Mythbusters, the TV show, isn't trying to discover new scientific principles. They just test the coolest myths using basic scientific ideas. However, the show is only an hour long and they don't have time to go over all the awesome science. But guess what? That's why you have me. Let me show you some of the best scientific ideas from the "Flights of Fantasy" episode in which Adam and Jamie looked at the U2 spy plane and investigated the dangers of drones.
As Adam and Jamie are training to ride on the U2, they get to practice landing with a parachute—you know, just in case they have to jump out of the aircraft. The proper technique has the jumper land with feet on the ground and then continue to fall over and roll during impact. This is called a "parachute landing fall". But what's the point?
The problem with falling off some height is that landing with high accelerations damage (and can even kill) a human. Let's define the vertical acceleration as the change in velocity divided by the time interval over which this velocity changes.
After opening the parachute, a jumper essentially moves with a constant downward velocity. The goal during the landing is to change this downward velocity to a zero velocity. Here you can see that if the time interval is small, you can get a large acceleration. The goal of the parachute landing fall is to increase the time over which the jumper interacts with the ground thus decreasing the acceleration and reducing the probability of sustaining an injury.
Another part of the U2 training was a low pressure test. Adam and Jamie put on the U2 spacesuit and then sat in a room with a reduced pressure to simulate a 70,000 foot elevation. One of the things in this room was a glass of water. At some point, the water started to boil—at room temperature. Yes, that's awesome.
But how can water boil at temperatures below 100°C? Let's start with an example of a glass of water. In this glass, there are molecules of water in the liquid phase—but there are also water molecules in the gas phase (called water vapor) just above the glass. Some of the liquid water molecules on the surface of the liquid have enough energy to escape into the gas phase. Below the surface of the water, the molecule energy would have to be great enough to produce a vapor pressure equal to the ambient pressure. This means that at the boiling point, you can get bubbles of water vapor in the liquid—the tiny bubbles you see in boiling water.
So, there are two things that can make liquid water boil. First, you can increase the temperature of the water (and thus increase the average energy of the water molecules). Second, you can decrease the ambient pressure so that lower energy molecules can form bubbles. In the case of the simulated 70,000 foot, the decrease in pressure is enough to cause boiling.
First, let's start with a practical example. When you are riding in a car, you can stick your hand out the window. If you tilt your hand a little bit, you can feel the air pushing up on your hand. This is essentially the same thing that happens with a flying wing. A tilted wing crashes into air molecules and pushes them down with some force. Since forces are an interaction between two objects, the air must also push back on the wing. We call this force the air exerts on the wing "lift".
In order for this plane to fly, the lift would have to be equal to the weight of the aircraft. That might seem like an overly simple model for flight—because it is. However, it's good enough for us to look at the important factors that can change the lift. You could have greater lift by:
- Moving the wing at a greater speed through the air.
- Increasing the wing angle.
- Increasing the wing air (you hit more air).
- Increasing the density of air.
When the U2 flies at very high altitudes, the density of air becomes very small and would decrease the lift. Clearly, you would need to compensate for this decreased air density by changing some other parameters. Increase the wing angle isn't one that you would want to change. Yes, this could produce more lift but it also produces more drag (air friction). That means you should probably just increase the wing speed and the wing area. Boom. That's exactly what the U2 spy plane does. That's why it has such large wings.
The sky is blue because it scatters blue light. Well, air is mostly transparent. Most of the light passes through the air, but some light interacts with the air and is scattered. This air tends to scatter shorter wavelengths of light more than longer wavelengths. Blue and violet have shorter wavelengths than red and green light and thus are scattered more.
But why does it get dark when you get to higher altitudes? Well, at higher altitudes there is less air. Less air means less scattering of blue light so you don't see the blue. And yet the Sun is still shining and the background looks black. This is because it's black in outer space. This is just like being on the moon during "day time"—black sky. But why is it black? Minute Physics has a great answer to this question.
Humans like to be in air that is similar to the air on the surface of the Earth. The pressure of air on the surface has a value of about 1 atmosphere (yes, that's a unit of pressure). So a human in a lower pressure environment needs to wear a space suit that keeps the body at around 1 atmosphere of pressure.
If you take a space suit that has a higher internal pressure than the outside pressure, it's difficult to bend your arms and stuff. Suppose I have a straight arm with a pressurized suit and I move my hand up.
A bent arm has a lower internal volume than a straight arm. This decrease in volume means an increase in internal pressure which requires effort on the part of the human. So, moving around in a pressurized suit can get tiresome—just like on the moon.
A hovering drone is a lot like a flying airplane wing. The difference is that the drone has small wings that move in circles instead of flying forward. Using the same lift principle as the wing, we could say that a drone hovers by "throwing" air down. Pushing down on the air means the air pushes back up on the drone. If this upwards lift is equal to the downward gravitational pull, it hovers.
How do you get more lift? You could throw the air down at a faster speed, or you could throw down more air. It turns out that it requires less power to throw down more air at a slower speed than just a little bit of air at a large speed. This is why you want bigger rotors for your aircraft and why a human powered helicopter has GIANT rotors.
So, if you just had one small rotor the drone would require a larger battery or it wouldn't be able to fly for a very long time. But why not just one big rotor? With multiple small rotors (usually between 4 and 8), the drone can adjust the power on different sides to adjust the orientation of the aircraft. This can make it both more stable and more maneuverable. Of course if a human was in control of these multiple rotors, it might be difficult to pilot—this is why the drones have on board computers to assist in the distribution of power.
So there you have it. There are six scientific ideas you can see from just one episode of MythBusters. But is there science just in this show? Of course not, scientific examples can be found everywhere—MythBusters just happens to be an entertaining way to see some cool examples.
