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.
PURPOSE
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.
TYPE, SIZE & WEIGHT OF MULTIROTOR
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.
ARM:
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.
CENTER PLATE:
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.
LANDING SKID:
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.
FASTNERS:
Use M3 nut-bolts of different length to assemble frame and motor mount.
SIZE:
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.
MOTOR
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
7x3.5
8x4.5
9x5
10x3.8
10x4.5
10x6
11x4.7
12x3.8
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.
BATTERY
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.
mAh
C discharge rating
2S, 3S or 4S.
mAh:
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.
even more interesting. So stay tuned.
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