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 -
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
Hitec HS-5085MG - Digital
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.
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: 10-2012 - Iadded a servo calculator spreadsheet to this post. You can use it to estimate what size servos you need for most applications. Isuggest at least a30%+ margin over anything recommended by the calculator to insure you have what you need in terms of servo strength.