Sunday, October 25, 2015

Choosing Your Parts (Multirotor)

Choosing Multirotor for your Quadcopter

      Multirotor becoming very popular nowadays. It's real fun and pleasure to see them flying, however making them at home from scratch is tough job.

        If you are thinking to make one then this tutorial might help you. You don't need to ask someone or search over forums or blogs to troubleshoot your issue. Here i am trying to put everything together for you. It comes from my personal experience and time i spent searching over google so it might help you to build your multirotor without any hassle. I would like to thank RCGroups, RCIndia and buddies LazyZero, Ron, er9xuser, rajath and roopesh sir for their help and support. Here i will post some basic tips of selecting motor-prop combinations, battery, esc etc.

If you are making it for school, college or university projoct make sure you have enough time for testing and debugging. KK boards are known for plug and play functionality but still fine tuning your multirotor needs time. I would also like to warn you that don't take multirotor lightly, they are monsters and can ruin your day within seconds. So be careful when you are testing them. So where to start and how to select parts for multirotor? Here i am trying to put everything step by step.

Before you start buying parts you should have rough idea, for what purpose you want to make your multirotor? People make them for fun flying and hovering, stunts, AP(Aerial Photography), FPV(First Person View) and Heavy Lifting. You just can't make one multirotor that can do everything for you and if you are completely new to this thing you should first learn hovering and some basics of multirotor flying. So here we assume that we are making multirotor for beginner flying that can probably lift 200-300grams payload so you can hooked up light weight camera for onboard video recording(not FPV) in future.


There are many types of multirotor people make nowadays. Some of them are Monocopter, Twincopter, Tricopter, Quadcopter, Y-6, Hexacopter, X-8, Octocopter. I found that less rotors more precision requires to build them. That's why FPV lovers use hexa or octocopters so if one motor fails, they can safely bring it down to the surface. But more rotors means you have to invest more money. As a beginner quadcopter is good to start. Tricopter is good as well but you will have to deal with yaw mechanism which is tricky if you are building it from scratch. So here we will start with quadcopter. Quadcopter or also called Quadrotor has 4 rotors and it has two flight configurations. X config and + config. In X config two arms will face front side while in + config only one arm will face front side. If you are making Quadcopter with plus config you can tune your quadcopter easily by single axis tuning method which we will learn later. Small and light weight quacopters can do stunts better while big quadcopters can lift heavy payload like cameras with gimbal and fpv gear. So here we assume we are making normal medium size Quadcopter with plus config and 1000-1100grams AUW(All Up Weight) for funflying. AUW includes battery and everything. Basic formula you should keep in mind that weight of multirotor should be half of the total thrust.
Total Thrust = 2x AUW
In short to lift 1000gram quadcopter we need 2000gram thrust. We will learn about thrust later.

One of the most important part of multirotor is its frame because it supports motors and other electronics and prevents them from vibrations. You have to be very precise while making it.
You can make arm from any material like CF, PVC pipes, Aluminium or Wood but make sure it has enough strength to withstand impact and rough landings. Frame weight should be around 200-250grams. I would recommend you to go with aluminium channels/beams as they are cheap, strong and easily available at any local hardware shop. You can cut, drill them easily at home.

Ceter plate holds the arms and supports FC, receiver and other sensors. You can use 2-3mm glass fibre, plywood, aluminium sheet or any plastic sheet material but make sure they are stiff, strong and light weight. You can put some holes to reduce weight.

If you want to attach landing skid, buy any RC helicopter landing skid or you can build your own skid from scratch. It's all upto you.

Use M3 nut-bolts of different length to assemble frame and motor mount.
There is no thumb rule for size of the frame but for medium size quadcopter 450mm to 550mm motor to motor frame is enough. If your frame is too big compare to your power setup(motor-prop, esc, battery)it will add extra weight while if it's too small props will fight with each other.
   Making frame is truly depends on your imagination, skill and availability of material. But it's fun building frame from scratch and you will learn many things from it. If you want to get rid of all these diy things you can simply buy readymade frame from the market but it might put big hole in your pocket. In our case we want to build quadcopter so make sure the angle between two arms is 90 degree and also check that arms are not twisted.

FC (Flight Controller)

There are many FCs out now in the RC world. Some of them are KK, MultiWii, Ardupilot, Naza, Naze, Rabbit, WKM and many other. Advanced FCs have more features and advanced sensors like gyro, accelerometer, sonar, GPS, Magnetometer. But as we know "more power comes with more responsibility and difficulties" so if you are newbie you should start with gyro only. Hobbyking v1, v2.1, v3.0 boards/KK boards have gyro sensor only but still they are best beginner boards in the market to learn and undestand science behind multirotor. So here we are assuming that we are going to use HK board. You can buy any board v1, v2.1 or v3.0. The only difference between them are ATMEGA chip. V1 has 4kb flash memory, V2.1 has 16kb memory while v3.0 has 32kb flash memory. One more reason to use these boards is they are cheap, reliable and easily available at any hobby store. It won't burn your pocket. If you crash and damage your board you can buy another and there are lots of posts and videos available describing how to use them.


One of the important part of multirotor is its motor. It's a part of power system. Infact whole power system depends on selection of motor so you should be very careful while selecting motor. We use Brushless motors for multirotor. Brushless motor comes with some important specifications. You will see these specifications on the page if you are purchasing it online. So it makes our job simple. For motor selection some important specifications are.
  + kV
  + Max current(A)
  + Shaft dia
  + Thrust
  + Weight
  + Lipo(3S-4S)
  + Suggested prop
For multirotor application 600-1200kV motors are good. Below 600kV even better. Low kV means you can swing big prop. Big prop means it can move more air and you will get more thrust.
kV = RPM/V
If you have 600kV motor and 3S battery to supply power, RPM of of motor at NO LOAD would be
600 x 11.1(3S battery) = 6660 RPM
Max current rating is another important factor while selecting motor. Selection of ESC and battery depends on this value(we will learn it later). It should be able to run on both 3S and 4S lipo battery. Shaft diameter helps you selecting prop adapter. Now we are coming to the thrust. In most cases you will see value of the thrust motor can produce with suggested prop on the website. If you remember we learned that to lift 1000 grams quadcopter we need total 2000grams of thrust. And quadcopter has 4 motors, so each motor should be able to produce atleast 500grams of thrust to satisfy our need.
4 motors x 500grams thrust = 2000grams thrust.
One more thing is Watt.
Watt = V(Voltage) x I (Ampere)
More Watt more power so you should also consider that while selecting motor.
But when it comes to selecting motor you will notice that there are many options available for motor selection and it's really confusing, so which motor is best for multirotor application? Well you should consider few more things other than specifications. Some of them are
Motor Mount: Well motor mount is one more important factor you should keep in mind while selecting motor for your multirotor. Motor mount comes under tensile force especially when you are swinging big prop and if motor is not fitted properly it might come off during flight and i am sure nobody wants to see such things happen. So make sure that motor you are purchasing has good mount that can hold motor properly under heavy load and same way you can fit that mount easily on the frame. However you should always precheck before flight that all connections and fastners are properly fitted. Get one spare motor. So i hope i have covered everything for motor selection.


We always neglect this plastic piece. Just because it's cheap? Who knows!! But in multirotor application contribution of prop is remarkable. Specifications of prop are easy to understand and they are dialmeter and pitch. Type of prop is important as well but we will see effect of diameter and pitch on flight of multirotor. Generally we see prop with the specification of
First value is diameter of prop and second value is pitch. Both are in inches.
Diameter: Virtual circle that prop generates.
Pitch: Amount of travel per revolution.
As we see above our motor runs at 6660 RPM at NO LOAD. But when you mount prop on it, RPM will be reduced. Here we will take example of two props 10x3.8 and 10x6. When you mount 10 inch diameter prop RPM of motor will be reduced to 3600 RPM (Revolutions Per Minute).
60 Revolutions Per Second.
Our 1st prop has 3.8 inch pitch. Means per revolution it will travel 3.8inch. So
60 x 3.8 = 228 Inch/Sec = 5.7 m/sec
For 2nd prop, it has 6 inch pitch.
60 x 6 = 360 Inch/Sec = 9.1 m/sec
So we can say if we have 10x3.8 prop our quad will climb in the air at 5.7 m/sec, while with 10x6 prop climb rate will be incread to 9.1 m/sec.
Bigger dia prop can produce more thrust.
So which prop is best for our multirotor?
Generally you will get suggested prop value in motor specification, so you should go with it and buy 1-2 pair extra. But what if prop value is not given. You will see kind of table with different props, Volts, Amp, Thrust and Efficiency. Here you will have to try trial and error method. But it doesn't mean you swing 13x3.8 prop on 1700kV motor.
Lower kV motor can deal with bigger prop. With increasing kV value size of prop will be decreased. So you will have to keep this in mind. For multirotor you should go with low pitch prop if you need more stability and less vibrations. How to balance prop? We will see in next part.

ESC (Electronic Speed Controller)

ESC supplies power from battery but not constant, it varies according to input signal. ESC also has BEC(Battery Eliminated Circuit). BEC is nothing but 5V output from ESC that can power up receiver, servomotor(for camera gimbal) and FC. But how to select ESC for our multirotor? Well it's really simple. You only need to keep in mind that Ampere rating of ESC should be higher than max amp rating of motor. For example the motor we selected draws maximum 15Amp so your ESC rating should be higher than 15amp. Say 18-20Amp. Still confused? Here is simple forumla. Thanks to Sai sir(rcforall) of RCI.
ESC = 1.2-1.5 x max amp rating of motor
1.2-1.5 x 15 = 18-22.5A
So you can select ESC between range of 18A to 22A. But please note it's note thumb rule. You can go with either 18-20A or 25A (it might be bit over powered). Whichever ESC you choose make sure it has programing facility(throttle range, battery mode etc). Go with quality ESCs check user reviews and buy 1 spare.


This beautiful monster eats quality food. If you don't feed them they refuse to fly. So how to select battery for your multirotor?
For that maximum Amp rating of motor comes first. If you remember our motor draws max 15amp. We are working on quadcopter and it has 4 motors, so all 4 motors will draw
4 x 15amp = 60Amp.
Now let me explain few specifications of battery. You will see them on battery and website as well.
C discharge rating
2S, 3S or 4S.
1000mA = 1A
You can compare mAh rating of battery with petrol tank of your vehicle. Big tank more petrol you can fill and more you can drive. The same way more mAh rating gives you more flight time.
C Discharge Rating:
Maximum current(A) at which battery can be discharged at particular time.
1S, 2S, 3S & 4S
1S = 1 cell of 3.7V
2S = 2 cells in parallel x 3.7V = 7.4V
3S = 3 cells x 3.7V = 11.1V
4S = 4 cells x 3.7V = 14.8V
4S means more power than 3S. If you remember we discussed that
Watt = V x I
Here we are increasing value of Volt so watt will be increased. Our motor draws max 15A. So watt value for 3S and 4S will be
At 3S battery 11.1 x 15 = 166.5 Watt
At 4S battery 14.8 x 15 = 222 Watt
but make sure your motor and esc are capable of handling 4S.
Generally you should go with 3S battery only.
Now as we know we need atleast 60A current. So here we will take example of two batteries.
2200mAh 25C
2200mAh 40C
Which battery is good for our multirotor? Let's see
2200mAh = 2.2A
2.2 x 25C = 55A
2.2 x 40C = 88A
We need atleast 60A so 2200mAh 40C is good for us. Go ahead and get it.
  I hope you have gathered enough confident to build your own multirotor from scratch. Next part will be 
even more interesting. So stay tuned.