Why are there 2 voltage specs listed for servos? How do I determine which voltage to consider? Also, what is single versus dual rate mean? I am a newby and am confused.

Most servos were designed to effectively use the 4 cell batteries (4.8V nominal) but some wanted faster response and more power so started adding 1 cell after they found the servos would usually withstand the higher voltage. They do this at a slight risk of earlier servo failure and faster wearout time. The capacitors used in the servo are usually the weakest length as their breakdown voltage is quite low as, to get the necessary capacitance in a small package, insulation between plates is the bare minimum to minimize bulk. Until you get very proficient at flying, I'd recommend staying with the 4.8 volt batteries. As to the dual rate, that is usually a function built into the transmitter so that with the rate switch in one position, you get maximum throw on the servo. By throwing the dual rate switch the other way (and adjusting the amount of rate change) the throw of the servo is reduced. This is supposed to help the beginner by not allowing him/her to overcontrol the model.

Some folks run 5 cell packs to get more speed and torque out of their servos, some will take it some wont, those w/o 6v specs wont. Also the thoery is on a 5 cell pack if a cell fails there will be enough voltage to run the reciever, if a 4 cell pack (4.8v) has a dead cell then you collect your plane with a dust buster.

Dual rates are a way of giving a lesser control throw with the flip of a switch making the plane less sensitive, many aerobatic planes use dual rates to land when over control would be a bad thing for example.

G'day Mate,
Welcome to this great hobby.
The usual voltage for Recievers & servos is 4.8Volts, ie 4 cell battery packs, & this is what you get when you buy a radio system, but you can safely use 6Volts, ie 5 cell battery packs, that is why they list 2 voltages, & speed/torque ranges for servos. I won't go into detail about the pro's & cons of the different voltages.
On most radios these days, they have a switch on the Elevator, & Aileron axis, & some on rudder too, which allows the pilot to set different amounts of servo travel, at full stick deflection.
On the lower deflection, or RATE, the plane's controls will be less sensitive to control inputs, & thus make it easier to control, if more control response is needed, then a flick of the switch, & instantly you have more, or less control surface deflection, depending on which way you move the switch.

grraphal,
First of all, welcome to RCU. Hopefully we can help you get up and running in this great hobby.

They list the specs of servos at two different voltages because you can run your receiver setup in the plane at either voltage, either 4.8 volts or 6 volts. Each setting has advantages and disadvantages. For 6 volts your get more power and quicker movement out of your servos, but you batteries don't last as long. For 4.8 volts you get better battery life, but you don't get as much power or the servos don't move as fast. In the context of beginner's planes almost all of the radios that you will see will run at 4.8 volts. Most people don't switch over to 6 volt until they get into bigger planes.

Dual rates is a term used for your radio which determines how far the servo will move on a given control surface. At a normal setting a control surface will move a given amount of travel. But let's say that we don't want that surface to move so far, we will set our dual rate as a percentage of the full rate. So for instance we choose 50% on the dual rate, when it's turned on the control surface will move only half as far as it will on full rate. These are usually used when you need to a control be less sensitive for certain situations. One common use that is a good example is for elevator control. When in normal flight the elevator will be kept at full rate so that you have as much control over the plane as possible, but when it's time to land having that much throw is a problem because it causes the plane to jump around when you're applying elevator corrections. At this point the dual rate (or low rate) will be used so that the elevator becomes less sensitive and is easier to control for a landing.

Hope this helps

Ken

Everyone else has already answered your question.
To expand on dual rate a bit. This is a function of your transmitter (not all transmitters have dual rate capability) not your servos.
Another good use of dual rates is when you can't get the throws as low as you want mechanically. This happened on my Kadet Senior with the aileron servos.It had too much movement on the ailerons but I couldn't move the clevis in any due to binding, dual rates at %50 took care of the problem.

Wow:

What a fast response to my questions. Many thanks to each of your great RC'ers that replied. There is so much I have to learn, and I now know where the experts are. Thank you so much. Hal in Grand Rapids

If you haven’t already found local RC folks, plug your location in on the AMA club locator to see what’s around.
http://www.ama-dist-8.org/Clubs/Locator.php
You can get a huge amount of info and help on RCU. For some things, there is no substitute for local, hands on, assistance.