First, we will discuss the basics of
a multirotor propulsion system, and how to produce the desired thrust based on application
requirements. In the process, we will prove why using basic motor / propeller
properties to determine parts in a propulsion system is better than merely
guessing a system ‘would work’.
The Propulsion System
To start, we need to
determine the platforms’ OEW (Operating Empty Weight) and Payload Allocated
Weight, both previously mentioned variables sum up to the MTOW (Maximum Takeoff
Weight) of the aircraft. Using the MTOW, and the type of aircraft with the type
of components we intend to build, we can determine how much thrust we need to
produce, and what thrust to weight ratio we need to aim for.
KV determines how big the propellers can be
and how much thrust we can produce. KV is the velocity constant. Higher KV
motors spin the propeller faster, but lower KV motors normally generate higher
torque. Wider and shorter stators result in lower KV, and the opposite is true
for higher KV motors.
A Propeller converts rotational
motion into thrust. Due to the airfoil-shaped blade there is a pressure
difference produced between the forward and rear surfaces of
the airfoil-shaped blade, and a fluid is accelerated behind the blade.
Propeller thrust can be calculated using the following:
k = Constant, which depends upon numbers of parameters (Frontal
area of propeller, density of air, etc.)
? = Angular velocity of prime mover to which propeller is
The prime mover is the brushless motor in a multirotor. Propeller
sizes come in pitch and diameter.
Electronic Speed Controllers
Electronic speed controllers (ESCs)
are devices that control brushless motor speeds. They come with different
amperage ratings. The bigger the thrust the more the amperage needed. A motor /
propeller / battery setup that requires 100A at 100% throttle requires an ESC
that is rated 200A.
In order to get the best flight
time and performance it’s important to know how to go about choosing the best
Lipo battery. The correct battery mainly depends on the size of your drone, and
the type and number of motors you use. Usually, the higher the voltage the more
the power, thrust, and weight. Weight also depends on capacity.
To conclude, a frame with an MTOW of 10 kg would need minimal
thrust of 20 kg to have a 2 to 1 thrust to weight ratio. Our motors will have
low KV ratings and high max power outputs. Our propellers would need to be large
to fit those low KV motors, and our ESC’s have to be able to withstand high potential
differences from the Lipo batteries onboard. As we intend to produce as much
thrust as possible, the more power we get from our batteries the merrier. Higher
potential differences equate higher power outputs.