Introduction

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.

Motors

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.

Propellers

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:

T= Thrust

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

connected

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.

Batteries

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.

Conclusion

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.