Servos for Radio Control Model Aircraft

Servos are to the radio system what you are to your company — the under-appreciated worker.

The transmitter is like the president whom everyone believes makes all the decisions but is actually having his levers pulled by someone else.

The receiver is the middle manager whose only function is to accurately pass information from the boss to worker, while being incapable of independent thought or doing any real work.  Fortunately, receivers pass information along much more accurately than your average manager.

The lowly servo ends up having all the work dumped on it and gets all the blame when anything goes wrong.

I should stop reading Dilbert before writing web pages.

Servo Specifications and Properties

With the exception of retract servos, all modern servos are digital-proportional.  What that means is that the servo moves in proportion to stick movement.  if you move the stick a little, the servo moves a little.  If you move the stick a lot, the servo moves a lot.

Retract servos (servos for mechanical retractable landing gear) move from one extreme to the other and can not stop in between.

When a person buys their first radio, they almost invariably use the servos that come with the set and do not think about it too much.

As your time in the hobby increases, you end up with more planes which usually means buying more flight packs.  The other option is to constantly swap the radio between planes which gets old real fast.

This is when most people start noticing the wide variety of servos and get very lost in deciding what to buy.  Many companies put together flight packs for various types models to save us the trouble of figuring it out ourselves.

For example, a company might offer a flight pack for large scale models, another for sailplanes and still another for park flyers.

Assuming you want to optimize your plane and get the most appropriate servo for each control, you will eventually need to understand what all the specs mean so that you can assemble your own flight packs for best efficiency, optimal performance and economy.

Analog vs. Digital

When digital servos were first released there were claims made about "holding power," "resolution" and "centering."  Frankly, I thought it was a lot of marketing hype.  While I see a lot of "experts" online talking about this and that, in person I don't know too many people who are good enough to tell the difference in one servo or another.  I'm one of those pilots.

However, when I rebuilt My Stik 30 I built a "3D" tail that was so sensitive I had to dial down the end-points to make the plane controllable.  That is a really bad thing to do because it lowers the overall resolution.  I used a good servo for the elevator but it just couldn't center well enough with the setup I had and it only takes a hair of up or down elevator to make the plane respond.

So I tried a digital mini servo meant for R/C car steering.  I noticed the difference immediately.  The servo centers so well that I'm not constantly bumping the stick to get the plane to stop moving in the last direction I moved the elevator.  In other words, if I gave "up" elevator and then released the stick the plane would continue to climb slightly.  Same for down elevator.

There are only two drawbacks to digital servos that I can see: price and current drain.  Digital servos cost more than traditional analog servos but I suspect the price will come down.

The current drain is higher because (as I understand it) there are more pulses to the servo to make it hold its position.  I don't think that's a bad thing because I have a good field charger so I can top off my batteries every few flights.  That means I don't need to put in a larger battery for planes that use digital servos.

Torque

Torque is expressed in ounce-inches (oz/in).  What it means is how much load the servo can handle 1" from center.

For example, if a servo has a 42 oz/in rating, then the servo should be able to lift a 42 ounce weight that is attached to the servo arm 1" from the center of the output shaft without stalling.  Servo stalling (not the same as aerodynamic stalling) occurs when the load over-powers the servo.

Note that torque is linear.  Using the same example of a servo rated at 42 oz/in, if the weight is moved 2" from the output shaft, then the servo can only lift 21 ounces.  If the weight is moved to 1/2" from the output shaft, then the servo can handle 84 oz.

Speed

All else being equal, faster servos are better.  Often manufacturers make pairs of servos that have all the same parts inside and are in the same cases.  One is a high-torque servo and the other is a fast servo.

The only difference is that the servo gears are arranged differently.  To make a servo faster (using the same parts) the gearing is such that the servo is weaker.

Motor

Some servos have standard motors and others have coreless motors.  Coreless motor servos start and stop faster due to less mass in the motor.

A body in motion tends to remain in motion and one at rest tends to remain at rest, yada, yada.

Coreless motor servos are better and also cost more.

Voltage

Traditional servos all work with 4.8 volt battery packs.  Many servos also work with 6 volt packs.  Higher voltage equates to faster servo movement and more power.  It also means shorter servo life.

Size

Servos come in a variety of sizes.  However, size is not always an indicator of strength.  For larger models, size is not much of an issue as far as the servo fitting inside the airplane.

Conversely, small models often are limited for space and require small servos.  There are many choices of small servos available today so finding one that will fit the application and be strong enough is not a problem.

Weight

Weight is the arch-nemesis of model airplanes.  All else being equal, a lighter servo is better.

Gear Train

Servos can have either nylon or metal gears or a combination of both.  Nylon gears are lighter, smoother, quieter and maintain a good gear mesh for a long time.  Their disadvantage is they break or strip more easily than metal gears.

Modern 3D planes can strip gears easily due to the forces fed to them from extreme control throws.

Metal gears have the advantage of being stronger and that is about it.  They get sloppy due to wear much sooner than nylon gears do.  They are also much heavier and noisier.

Don't get stressed about gears.  We have used nylon gears for a long time with very few problems.  Gears normally strip only because the servo was abused somehow.  Abuse is usually one of three things:

• Flutter
• Extreme control throws (3D capable aircraft)
• Crashes

Bearings

Better servos have one or two ball-bearing races supporting the output shaft.  The bearings reduce slop in the system as well as battery-draining drag.

Bearings make the servo heavier, but it's usually worth it.

Compatibility

Not all servos are compatible with all manufacturers equipment.  Be sure to check that a servo will work with your radio before buying it.

Selecting a Servo

An often asked question is how to figure out how much power a servo needs for any given application.  As far as I know, there are no formulas to figure this out.  The reason being is that nobody knows how much force is actually on a control surface in any given flight condition.

Most of us simply learn from experience how much servo we need.  You can not go wrong by having too much power, but that usually means a bigger, heavier servo.

If you think a servo may be questionable, but you want to try it anyway, then ease into maneuvers during test flights.  If the plane does not respond as quickly as you think it should, then it is possible that the servo is being over-powered by loads on the control surfaces.

One thing you can do is turn on the radio and push on a control surface until you think you are getting close to breaking something.  If the servo does not give up, then it is probably ok.

I really wish I had a simple equation to answer this question, but I am sorry to say that I do not.

Back to Radio Controls
Airfield Models Home

Copyright © 2004 Paul K. Johnson