Have you ever ridden in an open-top car
and felt the wind pushing past your face? It's exhilarating and you feel really
alive, but it's also surprising, because we don't normally feel
the air at all. Although we're surrounded by this mysterious gas, and life is impossible without it, we hardly
ever give it a moment's thought. Understanding how air behaves when
we slice through it at speed is incredibly important: without the
science of aerodynamics, as it's known, we'd never be able to
design planes or spacecraft, land-speed record cars, or
bridges that can survive hurricanes. So what exactly is aerodynamics? Let's take a
closer look!
Photo: A NASA wind-tunnel test of an experimental strut-braced wing
as part of a project to develop more environmentally friendly aircraft.
Photo by Dominic Hart and Lynn Albaugh courtesy of
NASA Ames Research Center.
One of the most obvious differences between solids, liquids, and gases
is their density: how many
atoms of "stuff" there are in a given space. Solids and liquids
are much more dense than gases—and you'll know this is if you've
ever tried walking through a swimming pool. Compared to walking
through the air, it's incredibly hard work to propel your body through
water. You literally have to push the water that's in front of you
out of the way; as you move forward, the water sloshes around you
into the space you've just left behind. It's much faster to swim
through water than to walk through it because you can make your body
into a long, thin shape that creates less resistance: you glide
through the water more smoothly, disturbing it less, and because
there's less resistance, you can move faster. (Find out
more in our article about the science of swimming.)
Moving through air is much the same. Like water, air is a
fluid (the name we give to liquids and gases that can easily
move, or flow) and, generally speaking, most fluids behave the same
way. If you want to speed quickly through the air, you're better off
in a long, thin vehicle—something like a plane or a train—that
creates as little disturbance as possible: planes and trains are
tube-shaped for exactly the same reason that we swim horizontally
with our bodies laid out long and thin.
Thinking about how to move through a fluid quickly and effectively is really what
aerodynamics is all about. If we want a more formal, scientific
definition, we can say that aerodynamics is the science of how things
move through air (or how air moves around things).
Photo: You can swim faster and for longer by ensuring your body
disturbs the water as little as possible as it moves along. You can't see air, but exactly the
same principle applies. Notice how the "bow waves" this swimmer creates stretch back from his body
in a similar way to the shock waves created by the airplane in the photo above. Like all waves,
these carry energy away from whatever creates them. To swim or fly efficiently, it pays
to create as little wave disturbance as possible. Photo by Joseph M. Clark
courtesy of US Navy and Wikimedia Commons.
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The science of aerodynamics
Aerodynamics is part of a branch of physics called fluid dynamics, which is all about studying
liquids and gases that are moving. Although it can involve very complex math, the basic
principles are relatively easy-to-understand; they include how
fluids flow in different ways, what causes drag (fluid resistance),
and how fluids conserve their volume and energy as they flow. Another
important idea is that when an object moves through a stationary
fluid, the science is pretty much the same as if the fluid moved and
the object were still. That's why it's possible to study the
aerodynamic performance of a car or an airplane in a wind tunnel:
blasting high-speed air around a still model of a plane or car is
the same as flying or driving through the air at the same speed.
Photo: Flying with less energy means flying more aerodynamically, and that means developing better plane and wing shapes. This is a NASA wind-tunnel test of an experimental plane design called the joined wing.
It's a type of completely closed wing that does away with wing tips, where disruptive, energy-wasting vortices occur.
Photo by Sean Smith courtesy of
NASA Langley.
Laminar and turbulent flow
When you empty water from a plastic bottle, you've probably noticed you can do it in two very different
ways. If you tip the bottle at a shallow angle, the water comes
out very smoothly; air moves past it, in the opposite direction,
filling the bottle with "emptiness." If you tip the bottle more,
or hold it vertically, the water comes out noisily, in jerks; that's
because the air and the water have to fight at the neck of the
bottle. Sometimes the water wins and rushes out, sometimes the air
wins and rushes in, briefly stopping the water flow. The fight between water exiting
and air entering gives you the characteristic "glug-glug" sound as you pour.
Photo: Pour water slowly from a bottle and you get smooth, laminar flow. Tip the bottle up more and
the flow will become turbulent. Also, can you see how the spout of water dripping down from this bottle gets narrower toward the
bottom where the water moves faster (after being accelerated by gravity)? That's an example of fluid continuity, which is
explained below.
What we see here are the two extreme types of fluid flow. In the first case, we have the
water and the air sliding very smoothly past one another in layers,
which is called laminar flow (or streamline flow
because the fluid flows in parallel lines called streamlines).
In the second case, the air and water move in a more erratic way,
which we called turbulent flow. If we're trying to design
something like a sports car, ideally we want to shape the body so the
flow of air around it is as smooth as possible—so it's laminar
rather than turbulent. The more turbulence there is, the more air
resistance the car will experience, the more energy it will waste, and the slower it will go.
Boundary layer
The speed at which a fluid flows past an object varies according to how far from the object you are. If
you're sitting in a parked car and a gale-force wind is howling past
you at 200km/h (125mph), you might think the difference in speed between the
air and the car is 200km/h—and it is! But there's not a sudden,
drastic discontinuity between the stationary car and the fast-moving air. Right
next to the car, the air speed is actually zero: the air sticks to
the car because there are attractive forces between the molecules of
the car's paintwork and the air molecules that touch them. The
further away from the car you get, the higher the wind speed. A
certain distance from the car, the air will be traveling at its full
speed of 200km/h. The region surrounding the car where the air speed
increases from zero to its maximum is known as the boundary layer.
We get laminar flow when the fluid can flow efficiently, gently and
smoothly increasing in speed across the boundary layer; we get
turbulent flown when this doesn't happen—when the fluid jumbles and
mixes up chaotically instead of sliding past itself in smooth layers.
Photo: Wind speed increases with distance from the ground. In theory, a wind turbine tower has to be high enough to ensure the rotors are operating outside the boundary layer. In practice, turbine designers have to compromise: very high turbines may be unacceptable for all kinds of environmental and safety reasons.
The idea of the boundary layer leads to all kinds of interesting things. It explains why, for example,
your car can be dusty and dirty even though it's racing through the
air at high speed. Although it's traveling fast, the air right next
to the paintwork isn't moving at all, so particles of dirt aren't
blown away as you might expect them to be. The same applies when you
try to blow the dust off a bookshelf. You can blow really hard, but
you'll never blow all the dust away: at best, you just blow the dust
(the upper layers of dust particles) off the dust (the lower layers
that stay stuck to the shelf)! The boundary layer concept also
explains why wind turbines have to be so high. The closer to the
ground you are, the lower the wind speed: at ground level, on
something like concrete, the wind speed is actually zero. Build a
wind turbine that's way up in the sky and you're (hopefully) reaching
beyond the boundary layer to the place where the air speed is a
maximum and the wind has higher kinetic energy to drive the turbine's
rotors.
Drag
Photo: The faster you go, the harder you have to work against the air. Air resistance doesn't really matter when you walk, because you're not traveling fast enough. But if you cycle, you need to think about aerodynamics much more and adopt a streamlined posture that disturbs the airflow as little as possible. Like this rider, you might want to get a tear-shaped cycling helmet and wear tight-fitting clothes?
Air resistance—drag, as it's usually known—follows on from the distinction between laminar and
turbulent flow. When a sports car speeds through the air, the flow
remains relatively laminar; when a truck plows through it, there's
much more turbulence. Drag is the force that a moving body feels when
the flow of air around it starts to become turbulent. If you ride a
bike or you've ever run a sprint race, it'll be very obvious to you
that drag increases with speed. But a very important point is that it
doesn't increase linearly as your speed increases but according to
the square of your speed. In other words, if you double your
speed, roughly speaking you quadruple the drag. Fast-moving vehicles
use most of their energy overcoming drag; once you reach about 300km/h (180mph),
you're using virtually all your energy trying to push the air out of the way.
This doesn't just apply to land-speed record cars but to ordinary drivers as well:
for stop-start city driving, you waste most of your energy in braking;
when you speed along on the highway, most of your energy is being lost pushing aside the air.
(To see the simple math behind this, take a look at David MacKay's discussion in his book Sustainable
Energy Without Hot Air.)
Photo: Top: Friction drag: The aerodynamic shape of this car allows the airflow around it to remain reasonably laminar. There is drag, but mainly caused by friction between the layers of air moving past one another at different speeds. Notice how the air becomes more turbulent behind the car and vortices start to occur in the wake. Bottom: Form drag: A box-shaped vehicle (such as a large truck) makes no attempt to divert air around itself. As soon as the air smacks into it, it starts to become turbulent. Photo by Eric James courtesy of NASA Ames Laboratory.
Why does drag happen? There are two types called friction drag and
form drag and they have different causes. Imagine a
car sitting still as the wind speeds past it. If the car is smoothly
shaped, the air next to its paintwork isn't moving at all. The layer
just beyond that is moving a little bit, and the layer beyond that is
moving a little bit more. All these layers of air are sliding past
one another in exactly the same way that your foot might slide across
the floor: they have to overcome the mutual attraction between one
another's molecules, which causes friction. Friction drag happens
because it takes energy to make layers of air slide past one another.
The rougher or more obstructive the object, the more turbulent the air flow becomes,
the greater the friction between the layers, and the greater the
drag. At low speeds, the air flows splits when it meets an object
and, providing the object is reasonably aerodynamic, flows right
around it, closely following its outline. But the faster the air flow
and the less aerodynamic the object, the more the air flow breaks
away and becomes turbulent. That's what we mean by form drag.
Photo: Minimize that drag! In 1981, engineers at NASA's Dryden Flight Research Center
experimented with streamlining this standard truck by bolting sheet metal sections to its outside.
Note the rounded corners and the "boat tail" rear end.
Photo courtesy of NASA
Supersonic!
The faster you go, the harder it is
go faster still—that's the tricky science that makes it so difficult
to break speed records in cars, boats, and planes. In theory, the
laws of fluid dynamics (of which aerodynamics is a part) apply in
much the same way, whether you're speeding over salt flats in a
rocket-propelled car, skimming over the waves in a hydrofoil boat, or
screaming through the air in a military jet. Planes, however, are in
a different category to cars and boats because they can go 5–10 times
faster. Once they hit a certain speed, the speed of sound, different
rules of aerodynamics come into play. Push your jet plane through the
sound barrier and huge, cone-shaped shock waves form at the nose and
the tail, where they can greatly increase drag. That's why
supersonic (faster-than-sound) jet planes
have sharp noses and sharp, swept-back wings. Go faster still and the
aerodynamic rules change once again. At hypersonic speeds (around five
times faster than sound), shorter wings work best and they need to
be positioned further back from the nose than on a supersonic plane.
Photo: Supersonic planes create shock waves and a "sonic boom" when they break the sound barrier.
Although we can hear the sonic boom, it takes a special kind of photo—a Schlieren photo like this—to
make the shock waves visible.
Photo courtesy of NASA.
Continuity
It might seem obvious, but if fluid's
flowing through or around an object, the amount of fluid you have at
the end is the same as the amount you have at the start. Write this
in math form and you get what's called the continuity equation.
More formally speaking, it says that the volume of fluid flowing in
one place is the same as the volume of fluid flowing in another
place. It follows from this that the area through which the fluid flows multiplied by the velocity
of the fluid is a constant: if a fluid flows into a narrower
space, it has to speed up; if it flows into a wider space, it has to
slow down. That helps to explain why wind really whistles down alleys
between buildings and why, if you pinch the end of a hosepipe, the
water squirts out faster. (It's also the reason that water being poured from a bottle, or falling from a faucet/tap, goes from
a wide stream at the top to a much narrower one—because it's speeding up due to gravity and the
release of pressure. You can see that clearly in the photo of water pouring up above.)
We can use the continuity equation to help understand two other very useful
bits of fluid dynamics: Bernoulli's principle and the Venturi effect.
Bernoulli's principle
Make yourself a rectangular tube of
paper, put it on a table, and blow through it. As you do so, the
paper will collapse down, then spring back up again when you run out
of breath.
Photo: Bernoulli on your tabletop: fold a sheet of paper into a tunnel shape and blow through it (top) and you'll see the paper squash down (bottom). The air speeds up inside the tunnel, the pressure goes down, and the paper collapses because atmospheric pressure pushes down on it from above.
What's happening? When a fluid flows from one place to
another, it has to conserve its energy. In other words, there has to
be as much energy at the end as there was at the start. We know this
from the fundamental law of physics called the conservation of
energy, which explains that you can't create or destroy energy, only
change it from one form into another. Think about the air flowing
through your homemade tube. The air just outside the tube, just where
you're blowing, has three types of energy: potential energy, kinetic
energy, and energy because of its pressure. The air in the middle of
the tube has the same three types of energy. However, because the air
is moving faster there, its kinetic energy must be greater. Since we
can't have created energy out of nothing, there must have been a
reduction in one of the other two types of energy. You're blowing
straight across a table so the air doesn't rise or fall—and doesn't
change its potential energy. The only place we can compensate for the
extra kinetic energy is in the fluid's pressure. As the air speeds
up, its pressure goes down. Since the air inside the tube is at a
lower pressure than the air above it, the tube collapses until you
stop blowing. Stated simply, Bernoulli's principle (pronounced
Bur-noo-ee's) simply reminds us that the total energy in a moving
fluid is constant. But you're likely to see it described a different
way: if a fluid speeds up, its pressure goes down (and vice-versa).
How wings really work
A lot of science books tell us that Bernoulli's principle is the key to understanding how airfoils (curved wings on airplanes, also known as aerofoils) generate lift.
The standard explanation goes like this. As air hits an airfoil, it splits into two streams, one of which shoots
over the wing as the other dives underneath. People used to think that a simple difference
in the speed of the two air streams caused the lift on the wing, but we now know this is wrong.
The argument went like this: the upper surface of an airfoil is curved, while
the lower surface is straight. We know from the continuity equation that there's as much air coming out
from behind an airfoil wing as there is going into it at the front.
So, theoretically, the air going above the stream has to go faster than
the air going underneath it, because it has to go further.
Bernoulli's principle tells us that fast-moving air is at a lower pressure than slower-moving air, so
there's less pressure above the airfoil, and this is what generates the lift (upward force) as it travels through the air.
Unfortunately, this turns out to be false, both experimentally and in theory.
With simple experiments, we can show that a plane can fly if its airfoils have identical upper and lower profiles (if they're symmetrical, in other words): a paper airplane with flat wings will fly perfectly well.
The theoretical explanation is also easy to understand: we're talking about two continuous streams
of air, one above and one below the airfoil, and there's absolutely no reason why two air molecules that separate at the front of an airfoil (one taking the upper route, one the lower) should neatly meet up again at the back, having traveled different distances in the same time; one molecule could easily take longer than the other and meet up with a different air molecule at the back. The real explanation of why airfoils create lift is
down to a combination of pressure differences and Newton's third law of motion. An airfoil wing generates lift because it's both curved and tilted back, so the oncoming air is accelerated over the top surface and then forced downward. This creates a region of low pressure directly above the wing, which generates lift. The wing's tilted angle forces the air downward, and that also pushes the plane upward (Newton's third law). Find our more in our article on airplanes.
The Venturi effect
Artwork: The Venturi effect: When a fluid speeds through a narrow pipe, its pressure drops.
Have you ever been on a canal barge as it sails upstream through calm water next to another,
similar boat? As the two boats
whizz along, they're very likely to drift and bump together. This is an
example of the Venturi effect, which follows on from the continuity
equation and Bernoulli's principle. The basic idea is that when a
fluid flows into a narrower space, it speeds up and the pressure
drops. So the speeding water between the two boats creates a low-pressure
zone between them that moves them together. This is one of the reasons
why wind farms are sometimes built in valleys
between hills or mountains, where the wind speed is higher. You can also
see the Venturi effect (and other aerodynamic principles such as Bernoulli's) at work
in carburetors and
in household fans like the Dyson Air Multiplier.
Why aerodynamics matters
Photo: A simple plastic fairing mounted on top of a truck's cab can save a huge amount of fuel. It works by diverting the airflow (yellow arrows) more smoothly above and around the sides of the huge, boxy cargo container behind the cab.
Why should we care about aerodynamics? Why does it matter? Suppose you run a haulage firm and you
have 500 trucks driving around the country delivering supplies to
supermarkets. Apart from the trucks themselves and the wages of the
drivers, the biggest cost your business faces is fuel. If you fit a
relatively inexpensive fairing (a sloped piece of plastic) to the top
of your trucks so the air is deflected smoothly up and over the cargo
container behind, you'll cut the fuel consumption by 10–20 percent
and save a huge amount of money. Fitting side-shields to the underside of the
cargo container (to stop turbulent airflow underneath them) will save more. The same goes for cars. Driving
around with a roof-rack in place when you're carrying nothing on it
will increase the fuel you use (and the amount you have to pay in
gasoline/petrol) by about five percent. Why? Because the rack drags in
the air and slows you down.
For airplanes and
space rockets, aerodynamics is even more important. When spacecraft return to Earth,
they pass from the virtual vacuum of space into Earth's atmosphere at
high speed, which heats them up dangerously; in February 2003, the Space
Shuttle Columbia was tragically destroyed, killing all seven astronauts onboard, when it overheated on reentry. Understanding better how air
moves over a spacecraft is essential if we want to avoid such things
happening in future.
Aerodynamics matters for the rest of us too. If you're a keen cyclist and you want to win a race, you need to use your energy as efficiently possible, losing as little to the air as you can. If you're a motorist who travels reasonably long distances on the freeway (motorway), minimizing air resistance is one of your best ways of saving fuel, saving money, and helping the planet.
A brief history of aerodynamics
Here's a quick tour through some of the major moments and key figures in the history of aerodynamics.
Science in motion
c250 BCE: Aristotle describes how objects float and move in fluids.
1490: Leonardo da Vinci considers the aerodynamics of flight and sketches the detailed anatomy of bird wings in his notebooks. He notes the importance of air resistance (drag) as a force that slows down moving objects and figures out the equation of continuity by watching rivers flow.
1600s: Isaac Newton studies air resistance, noting that it's much the same whether air moves around an object or an object moves through the air.
1673: French scientist Edme Mariotte shows that drag increases with the square of speed. Christiaan Huygens and Isaac Newton reach the same conclusion at about the same time.
1738: French scientist Daniel Bernoulli works out the connection between the speed of a fluid and its pressure.
Pioneers of aerodynamics
1840s: Englishman Sir George Cayley makes pioneering aerodynamic studies with model gliders and identifies the four forces of flight (thrust, drag, weight, and lift).
1852: German physicist Heinrich Magnus explains the Magnus effect, which explains why spinning soccer and tennis balls curve through the air.
1880s: Osborne Reynolds notes the difference between laminar and turbulent flow. A concept called the Reynolds number is used to describe and explain different kinds of fluid flow.
1880s: An Austrian physicist and philosopher named Ernst Mach takes pioneering
aerodynamic photographs showing disrupted air movements, including the shock waves produced when objects move through air at high speed.
1890s: Frederick Lanchester starts to study aerodynamics and figures out the circulation of air around airfoil wings.
The age of aerodynamics
Photo: Streamliner! In the early decades of the 20th century, engineers adopted the principles of aerodynamics and developed radically streamlined locomotives. This one is a preserved Class A4, the same design as Mallard, the locomotive that set a world speed record for steam locomotives of 203 km/h (126 mph) in 1938. Note how the normally tube-like boiler is hidden behind sleek, aerodynamic panels, and long, curved wheel arches guide the airflow smoothly past.
1903: After making their own detailed scientific studies of aerodynamics, the Wright brothers make the first powered flight.
1900s: German physicist Ludwig Prandtl derives the mathematical equations of airflow, figures out how drag occurs in the boundary layer, and effectively invents the modern science of aerodynamics.
1930s–1950s: The principles of aerodynamic
streamlining strongly influence the design of locomotives, automobiles, and other vehicles.
1930s–1960s: Hungarian Theodore von Kármán makes complex mathematical models of airflow and makes pioneering contributions to the science of supersonic and hypersonic flight, including the development of swept-back wings.
1934: Henri Coandă discovers what becomes known as the Coandă effect—that moving fluids bend toward nearby surfaces.
1947: Chuck Yeager makes the first supersonic flight.
1967: NASA and USAF's experimental, hypersonic X-15 plane sets a world record speed of 7274 km/h (4520 mph).
For another perspective, and later developments, see our article on the history of flight.
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Find out more
On this website
Airplanes (including airfoils/aerofoils): The basic theory of how planes stay in the air.
Forces and motion: Newton's laws of motion explain why and how things move around our world.
NASA Langley: Home of wind-tunnel testing and cutting-edge aerodynamic research.
Rod Cross: Physicist Rod Cross explains the science (and aerodynamics) behind various sports, including cricket, baseball, and tennis.
Books
Fundamentals of Aerodynamics by John D. Anderson. McGraw-Hill, 2016. Classic, comprehensive, readable textbook covering the theory, math, and history of flight.
Plastic Fairings Could Cut Truck Fuel Use by Saqib Rahim and ClimateWire. Scientific American, February 10, 2011. How plastic fairings fitted in front of a truck's container wheels can bring extra energy savings.
Edgy, Yet Still Aerodynamic by Phil Patton, The New York Times, 19 December 2008. Aerodynamic cars don't necessarily have to have smooth curves, wind tunnel tests have found.
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