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Old 11-04-2011, 01:20 PM   #51
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Another option for installing servos is hotglue. Especially with EPP foam. Works on all of my foamys.

Gord.

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Old 11-04-2011, 07:21 PM   #52
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I have never tried hot glue. How do you remove the servo if you have to take it out?

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Old 11-04-2011, 07:24 PM   #53
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You wrap the servos in blue painters tape first then hot glue them in. Makes it so much easier to remove them after. You don't need much glue to hold them in place I do this with my foamies.

Just cut the tape and work the servo out of the hole or how ever they are put into the plane. I heat up the hot glue with a junk soldering iron tip just enough to soften the hot glue.
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Old 11-04-2011, 07:36 PM   #54
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Thanks

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Old 11-05-2011, 02:23 PM   #55
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A note on servo arms.

When you look at servo ratings they report ounce/inches of torque.

If you put a 1" servo arm on the servo, it should be able to deliver its rated ounces to that arm at 1" out from the hub of the servo.

If you put a longer arm on the servo, say a 2" arm, a 2" long servo horn, like the 3D pilots do, then you expect less force avaliable at the end of that arm. This is due to the principal of leverage.

So a servo rated for 10 inch ounces can deliver that at 1" out from the servo hub. At 2" out is should deliver 5 inch ounces. So if you need 10 and you put on the long arms, then your servo better be rated for 20. Add to that our comments about inflated ratings and some room for safety you probably want a servo rated at 25 to 30.

Engineers, did I get it right?


The impact of using a differnet hole.

Servo side - The closer you place the control horn to the servo hub the stronger the force but the shorter the distance the control rod will move. If you look at the "circle" traveled by that control rod you see that the further out from the servo the bigger the circle. All the holes move through the same angle but the ones that are further out on the arm move over a longer distance. So, if you need more surface movement you move the control rod further from the servo.

Surface side - Here we have the opposite. The closer your control rod is to the surface the more the surface will move.

So, to get the maximum surface movement you place the control rod on the hole closest to the surface and farthest from the servo.

You should ALWAYS adjust your control throws mechanically first, getting as close to your desired maximum throws and your surface centers. Place the servo arm on the servo at the tooth that gets you as close as possible to a centered surface. If there is a adjustment on the rod ends, use them to get that final center.

Select the holes that give you the final end points first. After you have it as close as possible, then go to the radio if your radio offers end point adjustments or variable travel volume. By adjusting what holes you can often set your control throws without the need for adjustments in the radio. This allows for the greatest flexibility in set-up. It also gives you the finest resolution on the servo.


My preference:

I always look for the the set-up that uses the lowest hole on the control horn on the surface that will give free movement to the rod without binding and allows me to set the throw I want. Once I get it all set I cut off any excess on the control horn at the surface as this can catch grass, getting stuck on things in the shop or in my car. Then I file the control horn end into a smooth rounded shape, again to reduce the tendency to snag on things.

As a result I can use the inner most hole on the servo arm that will give me the surface movement I need giving me the strongest forces from the servo. It also palces the control rod as close as possible to the plane so that it is less likely to hit something.

After I have gotten finished adjusting the control rod holes to get as close as possible to my desired maximum throws, then and only then do I go to the radio to make the smallest possible adjustments from the radio in order to adjust my throws and centers.

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Old 11-10-2011, 10:54 PM   #56
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Seems we have answered everyone's questions. I hope this discussion has been helpful.

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Old 11-11-2011, 12:39 AM   #57
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Originally Posted by AEAJR View Post
A note on servo arms.

When you look at servo ratings they report ounce/inches of torque.

....
If you put a longer arm on the servo, say a 2" arm, a 2" long servo horn, like the 3D pilots do, then you expect less force avaliable at the end of that arm. This is due to the principal of leverage.

So a servo rated for 10 inch ounces can deliver that at 1" out from the servo hub. At 2" out is should deliver 5 inch ounces. So if you need 10 and you put on the long arms, then your servo better be rated for 20. Add to that our comments about inflated ratings and some room for safety you probably want a servo rated at 25 to 30.

Engineers, did I get it right?

....
AEAJR,

I don't think this is correct. It's the motor torque or force that servo’s list. That same amount of force is available everywhere on the control arm. Think of you pushing with the same force on a teeter-tauter. You can lift something heavy if the weight is close to the fulcrum (tipping point) and you are far away. If you are close to the fulcrum and the heavy weight is far away and you push with the same force, you will not be able to lift it. This is leverage. The servo can only apply the same amount of torque/force everywhere along the control arm.

So, if you hook your control rod to the inner most holes of both the servo and control side and get say 1 in of deflection and if you move the control rod to the outermost hole on both and still get 1 in of deflection, the force available to move your control surface is still the same. One is not any better than the other as far as force goes. The difference is that you have more control rod movement back and forth.

Does this make sense? (I think this is correct!)

Steve

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Old 11-11-2011, 12:44 AM   #58
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Very well done!
The added bits to the control arm lengths is something I never thought of when setting up longer servo arms that it would take away from the torque the servo puts out.
Something to think about next time I setup a 3D plane.

This should be made a sticky like it was suggested before very good info and many things I did not consider, or know about servo selection for a given plane.
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Old 11-11-2011, 12:53 AM   #59
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Originally Posted by Stevephoon View Post
AEAJR,

I don't think this is correct. It's the motor torque or force that servo’s list. That same amount of force is available everywhere on the control arm. Think of you pushing with the same force on a teeter-tauter. You can lift something heavy if the weight is close to the fulcrum (tipping point) and you are far away. If you are close to the fulcrum and the heavy weight is far away and you push with the same force, you will not be able to lift it. This is leverage. The servo can only apply the same amount of torque/force everywhere along the control arm.

So, if you hook your control rod to the inner most holes of both the servo and control side and get say 1 in of deflection and if you move the control rod to the outermost hole on both and still get 1 in of deflection, the force available to move your control surface is still the same. One is not any better than the other as far as force goes. The difference is that you have more control rod movement back and forth.

Does this make sense? (I think this is correct!)

Steve
Thing is when you mention a teeter tauter that is not the same at all. Picture trying to push a car with your arms extended to the side you couldn't your arms would get tired and you would loose the leverage. But if you stand behind the car with your arms in front of you you have great strength to push it. I know this is extreme example, but you are comparing a focal point for leverage to a servo arm the is not designed to be used that way. It would be weaker as the servo arm is made longer more force, or in this case torque would be required to move it.
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Old 11-11-2011, 01:23 AM   #60
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gramps and AEAJR,

Everywhere else that I look reads just as you guys are saying, so I guess I'm wrong. I must be getting torque and force mixed up....

Steve

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Old 11-11-2011, 02:17 AM   #61
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AEAJR,
I humbly request a sticky again just because there was ton of info that I still have not read and wish to. I figure I cant be the only one who still wishes to read the entire thread or refere back to it. As a beginner in the hobby it the threads like these that are so important. I would hate to see this thread get dated thus becoming buried. Especially at the speed of technology I am sure there will be more to add here in the near future. I dont mind saying your threads have tought me tons. In fact I think I would still be trying to figure out my cub if it had not been for your advice

Happy flying may your crashes be limited and if they are not limited let them be cool.
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Old 11-11-2011, 02:26 AM   #62
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I agree, Ed. This is a thread whose subject is unlikely to come up again because it has to be raised by a person experienced enough to know you have to talk about it. This is definite sticky material.
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Old 11-11-2011, 02:31 AM   #63
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Hey that's the great thing about this place reading and learning.
Heck I was off, or I should say wrong on the way I was picking my servos for my planes I based it on the AUW then picked a servo. Never knew there was a program much like Motocalc to help select the correct servos for a given plane.
I am going to use it to help me pick out servos for a up coming plane.
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Old 11-11-2011, 02:48 AM   #64
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Another way to look at it. hold the servo up with the servo arm out sideways. Tie a string on it 1 in. out from the shaft with say, a two oz. weight hanging off the end of it. Put power on the servo and it should be able to hold the weight, or even lift it if the servo rating were a little lower than two oz. in.

Gord.

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Old 11-11-2011, 03:02 AM   #65
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Cool how the servo knows where it is.

Since electronics has been a hobby of mine since the late 60's I will attempt provide an answer. Attached to the output shaft of the servo,is the sliding arm of a potentiometer. This arm contacts the circular resistor at some point along its length and creates a value of resistance from itself to each end of the circular resistor. Each end of the resistor and the arm are wired to a circuit inside the servo which detects the value from one end of the resistor to the arm and compares that to the value from the arm to the other end of the resistor . It then compares that to a value given it by the signal from the receiver . For example; if the signal tells the servo that it should be at the exact center of it's throw; and the servo circuit sees that there is more resistance on one side of the potentiometer's arm than on the other side; it will instruct the motor to turn in such a manner so that the arm will end up in the very center; thereby equalizing the resistance on each side of itself. This is a very simplified explanation ( and, yes I know it isn't so simple)!! Todays high tech servos use other devices to detect that positioning,but basically it works the same way. I hope this helps clear up what goes on inside the servo. See you at the meeting ED! By the way there is NO feedback to the transmitter itself;at least not yet. The transmitter tells the receiver where it wants the servo to go and this signal is fed to the internal circuit of the servo as a reference. It is up to the servo to match that reference point with appropriate movement and positioning; using the transmitters command ; but the transmitter doesn't really know what the servo is doing. I guess that is why we can have crashes caused by equipment failure.... the transmitter ( and sometimes the dummy behind the transmitter) don't know what the servo is doing until it is too late!!
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Old 11-11-2011, 04:37 AM   #66
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OK, let's keep this going as I sense there is more to discuss.

And I will make this a sticky, at least for a while.

Keep the questions and the inputs coming.

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Old 11-12-2011, 03:48 PM   #67
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Build Review jittery or "stuck" servo

Some years ago I found a fuselage in a trash bin and took it home. I found an old airtronics servo buried inside the fuse and extracted it.After I plugged it in I found that it moved very erratically and refused to move over it's full range. I decided to open it up to get a better understanding of it's insides. Inside I found the cup which contained the 'feedback' potentiometer and the arm which rotated when the output shaft rotated. I looked inside the cup of the pot and saw that there was some powder from the carbon resistor inside the cup and some tracks which were worn down into the carbon from the arm's 'fingers' or wipers. I cleaned the powder out with some cotton soaked in rubbing alcohol and slightly bent the fingers or wipers over toward an area where the carbon was still not worn down. After cleaning the gear train and inside of the servo I reassembled the servo & it worked as good as new.! Granted I wouldn't want to trust it in a critical function but I do use it as a test servo to test out receivers and as a standard to test against other 'suspect' servos. MOTTO : IF YOU ARE GOING TO THROW IT OUT;OPEN IT UP...YOU MAY LEARN SOMETHING & YOU HAVEN'T LOST ANYTHING!!!
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Old 11-12-2011, 08:51 PM   #68
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I still have a about 40 or so new HS81MG & HS225 that I bought at a estate sale for less than $4.00 each NIB. I love HS81's.
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Old 11-19-2011, 10:07 PM   #69
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Originally Posted by AEAJR View Post
When I was a new pilot I was confused about servos, how they were used, how they were mounted and how to pick them. So let's start a discussion about servos for all the new pilots.

Let's talk about types, sizes, how to pick them, etc., whatever you like.

Here are some general comments to get things started.

Servos are rated by:

Size - Length, width and depth
Strength - Measured in inch/ounces of torque
Speed - Measured in degrees per second
Weight - Ounces or grams (28 grams to the ounce)

For illustrations I will use Hitec's servo line but my comments apply to other brands as well.

Hitec web site -
http://www.hitecrcd.com/


Bearings

Some servos have no bearings. The plastic case essentially holds the shaft and supports it against pressure in use. The Hitec HS-55, HS-81 and HS-82MG servos would be good examples of servos with no bearings. They are very inexpensive and work quite well when new, but over time they wear and tend to lose their centering ability. However for light duty sport use they are fine.

I have many planes with these and similar servos of other brands. However today I generally buy servos that have some kind of bearings. These hold center better over time and move more smoothly over time.


For example a Hitec HS-85 BB is about the same size but a little more expensive than a Hitec HS-81 or 82. If I were putting a new sport plane/glider together I would chose an HS-85 over an 81/82 because the HS-85 has bearings to support the shaft.


Another example would be the HS-55 which does not have bearings. The Hitec HS-45 and HS-65 have bearings and can be used in similar situations. Partially because of the bearings they should center better and run smoother over a longer useful life.


Analog vs. Digital Servos

There are standard or analog servos and and there are digital servos. I will use Hitec brand again as examples but I also use JR, Airtroncis, Spektrum, Futaba and other brands.

Analog servos are typically your lower priced servos. They work well in most sport applications.

Digitals would be my preferred choice for any form of a competition plane or high speed planes.

For me the main benefit of digitals is more precise centering. They can also tend to hold position better under force, but they will pull a lot of power to do it.


If you are flying at 150 mph you don't want those control surfaces being blown back because the servo can't hold position. And in any kind of competition you want those servos to go to the correct center or offset EVERY TIME and hold there solidly. Small variations can be a real issue in competition.

Some brands, like Hitec, have basically the same servo in analog and digital. For example:

Hitec HS-85MG - Analog
http://www.hitecrcd.com/products/ana...i/hs-85bb.html

Hitec HS-5085MG - Digital
http://www.hitecrcd.com/products/dig...hs-5085mg.html

They are the same size, same spline, same ball bearings, same gears, but have different control boards and maybe a different motor. You can swap one for the other in your plane and they will fit.

In my competition planes is it all digital servos with bearings. I use Hitec, JR and Airtronics digitals. I have to have solid centering in these ships.


Spare Servos

I always keep spare servos in my field box. If I am going to an away contest I will have the servos that match the digitals in the plane along with me. But some of my servos cost $50 or more, so keeping lots of spares spares in the box, hopeful that I will never need them, can be expensive.

I still have some of the Hitec HS 81/82 and HS-55s. I at least one of each in my field tool box as emergency spares. They are cheap spares to get me through the day at the home field and work well enough and can cover a wide range of applications. Best of all they are low cost so they do not represent a big investment to just toss in the tool box for unexpected field repairs.

Just this past weekend I had a servo strip in my Radian. I pulled out an HS-81 and in 20 minutes I was back in the air. They will keep me flying if I need one at the field. And if a friend is in need I can offer them a servo without breaking their bank or mine.



So, what do you know about servos that you would like to share?


What questions do you have about servos?

Let the discussion begin!
I was looking through my toolbox and low and behold I found 4 aero sport GS-ICR servos from my first Joe Bridi trainer which was 30 years ago. Don't have a scale but they are noticably "heavy" as compared to the htx 900's . Was wondering if I can find a home for them in anything. Anyone old enough to be familiar with these? A white, black and red wire.

Just trying to avoid another order with the "King"

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Old 11-19-2011, 11:02 PM   #70
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Originally Posted by dahawk View Post
I was looking through my toolbox and low and behold I found 4 aero sport GS-ICR servos from my first Joe Bridi trainer which was 30 years ago. Don't have a scale but they are noticably "heavy" as compared to the htx 900's . Was wondering if I can find a home for them in anything. Anyone old enough to be familiar with these? A white, black and red wire.

Just trying to avoid another order with the "King"

-Hawk
If you wires are white, black then red you can not use them with today's receivers as the standard today is red in center. You will likely damage the servos if you connect them to something. Chances are the plugs won't fit either. So if you reconnect them put the red in the center.

If they have been sitting in your toolbox for 30 years, I would not trust them. Rust, dust and corrosion could make them unreliable.

Time to toss 'em buddy, or maybe use them on an RC car or boat that won't fall out of the sky.

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Old 11-20-2011, 12:44 AM   #71
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I would like to see you extend this discussion - or start a new one - on how you can tell how many servos a BEC on an ESC can safely operate. I have never been able to understand that

Wolfe
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Old 11-20-2011, 12:52 AM   #72
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Originally Posted by AEAJR View Post
If you wires are white, black then red you can not use them with today's receivers as the standard today is red in center. You will likely damage the servos if you connect them to something. Chances are the plugs won't fit either. So if you reconnect them put the red in the center. If they have been sitting in your toolbox for 30 years, I would not trust them. Rust, dust and corrosion could make them unreliable. Time to toss 'em buddy, or maybe use them on an RC car or boat that won't fall out of the sky.
Not too fast there Hoss, folks might be looking for the older servo. Try selling them on ebay, or in a NYC add. Plenty of collectors about. You might also think about the motor and gears as recyclable. Is the motor the old two wire, or three wire brushless type? If either, you can also use the motor on a small indoor model with a propeller attached. All sorts of things can be done with an old, small motor, including a tiny electric screwdriver or drill with parts. Who knows, you might already have a tiny drill or screw driver that needs a better motor.
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Old 11-20-2011, 02:10 AM   #73
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Originally Posted by wolfewinde View Post
I would like to see you extend this discussion - or start a new one - on how you can tell how many servos a BEC on an ESC can safely operate. I have never been able to understand that

Wolfe
You know, that is a REALLY good question. I really have no idea other than to read the MFG recommendations.

I believe, for example, that the ESC in the Radian Pro PnP has a BEC that is only rated for 800 mah and normally they say no more than 5 micro servos, but the Radian Pro has 6.

There have been those who have tested and seen the 6 servos exceed the rated capacity of the BEC.

At 800 mah and 6 servos that would be about 130 ma per servo. But I could never tell you if that is the max the could pull or just a sample.

I have always used 200 ma as a personal guide for micro servos, like the HS-55. But I really have no documentation to support that number.

I beleive that large digitals can exceed 1 amp under heavy load.


Anyone have any good reliable info on this topic?

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Old 11-23-2011, 02:15 PM   #74
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You would expect that servo draw would be a natural thing for the new generation of data logging electronic speed controls to report. However, none of the Castle Phoenix Ice or Hobby King Super Brain data logging ESCs track that. Just imagine how useful having actual in-flight servo drains could be!
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Old 11-23-2011, 02:37 PM   #75
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My only reference to test data of any kind was a post in a Radian Pro thread that was driven by a concern about the BEC on the ESC. Someone tested and found that the servos could hit as high as 800 ma which exceeded the 700 mah BEC in the ESC. That would be for 6 HS-55 class servos.

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