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Old Jun 16, 2012, 03:54 PM
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Another beginner question: How exactly do servos work?

Sorry for yet another noob question, but how do servos actually work? The pushrods (I think) go in the servo horns, but the servo horns only seem to be able to rotate parallel to the ground (if that makes any sense), so how does it manage to push/pull/move the control surface up and down? Well actually, I can imagine that working if there was a servo and pushrod at the top and bottom of the plane (for an aileron for example) so as the servo rotates back, it pulls the pushrod back which is connected to the actual aileron, which will move it up or down, depending on whether it is on the bottom of the plane, or the top of the plane (I hope that made sense), but I don't think that is actually how they work (is it?). Sorry if it's obvious, but I really don't get it and I can't find any help elsewhere. Do you think I'd be able to get more help if I post this on electric? Thanks in advance
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Old Jun 16, 2012, 04:34 PM
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If you Look at the ailerons which are hinged into the wing and see the horn on the aileron also it will be crystal clear how the rotation of the servo pushes and pulls and it is easy to understand how the control rod moves the control surface.
You need to have a plane to look at
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Old Jun 16, 2012, 04:46 PM
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Do you know any links that would be helpful such as diagrams or pics? Also, one servo can be used for different surfaces, right, as long as the torque is sufficient? And is an extension lead only needed for the elevator (on the Wot 4) and does that actually differ on different planes? And if no one can provide any links than I would really, really appreciate it if someone could take a pic of there's and show me, using the pic, how it works. Thanks for taking your time to help me!
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Old Jun 16, 2012, 09:52 PM
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Originally Posted by absandams View Post
Do you know any links that would be helpful such as diagrams or pics? Also, one servo can be used for different surfaces, right, as long as the torque is sufficient? And is an extension lead only needed for the elevator (on the Wot 4) and does that actually differ on different planes? And if no one can provide any links than I would really, really appreciate it if someone could take a pic of there's and show me, using the pic, how it works. Thanks for taking your time to help me!
Do you have the manual for the WOT4? It will show you all about hooking up the control rods. Also it will tell you exactly what you need for extensions. Generally there are extensions needed for the ailerons and on some planes the elevator and rudder. The manual has this information. Or you can just measure the needed length to make the servo wire reach the receiver and buy what you need at your LHS.
BTW: Who makes the Wot4?
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Old Jun 16, 2012, 11:18 PM
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absandams,

I don't think you understand how servos work. A servo doesn't move an arm from left to right. It moves an arm from a center "neutral" position to either left or right. When the servo is connected to a control surface, it is connected so that the servo is in the center "neutral" position when the control surface is in its centered neutral position. Then when the servo moves from its neutral position the control surface also moves from its neutral position. The direction the servo moves from the neutral position controls which direction the control surface moves. The servo will pull the control surface in one direction, and push it in the oposite direction.

Ken
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Old Jun 17, 2012, 04:01 AM
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Oh I get it now, depending on which way it rotates, it will pull or push the control horn that the rod is attached to, pushing it down or pulling it up, right? But if one servo can be used on more than one control surface, then how can you adjust each servo so it's arm only moves 90 degrees either way from neutral, and not the full 360 degrees, unless each servo arm on each servo only rotates 90 degrees from neutral either way, or each servo is designed for one control surface? Thanks for all of the replies so far!
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Old Jun 17, 2012, 10:01 AM
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And with servo extension leads, I understand that one end of it connects to the servo, but what does the other end connect to (sorry if it's an obvious question). Thanks!
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Old Jun 17, 2012, 11:05 AM
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and with servo extension leads, i understand that one end of it connects to the servo, but what does the other end connect to (sorry if it's an obvious question). Thanks!
the receiver.
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Old Jun 17, 2012, 11:09 AM
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Oh I get it now, depending on which way it rotates, it will pull or push the control horn that the rod is attached to, pushing it down or pulling it up, right? But if one servo can be used on more than one control surface, then how can you adjust each servo so it's arm only moves 90 degrees either way from neutral, and not the full 360 degrees, unless each servo arm on each servo only rotates 90 degrees from neutral either way, or each servo is designed for one control surface? Thanks for all of the replies so far!
One servo per control surface PERIOD. ( in your type plane). Servos have a limited rotation that is typically 60 degrees. They do not rotate 360 Ever!. NEVER do contrpl surfaces require servos to rotate anywhere close to 90 degrees.
You need to find somebody with a plane to SHOW you how these things operate. The forum cannot explain it nearly as well as SEEING an airplane.
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Old Jun 17, 2012, 11:52 AM
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1000 words ...

The type of servos used for fix wing RC craft typically operate between plus or minus 60 degrees. Some to it gradually and smoothly. Some have 3 positions - neutral, full one way, full the other way. Some transmitters can overdrive the servo so that it can be pushed beyond 60 degrees.

Other servos used for many many different applications surely CAN rotate 360 degrees.

For some more detailed info see ServoCity

BTW - the pictures below can be extended to include the rudder, elevator, ailerons and flaps. This example uses a simple push rod using a Z-bend at both ends. I'm just too damn lazy to draw clevis', quick disconnects, turn buckles, etc.

Added a pull-pull control of the rudder. As the servo arm rotates one arm pulls the rudder's control horn in that direction. There is no effort applied by the other control horn as the control lines are very light strong flexible wires - not rigid control rods.

Who can point out the "issue" with the pull-pull picture?
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Old Jun 17, 2012, 12:52 PM
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Thanks for the diagrams, I understand it a LOT better now, and thanks to all of those who took their time to help me understand, it's much appreciated, as always
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Old Jun 17, 2012, 02:09 PM
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Here's a top view of a SIG LT-40 which has a single aileron servo. The servo is mounted where the wing's root ribs meet, in a box. Each of the ailerons have a torque rod that transfers servo movement to the control surface. When the servo arm rotates, one servo arm attachment point moves forward while the other servo arm attachment points moves backward an identical amount. As a result one torque rod is pulled while the other is being pushed. The effect is that one aileron is rotated up and the other is rotated down.
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Old Jun 17, 2012, 04:07 PM
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Haha, I find that pretty clever! Is that how the aileron servos normally work together to move up and down, if not, what is the normal way, and would I have to purchase anything else for the aileron servos if I was sticking with normal servos?
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Old Jun 17, 2012, 07:32 PM
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Depends. Some ARFs use a single aileron approach. Some kits (like the SIG LT-40) instructions have you mount a single servo in the middle of the wing's bottom. But, many build the LT-40 kit with dual ailerons (like me). Most sport & aerobatic planes use dual aileron servos. Bi-planes can use from 1 to 4 aileron servos. Tri-planes? Gotta defer to Someone that has built one.

If it's a single aileron servo configuration, then yes.

If it's a dual aileron servo configuration then there is servo mounted in each wing panel just behind the wing's main spar about in the middle of the aileron length.

On wiring.

The single aileron servo lead is plugged into the Rx's aileron channel. Usually you want to use a short extension (3" to 6") that is "permanently" plugged into the Rx. And then, when you mount/remove the wing at the field, you just plug/unplug the servo lead to/from the easily accessible extension.

The double (dual) servo approach can be wired in one of two ways.

You can wire both servos into a single channel on the Rx by using a "Y" extension. The "Y" can either be connected to the wing servo wires "permanently" and then just plug the "Y" connector to the Rx (or Rx extension) when the wing is installed. Or, the "Y" can be installed into the Rx and then plug each servo into one of the "Y" connectors. If you do this LABEL the wires - LEFT & RIGHT.

Or you can plug each aileron servo into its own channel in the Rx. Usually the right wing's aileron servo gets plugged into the AILE channel on the Rx and the left wing's aileron servo gets plugged into an AUX(iliary) channel. Then you use your radio's (TX) mixing capabilities to mix the left and right aileron servos.

HTH
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Old Jun 17, 2012, 07:38 PM
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Your next post should probably give us "I'm thinking about a specific plane" so that specifics can be used rather than endless hypotheticals ..
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