How do Multirotors Actualy Fly?
So we are all pretty used to seeing quadcopters and other multirotors whizzing around, but have you ever wondered how exactly it is that they fly in the first place? It’s not just a question for interest’s sake, either. If you understand how your quadcopter works you’ll be able to fly it better. Also, you have a better chance at figuring out any problems if it behaves strangely, and you’ll be able to bore your friends by telling them all about it,i too.
Before we get on to quadcopters, let’s have a quick and (very) simple look at how planes and helicopters fly, so that you can see the differences and similarities.
Biggles Flies Again
As you probably learned in school at some point, the key to getting a fixed-wing aircraft like an airplane up into the sky has to do with the shape of its wing. That shape causes lift by producing pressure differences on the top and bottom of the airfoil, or wing.
Modern planes have various flight control surfaces on their wings (such as ailerons and flaps) that alter the shape of the wing temporarily and allow the craft to be maneuvered in various directions.
Although planes can have rotors, the thrust from the rotor is not used to lift the plane directly, but to pull it along a horizontal plane (excuse the pun), dragging the wings through the air and allowing them to generate lift. This is a very efficient design for a flying machine, and that’s why long distance air-travel is done by plane.
Get to Da Choppa!
In the case of helicopters, which are otherwise known as rotary-wing craft, it works a little differently. Instead of having an engine generate thrust along the length of the craft and pulling its wings through the air, it has small wings mounted on a rotor head which spins at high speed, generating lift that pushes the helicopter up into the air.
A plane needs to keep moving to stay airborne, but a helicopter can hover, letting it do jobs and get to places planes have no hope of matching. So a helicopter is basically balancing on a shaft of thrust and moves by tilting that thrust in different directions – a sort of controlled forward falling. You can get the idea if you balance a broom handle on the palm of your hand. Imagine the head of the broom is the helicopter. To make it stay in one place you have to constantly adjust your hand to compensate for gravity and other physical forces that are trying to topple it.
The rotor head spins at a more or less constant speed, so thrust is increased or decreased by varying the pitch of the rotors. The sharper the angle of pitch the more the wings “bite” into the air and the more thrust is generated. The blades are also capable of negative pitch, which means the helicopter can push itself back towards the ground and in some special cases, with the right type of aircraft, perform loops or even fly upside down, in principle at least.
Four or More
So the typical quadcopter that you’ll buy uses some of these principles, but in somewhat different ways.
If we look at the basic quadcopter, we have four rotors but, unlike the rotors on a real helicopter, the pitch (or angle) of the blades is fixed. In other words, the amount of thrust the rotors produce is directly related to how fast they spin. Obviously this also means that they can’t reverse their thrust, so they can’t descend faster than free fall would allow.
In a helicopter, thanks to Newton’s third law, the torque from the main rotor would spin the body of the helicopter in the opposite direction; basically turning it into the world’s least-fun vomit comet. That’s why helicopters employ a tail rotor to counteract this force and keep the nose straight. It turns out this is also useful for letting the helicopter turn left and right (or “yaw”) by reducing or increasing the anti-torque thrust of the tail rotor.
The rotors on a multirotor craft have the same problem – each rotor is trying to turn the whole craft in the opposite direction, so each one must have a partner that spins in the opposite direction.
In quadcopters this is usually achieved by each rotor directly across from another spinning in the same direction. In other words, each rotor will spin in the opposite direction of its neighbor. There are, of course, many different designs for multi-rotor craft, and I’ve written a separate article about it, but the principle remains – the torque from each rotor needs to be canceled out by another rotor.
In theory, if all four rotors are giving the same thrust and are canceling out each other’s torque perfectly, then the craft should hover in place.
So unless that’s all you want it to do, you need some way to make it move, that’s where the magic happens.
(More Than) One Direction
An aircraft has three basic movements it can make: yaw, pitch, and roll.
Yawing is when the craft turns left or right. Rolling is when it rolls side to side. Pitching is when the whole body pitches up or down. By combining these three movements, pilots can complete complex maneuvers and aerobatics.
To roll or pitch, a quadcopter slows down two motors and speeds up the other two, tipping the body in the desired direction. To yaw, it does the same thing, except the pairs of motors are diagonally across from each other. By doing this, it changes the anti-torque balance and lets the craft turn.
To gain or lose altitude all four motors simply thrust less or more.
Of course, without the onboard flight control to coordinate all of this, a quadcopter would be impossible to fly, since the computer control must coordinate the speeds of the four motors many times a second. It’s no wonder that early quadcopter attempts were doomed to fail; you can read all about that in my “history of quadcopters” article.
Remember I said that proper helicopters have the ability to change the pitch of their rotors? Well, “proper” model helicopters have this too. Beginner helis have fixed-pitch rotors, but skilled pilots use something called “collective pitch” rotor systems. These are harder to fly, more expensive to fix, and the ultimate in performance.
Lately there have been some collective-pitch quadcopters as well, although they are not yet widely available. They are definitely not for beginners and are less efficient for normal flight, but can’t be beaten for aerobatic ability.
So there you have the basics of how a quadcopter flies. Craft with more rotors use the same principles; they just split the workload differently while the basics remain the same. Now you may be interested in reading my other article on how to actually fly your quadcopter, armed with knowledge such as you are. Well? Get over there and read it!