings would be able to handle that without stress.

I'm not trying to argue, your hangar is MUCH larger than mine, you probably have much more experience than I, but I do know that I want my cables tight enough to negate play while the surface is centered (and minimize flutter possibilities). Any minor slack that occurs when the surface is moved off center doesn't concern me because IMHO, flutter won't occur if there is pressure on one side of the surface (as there is when the surface is deflected). I'd be more concerned about the cable getting tighter, rather than looser.

That is exactly right!



The servo-arm/control-horn size ratio DOES determine throw, but it's the mismatched pivot offset that causes slack in the cables. Matching the offsets controls slack in the cables.



Again, offsets mainly control slack in the cables, (don't want to open up another can of worms about the travel actually not being linear).

Another thing, in your picture, since the offsets are the same, cable tension variations are minimized (rather than your feeling that one WILL go slack). Your ratios are also wrong, in that 45 degrees of servo throw will yield LESS than (not more than) 45 degrees of rudder throw. This is because in your example, the servo arm is shorter than the rudder arm. To get more surface throw than servo throw, the servo arm has to be LONGER than the surface arm. For example, a 3" servo arm (1.5" from pivot to clevis point) attached to the rudder's 1.5" control horns (.75" from pivot to clevis) will yield twice the throw at the rudder than the servo. At the same time it cuts the mechanical advantage in half, so that if you're using a 100 oz/in servo, the rudder only sees 50 oz/in. That's the reason that proper setup dictate you first maximize the servo travel with your transmitter, then change your ratios (servo arm length and control surface arm length) to give you the travel you need. this way, you minimize your loss of mechanical advantage to the surface.

Tim:

Thanks for taking the time to explain this. I think I understand now, please check the new pic below and let me know if I'm on the right track. Another question:

Does this mean servo throw to surface throw is the inverse of servo torque to transmitted torque? I.E. if you have a 50 oz/in servo and want to transmit (through mechanical advantage) 100 oz/in to the control surface, the control surface can only deflect 30 degrees if the servo is deflecting 60 degrees? (cet. par.)

Once again, thanks for taking the time to help.

Yep, THAT'S the ticket!!!

Yep, that's what it means. Guys get into trouble not having enough strength to move control surfaces after setting up for large surface travel, because they don't take into account that the loss of mechanical advantage. The back half of a knife-edge loop is NOT the time to find your rudder is blowing back from the commanded position!

Thanks a lot for your help!!!! (now if I can just commit it to memory...)

Memory? That's something that my computer has more of than I do!! Put that stuff in a file, and pull it up whenever you need it!!!

Yes, I agree with this statement.

Again Yes, I agree with this statement. I do not think I have stated anything to the contrary... One thing that I am doing differently is matching the rudder post/stud cable attaching pivot points width or spacing to that of the tiller bar and or the servo output arm. In the photo examples provided by others the spacing here is noticeably smaller.

On a small plane with a standard servo I would agree. On anything with two or more servos I would use a tiller bar system. I have personaly observed rudder deflection blow-back in knife edge loops with a pair of binoculars. Utilizing two 8411's the rudder folded up like a camp cot an a Morris Giant Su-Do-Khoi at 16lbs with a BME 44. The load is actully quite large, or at least it appears to be...

Again we agree.

Like others have stated in other threads ther eis more than one way to skin a cat.

Having Fun

Wow,
Talk about answering a question!
I guess I wasn't the only one that was confused about this.

Thanks very much, Guys!

Joe Petro

A better pull-pull system.

I'm shocked no one has mentioned this, but far and away the best pull - pull system for a single servo set-up is the wheel.

You don't have the issues of correct geometry and you are able to increase the servo travel, and gain significant mechanical advantage over using a servo arm.

For the single servo setup, it's cheaper, easer, and offers significant mechanical advantage over using a servo arm.

Okay Wings... so the wheel is the ticket.... How do you implement your fix? Offset the holes from the center line?? Give some more details... I'm interested in your idea.

Paul,
Here is a picture of a H-9 pull-pull wheel.
You run one cable from the rudder horn around the servo wheel back to the other rudder horn. The cable is kept from slipping on the wheel buy a set screw. It's really quite simple, has less linkage to assemble and you can run your servo at 140% ATV (a full 100 -120 degrees) and gain significant mechanical advantage.

how does this correct for geometry?

Probably could do the same with the wheel they give you in the servo mounting hardware pack.... What do ya think?