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Servo Questions?

Old 01-09-2009, 03:13 AM
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Default Servo Questions?

I know the general aspects of RC planes but servos are my main weakness, my question is can I use any servos for most RC planes? How do I decide on what servos to use for example my p-51 from top flite has 3151s and 3152s, and my throttle servo has enough torque for the recomendations, also what is the difference and is there a differnce in performance in digital servos besides the response time? I know I have to use certain rx for my tx but for servos is my problem in understanding. Since I have someone who builds my planes to save me time as I have no time with work and my fiancee to build my own I just would like to learn the basics of servos and there function and how to choose which ones for a certain plane. Thanks again fellas.
Old 01-09-2009, 03:44 AM
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Default RE: Servo Questions?

Well, that's a pretty straight forward set of questions.


servos are going to be determined by the amount of force necessary to drive the control surface. You go all the way from 9g micro servos, to the monstrosities built for 1/2 scale stuff. Gearing can be important as well, some servos are metal geared while some are plastic.

Digital I will leave for someone else, I don't have a brilliant answer for that.

Yes you can mix and match servos. My tiger is stacked out in all hobbico and futaba servos(because I used futaba for 4 years), but I'm using a JR based radio. There are a few exception for much older airtronics servos, but I doubt you will encounter that. The because hurtle to using a futaba/jr mix and match is trimming the alignment tab of the futaba plug....
Old 01-09-2009, 08:23 AM
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Default RE: Servo Questions?

There is an article in the March (?) 2009 Model Airplane News magazine titled "Servo Essentials, Everything You Need to Know."

Not sure why they publish a "March" magazine in early January tho... Maybe it IS March and I've forgotten to flip the calendar, I dunno...
Old 01-09-2009, 08:26 AM
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Default RE: Servo Questions?

a servos job is to convert electrical power from the receiver to mechanical force....some servos aren't for aircraft use ..as they don't always center in the same spot or "hunt" for center...what servo you need is determined by size of the servo, or speed the servo travels and the torque required.....some servos will not tolerate 6 volts, most will...6 volt over 4.8 is termined by the usage of the servo....6 volts will increase the speed and torque of the servo....
servo can fail, in different ways...gears strip out...one side can open causing a hard over servo...the servo can hunt for center....
Servos are for the most part reliable..but if you have only one on a control surface and it goes bad in flight...you're gonna have your hands full...
Old 01-09-2009, 10:07 AM
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Default RE: Servo Questions?

Digitals give a better "return to zero" than analog. A cheap servo may have 128 positions that it stutters through as the arm swings, a good one 512, and high end almost unlimited positions (1024 & up) within the arc of travel.

In addition to the strength in torque (inch-ounces) the more you pay generally the better the response time, repeatability of travel.

But it makes no sense at all to buy digital servos and attach them to the control surfaces with sloppy or "bendy" control rods and actuators, or with slop in the attachments to the servo arms. This is why you see the ball-joint connectors and aluminum replacement servo arms. For most of us a plain 'ol servo is sufficient for sport flying.
Old 01-09-2009, 10:13 AM
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Default RE: Servo Questions?

Yeah, Charlie is right. I've amassed quite a collection of servos, from micro to high torque ball bearing coreless servos, ranging in price from $9.00 to $75.00 each... and I have found, that for most of my applications, well, the good old standard $9.00 servo does just fine.

There are a couple of planes that I will justify the expense for, My Venus II pattern plane is one. Because of the specialized use, that plane gets top notch components, including servos, linkage, rods, and so on. The throttle is a basic $9.00 servo. The elevator, rudder, and elevator get top notch components because of what we put them through when flying pattern, and what we demand on them for repeatability and precision of maneuvers.

CGr.
Old 01-09-2009, 10:27 AM
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Default RE: Servo Questions?

One note on digital servos. But they do have a downside. They use more power than their equivalent analog servo. If you use digital servos you'll want to make sure you have a larger battery in the plane to handle the extra power demand

Ken
Old 01-09-2009, 07:38 PM
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Default RE: Servo Questions?

Not to high-jack but another servo question...control arms.

I have three types in the box: circle with 12 holes; 3-hole arms (6 arms) and 4-hole arms (4 arms). Which ones to use? Does it vary depending on control surface?

Yes, I know to cut off unused ones but not sure which one to start with....I assume the 4 hole if only because the instructions said to connect the control rod the the second last hole which would be silly for 3 holes!
Old 01-09-2009, 08:17 PM
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Default RE: Servo Questions?

Is there any tricks to slow down the speed of a HS-55? My radio is a Spectrum DX6i. I don't think I can do it with the radio.
Old 01-09-2009, 10:50 PM
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Default RE: Servo Questions?

ORIGINAL: AircamperAce

Is there any tricks to slow down the speed of a HS-55? My radio is a Spectrum DX6i. I don't think I can do it with the radio.
Not familiar with Spectrum, but my Futaba has the clever option of bypassing the switches and using the joysticks that can be moved at either fast or slow speeds and in small increments if desired.

Seriously, you can either program in exponential so the response is "dulled" near the center of the throws or you can adjust the endpoints so you are moving through broader or narrower arcs in a given time. You can also effectively slow a servo by moving in on the servo arm and out on the control horn. But short of adding a resistor to the servo circuitry you cannot change the motor response time. Would that we could speed them up sometimes.
Old 01-10-2009, 12:27 AM
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Default RE: Servo Questions?


ORIGINAL: AircamperAce

Is there any tricks to slow down the speed of a HS-55? My radio is a Spectrum DX6i. I don't think I can do it with the radio.
I don't think servo speed is an option on that radio. Why do you want to slow it down? If it just due to control sensitivity, try lower throws or program in some exponential.

There are devices on the market for slowing servos down on things like flaps and retracts.
Old 01-10-2009, 06:56 AM
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Default RE: Servo Questions?

chemie: For the servos with arms, well, first you have to center the servo using the receiver, a battery, and your TX. Here's how:

1. Center the control you intend to use that servo for. Meaning, if it is for the aileron, for instance, center both the stick and the trim.

2. Plug the servo into the aileron slot on the RX.

3. Power up the RX.

4. Take one of the servo control arms and push it on the servo shaft. Don't screw it down. See if it is perpendicular to the sides of the servo. If it is slightly off, then remove the servo arm and rotate it to the next position (the next arm) and see if THAT one is perpendicular. One of the four will be perpendicular. When you find the one that is, well, mark it or cut the other arms off, and keep that combination servo and servo arm together as a matched pair.

For the round ones, well, those can be used pretty much however you want, pretty much using the holes as guides as to the position you want them to be in, and centering, if necessary, on that hole.

There was a thread here on RCU about setting up a throttle servo and it pretty much describes that setup, as it is somewhat different than it is for other applications in that it is easy to over-rotate the servo arm and put the servo into a binding situation. If you want that information, well, either you can look it up or let me know or one of us know here on RCU and we can re-post that information.

CGr
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Old 01-10-2009, 08:13 AM
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Default RE: Servo Questions?

Thanks CG. I was more wondering if I should use the 4 hole arm or the three hole arm...or if it mattered or if it changed between usage.

Yes, if you have it, I would like to see throttle setup as I was wondering about that too (although I assumed with computer Tx, you can adjust end-points anyway).
Old 01-10-2009, 09:37 AM
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Default RE: Servo Questions?

ORIGINAL: Charlie P.

ORIGINAL: AircamperAce

Is there any tricks to slow down the speed of a HS-55? My radio is a Spectrum DX6i. I don't think I can do it with the radio.
Not familiar with Spectrum, but my Futaba has the clever option of bypassing the switches and using the joysticks that can be moved at either fast or slow speeds and in small increments if desired.

Seriously, you can either program in exponential so the response is "dulled" near the center of the throws or you can adjust the endpoints so you are moving through broader or narrower arcs in a given time. You can also effectively slow a servo by moving in on the servo arm and out on the control horn. But short of adding a resistor to the servo circuitry you cannot change the motor response time. Would that we could speed them up sometimes.
Thanks Charlie,

I should have explained myself a little more. I am using a HS-55 in a .40 sized Hangar9 Cub for the throttle. It is powered by an old Saito 65. I've been known to overreact and chop or firewall the throttle, causing the engeine to quit. I remember reading somewhere someone suggesting slowing the servo down with the radio (a function that my radio does not have) to remedey this. Some better engine tuning may help but I really need to condition myself not to be so abrupt with the throttle. Adding a resister is kind of what I had in mind when I posted. Thanks for all the good ideas, unfortunately the engine requires the full travel of this little servo, otherwise I would reposition the linkage.
Old 01-11-2009, 11:30 AM
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Default RE: Servo Questions?

What CG has said is most important. The decision on which arm to use depends mainly on the amount of throw required, as does the hole you select to use for linkage. The 4 hole arm will give you the most throw since it's longer therefore the distance it travels will be greater. Of course the hole location chosen on the control surface horn enters into this also.
Old 01-11-2009, 01:30 PM
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Default RE: Servo Questions?

And, it comes down to resolution (precision) of the control. Generally, the longer arm hole will give you less resolution. There is a whole discussion about that in another forum. I will try to find it and post the link.

Meanwhile, as far as the throttle setup is concerned, well, the discussion was about the Futaba Skysport, which does not have end-point adjustments (as in your typical computer transmitter). Here is what I posted in the other thread:

Because the Skysport does not have end points (not a computer radio I believe) you have to do all this mechanically. The stick movement relates to 100% servo movement. So, you have to work with the mechanical settings of the "levers and bell-crank" action of the servo arm and the thrlttle arm.

First, zero the servo . You can only do this using one of the other channels with trim set to the mid point or at zero (no plus or no minus trim, just set it to the mid point on the TX). Connect the servo to that channel (rudder, elevator, aileron... whatever, but not the throttle ). This will set the mid point of the servo . If the servo arm is not 90 degrees to the side of the servo , then remove the servo arm and rotate it 90 degrees. Each arm is offset somewhat to enable you to find the ONE of the four arms that are the correctly centerd ones for that servo . If you do not have a four arm servo , get one.

Next, center the throttle on the TX and again, center the trim. Then, once you have the servo centered and the correct servo arm selected, then you can cut the other arms off... or not, your choice. Install the servo , or if installed, plug it into the throttle port on the RX. If it does not "rotate" correctly, then use the throttle reverse switch on the TX and get it right. The throttle arm on the servo should now be centered. Do not touch anything on the TX. Set it aside, yeah, leave it on.

Open the throttle on the engine to what you think is mid throttle setting. Set the trim to fully closed (all the way in) Now, use the hole on the servo that is the furthest away from the center and the same on the engine throttle arm. Adjust your linkage so that they match up. Lengthen or shorten as necessary. Connect the throttle linkage. Now, slowly move the throttle until you go to full open. IF no bind, then go to full closed. If no bind, then you are ok. If it binds, then re-center it, chooose a next hole on the serov that is closer to the center. Leave the one alone on the engine. Re-check for binds and that the throttle opens and closes fully with no binds. If no binds then you are ok. If it binds, then more adjustment is necessary. Continue to adjust as necessary until you are happy that it opens fully and closes fully wiht no binds. NOw, you should have correct adjustment and trim settings so that you can close the throttle with the trim to shutdown the engine when finished.
Old 01-12-2009, 07:03 AM
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Default RE: Servo Questions?

Thanks CG. I had not thought of using another channel to get center...
Old 01-12-2009, 07:55 AM
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Yeah, any channel that will center the servo is fine. The problem with using the throttle channel is that you really don't have a 'zero-ing' point on the stick. The others will go to center, and as long as you center or zero the trims, you are all set. The Spektrum, for instance, has an LCD display that shows the trim position, which makes it real easy to both zero the stick and the trim. I'm sure others have that feature, but I am not familiar with them.

There is another gadget that I bought a while back which uses a gizmo that behaves like an RX but is actually a USB device which allows you to control the servo on a PC via the installed software. TXServo is the device and here is the link:

http://www.txservo.com/

This may be more than an average RC'er would use, but I find mine pretty helpful in setting up servos and doing such things as setting up the throttle, and other controls. And, it fully exercises the servos.

CGr
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Old 01-12-2009, 12:08 PM
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Default RE: Servo Questions?


ORIGINAL: AircamperAce

ORIGINAL: Charlie P.

ORIGINAL: AircamperAce

Is there any tricks to slow down the speed of a HS-55? My radio is a Spectrum DX6i. I don't think I can do it with the radio.
Not familiar with Spectrum, but my Futaba has the clever option of bypassing the switches and using the joysticks that can be moved at either fast or slow speeds and in small increments if desired.

Seriously, you can either program in exponential so the response is "dulled" near the center of the throws or you can adjust the endpoints so you are moving through broader or narrower arcs in a given time. You can also effectively slow a servo by moving in on the servo arm and out on the control horn. But short of adding a resistor to the servo circuitry you cannot change the motor response time. Would that we could speed them up sometimes.
Thanks Charlie,

I should have explained myself a little more. I am using a HS-55 in a .40 sized Hangar9 Cub for the throttle. It is powered by an old Saito 65. I've been known to overreact and chop or firewall the throttle, causing the engeine to quit. I remember reading somewhere someone suggesting slowing the servo down with the radio (a function that my radio does not have) to remedey this. Some better engine tuning may help but I really need to condition myself not to be so abrupt with the throttle. Adding a resister is kind of what I had in mind when I posted. Thanks for all the good ideas, unfortunately the engine requires the full travel of this little servo, otherwise I would reposition the linkage.

The Futaba 9C has just that feature. It's a programmable throttle delay (I think the intention is for jet turbines, or to simulate jet turbines). I played with it when I had a very cranky Kangke SK-50 engine that stalled if goosed. It was the wrong way to try and correct the probem. (Better to get the engine tuned - which involved a Perry carburator with that engine).

I have found the best way is to assign two switches. I use the two over the throttle. I set my throttle lever so it is at a solid idle when lowered. There is one switch called "taxi" that is lower idle that I know the engine can hold briefly when sitting. So that becomes lowest idle when that switch is flipped, great for final approach with a speedy model. And then there is a designated "kill" switch. Flipping this one closes the carb and stops the engine. This way I never have to worry about killing the engine in a flight situation.

I like biplanes and I can program those models easily so that the throttle idle is higher, as some tend to glide like clumps of mud.

On my old 6DA I just flew with the throttle trim slider up and set the throttle so that at low lover it still idled and to kill the engine I had to thumb the trim slider down. That is the only feature I miss with the digital trims of modern transmitters.

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