How Drones Work
HOW DRONES WORK
Flying, building, and playing with multi rotors can be an amazing hobby. However, it can be a bit overwhelming at first if you have no prior knowledge. Do not let that discourage you! There is a lot to learn, but everything you need to fill your brain with can be found here.
If you are in the market for your first multirotor, visit our “Best Beginner Quadcopter and Drones for 2015” article.
What is a drone? Multirotor? UAV?
There are a lot of loose terms thrown around in this industry. So let’s clear the air a bit…
This is probably the most overused, and incorrectly used word. True RC hobbyists really dislike this term, as it’s meaning doesn’t really belong in the RC genre. Drone is more correctly used for an unmanned aircraft flown beyond line of sight for the military. These drones are capable of dropping missiles and performing spy tactics hundreds of miles away. A little different than the DJI ones we are flying in our backyard… It is now becoming used so often, that the term “drone” is slowly becoming synonymous with “mulirotors.” It really doesn’t bother us, as we happen to use it quite often ; ).
A multi rotor is a rotary aircraft with more than two rotor systems, typically unmanned. Multirotor is the general term for any unmanned system that isn’t a traditional helicopter. Examples being: tricopter, quadcopter, hexcopter, octocopter, etc…Since there are such a wide variety of rotor systems, it’s easier to classify everything into a common group called multirotors.
UAV – “UNMANED AERIAL VEHICLE”
UAV is any machine capable of unmanned flight. Drones and multirotors fall into this category as there is no physical pilot onboard. They still can be directly operated by a pilot elsewhere. UAV’s are slowly transitioning to full size fixed-wing and helicopter models. Airbus has been developing unmanned platforms for a few years, while the military has been doing this for quite a while. It is no question that UAV’s will slowly start to become the new norm. Not only does this solve a pilot shortage problem, but will help lower insurance requirements and costs for civilian operators and put less military pilots in the crosshairs.
What are the parts of a Multirotor?
Frame – the frame of a multirotor works in much the same way the frame of a car works, there to support and add strength to the vehicle. Frames are typically made from plastic or carbon fiber, and can be arranged with different arm variations (tri, quad, hex, octo). The end of each arm houses the motor and propeller, while the center holds the flight controllers, gimbals, and other electronics. Most of the weight should always be towards the center of the ship, as this leads to the best flight characteristics keeping the center of gravity (CG) towards the center.
As with each part listed below, weight is an extremely crucial element. The heavier the frame, the less lift can be achieved. However, you don’t want a super lightweight frame that will break on you. Carbon fiber tends to be a favorite because of its strength and light weight.
Motors – there is a separate motor for each blade/arm. Motor decisions are based on required power, and what you want the motor to do. If the multirotor is being built to carry heavy payloads and have the best possible flight times, then a slower spinning, higher torque motor is ideal. This is in contrast to an aggressive, fast system with lots of maneuverability, having faster spinning rotor systems. The measure of the RPMs, or speed of the rotors, is its kv value. Faster builds will be upwards of 1400kv, while slower, longer battery life builds are roughly 300-900kv. These figures only work properly if the correct battery and propellers compliment it accordingly.
Be sure the motor is compatible with the ESC and battery you have chosen.
Propellers – can also be made from plastic or carbon fiber. Carbon fiber being the higher end choice, but also more expensive. When picking propellers, be sure to check that your frame can house the size you are selecting. Most frames will have a max propeller size to them. The size of the propeller should match your intended purpose. If you wish to have a more aggressive build, then choose propellers on the smaller end of the spectrum. Vice versa with higher payload, longer flight time builds. Propellers generally come packaged as a pair, one being a CW (clockwise) spinning prop, and the other being CCW(counter-clockwise). If you are building a quadcopter, you will have to order two.
Propellers need to be balanced before precise flight can be achieved. Although a select number of props can be purchased already balanced, most should be hand balanced when you get them. Check out our prop balancing video to learn how to balance the props and the hubs.
Battery – batteries come in a wide variety of weights and capacities. It seems intuitive to always pick the largest capacity battery to achieve the longest flight time, but this is not always the case. As the capacity of the battery increases, so does its weight. There is a certain point where more capacity is no longer beneficial, and the benefits are no longer there. Ecalc is a great tool to play around with and find your ideal battery weight and capacity.
It is important to play close attention to the number of cells your battery has and what your motor, ESC, and flight controller requirements are. This is often an overlooked detail that can get people in trouble. If you want a high capacity 10,000 mah 6s batttery, be sure your motors and ESC are 6s, and that your flight controller can support it.
ESC – “Electronic Speed Controller” run the motors you have. ESCs are rated for the amount of current they can consistently supply to the motor system. Since the motors are constantly spinning at different speeds, they need a speed controller to dictate that to them. If the motors all ran at the same speed, you would always be hovering. Since we are not changing the pitch of the rotors, pitch of the system is through difference in motor speeds.
It is highly recommended to use four identical ESCs. Although this doesn’t necessarily need to be true, you will more than likely have a better running system using identical parts. Also, running ESCs flashed with SimonK firmware is highly advised. There are more efficient and smoother operating ESCs and have built up quite a reputation.
Transmitter and Receiver – this combination is the “remote control” to operate your multirotor. The transmitter “transmits” the signal, and the receiver “receives” it. The receiver is connected to the flight controller delivering these inputs and outputting the responses to the motors.
The choice of transmitters is a little more basic. Transmitter choice is most often based on the number of channels required for operation. For multirotors, the bare minimum is four (roll, pitch, yaw, and throttle). More is always a nice convenience. A separate channel can used for autopilot, operating a camera gimbal, retractable landing gear, etc…
Setting up the transmitter in either mode 1 or 2 needs to be considered. Mode 2 is set by default for the US, but lots still prefer mode 1. Mode 1 vs Mode 2 is the difference in what the control inputs on the transmitter do.
How Does A Multirotor Fly?
Now that you have a basic understanding of the different parts of a multirotor, it’s time to discuss how these all come together to achieve flight.
Let’s use a quadcopter for the following examples. A quadcopter uses four different propellers, powered by four different motors, on 4 separate arms. Easy, right? Each spinning propeller creates its own torque. Newton’s Third Law states “to every action there is an equal and opposite reaction.” So if that propeller is spinning, the arm holding it will want to spin the opposite direction. This is the law of Torque Reaction. This is why a traditional helicopter has a tail-rotor, to compensate for that fuselage torque.
In a quadcopter, we don’t need a tail rotor. Why? Because we can combat that per-propeller torque with an equal and opposite torque (the propeller opposite it). Notice in the picture how the opposite propellers are spinning in the same direction. They are canceling out each other’s torque effect.
This allows quadcopters to hover extremely well. If you were to hover a helicopter and apply more power, then there will be more torque, and more power will be needed from the tail rotor. This takes lots and lots of hours to master, especially to make it look smooth. These problems obviously don’t exist in multirotors because power increases are always done equal and opposite in the propeller system.
We want to do more than just hover our multirotor. If we wanted a forward motion, then both forward propellers will apply less power, while the back propellers add more. This principal applies to all roll directions.
A yaw movement is different, however. The picture below depicts what would happen if we wanted our craft to yaw left.
We want that torque reaction to be more prominent in the left direction. So the quadcopter gives more power to the propellers that have a left torque direction (clockwise spinning propellers).
Ascending and descending are very simple. Power is either increased to the entire propeller system to ascend, or decreased to make a descent. Now you can fly your quadcopter in a multitude of ways. You can be in a slow yaw while ascending, and at the time applying a slight roll. The algorithm and codes built into the flight computer takes these into consideration and apply the necessary power to each propeller.
What are flight characteristics? Do they matter?
In addition to understanding the individual parts, it’s equally important to understand flight characteristics to learn how drones work.
Coming from a commercial helicopter pilot background, I am very surprised by the number of RC hobbyists that do not understand even the most basics of flight dynamics and how drones work.
The goal with any flying aircraft is balancing the four different principals of flight. These are: lift, weight, drag, and thrust.
For any aircraft to leave Earth, lift and thrust need to be greater than weight. In the case of the helicopter above, lift is achieved through the rotor system. If you put too much weight in the helicopter, there might not be enough power in the rotor system to overcome the weight. Thrust is generated through the power plant and overcomes the force of drag.
In a multirotor, this is essentially happening on each separate motor/propeller system. If there is too much weight, flight performance is going to take a major hit. The lighter the system is, the longer flight time and higher performance characteristics it will have. However, the goal for many is to carry heavy cameras and other equipment. To compensate for this, stronger motors and propellers need to be added. The frame needs to be able to support that weight as well.
The same principals that helicopter pilots mind, also need to be minded by multirotor pilots. But many are uneducated, causing unnecessary crashes or heavy strain on their frame/parts. The most obvious and basic example is having an understanding of how wind affects flight performance.
Airspeed vs. Ground speed
This is one of the more basic concepts, but extremely important to understand. Part of the lift equation is velocity. Velocity is squared, showcasing its importance.
Lift = Coefficient of lift * 1/2 Density * Velocity Squared * Surface Area
Coefficient of lift = is the angle of attack and shape of the wing.
Density = density of the air and conditions being flown in.
Velocity = AIRSPEED of the wing through the relative wind.
Surface Area = Surface area of the wing.
Take note that velocity is measured in airspeed, not ground speed. Airspeed is the speed of the air hitting the leading edge of the wing or propeller. This airspeed is the most important part of the lift equation.
So what is the difference between ground speed and airspeed?
Let’s use a car for this example. If you’re driving along the interstate going 75mph, your groundspeed would be 75mph, obviously. But what is your airspeed? In other words, what is the speed of the air if you stick your hand outside the window going 75mph? It’s a trick question, without knowing the wind speed and direction, it’s impossible to know.
On a zero wind day, perfectly calm, the airspeed would also be 75mph. But what if you had a 20mph head wind. The ground speed would still be 75mph, that is unchangining, but the airspeed is 75mph + 20mph (wind) = 95mph. We wouldn’t be getting very good gas mileage anymore…
Opposite is true with a 20mph tail wind. Ground speed is 75mph, but now the airspeed is 55mph. Now we are getting better than normal gas mileage.
This example is true for aviation. The only difference is that airspeed is staying the same, and ground speed is changing. Helicopter pilots have an airspeed of best lift, usually around 60 knots. They are going to fly that speed to climb to altitude regardless of what the wind is doing. But on a 20 knot headwind day, there groundspeed is now 40 knots. In contrary, that 20 knot tailwind would give them a 80 knot ground speed. The difference between having a 20 knot tailwind or headwind changes the groundspeed from 40 to 80 knots, pretty big difference. Now assume that helicopter is coming in to land at the hangar for the day, they would MUCH rather come in and land with a 40 knot ground speed versing 80 knots. It is simply much safer.
So what happens when that helicopter pilot is coming in to land unknowingly in a tail wind? It might be a complete accident, but the pilot will look outside and see they are going very fast and want to slow down more. They will continue to slow down, and if they aren’t careful, could come extremely close to stall. Stalling a helicopter is a little different than stalling a plane, but we don’t have time for those details.
The same thing can happen in a multirotor. In strong winds, if your flight patterns don’t have you coming in to land facing a headwind, you could have serious problems. Have you ever wondered why your multirotor shakes coming in to land sometimes? Most the time you are probably extremely close to stall.
Now the real beauty of a multirotor is that there is no real “forward” direction. So most importantly, when making your final approach, be sure your forward movement is into the wind. Newer drones have lots of algorithms built in that they can make complete vertical approaches. They will shake, and at times it can be violent, but they are designed to accomplish this. However, this would have been very bad practice years back before DJI and other companies really fine tuned these algorithms. There were many more crashes from incorrect landing patterns back then.
What these DJI algorithms are trying to compensate for is a process called “Vortex Ring State.” This is something you learn on day 1 when you become a helicopter pilot. The downward air from the propellers creates downwash (turbulent disturbed air), and during a quick descent, you are descending into that downwash. It is very hard to create lift in this air, and in a helicopter, it would create an uncontrollable descent. The longer you sit in this air, the harder it is to recover from it. The only way out of Vortex Ring State is to get into clean air. Aka, lateral movement. While flying your drone, if it ever starts shaking very violently in a descent, just roll it laterally into clean air and it should solve your problem. Or develop better descent habits and don’t make vertical descents ;).
As for me, I like to maintain good forward airspeed, even if it’s not entirely necessary all the time. It might save me some day if I’m not paying attention and make a downwind approach when it’s 20mph wind. It could be an expensive mistake that I will try to avoid through habit.
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