drone basic physics

The Kv (kilovolts) – rating in a motor demonstrates how various RPMs (Revolutions each moment) the motor will do per volt. The higher the kV rating is, quicker the motor rotates at a steady voltage.

The relationship between this thrust and the RPM of the motors or the angular velocity (Av) is approximately quadratic. That is, FRes = K * Av^2 (where K is some constant).
How quadcopters work: Resultant forcey
We have also established what a drag is. Each rotor in a quadcopter has to overcome drag every time it spins. And as you may already know, a moment is the ability to make an object rotate (torque).  The relationship between the RPM and the drag moment (T) is also quadratic. That is, T = K * Av^2
How multirotors work: Moment
Since the quadcopter has four rotors, every rotor has to account for 1/4th of the total weight of the quadcopter, in order to hover. Hence, if you go back to the curve from the first graph (thrust vs RPM), you can elicit the speed that each rotor needs to spin in (S = ¼ Mass of the quadcopter * acceleration by gravity).
How drones work: force of each motors
How drones work: force of each motors
Quadcopter working principle: RESULTANT FORCE
The resultant force of the spinning of the rotors on the quadcopter is the sum of all the thrust forces by the four rotors, minus the force of the gravity (Fg = mg), where m = mass of the quadcopter and g = acceleration by gravity (9.8 m/sec^2).
FRes = F1 + F2 + F3 + F4 – mg
Propeller Comparison

AcronymsStands ForExplanation
2.4GHz, 5.8GHzFrequencies used in RC and FPV.
ACCAccelerometerA device that measures acceleration forces in a specific direction.
AcroAcrobaticIt means multiple things, when referring to flight mode, it’s means “rate mode”; when referring to hardware, it’s more or less saying “free style” or “acrobatics”
Active BrakingAKA Damping Light. A feature in BLHeli that improve throttle responsiveness.
AHAltitude holdA flight mode that maintains height of the drone using a “BARO” sensor, or ultrasonic sensor
AILAileronsAlso known as roll
Air ModeFC firmware feature that keeps PID controller working even when throttle is at zero, what is air mode
ALAuto LevellingA flight mode that allows the copter to stay level. It uses “ACC” sensor
ALTAltitudeThe vertical distance you are from ground
APMArdu Pilot MegaA type of flight controller board
ArducopterA flight controller firmware software
ARTFAlmost Ready to FlyAn ARTF aircraft package usually includes everything you need to fly, but it requires simple assembly.
AUWAll up weightThe total weight of a copter including LiPo Battery and other necessary parts in order to take off
AUXAuxilaryAdditional switches/potentiometers/sliders in a radio transmitter
BAROBarometric AltimeterIt measures altitude by using barometric pressure
BECBattery Eliminator CircuitA voltage regulator found in some ESCs. Designed to provide constant 5V voltage for RC equipment such as Flight controller and radio receiver
BNFBind and FlyA BNF plane, helicopter, or quadcopter requires only one extra thing to fly, a radio transmitter.
BindThe process of pairing radio transmitter and receiver when first setting them up to work together
BootloaderSpecial code stored in non-volatile memory in a microprocessor that can interface with a PC to download a user’s program.
Brushed MotorA type of motor that is commonly used in small multirotors, they are generally powered by 1S batteries, and weaker than brushless motors
Brushless MotorA type of electrical motor that is commonly used in multirotors. They have a wide range of voltage input, more durable and powerful than brushed motors. How to choose mini quad motors

BuzzerA very loud speaker commonly used with drones, for the purpose of low votlage alarm and lost model
CC3DCopterControl 3DA type of flight controller
CellIn mini quad, cell is normally refered to the nominal voltage level of a LiPo battery, each LiPo cell has a nominal voltage of 3.7V
CFCarbon Fiber
It can mean 2 things in quadcopter: carbon fibre, the material used in quadcopter frames; or Cleanflight, a flight controller firmware
ChannelVideo transmitter broadcast signals on a particular frequency, and this frequency is a channel, each VTX can support multiple channels; Channel can also refer to the different sticks and switches on a radio transmitter
COG or CGCenter of GravityThe average location of the weight of a copter. It’s important to make sure the COG of your copter is right in the middle (or close enough)
CriusA type of flight controller board
CW / CCWClockwise/Counter-CWDescribing the rotation direction of motors or propellers
DDDirect DriveA type of prop designed to go straight on a motor shaft without a gear reduction.
Diversity ReceiverA multiple receiver FPV system picks the strongest signal from multiple video receivers and antennas to display the best picture
DOFDegree of FreedomIt give clues as to whats on the “IMU”. For example 6DOF = 3 axis Gyroscope (gyro) + 3 axis acc; 9DOF = 3 axis Gyroscope + 3 axis Accelerometer + 3 axis Magnetometer; 10DOF = same as 9DOF and adds a Barometric Altimeter
DroneAn unmanned aircraft that is guided remotely by a pilot. For military use, these aircraft have weapons and/or surveillance gear. For personal/recreational use, these aircraft often have HD cameras. The term is popular in the media and can be misleading sometimes.
DShotDigital ShotA type of ESC protocol, what is DShot
DSM / DSM2 / DSMXThese are the proprietary technology of an RC equipment maker, Spectrum, and DSM stands for “Digital Spectrum Modulation”.
DSSSDirect-Sequence Spread SpectrumDSM uses Direct-Sequence Spread Spectrum (DSSS) modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that modulates the carrier or broadcast frequency. The name ‘spread spectrum’ comes from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device’s transmitting frequency.
Dry WeightThe weight of a multirotor including everything necessary to take off except the battery.
EEPROMElectonically Erasable Programmable Read Only MemoryA type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g.,static calibration/reference tables. Unlike bytes in most other kinds of non-volatile memory, individual bytes in a traditional EEPROM can be independently read, erased, and re-written.
ELEVElevatorAlso known as pitch
EMIElectromagnetic  InterferenceEMI is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source.
ESCElectronic Speed ControlESC is used to convert signal from Flight controller or radio receiver, and apply the right current to the  electric motors. It also usually includes a Battery Elimination Circuit (BEC), which provides power for the RC system and other onboard electronics.
FAAFederal Aviation AdministrationAn United States Department of Transportation Agency, it has authority to regulate and oversee all aspects of American civil aviation.
FailsafeA mechanism to minimize the damage to the multirotor, and nearby objects in case of system failure; what is failsafe
FCFlight ControllerFlight Controller is the brain of your multirotor.
FETField-effect transistorFET is a device which enables us to use one smaller electrical signal to control another bigger one.
FlowFlying fast and smoothly with style
FOVField of ViewA measurement how much environment you can see through a camera lens. Usually measured in degrees.
FPVFirst Person ViewA technique that uses an onboard FPV camera and wireless connection, to allow a pilot on the ground to see a live stream video while flying, throught a FPV goggle or monitor.
FTDIFuture Technology Devices InternationalThe name of the company that makes the chips. A standard to convert USB to serial communications. Available as a chip for boards that have a USB connector, or in a cable to connected to breakout pins.
FWFirmwareA firmware is basically the software designed specifically for a piece of equipment.
GCSGround Control StationSoftware used to monitor a Flight Controller Board. The OpenPilot GCS for example.
GimbalI can mean two things: 1. camera stabilizer; 2. the stick controls in a radio transmitter
GITA version control system for software developers.
GPSGlobal Positioning SystemUsed to track movement or hold position at pre-defined coordinates
GNDGroundGround of a electrical circuit
Ground EffectAir turbulence from propellers bouncing off the ground when the copter is hovering at really low altitude
GYROGyroscopeProvides the angular velocity around 3 axes of space in degrees. Assists with keeping a quadcopter level and facing the same direction.
HEXAHexacoptertype of multicopter, that has 6 motors, no servos, all motors on same level.
HKHobbykingA famous RC Vendor based in China but with EU, US, AU, RU warehouses as well.
I2CInter-Integrated CircuitA serial bus that allows multiple low speed peripherals, such as sensors, to be connected to a microprocessor.
IMUInertial Measurement UnitA Break Out Board containing sensors that reads and sends data to the flight controller
JelloUndesirable high frequency vibration in flight footage
JST-SHA type of Connector
Kalman FilterThe algorithm used in Multicopter Flight controllers. It is primarily used to combine accelerometer and gyro data to provide an accurate description of aircraft attitude and movement in real time.
KKKaptein KukMulticopter controller by Rolf Bakke
LC FILTERAn LC Filer is a circuit that removes frequencies (noises) from a power supply and smooths / cleans it so the equipment being powered
LEDLight Emitting Diode
LIPOLithium Polymer BatteryAka LiPoly. Most used power source for RC Hobby these days because of its high energy storage density-to-weight.
LOSLine of SightWhen you are flying your aircraft by watching your aircraft directly, you are flying LOS. Another way of flying is FPV.
LPFLow-pass FilterA technique that filters out high frequency signal
mAhmilli-amp per hourAn unit that measures the capacity of a LiPo battery
MAGMagnetometerAn electronic compass – used to let our FC ( Flight Controller ) know which direction it is pointed in, compared to the Earth’s Magnetic field
MegaPiratea flight controller program software that copies Arducopter
Mode1 / Mode2These are the different transmitter stick configurations: mode 1, throttle stick on the right; mode 2, throttle stick on the left (more widely used)
MTOWMaximum take-off weight
MultishotAn ESC Protocol, what is multishot
MWCMultiWii CopterMultiWii is a general purpose software to control a multirotor RC model. It can use various sensors but was initially developed to support Nintendo Wii console gyroscopes and accelerometers
OCTOOctocopterA type of multicopter, that has 8 motors, no servos, all motors on same level.
OneshotOneshot125A faster way of communication between FC and ESC.
OSDOn Screen DisplayA piece of hardware that overlays flight data in text or graphical form over an existing live stream video.
P“Battery cells in parallel”For example, a LiPo battery of 4S2P, has two 4S batteries connected in parallel.
PayloadWhat the drone can carry/lift, such as a camera, or package
PAL / NTSCThese are video formats in FPV cameras and your video display.
PCBPrinted Circuit BoardSillicone board with copper traces and components soldered on it. Most electronics in this hobby are using PCB, such FC, VTX, TX, RX….
PDBPower distribution boardA board designed to carry high current, it’s built with copper traces used to connect various electronic components such as LiPo battery, ESCs, Video transmitter, LEDs etc
PIDProportional Integral DerivativeThe three parameters that you can adjust to change the copter’s flight performance. In theory PID is a control loop that attempts to minimize the difference between a measured process value and its desired value by adjusting the control input of a process.
PNPPlug and PlayA PNP quadcopter kit usually comes without the receiver, transmitter, battery and FPV Goggles.
PPM SUM (CPPM)Pulse Position ModulationA way of sending sequence of multiple signals down one signal wire
PropPropellerShort for propeller
PunchSudden burst of thrust
PWMPulse Width ModulationThe square-wave signals used in RC control to drive servos and speed controllers.
QuadQuadcopterShort for quadcopter, which is a type of multicopter that has 4 motors on the same level
RateRate changes how fast your copter rotates with the sticks, what is rate
Rate ModeA flight mode that only use gyro sesnor, also called manual mode. What is rate mode
RCRadio ControlThe foundation of our hobby, it’s also used to refer to the hobby as a whole, radio controlled cars, multirotor, planes boats and more.
RCGRCGroupsA very popular RC forum
RevoRevolutionThe 3rd generation of OpenPilot Controllers
RFRadio FrequencyCommon frequency bands RC hobby use are 5.8gHz, 2.4gHz, 1.2gHz, 433mHz, 900mHz Not all bands are available in all countries, and some require a HAM (Amateur Radio Operator) license.
RPMRevolutions Per Minutethe number of times a motor shaft  rotates a full cycle in 60 seconds
RSSIReceived Signal Strength IndicationRSSI is a measurement of how good your radio signal is.
RTFReady to FlyA multicopter that’s sold with everything you will need in order to fly right out of the box.
RTHReturn to HomeA GPS feature that when enabled, it returns the aircraft to the “home” position where it took off.
RTLReturn To LaunchSame as RTH.
RUDRudderAlso known as yaw
RXRadio ReceiverA device that receives commands from our radio Transmitter, and sends them directly to the servos or to the Flight Controller.
SCell Count – “Cells connected in series”For example, a 4S LiPo is a LiPo battery with 4 cells connected in series, nominal voltage is 14.8V (4×3.7).
SBUSSBUS is a type of serial communication protocols, shared by Futaba and FrSky, that supports up to 18 channels using only one signal cable. Check out this post about different radio signal
SFSlow flyA type of propellers that are designed to go on a motor using a gear reduction to lower prop speed
SimonKA famous ESC firmware optimized for multirotors
SMA and RP-SMASub-Miniature Version A – Reverse Polarity SMADifferent Connectors for FPV gear.
TAER1234A type of radio channel mapping, it means mapping throttle as channel 1, Aileron as channel 2, Elevator as channel 3, and Rudder as channel 4
TelemetryA feature that allows you to receive real time flight data from the aircraft back to you; It’s commonly seen in radio TX and RX, there are also standalone devices for telemetry
ThrottleControls on your radio transmitter, it changes the speed of the motors
ThrustThe force generated by the propellers and motor, measured in gram
TOWTake off weightSimilar to AUW.
TRITricopterA type of multicopter that has 3 motors,  one servo for yaw, all motors on same level
TVLTV LineTVL (TV Lines) is basically a measure of how good the FPV camera resolution is. The number is based on how many alternating black and white lines can be displayed in its image horizontally.
TXTransmitterA device that sends our commands to the Receiver, controlled by the pilot.
UARTUniversal Asynchronous Receiver/TransmitterSerial communication port. A piece of computer hardware that translates data between parallel and serial form.
UAVUnmanned Aerial VehicleAircraft without a human pilot onboard. Control is provided by an onboard computer, remote control or combination of both.
UBECUniversal Battery Elimination Circuita DC voltage regulator
VccVccPositive power supply input
VRXVideo ReceiverA device that receives video signal from our video Transmitter, and display the live footage on our FPV goggle or screen.
V-tailA type of multicopter that has 4 motors, 2 tail motors form a V shape, has no servo
VTOLVertical take off and landing
VTXVideo TransmitterA device that sends video signal to  our video receiver, video captured from a on-board camera. How to choose VTX
WOTWide Open Throttle100% throttle
X8A type of multicopter that has 8 motors and of a shape of X,  4 motors on top, 4 motors at bottom, no servo
XT60A type of battery connector type
Y4A type of multicopter that has 4 motors, 2 tail motors one on top of ther other, it looks like a tricopter but has no servo
Y6A type of multicopter that has 6 motors and of a shape of Y,  3 motors on top, 3 motors at bottom, no servo
YawFlight term used to describe the rotation of a drone around it’s center axis. Controls which direction the quadcopter is facing.

Controlling the motors

Motors are controlled through the Electronic Speed Controllers (ESCs). They work on pulse widths between approximately 1000us and 2000us like the RC radio receiver – sending a pulse of 1000us typically means off, and a pulse of 2000us means fully on. The ESCs expect to receive the pulse at 50Hz normally, but most off the shelf ESCs average the last 5-10 values and then send the average to the motors. Whilst this can work on a quad, it behaves much better if we minimise the effect of this averaging filter to give near instantaneous response. Hence, the APM HAL library sends the pulse at 490Hz, meaning that the 5-10 pulses which are averaged occur very quickly largely negating the filter’s effect.
In setup(), let’s enable the outputs:
hal.rcout->set_freq(0xF, 490);
After your includes, let’s define a mapping of output number to motor name – this mapping is the same as the ArduCopter uses but the numbering starts from zero rather than one.
#define MOTOR_FL   2    // Front left    
#define MOTOR_FR       // Front right
#define MOTOR_BL   1    // back left
#define MOTOR_BR   3    // back right
In your loop, after reading the radio inputs, let’s send the radio throttle straight to one of the motors:
hal.rcout->write(MOTOR_FR, rcthr);
You can now program your quad and try it, WITHOUT propellors. Slowly raise the throttle and the front right motor should spin up. By repeating the last line for the remaining three motors, all your motors would spin up although the quad will just crash if you have propellors on because we have to do stablisation – slight differences between the motors, props, ESCs, etc mean that slightly unequal force is applied at each motor so it’ll never remain level.
** Comment out the write line before proceeding for safety reasons **



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