I was intrigued by this demonstration of a plane that has no moving parts and moves silently through space, accompanied by just a blue glow. The abstract of the paper describing it can be read here but the paper itself is behind a paywall.
As Steven Barrett, one of the MIT scientists behind the design of the plane, said:
The plane weighed about five pounds (2.45 kilograms) and had a wingspan of 15 feet (5 meters), and traveled about 180 feet (60 meters), so it’s a long way from efficiently carrying cargo or people long distances. But we have proved that it is possible to fly a heavier-than-air vehicle using ionic winds. It even has a glow you can see in the dark.
How does it work? The basic idea behind Electroaerodynamic (EAD) propulsion is quite simple really.
Getting a plane to fly requires it to move forward and that depends on the law of conservation of momentum, where the momentum of an object can be defined (at speeds that are not too close to the speed of light) as the mass of an object times its speed. This law says that the total momentum of a closed system remains constant even as the momentum of each component of that system changes. So, for example, if you are on a small boat that is stationary on a calm lake, if you throw some object backwards over the back of the boat, the boat will try to move in the opposite direction in order to conserve momentum, its motion only constrained by the friction due to the water.
Planes use this same principle. By means of propellers or turbines, it takes in stationary air in front of the plane and emits it backwards at high speeds, and to compensate for the creation of high momentum in the backward direction, the plane moves in the forward direction. Rocket engines burn stationary fuel and emit the produced gases in one direction at high speeds, causing the rocket to move in the opposite direction. What causes the plane to lift off the ground and stay in the air is a little more complicated, involving both the law of conservation of momentum plus some other laws of hydrodynamics that I will not get into.
But the important point is that it does not really matter what the substance is that is propelled backwards. As long as it has mass and speed, that is sufficient. To get more power, you need more mass and higher speeds and this is why normal plane engines have huge engines so that they can give very high speeds to large amounts of air.
Scientists have developed an ingenious engine that does not contain any moving parts. The basic idea used here is that the molecules of air are made up of positively charged small nuclei and negatively charged electrons surrounding it so that the net charge on the atom is zero. If you apply an electric field to a neutral atom, the positively charged nucleus will go try to go one way while the negatively charged electrons will try to go in the opposite direction. But the force of attraction between the nucleus and the electrons is usually large enough to hold the neutral atom together.
But if you apply a large enough electric field, you can split the atom into the oppositely charged parts, positive ions and negative electrons. It takes an electric field of about 3 million volts/meter to cause the nitrogen molecules in the air to break down. For a given voltage produced by a battery (they use 20,000 volts), you can increase the field by making the object to which the voltage is applied have a sharp point, which is why lightning is more likely to strike sharply pointed objects like a steeple or a tree in a field. Another electric field can then accelerate the positively charged parts, giving them a momentum in the backward direction. This results in the plane moving forward, as shown below.
Here Barrett explains the history of the project and how they set up the fields to get the required effect.
There is still a long way to go before this can be scaled up. It is likely that it will be first used to power small drones.
But it is undoubtedly a cool idea.