The Flettner Airplane made its debut in the 1930s. Instead of wings, it flies using rotating cylinders that stick out from either side of its body. We'll tell you about its inventor, and show you how it flies.
Anton Flettner and His Rotorcraft
Anton Flettner was a German engineer who didn't see why things needed wings to fly or sails to sail. Eventually Flettner played a major part in the development of the helicopter, but he had to get some personal and professional weirdness out of his system first. In the 1920s, he invented the "rotor ship." This ship, instead of sails which harnessed the wind, used rotors that would turn and use the resulting air redirection to move the vessel. One such ship, called the Baden Baden, crossed the Atlantic ocean in 1926.
Flettner wasn't content with simply putting rotors on the sea; he wanted them to take to the air. Inspired by his ship, he designed a plane based on the same principle. This was a plane shaped in body like any other — two rotating cylinders, called Flettner rotors, were attached to the sides of the body, roughly where the wings would be. The plane took off somewhat like a regular airplane. It accelerated forward until it picked up a good deal of speed. It also rotated its rotors faster and faster until it was drawn upwards into the clouds.
A Quick Flettner Experiment
To understand, viscerally, how this plane works, take a piece of paper and roll it into a cylinder, securing the shape with tape or staples. Now take a large book book, or better yet, a piece of cardboard about 18 inches long. Hold the board on an incline well above the ground. If you've set it up right, you can expect the paper cylinder to roll down the board and then fall through the air for a few feet before hitting the ground.
The paper cylinder does roll down the ramp, picking up speed as it goes. When it drops down, it falls through the air. But it doesn't fall through the air outwards in a curved path, the way you would expect. Instead, it turns "backwards," heading back under the ramp it just rolled off.
What you're seeing is the Magnus Effect. It happens with every spinning object, but only light objects let us see the effect clearly. A paper cylinder is light enough that its trajectory is visibly affected. If you don't see the effect the first time, give the cylinder a little spin as you roll it, so that it's spinning quickly when it leaves the ramp. Check out a video of the experiment here.
The Magnus Effect
The paper cylinder is demonstrating the effect that the Flettner plane uses to fly, and that baseball players use to make balls curve in mid-air. The Magnus Effect, because it involves a spinning object that's moving in one direction and then curving off in a second direction, can be a bit confusing. Think about it from the point of view of a single atom of air moving over the spinning cylinder. (Technically, the plane is moving through the air, not the air around the plane, but it's the same effect.)
You come speeding towards the cylinder. You hit the bottom of the cylinder, and it's spinning against the direction of your motion. Basically it's like being clobbered; you get hit, you skid along the outside of the cylinder, and you break away from the spinning object as fast as you can.
Now you come speeding towards the top of the cylinder, which is spinning with the direction of your motion. It's like getting on one of the moving sidewalks at the airport. It's a nice easy ride, and you adhere to the side of the spinning cylinder for as long as you can before you're torn away. This adherence changes your path, slightly. You've curved around with the spin of the cylinder, and now you're moving slightly downwards. In return, you impart a slightly upward momentum to the cylinder. The cylinder moves up.
So if you're thinking about spin, motion, and which way the Magnus effect will make an object move, just think about a particle of air "riding" the spin of an object, and which way that particle of air will be going when it leaves the object. Because the particle of air, and the object, exert equal and opposite forces on each other, if the air goes down, the object goes up, if the air goes left, the object goes right.
Flettner planes did not catch on in commercial aviation, but model plane enthusiasts do still construct little working Flettner aircraft. Have a look here at a Flettner model plane zooming around an airfield.