Part 1

By Jason Sagaidak

ARF aircraft!!!!!!! Where do I start ?? As most who know me, know I’m not really fond of the cheaper line of ARF model kits and computer radios for beginners. “Why?” I here you ask. Although ARF kits have been good for the hobby they have been equally as bad. Yes they have probably gotten more people into the hobby and yes they are cheaper than building one from plans or scratch, and are certainly quicker to assemble.

BUT  and it is a big BUT…….they have taken the skill and understanding away turning what was once known as aero modelling to simply production assembly. People are simply relying on the kit manufacturer in getting it right…with hardware , servo selection, assembly instructions and most important; centre of gravity (CofG). Apart from the basic trainer I’ve only come across a couple of the cheaper brand ARF’s that has had the CofG right. Most are way out.(Now don’t get me wrong there are some very good ARF kits out there but most of these start at a couple of grand and go upwards and are usually of composite materials.)

I’m seeing more and more ARF aircraft coming out to the field that are from being airworthy!!

Some of the hardware that’s supplied with these kits is atrocious and should not have even been supplied. There is a reason they are cheap people!!!! The cheaper kit manufacture s all cut corners and supply crap hardware, hinges and as for the airframe you pay for what you get if you assemble the aircraft as per instructions.

Now having said my piece, there are some ways with a bit of thinking, modifications and cleaver assembly work some arf’s can be fixed up to be quite good and more importantly last. But firstly toss the hardware they supply into the bin. And go buy some good quality gear like Du-Bro ,Sullivan or Robart hardware. And for god sake don’t even look at the assembly instruction photos they are only a guide use your common sense (usually if it doesn’t look right it’s not) and apply what you know,(and if you don’t know ask an experience old school builder how he would do it) don’t take for granted that these kit manufactures are getting it right….some of the kits I’m sure they don’t even test fly…They only want your money and hope you crash so you buy another one.

Now computer radios……….AAAAAAHHHHHHHH what a pain in the you now what!!!!!

With the basic four channel model all you need to know is servo reverse ,dual rates engine cut, that’s it. Forget about end point and servo travel adjustments you should not need them. It should all be done at the servo (mechanical advantage) I’m seeing far too many fliers installing radio gear as per the photos in the instructions this is usually the horn is in the outer most hole on the servo and in the mid hole on the control surface , then they reduce the endpoint or servo travels on the radio. (don’t do it your overloading your servos and when you build a large model it will fail)

I’ll try to explain why you should not set up models like this so let’s have a look at the servos and how they work and are rated in torque , speed. Now I don’t claim to be an expert but I’ll share and explain what I do know and have learnt over the years.


Servo Basics

All RC servos have a three wire connector. One wire supplies positive DC voltage – usually 4.8 to 6 volts. The second wire is for voltage ground, and the third wire is the signal wire. The receiver “talks” to the servo through this wire by means of a simple on/off pulsed signal.


Servos basically come in 3 different sizes (micro, standard, and giant or 1/4 scale) to accommodate the type of RC models they are being used in. There are slight variations depending on the specific application but for ease of explanation, these 3 sizes cover most of the RC servos out there.

Speed and Torque Ratings

Other than physical size, the next item that all RC servo specifications indicate is speed and torque.

Speed is a measurement of the time it takes the servo to rotate a certain number of degrees. This has been standardized in most specifications to 60 degrees; In other words, the time it takes the servo wheel to turn 60°. The smaller the number, the faster the servo is.

For example a 0.12 sec/60° servo rating means it will take 0.12 seconds to rotate the servo arm or wheel 60°. This would be twice as fast as a standard speed servo that is rated in the 0.24 sec/60° range. A RC helicopter tail rotor specific servo will have speeds as fast as 0.06 sec/60°.

Torque determines the maximum amount of rotational force the servo can apply. This specification is measured in ounces per inch (oz-in) or in kilograms per centimeter (kg-cm). The larger the number, the more force the servo can exert. A typical standard servo will have a torque rating around 40 oz-in 2.8kg-cm. A high torque specific servo can have torque values well over 200 oz-in.16.8kg-cm

So what exactly does 40 oz-in or 2.8kg-cm mean?

This means if you had a servo arm that was one inch long on your servo it would be able to produce 40 ounces of pull or push force at the end of the servo arm before stalling. Now be careful when reading the metric values it’s 2.8kg at 1 cm (10mm) along the arm before the servo stalls. This is always measured from the centre of the servo head.

So many people think that the servo rating is anywhere along the servo arm. That’s not the case people read and understand the values that are printed on the servo box.


If you had a 25mm servo arm what do you think the force would be?

You got it, 1.86kg of force. How about a 50mm arm, 0.7kg of force – is it starting to make sense?

I should also point out that both speed and torque specifications are usually given for the two common voltages used for receiver battery packs. 4.8 volts for a 4 cell battery pack and 6.0 volts for a 5 cell battery pack. This also translates over to the typical BEC’s or voltage regulator outputs if that is how you power your on board electronics. Obviously the 6.0 volt packs give slightly higher speed and torque ratings.

Even higher voltage servos are starting to make their way into the market with ratings up to 8.6 volts. These servos offer even more speed and torque and will continue to grow in popularity as 2S LiPo RX battery packs become more and more popular so no voltage regulator will be required. Assuming of course your receiver will operate at these higher voltages but most of today’s 2.4 GhZ receivers are able to handle it.

The limiting voltage factor in the RC heli world is generally the Gyro and or gyro servo, many of which are designed to operate at no more than 5 volts; but , that trend is changing and it seems every new gyro/tail servo or electronic flybar system are now rated at 6 volts.

Digital servos vs Analog servos

Right lets look at digital servo vs an analog servos, just a few years ago the only r/c servo available where analog, but now we digital servos too. To answer the question of which is better lets look at how each work and the choice will be pretty obvious.

Firstly there is no physical or main component difference between them, The servo case ,motor, gears, and even the feedback potentiometer all have the same functions and operations in both.

The difference is in how the signal from the receiver is processed and how this information is used to send power to the servo motor

Analog servo operation

An analog servo controls the speed of the motor by applying on and off voltage signals or pulses to the motor. This voltage is constant (the voltage of the receiver pack 4.8-6v)

This on /off frequency is standardized 50 cycles a second. The longer each on pulse is the faster the motor turns and the more torque it produces.

This is the same way speeds of most motors are controlled, for eg. Ceiling fan. The motor isn’t given lower voltage to adjust the speed.

The speed switch simply cycles the 240 volts to the fan motor on and off many times a second. The longer each on pulse is the faster the fan runs. This is also the same way electronic speed controllers work for brushless motors.

Ok back to our analog servo. At rest there is no voltage going to the motor. If a small transmitter command is given or some external pressure is applied to the servo horn forcing it off neutral, a short duration voltage pulse will be sent to the motor.

The larger the stick movement the longer this “on” pulse will be in order to move the servo quickly to the desired position.

Remember me saying that these voltage pulses are sent 50 times a second? This means that in one second there are 50 windows that last 20 milliseconds each (50×20=1000ms=1second). The longer each voltage pulse is in each of these fifty 20 millisecond windows the faster the servo motor turns and the more torque it produces.

I thought this info might be helpful for those of you who really wanted to know what makes a servo tick. And for a large number of you it will be a lot of mumbo jmbo. As long as you understand that during small amounts of stick movment or when external forces are applied forcing the servo off it’s neutral or holding position , a short duration voltage pulse will be sent to the servo motor every 20 milliseconds. With large stick movments a long voltage pulse will be sent every 20 milliseconds to the servo motor

As you can imagine, a short power pulse every 20 milliseconds doesn’t get the motor turning that quickly or allow it enough time to produce much torque. This is the problem with all analog servos. They don’t react fast or produce much torque when given small movment commands or when external forces are trying to push them off their holding position. This area of slow sluggish response and torque is called “dead band”.

Digital R/C Servo Operation

Digital servos to the rescue!! Like I mentioned before a digital servo has all the same parts as an analog servo , even the 3 wire plug. The difference is how the pulsed signals are sent to the servo motor.

A small microprocessor inside the servo analyses the receiver signals and processes these into very high frequency voltage pulses to the servo motor. Instead of 50 pulses per second, the motor will now receive upwards of 300 pulses per second. The pulses will be shorter in length of course but with so many more voltage pulses occurring the motor will speed up much quicker and provide a constant torque.

Also if you are wondering why digital servos “sing” or “hum”, when very light force loads are placed on them what your hearing is the short high frequency voltage pulses acting on the motor.

The result is a servo that has much smaller dead band, faster response , quicker and smoother acceleration, and better holding power. You can test this very easily by plugging in a digital servo and a n analog servo into your receiver. Try to turn the analog servo off centre.

You’ll notice the analog servo will move slightly before it starts to respond. Now do it to the digital servo it doesn’t move it’s like the servo is glued to centre. It’s response time is much quicker.

OK now with this higher pulse frequency with digital servos there is a trade off. With increased speed, torque and holding power and that is POWER CONSUMPTION

You got it, digital servos are power hungry. All those hundreds of power pulses per second use up more battery power than an analog servo would. This isn’t much of a problem these days as our battery packs are getting larger in capacity of what we use to use.

I hope this has been a help.

To be continued……..