How RC servos work

Your Reviewer

My Credentials
So who's doing this reviewing then?

Well I've been building and flying or driving radio controlled models for over 40 years and during that time I like to think I've built up a reasonable amount of knowledge.

I'm also a qualified electronics engineer who has worked in radio frequency, analog, digital systems and software for more than three decades. In fact I designed and built my first RC set back in 1969.

For the past nine years I've also been involved in the design and manufacture of some rather sophisticated engine technology and UAV flight control systems.

So, chances are I've been there, done that and have a huge pile of tee shirts to prove it.

Right now I'm heavily into 3D flying and enjoy all aspects of the RC hobby. I may be old but I don't feel it.

In the Pipeline

Here's just a little bit of what's to come on this site...

RC explained: Demystifying terms such as PCM, PPM dual conversion, single conversion, full-range etc., this feature will explain it all.

Cheap Chinese Engines: Just how good are those cheap Chinese glow and gas engines that sell for half the price of their "brand-name" equivalent? I put several to the test.

Build your own radio gear?: Back in the old days, building your own RC gear was not uncommon and now the arrival of 2.4GHz has made it practical again.

How do RC Servos Work?

THE BASICS OF RC SERVO OPERATION

Brand Wars

So who makes the best servos?

Well there's no easy answer to that question and asking it is likely to result in as many people as there are opinions offered.

The reality is that most of the big-name manufacturers produce perfectly good servos that, providing they're matched to the model properly, will provide long and useful service.

When it comes to the no-name servos, most of which originate from China then all bets are off.

Some time ago I began a very comprehensive review of every servo brand/model I could get my hands on and that will be online soon.

Choosing The Right Servo

More important than choosing the right brand of servo is actually choosing the right servo for your model and flying style.

With modern servos spanning the range from featherweight 3g units with only a tiny amount of torque to mammoth heavyweights that can exert over a thousand ounce-inches of power there's really no excuse for using a servo that's not matched properly to your model.

For small models (foamies ane the like) small, cheap 5-9g servos do the job just fine. Servos like the HXT900 are incredible value for money and are so cheap you can throw them away when/if they break.

Small heli-fliers can use these budget servos too but unless you're into just hovering around, there are some real benefits to be had from using digital servos instead. A number of companies make fast, strong digital servos that are well suited to small helis -- but you'll pay extra for that improved performance.

A sport/trainer model up to 5-6 lbs in weight will be ideal for the so-called "standard" servos such as the Futaba 3001/3003, Hitec 425 and a raft of others. While a ball-bearing servo will outlast a non-BB one, many people fly very low-cost (under $10) servos in these applications with no problems at all.

If you're into aerobatics or 3D flying then digital servos may offer you some benefits in terms of precision, speed and torque. It's also worth considering the use of metal-gears once models get above the 6lb mark as this will reduce the chance of stripping.

Larger helis are also a good candidate for stronger gears and digital amps since the force required (especially for collective/cyclic) can be very high.

Gas planes of 26cc and above really do need metal-geared servos due to the higher levels of vibration encountered and the size/weight of the control surfaces. Once you get into Giant Scale you should be looking at the top-end servos from the likes of Hitec, JR, Savox, Seiko etc. From a performance and safety perspective, it makes no sense to try and save money by using anything less than the best you can afford.

Final Thoughts

It's important to remember that servos are just one link in a long chain of components that effectively connect your transmitter sticks to the control-surfaces on your model.

The failure of any one of the components in that chain can cause disaster.

One of the most common issues is not having enough battery or BEC capacity to handle the servos you're using.

Hi-torque and hi-speed servos (especially digital ones) can draw enormous amounts of current from your system and if your battery or BEC isn't up to the job, bad things will happen.

Originally, Futaba advised against using NiMH batteries with some of its hi-torque servos because the early nickel-metal batteries just weren't up to the task. Fortunately things are much better now and a good quality NiMH pack (of adequate size) will work just fine.

However, anyone who tries to drive a bunch of hi-torque digital servos using nothing more than an AA-sized receiver pack is going to be sorely disappointed with the performance (and wreckage) that results from such folly.

Also make sure that your extentions (if you're using any) are up to the task.

Always use heavy-duty extensions if you can afford to carry the extra weight as these will produce the least loss of torque in your servos. In fact some servos (notably the Futaba 3000 series) will behave very oddly (oscillating and hesitating) if your servo extensions are not up to the job.