By Jason Sagaidak

It all started one blustery Sunday (16/2/14) as a few diehards where sitting round the table in the club house discussing issues of the world and of course the weather, when Scott from Pro Hobby turned up and moseyed over to the club house with a rather large cheeky grin on his face and said, “Jase have I got a deal for you!”.  “Oh no this usually cost me money!” I replied.

Scott put on the table one of E-Flights new Carbon-Z Cub’s. I replied to Scott, “Mate its foam; I told you I’m not buying foam any more especially 85in of it, it will be crap!”

After all the lads had their look and a lot of umming and aaah’ing I decided to have a look myself.

Well I must say first impression was how neatly it was packed, further inspection (as I look with my hands and not just my eyes) I started to unpack the Carbon-z cub and was starting to get slightly impressed with its quality of foam moulding. The finish, for a foam model is really quite nice, good paint and really shiny smooth finish. E-Flight have done a really, really good job of this one.

Ok lets look at the electronics; and ok, it’s got a Spektrum DSM receiver in it, not really a fan of JR or Spektrum but hey each to his own and for many would be happy to see that in there. The servos are mini digital with metal gear that look to be installed pretty well. Scott pointed out that it had a tow release pre installed just needed to supply your own servo. Now one would think E-Flight could have put one in surely for the few bucks it cost them.

Motor and  ESC are of course E-flight and we all know that’s all pretty good quality.

Right-o Scott how much???

I Looked at the boys (Frank , Bruce , Chris) and said we could use it for the Little Park Zone K8’s , Scott said, “It would be perfect for that job”. “That’s what its been designed for!”. The lads said, “ you’ll have to buy a K8 too then!”.  “Yeah, all right then”, I replied. More Foam!

We decided to form a syndicate and pay 1/4s, we paid Scott (Pro Hobby). I went and got my tools from the van and we started to build; well assemble the Carbon-Z Cub in the club house. An hour of assembly and we were ready to select a radio for it. The four of us have Futaba 14SG’s, so I put a futaba FASSTest  receiver in the cub. This way we could all use our own transmitter when each of us has a turn of towing.

I programmed my radio to it first and then down loaded the file onto Bruce’s SD card to transfer the program to the other 3 Futaba 14SG’s. This worked really well even though one of the syndicate members is a mode 2 flyer.

I have to say, love the Futaba for this and it’s features on transmitter to receiver linking. No need to, what’s known as, re-bind just simply re link the transmitter back to the receiver without having to get to the receiver.

As it was far too windy to test fly the new Carbon-Z Cub we had to wait till Monday after work. Bugger!!

You guessed it Monday was simply a picture perfect day not even a flutter of a leaf on a tree, Typical. All day at work, all I wanted to do was pack up and head down to the field to have a fly of the new Carbon-Z Cub.

At knock off time a quick TXT message to the Lads and we all met out at the field. The retired lads where already out there and their Park Zone K8’s at the ready.

The Cub took 5min to assemble and she was ready for her maiden flight. I test flew the cub first up and decided not to deploy any flap for the take off, I have to say text book rollout, tail slowly raised and she sat there on her rather large wheels running down the runway waiting for me to lean on elevator. As I slowly leaned on elevator the wheels broke free from the runway and she climbed out at about 30deg angle at ½ power. A couple of clicks of up elevator and that was it. After 2 circuits I had to make my self land to give Frank his go. What can I say, it’s a Cub and they are a joy to fly slow, docile and scale.

I decided to deploy flap for the landing as one would expect nice and slow magical touchdown and a perfection rollout. (Nothing to do with the pilot)

Ok, Frank’s turn. I turned off the receiver and transmitter and Frank turned on his transmitter got it into link mode and simply turned on the receiver after about 1 second he was ready to fly. Flight control check all ok so we put the same trim into his radio as mine and he was away.

Frank too performed a text book rollout and take off and of course he had a great flight as well. “She’s a rippa”, came from Frank as he touched down.  We had all cycled through having a turn. As Chris was the last one to have his test fly Frank put his Park Zone K8 behind the Carbon-Z Cub and here we go the aero tow test.

Chris took up the slack and then slowly applied power, what was to follow was the most graceful of K8 takeoffs followed by the Carbon-Z Cub lift off, both started to climb out ever so gently and gracefully. It simply looked really graceful. The late after noon air was simply magical, with not a breath of wind.

After Frank’s release, Chris pulled the power completely off and headed for the deck as we do with the tow planes and performed yet again a perfect landing, taxi back and then Bruce Hooked up his K8 all systems ready, “let’s go!”

Another awesome aero tow and flight.

We threw another Battery into the Carbon-Z Cub and she was ready for the next round of aero tows.

Pulling up a chair in the shade of the pits, I watched for the rest of the afternoon as Chris played tow pilot for Frank and Bruce’s K8’s, it was most enjoyable to watch.

While I was sitting watching the lads aero tow, I thought to myself, that for the price of a carbon prop from one of my other tow planes we could buy a complete ready to go Park Zone 2.25mtr K8 and have so much fun.

It really does go to show how little money one needs to put up to have a great afternoon of aero towing (don’t tell the wife I said that!). Sure the big expensive stuff is awesome (just a much higher level of awesome) and one can fly a lot higher, longer and faster but gee we sure had a lot of fun with foam K 8’s and an electric Carbon-Z Cub tow plane.

Thanks to Eflight, Parkzone,  Model Engines, Model Flight, Pro Hobby Great little package producing big fun.

Jason Sagaidak
President

BASIC R/C KNOWLEDGE

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.

Sizes

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.

Therefore,

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……..

BASIC ELECTRICS FOR R/C MODELS

BY JASON SAGAIDAK (MMAC education officer)

Ok so you want to try electric r/c model aircraft but don’t know where or how to start?

Been disappointed with the performance of that ARF electric model you purchased?

Got an old IC model at home that you thought might make a good electric one?

THEN READ ON

Right –o-then ,I don’t claim to know everything about electric r/c modelling however I can put you in touch with some good basic knowlage to get you confidently on your way, to power and purchase parts that will work for your application.

There are a couple of items you will need to purchase to put into your tool kit.

1. Rev / min counter (optical tacho) you may already have one
2. Watt-Voltage-current meter
3. Calculator (there is a bit of maths need to work it out)

What does KV on a motor mean?

Ok in electrical terms KV = kilo volts however in motor terms K = RPM thus meaning RPMV = RPM per VOLT

So this means that if you have a 1400kv motor and you intend to use a 3cell  li-poly battery you then take the source power voltage and multiply it by the motor KV.

• 2 cell li-poly = 7.4volts
• 3 cell li-poly = 11.1volts
• 4 cell li-poly = 14.8volts
• 5 cell li-poly = 18.5 volts
• 6 cell li-poly = 22.2 volts

Example

Formula is RPMV where RPM=1400 and V=11.1

Therefore  RPM x V = 1400 x 11.1

                                    = 15,540 rpm

Lets say you have a 3600 kv motor and use a 3 cell li-poly battery what would be the rpm?

RPM x VOLTS = 3600×11.1

= 39,600rpm

What if I don’t know the KV of the motor?

Ok using your optical tacho and the volt-watt-amp meter connect a li-poly battery run the motor up to full power and get the tacho and voltage reading.

The test motor I used gave a reading of 26,400 rpm and I use a 3cell li-poly.

Example

Using RPM / V=KV  , rpm = 13,400 and  v=11.1

Therefore  RPM / V = 13400 / 11.1

=12,070 KV thus meaning the motor is probably around the 1200kv area.

Why is KV important?

The KV is important when choosing the right motor for the airframe you wish to power. For example a large model such as a 80in scale warbird your going to want a low KV motor so it will swing a large propeller (22”x10”). If you where to choose a high KV motor chances are the propeller would not protrude out past the cowel.

• Low KV = high torque & lower rpm
• High KV = low torque & high rpm

Low KV motors are commonly used in larger slower aircraft and high KV motors are used in smaller quicker aircraft.

So to recap high KV are a generally small motors and low KV are generally large motors.

WHAT IS POWER

Ok this is where OHM’s law comes into play. OHM’s law states that V=I x R , where

• V = volts ,
•  I = current (amps) &
•  R = resistance (ohm)

Therefore using ohm’s law we also know that power is measured in watts thus giving W = V x A where

• W = Watts
• V = Volts
• I = Current (amps)

Now why do we need to know the power rating of the motor? This tells us how much power the motor is consuming to run flat out. The spec’s given on the motor spec sheet are usually the max rates. If the motor is pushed to use more power then burning out of the motor is going to result.

Now this is going to be important when selecting prop and speed controller sizes with the battery combination you wish to use, and of coarse the model your going to put it into.

We will come back to this bit later

HOW TO UNDERSTAND MOTOR SPEC’S INFORMATION

Here is an example of a brushless motor spec information.

D2830-11 1000kv Brushless Motor

Suggested Prop: 8×4 (4S) ~ 10×7 (2S)

So from the information  on the table above we can gather the following important information.

• The KV , (1000 rpm/v)
• Max current in amps , ( 21amps)
• Max voltage , ( 15volts)
• Power in watts , ( 210watts)

We can also get information on the shaft size and physical size of the motor as well. This info is equally important when matching motor to aircraft, ie will it fit in the cowel…etc

The motor manufacture usually provides the prop sizes at the nominal voltage used this is very important to take note of too. In this case on 4 cells they used a 8×4 while on 2 cells a 10×7 was used.

Lets look at the rpm difference between the to prop sizes

• 14.8 x 1000 = 14,800 rpm on the 8”x4” prop
• 7.4 x 1000 = 7,400 rpm on the 10”x7” prop

It’s most likely that each prop combo produced the same thrust , however the same plane with the smaller prop and higher rpm would be quicker than the larger slower prop as there is considerably more drag from the prop diameter.

Ok lets’ look at the next example

Required; 

6S~9S Li-Po

Test Data:
22v – 21×13 Prop – 40A – 5100g Thrust
22v – 19×12 Prop – 29A – 3500g Thrust

Suitable for sport and scale airplanes weighing 9 to 15 pounds (4–6.8 kg).

So from the information  on the table above we can gather the following important information.

• The KV , (210 rpm/v)
• Max current in amps , ( 60amps)
• Max voltage , ( 33volts)
• Power in watts , ( 2000watts)

So lets have a look at the tested rpm

• 22.2 x 210 = 4,662rpm now look at the current diff using the same voltage 40amps compared to 29amps this was achieved only by changing prop size. This is always good to remember when trying to work within the current rating spec’s.

LI-POLY BATTERIES

This seems to be the most confusing for people to come to gips with so here goes I’ll try to explain it as best I can.

Lets look at a commonly used li-poly battery used.

2200 25c 3S1P what does this info mean.

• 2200 = 2.2amps this is the C rating of the battery
• 25C = is the discharge rate the battery can supply without melt down
• 3S1P = 3cell series and the 1P = 1pack parallel

This means that a 2200  25C pack can deliver 2.2 x 25 = 55amps continuous

2200 30-35C 4S1P

• 2200 = 2.2amps
• 30-35C = discharge rate with it’s maximuim short burst
• 4S1P= 4 cell series 1pak

This means that a 2200 30C pack can deliver 2.2 x 30 = 66amps with short burst 77amps.

How long can I fly for on a charge

Right-O this seems to be the first thing that people ask when discussing batteries is ,” how long will I get out of this battery.”

Think of the battery as a fuel tank and we all know that we can buy different size fuel tanks for IC. It is all dependant on room in the model, well the same goes for li-poly batteries.

Think of the battery this way for a sec,

• 6oz tank = 850mh li-po
• 8oz tank = 1000mh li-po
• 10oz tank = 1250mh li-po
• 12oz tank = 1500mh li-po & so on

They are a store energy and in this case size does matter there is a trade off though with more capacity comes disadvantages such as

• Physical size (larger)
• Weight
• Longer to charge between flights

To work out how long a battery will last use the following formula

Ib / I x 60 where

• Ib = battery amps (ie 2.2A)
• I   = current draw (ie 12A)
• 60= seconds to put into min

Example

Lets look at a model  that uses a 2200mh 25C battery. The first thing you need to do is hook up your watt-volt-amp meter and run the motor flat out and get the reading, repeat again at ½ throttle and get the reading.

The plane I used is the axn floater (night flier) 2200mh battery at full throttle pulled 16.81Amps and at ½ throttle pulled 7.27amps therefore flight time are

• 2.2 / 16.81 x 60 =7.89 min at full throttle
• 2.2 / 7.27 x 60 =18.15 min at ½ throttle

So here we can see that the same size battery can give us a 8min flight and with a bit of throttle management can also give 18min flight.

Lets use the same plane but change the size of the battery to 1000mh 25C one.

• 1 / 16.81 x 60 = 3.56 min at full throttle
• 1 / 7.27 x 60 = 8.25 min at ½ throttle

So the flight time was reduced in proportion to the capacity size of the battery.

Lets look back at the motor data to see what size battery one would need to get a 8min flight on our aircraft we are going to bring back to life.

First thing we need to transpose the formula to = Ib

Therefore from Ib / A x 60 = T

We get Ib = T x A / 60 

T=8 , A=21  =>Ib = 8 x 21 / 60

Ib=2.8amp or 2800mh pack

From the hobbyking website this was the closest in mh

Take a look at the physical size of the battery to see if it will fit. If it will then lets work out what flight time one should expect.

3 / 21 x 60 = 8.5 min at flat out.

Now one can see that this battery is a 20C so 3 x 20 =60amps the motor is only 21amps so we know this will work well and not get too hot delivering its power.

Next time i’ll talk about prop’s sizes with motor KV

Hope this has been a help.