Last spring, my wife and I took a trip to Bali for our honeymoon. While the trip was an absolutely incredible, once-in-a-lifetime adventure, the journey to our destination, as I described previously, was not a walk in the park. After 24 hours in transit through 13 time zones, where daytime accelerated and nighttime shrouded the plane in darkness for the majority of the trip, paired with my inability to sleep on planes, I was starting to regret ever getting a passport in the first place.
Our vacation still many hours away, the only thing that kept me going on the plane was the screen in the headrest in front of me that featured a rotating set of maps and metrics about our flight in English and Arabic, in both English and metric units. In the middle of my insomniatic daze, however, I noticed something peculiar: the screen that told me how fast were going listed two different speeds! Looking closer, I noticed that we traveling at two different velocities at the same time: one was listed as airspeed and the other was listed as ground speed.
Despite my exhaustion, I still had enough sense to know that we didn’t suddenly enter some sort of time warp. At the time, I chalked it up to our elevation. If we had been driving, the arc we traveled around the Earth would simply lie on the Earth’s surface, and our distance traveled would be some portion of the Earth’s circumference. But since we were 39,000 feet in the air, we were traveling an arc on a bigger circle, which meant we were traveling a longer distance. To cover the extra distance and still get to our destination on time, we’d have to go faster. I thought that explained why our airspeed was faster than the ground speed. But is this actually what was going on? Nope. The plane takes elevation into account when calculating its speed. If it’s going 100 mph on the ground, then it’s going 100 mph in the air.
So, What is Airspeed?
As a a plane flies through the air, the airspeed is based on the direction and speed of the wind going past it. It relates to how much “lift” there is on the plane: the faster the wind is flowing across the wings of the plane (that is, front to back), the greater the force that pushes up on the plane. So when you’re trying to keep your plane in the air, it’s important to keep track of the airspeed.
Our airspeed was 27 mph greater than our ground speed, which meant that the wind was blowing at 27 mph in the plane’s face. And since air is composed of gas, and therefore distributes itself evenly in whatever space it is given, the airplane’s wings slicing through the atmosphere caused the air to flow in different ways around the wing that made the plane to go up.
In researching how lift works, I realized that no one really knows exactly how it happens, which is pretty incredible since, according to the Federal Aviation Administration, there are about 5,000 aircraft in the air at any given time. There are several competing theories out there; I had to go beyond normal resources on air travel and seek answers from NASA to figure out how they all fit together. They haven’t quite figured it out all either, but at least, they came up with a reasonable explanation. Here’s what I learned:
As a plane’s wings cut through the air, the air in front of the wings spits into two streams: one that rises and flows over the top, and one that flows along the bottom. This might seem obvious, but the way the air flows across the wing is not so obvious. Because of how plane wings are shaped, the air that flows across the top of the wing turns upward and curves around the top, while the air on the bottom stays pretty level, or sometimes turns downward (see figure to the right). As the air on top makes it around the curve and into the open air, it breaks free and expands, reducing the pressure on the wing. Meanwhile, the air on the bottom gets crowded along the downward-sloping bottom of the wing, increasing the air pressure below the wing. At this point, the air pressure below the wing is greater than above, generating lift.
At the same time, since the air above the wing is flowing with less pressure, it also moves faster. (A famous physicist named Bernoulli thought of that a couple hundred years before airplanes). Since the air above is moving faster, it pushes it downward on the air it meets on the other side of the wing.
Think of when you’re in the grocery store and you’re pushing a cart with a bad wheel. The healthy wheel wants to keep going, but it’s being held behind by the slow wheel. To keep going at a constant speed, its only option is to run circles around the slow wheel. The same thing happens in the air above the wing: it’s running a circle around the slow air, pushing it downward.
This is where another famous scientist comes in — Sir Isaac Newton. Sir Newton predicted in his third law of motion that every action has an equal and opposite reaction. Another scientist before his time, his principles also help an airplane stay airborne. That downward airflow I was talking about produces a downward force, which is met with an equal and opposite upward force on the wings, another form of lift.
Why does Airspeed Matter?
As I mentioned previously, the strength of this lift depends on how fast the air is flow, and the speed of the air is partially depends on wind outside. If there’s a strong headwind, that is, if the wind is blowing against the plane, the air is going to flow over the wing faster, giving the plane more lift.
Air traffic controllers pay attention to this when planes are set for takeoff. They want their planes to take off with as little runway distance as possible, so they try to arrange for planes to take off directly against the wind. This maximizes lift, minimizing the amount of runway needed.
When a pilot is in the air, their airspeed, as compared to their ground speed, tells them how much lift they have on their wings, allowing them to keep the plane at a constant altitude.
I don’t know about you, but the next time I am on a plane and see that I’m traveling at two different speeds at the same time, I’m no longer going to worry that we’re flying into a different dimension, but rather that the pilot is seeing what I’m seeing and using this information to keep the plane in the air.