LENShaving four motors of maximum 30A, our peak current
needs to be at least 120A. Our build has a 4S pack
with a capacity of 1800 mAH, or for a more useful
version of the same figure, 1.8AH. Its C rating is 75,
so the peak current is 75 × 1.8, which is 135A. It
certainly can’t supply this figure for any length of time,
but it provides a comfortable overhead beyond the
capacity of our motors.
POWER UP
Lithium polymer cells are both powerful and compact,
but that power comes at a price. They can be
hazardous if not treated with care, as when they
degrade they can build up hydrogen gas within them,
and if punctured they can catch fire. They must always
be stored somewhere fireproof and in a charged state,
and never overcharged or aggressively discharged. You
will find two connectors on a multi-cell lithium polymer
pack: the main power connector, and a multi-way
charge balancing connector, allowing the charger to
monitor the voltage in each individual cell. Always use
a proper lithium polymer charger with a receptacle for
this balance connector, and it is imperative that you
read the instructions and safety notes that come with
your cells.
Having picked a battery pack, there is one other
matter to attend to with respect to the power system
on your machine. The battery connector will almost
certainly be the popular ‘XT60’ design, and you’ll
have an XT60 plug and short lead to fit it. Something
is required to safely pass the high current from it to
the motor controllers, and that takes the form of a
power distribution board. This is a printed circuit board
with a master connection for the battery pack and a
set of solder pads for each of the motor controllers.
Sometimes a four-way motor controller doubles up
as a power distribution board, and in our build it is
incorporated in the frame, but if you don’t have one,
you will need to put one on your list of parts.
CONTROLLING IT ALL
The final large on-board component that we haven’t
covered is the flight controller. This is a small computer
that monitors an accelerometer and gyroscope on a
chip, and continually adjusts the power to the rotors
to keep the craft stable, in the desired heading and
altitude. Above all other components, this is the one
that has made multirotor flight possible – maintaining
stable multirotor flight unaided would have been
beyond the abilities of human pilots.
The state-of-the-art in-flight controller development
is a moving target, and with several years of
development now behind us in terms of affordable
models, there is a huge choice to be found. At the
lower end there are models based upon the technology
of a few years ago that will provide a basic flying
experience, but if you are building one in 2018 it makes
sense to use a controller appropriate to the present.
Of the many choices, we are going to direct you
to one of the recent models featuring the STM32
microcontrollers. You will see various different
specifications, usually something like ‘F3’, ‘F4’, or
‘F7’ – these refer to the different revisions of the
STM32 line. The higher the number, the more capable
a component, so we would suggest getting an F4 or
an F7. The other features you will be looking for are a
so-called BEC or battery eliminator circuit to provide
low voltage for the electronics, an input voltage range
to match your battery choice, and a barometer chip
that will allow the controller to maintain an altitude.Our choice of an Omnibus F4 controller also gave
us the option to monitor the battery current, and an
on-screen display system, should we upgrade with a
first-person video camera.
The controller itself is only half the story though,
because it is simply a piece of hardware. Its real
capabilities come through its software, and here at
the time this is being written, the choice we would
recommend is the popular Betaflight firmware.
This should be pre-loaded, but you will often want
to upgrade your controller over USB with the
latest version.THE HIGHER THE VOLTAGE, THE
LESS CURRENT REQUIRED FOR A
GIVEN POWER, SO IN OUR BUILD
WE OPTED FOR A 4S PACK