The (Disputed!) Principles of Flight

Whatever you want to call them — drones, UAVs, sUAS, flying robots, flying cameras, quadcopters, quadrotors — they fly. So we figured some of you might want to know exactly what’s going on while your Solo is aloft. What I’ve learned is that it’s probably not what you expect.

Afraid of flying? Then skip this next sentence. Even though we’ve been flying planes with people in them for well over a hundred years now, physicists and aviation experts still don’t agree about exactly why airplanes fly. The physics is quite complicated, and many experts say our understanding is still incomplete. But that doesn’t stop our frantic searching for the best deal on a value airline; most of us probably aren’t concerned with how flight works, as long as it does. But for the curious, here are the facts (simplified for readability) as we currently purport to know them.

Why airplanes fly

The short of it is that the traditional explanation for how planes achieve lift doesn’t tell the whole story. Lift is one of the four forces of flight, along with thrust, drag and gravity. As the popular story goes, lift is the upward force; thrust is the force that pushes the plane forward; drag is the friction that pulls backward; and gravity is the plane’s weight, pulling the plane down. Lift, that most miraculous of the four forces, is often attributed to the shape of the wing — a curved upper surface and a flat lower surface, called an airfoil. The principle here is that the airfoil shape causes air to flow over it in such a way that the air pressure above the wing is lower than the air pressure below the wing, a differential that lifts the wing up.

The four forces of flight.

This explanation is based on a theory called “Bernoulli’s principle,” which says that the faster a fluid moves, the less pressure it has. The airfoil theory, then, claims that the air (which is a fluid) moving over the curved surface has to move faster than the air on the flat surface — the curved surface is a little bit longer than the flat surface, so the air particles have to travel a longer distance to meet back up with the air that passes over the flat surface. This disparity in pressure creates the lift necessary for flight.

But there’s nothing in physics that says those air particles have to meet on the other side of the wing, and wind tunnel tests show that in fact, no, they don’t have to meet. They go pretty much wherever they please. That means this airfoil theory is incomplete — it doesn’t explain why the air on the wing’s curved surface should travel faster than the air below. Alternatively, if you’ve ever seen a plane fly upside-down, or folded a moderately successful paper airplane of your own, you’ll understand intuitively that the airfoil theory isn’t quite right. Bernoulli’s principle, though accurate in and of itself, doesn’t fully explain flight. Yet this is the story most of us have been taught — that the airfoil shape creates the lift that pulls the plane up — and it’s the story still taught in many flight schools and physics classrooms around the world.

So if that’s not a fully accurate explanation for lift, then what is? It’s thankfully a little simpler, and in some ways a little more familiar; it goes all the way back to Isaac Newton. To oversimplify it, Newton’s third law of motion tells us that every action has an equal but opposite reaction. It’s easy, then, to visualize air particles hitting the bottom of a wing and pushing upward, especially when you consider those wings bolted onto jet planes at a slightly upward angle — this is called the “angle of attack,” the angle at which the wing hits the air. So it turns out that the angle of attack has a lot more to do with generating lift than the airfoil shape does.

The Coanda effect.

But — sorry — it’s not quite so clean as that, either. It’s not that air simply bounces off the bottom of the wing and pushes the plane up — the shape of the wing, taken together with the angle of attack, actually pulls massive amounts of air downward. A great deal of this air passes under the wing, but some of the air also runs over the top of the wing, sort of suctioned to it. This is called the Coanda effect — and it means that yes, the curved surface does play a part. If you’ve ever tried to pour red wine from your wine glass into your friend’s, you’ve become devastatingly familiar with the Coanda effect. To see it, you can hold a coffee mug on its side under a running faucet — the water hugs the side of the mug until it reaches the very bottom. However, if the flow of air over the top of the wing slows down enough, the suction releases and the plane goes into a stall. Paper airplanes, having flat paper wings, don’t generate a high Coanda quotient, so once they start to slow down they tend to stall quickly and drop.

So: Air pushes up on wings thanks to the angle of attack; wings also pull air downwards thanks to their shape; and the Coanda effect ensures that air hugs the top of the wing, creating a powerful suction force. And that’s the best I can do to explain to you how airplanes fly. Or, as completely as the space I have here will allow. As David Foster Wallace once said, there’s a limit to what even interested parties can ask of each other.

BUT SOLO IS A COPTER!

Oh, right. Well, how do helicopters fly, then?

Where wings force air under them, helicopter blades literally beat the air down to generate lift. Like most plane wings, these blades are usually shaped like an airfoil. The pitch (angle) of the blades determines how much lift they generate. In a traditional helicopter, the pilot uses a control called a “collective stick” to regulate the pitch of each blade. As you can imagine, the rotor mechanism for a helicopter is quite complex: Not only does it rotate the blades, but the sockets where each blade attaches themselves contain mechanisms that swivel the blades’ pitch as they whirl about. And because the force generated by the swinging blades is so great, helicopters employ a small rear rotor that pushes back against this force and keeps the copter in line.

Solo, however, has four propellers — what’s called a quadrotor, or quadcopter. Quads aren’t a new invention: The first quadcopter was built in 1913 — it weighed tons, and needed a live pilot. This preceded Sikorsky’s helicopter, generally considered the first modern design, by decades.

Unlike in traditional helicopters, the propellers on today’s consumer quadcopters are set at a fixed pitch. In other words, their pitch doesn’t determine lift. Instead, a quadcopter’s position and the amount of lift it generates are regulated by the rotation rate of the propellers. (Two propellers rotate clockwise; two rotate counterclockwise.) When you want to make a certain turn, the autopilot feeds information out to the motors; depending what you’re asking the copter to do, some motors may speed up and some may slow down, and it’s the imbalance of these forces that determine your heading, direction and speed.

If you want to learn more about flight, there’s the whole internet out there. But I found this article incredibly helpful, and as you’ll see, I owe a lot of the information in this post to it. I’d also like to thank Scott Horn, from our flight ops team, for his time. And Wikipedia. Of course.

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