This module will cover basic principles from physics that affect flight. Although you do not have to be a scientist by the end of the lesson or course, you will need a conceptual understanding of how flight is possible. The background science will help you understand why some events or problems pursue a specific path forward in history, and to aid in understanding the evolution of technologies we will discuss in this course.
After reviewing this module, you will be able to describe and explain the following concepts:
general characteristics of Earth's atmosphere, including how air pressure and oxygen levels change when altitude increases
the flow of gasses around curved surfaces like a wing
the four main forces that affect flight: thrust, drag, lift, and gravity (also referred to as weight)
The first thing to understand is the atmosphere through which aircraft fly. The atmosphere above the Earth is held in place by Earth's gravity and consists of a variety of gasses along with water vapor and other things like molecules of pollutants (e.g. Carbon dioxide) or particles of dust. The atmosphere is most dense closest to Earth. This density helps us breathe (yay, plenty of oxygen!) but this density is also what creates air pressure to help lift aircraft into the sky.
Air pressure decreases as you go up in altitude, meaning there are less molecules and atoms of various gases the higher you go. You have experienced this if you have ever flown in an aircraft: your ears 'pop' as they try to equalize the pressure in your inner ear with the changing pressure around you at higher altitudes (or when you descend again). You also needed the cabin to be pressurized to make this change less dramatic on your body and to keep enough oxygen available for you to breathe. With fewer gas molecules in the less dense air of higher altitudes, it is possible for you to die without a separate oxygen supply. You do not have to go very high to notice this difference in available oxygen; travelers and especially athletes often talk about the difficulty of adjusting to less oxygen when training in mountains or even cities like Denver, which sits 1 mile (or 5,280 feet) above sea level. Sea level is considered "0" on the altitude scale, and the outer edges of the atmosphere where Space "begins" are generally recognized by scientists as being 100 km above the Earth (a little over 62 miles up, or just over 328,000 feet).
Without digressing too much into biology, the lack of oxygen at altitudes even in the ten- and twenty-thousand feet range was something that affected pilots flying in World Wars I and II as well as some pilots attempting to set altitude records (into the thirty-thousand feet range) in the 1930s. Early aircraft were not equipped with oxygen supplies (or even closed-cockpit designs, in some cases through the 1930s). Today, even with oxygen masks, this problem can still affect fighter pilots due to the extreme forces they encounter when turning and maneuvering in flight. Centrifugal force in these turns pushes the blood from the head and core of the body where the critical organs are located into the arms and legs. Without G-suits to counter that effect, fighter pilots can become unconscious from lack of oxygen to the brain.
Returning to our discussion of the atmosphere: air pressure, oxygen, and even temperature vary depending on the increase in altitude. Let's examine briefly the layers of the atmosphere as we ascend in altitude. Pay particular attention to the Troposphere and Stratosphere in the following resource, where most of the aircraft we will study operate (until we discuss space flight).
Visit the University Center for Science Education overview of the layers of the atmosphere page.
Now that you have a general sense of what the atmosphere looks like, including where weather and the jet stream occurs and where planes typically fly, let's look at those air molecules specifically and how the flow of air over a wing's surface can create lift and allow aircraft to fly.
You may be thinking, 'what does fluid have to do with flight?' In science, a fluid is generally anything that flows--so it can be a liquid, but it can also refer to a gas (or gasses, in the case of the atmosphere). In our class, we need to take a moment to talk about fluid dynamics, or how things like gasses and liquids flow around curved surfaces (like a wing) because that flow of air is precisely what creates a pressure difference on the top and bottom surfaces of the wing and creates lift.
Let's start with a NASA video that talks about flow, and something called Bernoulli's Principle. You can access it using the link or by clicking to start playing the embedded video below. https://www.youtube.com/embed/J4WRd7OAt0A
So let's take this concept one step further, and think about how the flow of air over a wing actually creates lift. We saw at the end of the video a discussion of the flow of air over a wing. The air flows faster over the curved surface of the top of a wing. Based on Bernoulli's Principle, we know that faster flow means lower pressure. The air pressure on the top of the wing is therefore lower than the air pressure of the slower-moving air underneath the wing. This difference is what creates lift, as the air pressure under the wing--along with the speed of the aircraft and other elements like the angle of the wing entering the air flow--are what help lift the aircraft into the sky and keep it aloft. Now let's look at all of these elements as they are coming together to create flight through looking at the four primary forces acting on an aircraft during flight.
The four primary forces acting on an aircraft in flight are thrust, drag, lift, and gravity. First, let's start with what "force" means in physics. A force is often called a push or a pull, meaning something is being given kinetic energy to move in a specific direction. But in its most generic sense, a force is any interaction that can move an object. We do need to keep in mind that the force may or may not be opposed by something else. In the case of the four forces we are examining for flight, two of the forces are each in opposition with one of the other forces. Thus lift is in opposition to gravity, and thrust is in opposition to drag. So let's look at this quick video about the four forces, then talk in a little more detail about the two sets of opposing forces.
By now we have a general understanding of lift as a force that is created on the wings of an aircraft by the flow of air over the wing. Lift counteracts gravity, which is a constant force here on Earth. You can create more lift by going faster, adding flaps on the back of an aircraft wing that increases the curvature of the wing, and by increasing the angle of attack. The angle of attack is the angle at which the wing meets the on-coming air. It turns out the optimal angle of attack is 6 degrees to create the most lift. If you continue to increase the angle of attack, there is a point at which air flowing over the top of the wing no longer follows the curve of the wing--this is when aircraft "stall" because there is insufficient lift.
The fuselage or body of an aircraft can also provide lift. In the case of the B2, something you may have seen fly overhead before, the virtually the entire body of the aircraft is a wing and contributes to lift. In the case of the Space Shuttle, the body of the spacecraft was designed to be wider both for payload transportation but also to create more lift for when the orbiter is returning to land (since it has a delta-shaped body with small wings). These are more exceptions than the rule for what we study in the course, but may help convey the idea that there are a variety of aircraft and wing designs based on the specific function of the aircraft. The basic concept remains the same, however; lift is the 'up force' counteracting gravity that allows the aircraft to fly.
Thrust is what propels the aircraft forward (or up, in the case of rockets). In our course, aircraft are generally generating thrust with a propeller engine, a turbojet engine, or a rocket engine. Thrust counteracts drag, which is resistance on the aircraft by the air through which it is passing. Remember the atmosphere is thinner at higher altitudes, so there is more drag at sea-level that there would be at 70,000 feet.
In this module we've worked through some basic science to talk about several important concepts. These concepts include:
The Earth has an atmosphere, which is fabulous if you enjoy breathing or flying, or both;
Most human activities take place in the Troposphere and Stratosphere;
Although temperature can increase or vary in the Stratosphere, generally speaking temperature, oxygen levels, and the density of the atmosphere decrease as you increase in altitude;
Bernoulli's Principle, as applied to gasses like those in the atmosphere, means that a faster flow of air over a surface creates a lower pressure, which for our course is important to understand as the source of lift;
There are four primary forces that affect an aircraft in flight: thrust, drag, lift, and gravity;
Thrust opposes the force of drag, and lift opposes the force of gravity.
With these conceptual tools, we are ready to talk about the history of manned flight! Feel free to refer back to this lesson as we talk about specific technologies or aircraft during the course.
This is not required, but a very cool look at the U2 and flying in the upper atmosphere can be seen in this Mythbusters episode. It can be purchased on iTunes, or a short clip about the show is also available on YouTube that shows some of the aerial shots.