Principles of Multicopter Flight

Alfred Gates

© Alfred A. Gates, 2014
Principles of Multicopter Flight and 

Multicopter Components

• Multicopter components

• How a multicopter is controlled 

• Myths about autopilots

• Flight in winds

• Lithium Polymer batteries

• Electronic speed controllers

• Brushless motors

© Alfred A. Gates, 2014
Autopilot

Receiver Electronic Speed Controller

GPS-Compass, battery and transmitter not displayed
Brushless Motors

GPS Compass in top cover

© Alfred A. Gates, 2014
© Alfred A. Gates, 2013-2014 5

Pilot: moves stick forward

Controller move aircraft 

forwards and minimizes change 

in ROLL or YAW movements, 

rate of change with the MEMS 

Gyros.
© Alfred A. Gates, 2013-2014 6

Minimum Amount of Control for a Multicopter

MEMS (micro electromechanical systems) accelerometer

Vibrating Structure Gyroscope

The Gyroscope can detect movement on an axis and send an 

electrical signal

Hands off hovering a multicopter in rate mode

The controller will detect a rate/rotation about an axis 

and correct it to a point. It will drift.

Flying a multicopter manually. The pilot sends a 

command to the multicopter to move forward. The 

controller interprets the command with the MEMS 

gyro and executes the command.

The correction response can be adjusted with the Gyro 

Gains.
© Alfred A. Gates, 2013-2014 7

Movement: Increase in altitude, Vertical Position Control

Method of Movement: Thrust 

Increase or Decrease RPM

Increase or Decrease RPM
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Movement: Rotate about a vertical axis, Yaw Control 

Increase RPM Decrease RPM

Method of Movement: Differential Torque
© Alfred A. Gates, 2013-2014 9

Movement: Forward Flight, Pitch Control

Method of Movement: Differential thrust 

Increase RPM

Decrease RPM
© Alfred A. Gates, 2013-2014 10

Movement: Sideways Flight, Roll Control

Method of Movement: Differential thrust

Increase RPM

Decrease RPM
© Alfred A. Gates, 2013-2014 11

Autonomous Control of a Multicopter

Additional Components

IMU

MEMS

Gyros, 3 Axis

Accelerometers, 3 Axis

Magnetometers, 3 Axis

Barometric Pressure, 1 Axis

GPS: Latitude, Longitude and Altitude 

position sensor

Magnetometer: Heading sensor

Autopilot Software:
Myths About Autopilots

• Autopilots will fly any aircraft

– The autopilot gains must be adjusted to fly each different aircraft.

• Once the gains are set in an autopilot it will fly the aircraft in any condition

– Autopilots are tuned to fly aircraft in calm conditions and levels of windy conditions.

• Once the gains are set in an autopilot for calm condition the autopilot can fly the 

aircraft

– A tuned aircraft may not be able to recover from a stall or spin or other maneuvers.

– Each aircraft must undergo a series of test to verify the autonomous flight capabilities. 

• Once an aircraft has successfully completed the flight testing and tuning the 

autopilot will be capable of flying it

– Other factors can interfere with an autopilot. They included, aircraft wear, to much free 

movement of control linkage, flexible airframe, poor GPS signal, moving ESCs or wiring, 

antennas to a new locations (EMF interference), replacing components on installing new 

components in the aircraft. (Additional factors not listed)

• A properly tuned aircraft can be flown on a new mission once the power plant is 

turned on and running

– The aircraft must undergo a preflight inspection followed by in flight verification testing 

before the autopilot can be switched to autonomous flight

© Alfred A. Gates, 2013-2014 12
Multicopter

• Flight limitations in GPS hold it can’t fly as well in 

ATT mode due to max allowed bank. Important 

to know this in winds (20MPH). The multicopter 

may not be able to hold position in GPS mode. In 

ATT mode an experience pilot could hold 

position in winds. 

• ATT mode best for flight in winds (20MPH)

• The best flight mode for high winds will be 

manual mode since the max bank it the highest 

and the throttle response is the highest to change 

altitude. An experience pilot is required.
Lipo Batteries-ESCs-Motors

• Max Lipo battery capability 

must be greater than motor 

capability

• Max ESC current draw 

capability must be greater 

than motor capability. ESC 

also must be able to handle 

the number of battery cells

• If not the battery and or ESC 

will be destroyed
Lithium Polymer, Lipo, Battery Technology

• Lipo batteries must be stored in a cool 

location

• In hot weather store the batteries is a cooler

• In freezing weather Lipo batteries will be 

damaged if frozen

• If the batteries are not used for an extended 

time store them at a half charged setting

• Store the batteries in a battery burn proof bag
Lipo Batteries

• Must be charged with a Lipo charger.

• Minimum loaded voltage (aircraft is in flight) must not go below 

3.2V/ cell. The battery will be damaged below 3.2V/cell. Use 

3.4V/cell as a minimum. 

• Minimum unloaded voltage must not go below 3.7V/cell. The 

battery will be damaged over time.

• Over time a battery pack will become puffy and the operating time 

will decrease

• ESC are programed to cut power to the motor at 3.2V/ cell for fixed 

wing aircraft.

• Must be disconnected from circuit when not used.

• They can explode if shorted out. They will definitely catch on fire.

• Discharge battery before throwing it out pr. manufacture 

recommendations.

Alfred A. Gates, Ph.D.,P.E., © 16
Lipo Batteries

New highest flight time Lipo battery Damaged puffy Lipo battery
Charging lead Aircraft power source and charging
Set number of cells Set the amperage

2 cells or 7.4V 4200mahr or 4.2A

This should be in a battery bag. If the battery puffs up stop charging immediately

Lipo Battery Charging



Lipo Battery Terminology

Power for 1 hour

5.0 Amps X 3 Cells X 3.7Volts/cell = 55.5 Watts for one hr

Amperage for 1hr of use Number of cells and voltage

Max continuous discharge amperage Max burst amperage
Lipo Battery Terminology

Max continuous power

5.0 Amps 35C X 3 Cells X 3.7Volts/cell = 1,942.5 Watts = 2.6 hp

Time for 2.6hp output = 5 Amps/ (5Amps X 35C) = .14 hr X 60min/hr = 8.6 min

Amperage for 1hr of use Number of cells and voltage

Max continuous discharge amperage Max burst amperage
Lipo Battery Terminology

Max continuous power ( This can only occur for a few seconds)

5.0 Amps 70C X 3 Cells X 3.7Volts/cell = 3,885 Watts = 5.2 hp

Amperage for 1hr of use Number of cells and voltage

Max continuous discharge amperage Max burst amperage
Alfred A. Gates, Ph.D.,P.E., © 26

1800 Mah X 1A/1000Ma X 11.1V = 20 Watt hr 

This battery can produce 20 Watts for 1 hr

C rating the battery can produce 30 times the amperage or power 

30C X 1800Mah X 1A/1000Ma = 54 Amps

How long will this battery last at discharge rate of 54 Amps

(1800 Mah X1A/1000Ma) /54Amps = .033hr X 60min/hr = 2 min 

3S battery pack (3 cells at 3.7Volts/ cell) = 11.1 volts

Power output for 2 min = 54Amps X 11.1 Volts = 599. Watts (.8hp)

Example 2
Electronic Speed Controller
Brushless Motors Basics

• Motor max current capability must be less than ESC 

max current capability

• Motor max current capability must be less than max 

Lipo battery current discharge 

• High KV rating = Low Torque and High RPM

– High speed flight, slow acceleration 

– (Use smaller diameter and pitch props)

• Low KV rating = High Torque and Low RPM

– Low speed flight heavy lifting (Multicopters)

– Higher acceleration 

– (Use larger diameter and pitch props)

Alfred A. Gates, Ph.D.,P.E., © 28
Brushless Motors Basics

• Motor efficiency: The percent efficiency goes 

into power to the propeller. The remainder 

goes into generating heat.

• Example 85% efficiency, 85% power goes to 

the propeller thrust, 15% power goes into 

generating heat. If the motor is large and the 

power demand is high the soldered wires 

could be melted.

• Always go with high efficiency.

Alfred A. Gates, Ph.D.,P.E., © 29
Max Current Draw From Motor < ESC and < Battery

Max current draw 

from motor is

20A

Max continuous 

current is 25A

Max continuous 

current 1.8A X 30C 

= 54A

950KV X 11.1V = 10545 RMP

Alfred A. Gates, Ph.D.,P.E., © 30

Acceptable/no fires
Example 3 RPM Estimation from KV, High Torque

Alfred A. Gates, Ph.D.,P.E., © 31

360KV * 3.7V/Cell * 6S/Cell = 7,992 RPM

Put a big propeller on this (15 inch)

Max thrust 2080 grams (4.6 Lb) 14.7Amps, 6S Lipo

Propeller size 15 inches
Alfred A. Gates, Ph.D.,P.E., © 32

• Max power 14.7 A X 3.7V/Cell * 6S/ Cell = 14.7A * 22.2V = 326.4W 

• 326.4W X 6 motors in a Hexcopter = 1951 W ( 2.6 Hp)

• Use a 5000mah 6S batteries

• Flight time = (Battery amps pr hr)5000mah/(14.7A *6)* 60min/hr = 3.4 min

• Lifting capability = 2080g X 6 motors /454 g/Lb = 27Lb for 3.4 min

• 27 pounds for 3.4 min