Multirotor Aerodynamics – Multirotor Principles

Main Components:

• Autopilot/Flight Controller: Takes control inputs from the RC Receiver and commands from the Data modem, interprets them, and sends out motor commands to spin the motors
• Speed Controllers: Takes motor commands and sends power to the motors to spin them to the correct speed
• Motors: 3-Phase motors (a bit fancier than the cheap DC motors most people might test in a physics class)
• Propellers: Matched with the motor (need to be the right load, otherwise the motor will struggle) and form our “rotating wings”
• Battery: Used to power everything on-board (notice that the speed controllers draw most of their power from the battery directly). Needs to be a very good battery (high current output) as flying is hard!
• GPS (Optional): A GPS unit that will figure out the location of the RPA in the world and send it to the autopilot
• Data Modem: Connection to the Ground Control Station (GCS) which allows the operator to send commands to the UAV
• RC Receiver: Connection to the Pilot (via the RC Controller) which allows the pilot to send flight control inputs to the RPA

The simplest multirotors can produce forces in four different ways:

• Thrust (upwards)
• Roll
• Pitch
• Yaw

Using these four control inputs, we can fly the multirotor freely!

• …If by freely, we think of the it as a “flying brick”
• In flight, the multirotor will maintain it’s motion, including rotating!

An object will not change its speed or direction unless an unbalanced force affects it

Newton’s 1st Law

• Aerofoil (rotor) is rotated by the aircraft power plant 
• Air is drawn down from above and accelerated though rotor disc:
  • This produces the reaction force of “lift” (in the sense of a fixed-wing aircraft)
• There will be lower pressure above the rotor, and higher pressure below the rotor:
  • This sucks in more air through the propeller
  • There will be a large blade tip vortex 
• Maximum downwash velocity will occur approximately two rotor diameters below the disc

Overview

• All multirotors will use an autopilot to control the “lowest level” of the flight controls – the motors!
  • Simple autopilots (i.e. “flight controllers”) can be used to interpret thrust/rotation commands
  • The motor numbering sequence and direction of rotation are linked to the orientation of the autopilot

• The autopilot assumes what motors will create the right forces to turn the multirotor!
  • This includes the propeller direction (CW/CCW)
  • If you set your multirotor up incorrectly, it will most likely flip on take-off (if you’re lucky!)

To facilitate flight control: 

• The autopilot firmware interprets any control inputs and commands the correct motor(s) in the correct sequence to change motor speed (RPM) 
• Simple autopilots (i.e. “flight controllers”) are sometimes used to simply co-ordinate which motors to spin up to make the multirotor perform a control input
• Autopilot orientation determines which way is forward, not the airframe!
• The motor numbering sequence and direction of rotation are linked to the specific autopilot used!

How it works

The multirotor spins it’s propellers faster and slower to generate different amounts of thrust:

• This allows it to create thrust forces, and a rotation moment around the C.o.G.
• The amount of force possible is limited by the maximum propeller speed

• All motors work together to create “lift”
  • In this sense, “lift” is a reaction force to the propeller thrust!
  • We can say that each rotor provides ¼ of the Total Reaction Thrust (TRT)
  • If TRT is equal to weight, then the multirotor hovers!
• Vertical movement is made by adjusting the TRT:

• A multirotor can achieve “lateral movement” by changing the direction of the TRT by:
  • Tilting the aircraft (roll or pitch)…
  • …while increasing thrust (to maintain hover)
• This can happen in roll and pitch directions, and in both at the same time!
• We can use this to fly sideways without turning, we call this “strafing”
• Remember, this gets us started moving, but won’t stop us!

• Turning “on the spot” is much simpler than moving laterally:
  • Recall “Yawing” from “Flight Controls”
  • The autopilot already allows us to yaw (and will do all the work for us!)
  • If we use this to turn in motion, we will have sideslip!

Coordinated turns are a bit tricker!

• First start to roll the aircraft…
• …while increasing thrust…
• …while yawing the acircraft!

• Drag:
  • Multirotors (like aeroplanes) experience drag
  • It is more noticeable for multirotors in slow flight due to their shape
• Rotations:
  • Remember, an object in motion will stay in motion.
  • Making more quick motions will use more energy, and will reduce your flight time (“stay smooth”)
• Operating in confined spaces:
  • Objects such as poles, walls or ceilings will interfere with the airflow around propellers
  • Airframe likely to be “sucked into” collision with object!