AEAJR

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!


Edit:10-19-2012 - I added a servo calculator. I did not write this but the results look reasonable. This may be helpful when fitting out a new plane where you don't have guide lines for servos.

Good video on the topic of servos. His focus is on small planes, but some good discussion and graphics. Watch the video and then ask questions.
http://www.youtube.com/watch?v=t_LVNUC3Y8o

Fishbonez

Wow, Just that short bit was really informative. I have often looked at servos to purchase but never knew the difference. I usually just went ahhhh that one will do I guess. Thanks that explains a lot for me. I just learned that I have been paying to much for servos. I have always bought digital because I thought that since you are only dealing with 0s ans 1s that you were guarenntee not to have an electical failure, That has always been my logic behind it. Thanks for the heads up I appreciate that now the next time I go into my LHS for servos I will be more informed

CHELLIE

HXT 9 gram servos for just about everything, parkFlyers up to a 30 size plane, they are strong and powerful, And they are Cheap like me But they do draw a lot of current, thats where they get there Strength at, use a ubec if using 4 or more of these servos, Hitec , Jr, Futaba Make a great servo too, They just cost more thats all LOL

AEAJR

A good start.

Let me suggest that if you talk about a specific servo that you like, that you hate or that you have used in the past, post a link so that people can see the information about that servo.

Chellie - can you post a link for those HXT 9 Gram servos? I would like to see if they are analog or digital and if they have bearlings or not. I would like ot see their weight, dimmensions and their torque rating so I can tell if they will work for me.

AEAJR

VOLTAGE - Then and now

Traditionally servos were rated for either 4.8V or 6V which was based on 4 or 5 cell NiCd receiver packs. You needed to make sure your servos would operate at the voltage you were feeding your receiver or you might damage then or they might not meet your torque needs.

As BEC started to appear in ESC the BEC were typically tuned to 5 volts to be in that range. Some of the after market BEC can be adjusted to the voltage you want. The after market BEC also allow you to use 2 or 3 cell lipos as your receiver packs. Note that there is no advantage to using a 3 cell lipo as the BEC just as to waste the extra voltage when it steps the output down to 5 or 6 volts.

Some of the newer receivers can operate at 7.4V or higher. So we are now seeing servos that can also operate at 7.4V so a BEC is not needed when using a 2 cell lipo pack. If you can eliminate the BEC that is one more thing you don't need to buy and one less thing to fail.

Rodneh

All servos have bearings, even the cheapest ones. Usually the less expensive are sleeve bearings and the more expensive have ball bearings. For most of us sport flyers, the sleeve bearings are very adequate and, in most cases, will hold up as well as the ball bearings. If you are going to be flying a trainer or typical sport flyer, the extra expense of the highly touted digital servos, super bearings etc. is just a waste of money. Metal gears can also be debatable, they wear out much quicker than the plastic gears do and develop slop or play much faster than the plastic gears do. In most cases, the plastic gears are more than adequate. For most of us, the least expensive servos produced by the major manufactures (Hitec, Futaba, Spectrum, etc.) are perfectly adequate. There is a place for the super duper units, it is just that most of us will not benefit from the extra cost unless you are doing precision flying.

AEAJR

While one could say that you can not build a servo without some kind of bearing surface, and I would agree, the servo MFGs clearly draw a distinction between their servos which have bearings and those that do not.

That is the basis of my post.

Here are examples of servos listing bearings and those that do not. Hitec, Futaba and HXt draw a distinction between them in how they list their products. And I have seen a degradation in the non-bearing servos over time resulting in dual centering. The ball bearing servos seem to hold up better over time.

Here is how 3 MFG list their servos.

HS-81
http://www.hitecrcd.com/products/ana...ini/hs-81.html

Bearing type = none.

You can find the same assessment for some of their other servos.


HXT 9g servo - no bearing referenced
http://www.hobbyking.com/hobbyking/s...cro_Servo.html

However this HXT servo does list bearings
http://www.hobbyking.com/hobbyking/s...ing_servo.html


Futaba S3003 - No bearing referenced
http://www3.towerhobbies.com/cgi-bin...&I=LXH288&P=ML

Futaba S3004 - Ball Bearing servo
http://www3.towerhobbies.com/cgi-bin...&I=LXVW07&P=ML

Stevephoon

Good Topic.... I'd like to learn what servos eveyone likes as well. I've only been into small (mini) sized servos so far...

I have used the following:

HXT-900s - All 8 I have used have been good. Used in parkflyer sized planes.

TP SG90s - Similar to the HXT-900, but cheaper in all ways... I've used these in a couple of parkflyer models, but the gears have been stripped on some less than perfect landings. I prefer the HXT-900s for this type of servo.
http://www.headsuprc.com/servlet/the...sh---9g/Detail

EMax ES08MA - I've had good success with this servo. Cheap, but more torque and slightly heavier than the HXT-900s and have metal gears. I've used these in my Daddy-O and Firestorm without any issues. I assume that these do not have ballbearings, there is no mention of them in any of the sparse documenation on them. Not the fastest, but center well and not to too current thirsty...

EMax ES08MD - The Digital Version of the previous servo... I have 4 of these and none of them will center properly. They twitch constantly.... At center or any other postion... I will not put these in a plane! I'm not sure if I got a bad batch or what, but mine are terrible.

Emax data is here:
http://test.yinyanmodel.com:7323/En/....asp?SortID=13

And here for a US Local Supplier:
http://www.valuehobby.com/products.php?category_id=20


Steve

Rockin Robbins

After the loss of my Radian I built a Slow Stick as a disposable test bed for equipment I was not familiar with, specifically the Orange RX Spectrum compatible receiver and the new Hobby King $1.69 HK15178 servos. Reviews indicated that they were just about identical in performance to the HXT 900s. The HXTs were out of stock so I tried out the HKs.

Published Spec:
Torque: 1.2kg/cm @ 4.8v, 1.4kg/cm @ 6v
Weight: 10g
Speed: 0.10/60deg @ 4.8v, 0.09/60deg @ 6v
Voltage: 4.8v~6v
Plug: JR

These have a sleeve bearing (no bearing by aeajr nomenclature). They are analog servos. Weight is right at 10 grams fully assembled with control arm.

I find that they are noticeably faster and center better than the eFlite servos that came stock in my Radian. But those servos have received more than their share of criticism and maybe are not the best standard of comparison. Each click of trim on my DX5e results in an evenly measured discrete step from the servo and centering is excellent.

I only have about 3 months flying them in the Slow Stick but they have been flawless so far. Oops, they're now $1.89.

It's not like the HXTs are not value priced already. But alternatives are good and it's been fun trying out these cheapies and finding that I like them.

Their speed and current draw as tested by reviewers on the HK website are so close to the HXT900 that you've got to wonder.......

AEAJR

Good stuff guys, keep it coming.


New pilots, do you have questions?

Do you understand servo speed? How about torque?

Do you know why we sometimes us metal gear servos and sometimes nylon gear servos. And did you know servo gears can be made out of other stuff?

How about mixing brands or models of servos? Any issues?



Let's touch on mixing servo models and brands.


Mixing brands and models of servos within the same plane is usually fine as long as they can all operate on the voltage that is being delivered to the receiver. And most servos today use similar plugs so plugging them into your receiver should be a non-issue. I think Futaba still uses the J connector which has an extra tab. You can cut/file/sand that tab off and and it will plug into the receiver with no problem.

My recommendation is whenever two surfaces have to work together, such as ailerons, then they should have the same brand and model of servo. In addition, if you have more than two of the same model and brand, it is a good idea to test them before installation. Sometimes one will be just a little off from the others.

That "off" servo, as long as it works smoothly, will likely be fine for a single surface but in the case of flaps or ailerons, where you want them to work together, that slightly off servo could thow off the coordination of the surfaces.

I always test servos before I install them to be sure they work evenly and smoothly. On rare occasion a servo can be defective, a gear can be bad or even a connector can be bad. Better to find out before in you install it.


Anyone got a good set-up centering procedure they would like to share?

Turner

I'd like to see more on figuring how much torque is needed for various sized planes and different flying categories. I know there are some calculators out there but are there any general rules that can be used?

One I have read is that servo torque in should be nearly equal the the plane weight. I have used this with good results for 15-30 oz. planes but as you go bigger it would probably be overkill unless the flight style or speed was more extreme.

As a side note, I've always thought it a glaring oversight that there was no servo specific forum here.

AEAJR

There are a few approaches that I use for sizing servos.

1) RTFM - read the friendly manual - Frankly this works most of the time.

Whether it is a kit or an ARF, the instructions typically suggest an example of the servo that should be used. It might say to use a Hitec HS-55 or similar. So you use an HS-55 or another servo of similar size and torque.

Sometimes the instructions will say to use a servo with at least 16 inch/ounces of torque, so you read the servo specs and get a servo that fits in the plane that has 16 or more ounces of torque.


2) Look at similar models and see what they are using. If you have a 24 ounce electric airplane that is kinda medium speed and other planes of simlar size and speed are using GWS Naro servos, then look for servos about that size and torque.


3) Use a calculator - There are calculators out there. Frankly I have had it for 4 years and have only had to use it once when setting up a new slope glider where I didn't even know the name. Here is one calculator http://www.flyinggiants.com/forums/showthread.php?t=900


95% of the time approach 1 and 2 get me the information I need. And when in doubt, I err on a few more ounces of torque.

Most of the time the servos are much stronger than needed for most sport flying. However if you tend to be into high speed flying where the surfaces are going to have a lot of pressure then the strength of the servo will be even more important.

If you are flying an F27 Stryker that came with a speed 480 and you are upgrading to a 300 watt brushless, you might want to consider upgrading the servos too. The speeds you will be hitting were not what the designers had in mind when that F27 first came out.


If you are into high precision fast aerobatics or aerobatic helis, you might need a fast servo.

Some servo makers offer the same servos but with different gear sets. GWS is a good example. Look at the charts at this link. You will see that same servo but with different torque and speed specs. As speed goes up, torque goes down. I believe it is the same servo with the same motor but with a different gear set. So if you need a strong but slower servo you get the micro SD servo. If you need a fast servo you get the micro XFD or EFD, not as strong but MUCH faster.
http://www.gwsus.com/english/product/servo/micro.htm

flypaper 2

For what its worth. That little oval sticker on the inside bottom of the HXT 900s, on the inside says Tower Pro 90.

Gord.

Rolling Thunder

They're different...at least the gears are.I tried swapping the gears & they dont fit.The HXT's are a little thicker hence they dont strip out as easily

CHELLIE

I Love these little servos, they use a Teflon Bushing Thats super tough, and when and if ever they do wear out, toss it away and buy another one, they are a good, strong throw away servo I have used them in a Funjet doing 103 MPH with no problems. and for $2.69 How can you go wrong

http://www.hobbyking.com/hobbyking/s...cro_Servo.html

HXT900, one of the most famous and the best value micro servos available.
Used by tens of thousands of hobbyiests world wide. The HXT900 is the #1 trusted low cost 9g servo.

Size : 21x12x22 mm / 0.74x0.42x0.78 in
Voltage : 3v ~ 6v
Weight: 9g / 0.32oz
Speed : 0.12 sec/60(4.8V)
Torque : 1.6 kg-cm
Working Temp : -30C~60C
Teflon Bushing, 15cm wire, coreless motor
Servo arms & screw included

rcers

I am so confused - so many rave about the HXT900's and love them. So I bought 8 of them a year or so ago.

Two OBF failures - never worked at all. Hmmmm Two others would not center for beans. Hmmmmm. So now I stick the other two in a cheap foamy I don't care about. They seem to work fine.

I place two in a plane a care a great deal more about. They too are performing well.

So I have never purchased more and feel really good about my extra money and Hitec servo purchases.

Perhaps I need to try again, but am thinking what if your $3 servo brings down your $300 airframe - is it worth a few bucks?

I still have yet to find the equivalent of the HS-65 servos anywhere. They are beastly strong, center very well and have super tough gears.

At any rate - I think I will pay more money and stick to Hitec. No brand is perfect (just had a brand new Futaba die on a bench test).

Just to add to the thread - one thing I highly recommend is the use of a servo tester, cycler. I use them to setup all planes and to test all new servos. I run them on a cycle (they just run back and forth until I stop them) for a few minutes. I have found MANY issues (like the XHT900's I speak of) with this device.

I use a small electronic one but there are many others on the market. They save a great deal of time with system setup.

http://www.york-electronics.com/

Mike

bbqflyer

Great info everyone! I've been in the hobby a little while, but I have always been clueless as to what servos to get. All this info is hugely benificial, so thank you!! I'll be printing this thread out so I can always go to it for reference.

AEAJR

Thanks for the report Mike.

When you contacted the retailer about the bad servos what did they say? Did they offer to replace them?

If you are happy with Hitec then why change? If it ain't broke, why fix it. I too am a Hitec servo fan.

And, yes, test all new servos. I use a bench receiver and pack to work all new servos using the excercise program on my radio, or just by working the sticks. From time to time I find a new servo that is bad, but it is pretty rare. Sometimes I find a problem with an older servo that I planned to reuse. Much better to find problems before I install them.

rcers

No it was HobbyKing and they don't bother with that pesky customer service.

Because the XHT's were getting such glowing reviews. And I thought for a few bucks it was worth a shot. I learned you really do get what you pay for...

But I still see many rave about them and swear by them.

One other thing to note. The cheaper servos very much tend to draw more operating current - some over 3x more. That too is a factor to consider.

Fully agree - most will work or not. But I have had one fail after a few minutes of cycling with the servo driver. Glad that happened on the ground.

Mike

AEAJR

Servo gear kits.

The thing that is most likely to break on your servos are the gears. So, if I am going to use a bunch of the same type of servos I often buy a spare servo or a spare gear set so I have them on hand if I need them.

An HS-5085 MG digital servo costs $44, but the gear set is only $16. So if I stip a gear I can swap the gears and be back in the air in 20 minutes without having to keep a spare $44 servo in my tool box. And the same gear set fits the HS-85 so it covers me for two different servos.

I have spare gears for HS-55s, HS-81s, JR servos and Airtronics servos too. 75% of the time it is the gears that go.

NoResults

I just replaced a servo in my radian using a Hitec HS-81. Unless I am reading something wrong the specs look much better than the HXT.
As Chellie shows in above post Torque is 1.6 vs2.6 for the Hitec also speed on the Hitec seems to be better. Do these spec mean anything to the average user of my fist plane?

CHELLIE

Hi Mike use these servos in your cheapy foamy parkflyer planes, in more expensive planes, use a good brand like a hitec servo, but for cheapy rc planes, you cant beat these HXT 9 gram servos, in the 3 years i have been using these servos, i have only had 2 go bad on me, one was my fault, i did not clear a shipping container and the rudder hit the edge of the container and i stripped a servo, the second failure was maybe me again as I should have put the elevon linkage on the top of the wing rather than on the bottom where it can catch onto things being a belly lander, for me, those are the 2 only failures I have had with them, Take care, Chellie

CHELLIE

Hi The Hitec HS-81 is a Great High End Servo, they are about $13.00 to $17.00 Dollars Each as compaired to $2.69 for a HXT 9 gram servo, the HXT 9 gram servo could be better compaired to hitec 55 or 65 servo, You cant go wrong with a hitec servo, you will just be paying more for them, the HXT 9 gram servos are great for your not so expensive parkflyers planes, but if you have an expensive plane, then it would not be a bad idea to use the more expensive servos for some extra insurance, check out the performance of the HXT 9 gram servo on my scratch built plane, my friend steve is doing the maiden flight as i am doing the filming

[media]http://www.youtube.com/watch?v=lTbir0Uc6Q8[/media]

http://www.amainhobbies.com/product_...81-Micro-Servo

bbqflyer

Nice video and plane Chellie, as always! I agree with your idea of cheap servos for cheap foamies and vice versa for our more expensive planes. I love my little cheapy tpros and turnigys, I have a workshop full of them and have never had a problem, but I just can't bring myself to putting them in my nicer planes.
I do have a question to all though, when should you use an external battery pack to run the servos and how is that properly setup?

flypaper 2

It's all choices. I run a Turnigy UBEC in the Eagle above. Don't trust myself to remembering to charge the separate batt. Don't grow old, your memory takes a dump.