I've taken many flights and have some understanding of aerodynamics, but why does it still feel incredible to see a hundred-ton aircraft take to the sky?

Understanding A380 Airflow and Engine Impact on Airports

When people discuss engine airflow, I use the A380’s wind diagram as an example to help everyone understand. I also take this opportunity to explain why the world’s largest planes choose smaller airports in China for landing.

For easier understanding, I will describe everything in layman’s terms without using technical jargon like “flight zone.”

Idle State Wind Map:

• Red zone: Wind speed over 46 meters/second, equivalent to a category 15 typhoon (rarely occurs on land, extremely destructive)
• Orange zone: Wind speed over 30 meters/second, roughly equivalent to a category 11 typhoon (rare on land, causes severe damage)
• Yellow zone: Wind speed over 15 meters/second, roughly equivalent to a category 7 typhoon (minor tree damage, substantial resistance when walking)

Rolls-Royce Trent 900:

• Red zone extends nearly 30 meters (about 98 feet)
• Orange zone extends nearly 40 meters (about 131 feet)
• Yellow zone extends over 60 meters (about 197 feet) and can affect the ground.

General Electric GP7200:

• Red zone extends to about 18 meters (59 feet)
• Orange zone extends to about 40 meters (131 feet)
• Yellow zone extends to about 90 meters (295 feet)

And notably, the yellow zone always covers the ground.

Key point to discuss later

Idle State Temperature Map:

Rolls-Royce Trent9000:

• Red zone above 60 degrees Celsius
• Orange zone above 50 degrees Celsius
• Yellow zone above 40 degrees Celsius

General Electric GP7200:

• Red zone above 200 degrees Celsius
• Orange zone above 100 degrees Celsius
• Yellow zone above 50 degrees Celsius

Takeoff State Wind Map:

Rolls-Royce Trent900

• Red zone extends about 40 meters (131 feet)
• Orange zone extends about 60 meters (197 feet)
• Yellow zone extends about 110 meters (361 feet)

General Electric GP7200 Wind Map:

• Red zone extends over 40 meters (131 feet)
• Orange zone extends about 80 meters (262 feet)
• Yellow zone extends about 135 meters (443 feet)

Takeoff State Temperature Map:

Rolls-Royce Trent900:

• Red zone above 60 degrees Celsius
• Orange zone above 50 degrees Celsius
• Yellow zone above 40 degrees Celsius

General Electric GP7200:

• Red zone above 200 degrees Celsius
• Orange zone above 100 degrees Celsius
• Yellow zone above 50 degrees Celsius

Discussion Point:

How wide does a runway need to be for different aircraft?

Runway classifications mainly include PCN (Pavement Classification Number) and RCL (Runway Classification Letter). When most people refer to 4C, 4E, 4F, they’re talking about RCL.

We know that runways capable of accommodating an A380, a class of aircraft, are called 4F and are 60 meters wide.

Why?

What happens if the runway is too narrow?

• Is it because the aircraft’s wings extend beyond the runway?
• A380 has a wingspan of 80 meters, which seems too wide for a 60-meter runway.
• Boeing 747, in its efforts to reduce fuel consumption, has repeatedly increased its wingtips, making the span from 59.6 meters in the 747-100 to 68.5 meters in the 747-8.

In reality, there’s a wide safety margin outside the runway, so the wings extending doesn’t cause issues.

Are the wheels running off the runway?

• The taxiway is much narrower than the runway, but aircraft taxi without problems.
• The footprint of an A380 isn’t much larger than that of a 747.

Why is such a wide runway needed?

• As previously discussed, the wind map shows that the engine’s exhaust and the wing’s airflow can kick up debris.
• If the runway is too narrow, it can blow debris onto the runway, which can then be sucked into engines of following aircraft.

A380’s wind zone is up to 60 meters wide

Thus, runways accommodating A380s need to be 60 meters wide to ensure the debris doesn’t end up on the runway.

Clarification on Engine Airflows:

• Inner engine jets are affected more by the ground due to their proximity, extending their influence.
• Outer engines have different shapes and interactions with the ground, thus differing characteristics.

The Special Case of the Antonov An-225:

The An-225 Mriya, known for carrying the Buran space shuttle, is often misunderstood to be solely for transporting the shuttle.

Origin of An-225:

• The Soviet Buran shuttle could be carried by smaller aircraft like the An-124 or even the Il-76.
• The An-225 was developed not just for the shuttle but for a more ambitious aerospace system, capable of carrying large rockets for a space program.

Changes made to the An-225:

• Switched from a single tail to a twin tail to accommodate the large rockets.
• Extended the fuselage and wings for increased lift and added engines for more power.

Why An-225 chose Zhengding Airport over others?

Despite being closer to the Tangshan manufacturers, the An-225 didn’t choose busier airports like Beijing Capital due to its wide engine airflow and the need for a less busy, accommodating airport with the right PCN values.

The Legacy of An-225:

The An-225 has a unique place in aviation history, serving as a symbol of super-heavy lift and transport capabilities, though its usage is specific and less frequent due to its size and maintenance needs.

The Feeling of Flight

You may think that flying is impossible because you can’t fly and you don’t have that kind of energy. But maybe I can make you feel a bit differently.

Have you ever experienced standing in strong winds that nearly knocked you off balance? In such moments, you instinctively tighten your body, right? If you were to go against the wind, spreading your arms wide, you’d probably get blown over instantly.

But what if you face the wind, lean into it, and spread your arms? Would you feel like you’re flying?

Of course, if you’re very heavy, the wind won’t budge you.

However, do you know how powerful airplane engines are? The airflow they generate can easily lift a small car off the ground. A commercial airliner weighing hundreds of tons typically has at least two such engines, sometimes three, four, or even six.

Once, I stood behind a wire fence at an airport, about fifty meters away. A commercial plane turned around with its tail facing me. In that instant, I felt a terror greater than a Category 12 typhoon, and I turned and ran. The plane wasn’t even in the takeoff mode yet; otherwise, I might not have made it out alive.

Even so, airplanes don’t take off instantly. They need to taxi on the ground for a kilometer or two, gradually gaining speed until they reach a speed similar to a speeding car… You can try opening the window of a fast-moving car on the highway and sticking your hand out (not recommended for women and minors) to feel the wind.

Airplane wings are over ten meters long and one or two meters wide. If you were to put them on a car and expose them to such strong winds, the car would lift off the ground.

Now, imagine yourself as a 100-ton giant with arms over ten to twenty meters long and wings like a bat, each equipped with an engine capable of flipping a bus. Under your belly, there are wheels, very smooth ones, even smoother than ice skating.

Alright, ignite the engines! A tremendous supersonic airflow bursts out with a sound like thunder, and your friends and family bidding you farewell behind you instantly scatter like petals… I won’t describe the gruesome details here.

Your massive body starts to move, faster and faster, eventually racing like a sports car (actually, it doesn’t need to be that fast).

The wind roars in your ears, but fortunately, you’re streamlined and made of metal, or else your face would be distorted beyond recognition. Your lips are forced wide open, and no matter how hard you try, you can’t close them. You also can’t have nostrils, as the wind would rush in and damage your lungs and chest.

You slightly tilt your wings to catch the wind, and you’ll start to glide… At this point, you can retract your wheels.

The Limitations of Human Perception and the Role of Science

As far as our current knowledge goes,

Science is absolutely rational, whereas human cognition is not.

Human cognition originates from our sensory organs, and the way humans perceive the world is limited.

When you think that an airplane cannot take off, it is based on your individual perception of the world.

In other words, your consciousness tells you, “I am so small that I cannot fly; it’s so large that it surely cannot.”

The limitations of individual perception lead to limitations in individual subconscious cognition.

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I don’t know why this response suddenly became popular.

In fact, there is quite a bit of counterintuitive scientific knowledge.

The laws of classical physics are well understood by humans.

In a recent lecture by an academician, he mentioned that there are still many unresolved issues in fluid dynamics, but that doesn’t prevent us from using it.

Moreover, our country is indeed in a leading position in the application of fluid dynamics, so our fighter jets can compensate for a significant portion of engine capability with excellent aerodynamic designs.

I believe that knowledge that can be applied is good knowledge.

Understanding without the ability to apply it is not as useful as using it in a more intuitive way.

Because You’re Too Far from the Engine

If you were standing right next to a fully operating engine, you would be even more amazed: such a powerful piece of machinery can only lift such a small airplane into the sky.

Of course, you would never have such an opportunity.

By the way, most airplanes are actually not even close to hundreds of tons. The most common 737-800 has a empty weight of only 41 tons, lighter than some heavy main battle tanks. Considering the massive size of airplanes, their density is incredibly low. You should be more amazed that airplanes don’t get blown away on the tarmac by the wind.

Understanding Aerodynamics and Aircraft Density

Since you have an understanding of aerodynamics, I won’t embarrass myself as someone who has only studied basic fluid mechanics. You can understand the effect of compressing fluid at the right angle by throwing a piece of metal at a water float. Let’s do some arithmetic here.

The Boeing 747 has a fuselage diameter of 6.5 meters, a length of 76 meters. Taking into account the narrowing of the nose, tail, and wings, we can roughly consider it as a cylinder with a radius of 3.25 meters and a length of 75 meters — a volume of 2500 cubic meters.

The maximum takeoff weight of the Boeing 747 is 400 tons, while its landing weight is just over 200 tons.

So, the Boeing 747’s takeoff density is around 0.15, and the landing density is 0.09, with an average of about 0.12.

What does a density of 0.12 mean?

It means that the weight of a heavy commercial airliner is only two-thirds of that of foam plastic packaging of the same volume. Don’t be deceived by the size of the aircraft; in terms of density, it’s closer to a decorative balloon or a plastic inflatable toy that kids toss around.

A kite can soar hundreds of meters with just a single string pulling it. With the thrust of several powerful engines and aerodynamic design, why would there be any doubt that an enclosed space with a density of 0.1-0.2 can reach the sky?

What’s worth pondering isn’t that the aircraft is so heavy, but why more than 100 tons of aluminum can support a structure with thousands of cubic meters of enclosed space and withstand hurricanes and the impact of takeoffs and landings.

Before the United States embarked on large-scale space launches, NASA’s “Super Peacock Fish” often wobbled in the sky. Everyone who saw it thought it was a cartoon character in the wrong scene. But if you do a quick calculation, your seat on a commercial airliner isn’t that different.

Aerial Perspective

As you hold a drone for aerial photography, even a tiny ant underground can’t help but marvel at the wonder of something weighing just over a hundred grams taking flight.

Is a hundred tons substantial? Relative to the vast expanse of the atmosphere, it doesn’t even register, not even for an ant.

Understanding the Power of Wind

When I was a child, I didn’t really comprehend it either.

It wasn’t until one year during a typhoon that I went out to buy some things and ended up being blown over while riding my bike.

Getting off and walking was just as challenging. It felt like climbing a mountain, pushing against the wind at an angle. If you weren’t careful, you’d find yourself involuntarily running back.

If it weren’t for the typhoon, I would have never realized how powerful the wind could be.

Later, I saw some footage of things that you could never imagine taking flight under normal circumstances, but they were lifted by tornadoes.

Some very large and heavy objects, which would typically require machinery to move, were toppled by a strong gust of wind.

And the most direct example is the airplane’s engines, but most people can’t experience that kind of power firsthand.

Your misconception actually stems from the fact that airplane photos and videos often make it appear slow and tranquil. The engines, unlike flapping bird wings, don’t give you the impression that they are working very hard.

Psychological Sensations

I may not understand aerodynamics, and I may not understand psychology, but I’ve had experiences that are somewhat similar in terms of psychological sensations:

For example, despite having been through numerous kidney surgeries, every time I touch the physical organ during an open kidney surgery or after a laparoscopic kidney surgery…

I find it incredible that I can hold in my hand the actual kidney of another person, which theoretically should be entirely foreign to me.

The Power of Aerodynamics

I might not understand aerodynamics or psychology, but I’ve had experiences that somewhat relate:

You’ve probably seen those bamboo dragonfly toys. Playing with them, you can feel the force of the air. The air around us is actually quite thick; it’s not empty. We need to take the time to understand this dense air, much like we understand the force of water.

The usual way to play with them is to push hard upwards, trying to make your bamboo dragonfly fly higher than others. It’s somewhat similar to the way we used to make paper airplanes. However, if you change your approach, you can feel the power of the air. A gentle push upward, a light flick, and the bamboo dragonfly still takes flight. This force doesn’t need to be very strong, sometimes even gentler than you’d imagine. A slight flick, and it’s off the ground.

Later, I conducted an experiment. I hung three oranges evenly under the bamboo dragonfly and gave it a good flick. At the moment of takeoff, you could see that those heavy oranges were actually lifted into the air.

The air around us is thick, which allows it to easily transmit substantial forces. Because it’s thick, when you apply a force, it readily exerts an equal and opposite force, just like when you strike a stone or swim. When we were kids, we used to wonder how a large centipede kite could just take off with a few tugs.

Consider rodents like mice or Teddy dogs, which walk on the ground every day. Yet, birds of prey like eagles and geese, which are heavier than them, can soar in the sky. The key lies in how they know to utilize the sticky air to generate upward lift. It’s about how they shape their wings to manipulate this thick air to create differences in pressure, thus generating lift and taking flight. It’s similar to how fish can change buoyancy by altering the volume of their swim bladders in the water.

In the end, we find it astonishing, but birds and fish don’t. Because we are two-dimensional beings, we don’t understand how to move in the dimension of height. We have to stay close to the ground to gain support. We are amazed by things we can’t do.

The most impressive are insects like butterflies. They can experience two-dimensional and three-dimensional spatial activities throughout their lives.

When you’re driving across a big bridge, and it’s windy, you may notice that the gas pedal, brakes, and steering wheel become less effective. That’s because your car has taken off. Especially when crossing long bridges over seas or rivers, strong crosswinds can essentially turn your car into an aircraft, without a rudder. Two-dimensional cars don’t know how to utilize the sticky air to provide the necessary opposing force in three-dimensional space, which makes it quite dangerous.

Furthermore, it’s important to note that when driving on highways, you should never transport wide boards or objects hanging outside your vehicle, as they might become wings, disturbing the sticky air and causing you to take off.

Many people emphasize power and money, believing that with enough power and money, anything can fly. I believe that power and money are not the most important factors. If you equip a Teddy bear with a BMW engine, it won’t be able to fly, no matter how powerful the engine is. The key is aerodynamics. A paper airplane can fly without any cost, while a crumpled piece of paper, no matter how powerful the engine, can’t glide effectively.

A wildcat and an eagle have similar body weights, as do a mouse and a flying bird, but the wildcat and mouse can’t fly no matter how much power or money you give them. In contrast, the eagle and sparrow can soar freely.

It all comes down to aerodynamics and how to harness the power of this dense air.

Speaking of “Lida Zhuanfei” a humorous phrase often used to describe the phenomenon of heavy objects flying in the wind, it is undeniably true, but the question is how they fly. Let’s break it down.

In a zero-gravity vacuum environment: Any force, no matter how small, will make an object fly.

In a gravity vacuum environment: The force must exceed gravity for an object like a brick to fly in the opposite direction of gravity. However, once it takes off, it cannot change direction because it cannot generate reactive forces in a vacuum. (Details aside, methods like radiation pressure or directional particles can be used for changing direction, but that’s a different discussion. This isn’t about airplanes; it’s about spacecraft.)

In an environment with both gravity and air: The force must overcome gravity for an object to take off. However, this force doesn’t have to be entirely generated internally; it can also rely on directional manipulation of the dense air to create lift—once airborne, it can change direction freely. This is undoubtedly achieved through directed manipulation of the dense air to gain directional reactive forces, and this is what makes an object capable of flight. Clearly, this is what an airplane does.

In Wenzhou, Zhejiang, if leather shoes get wet, they won’t gain weight.

Imagination Matters

This should be a matter of imagination.

If Zhuangzi were to time travel to the present day and saw an airplane flying in the sky, as a person from thousands of years ago, he probably wouldn’t be too amazed:

In the Northern Ocean, there’s a fish, the name is Kun. Kun is so immense, I don’t know how many thousands of miles it spans. It transforms into a bird called Peng. Peng’s back, I don’t know how many thousands of miles it stretches. When Peng becomes angry and takes flight, its wings extend like clouds hanging down from the sky. It’s a bird, and when it wants to migrate, it will journey to the Southern Ocean.

…Now, you can see that water doesn’t need to be very deep to support a large boat; a spoonful of water in a depression can carry a mustard seed. When water is shallow and the boat is large, it will stick. Wind doesn’t need to accumulate much force to lift a large wing; after covering a distance of 90,000 miles, the wind is beneath the wing. This is how it gathers strength. When it carries the blue sky on its back and doesn’t stop its upward journey, there’s nothing to obstruct it. This is how it proceeds towards the South.

“Wind doesn’t need to accumulate much force to lift a large wing; after covering a distance of 90,000 miles, the wind is beneath the wing.” While Zhuangzi definitely didn’t study aerodynamics, at least he could infer from his own life experiences that the thicker the “accumulation of wind,” the larger the things it can carry. If the wind could become “thick” to a certain degree, even a creature as long as several thousand miles like the “Peng” could freely soar in the sky.

Of course, in reality, there’s no such thing as such a “thick” wind, so airplanes have to create “thick” wind by using engine power and relative velocity. For vertical take-off and landing aircraft, as long as the thrust is sufficient, taking off from a standstill isn’t anything unusual.

If Zhuangzi saw modern airplanes, even the Antonov An-225 Mriya would probably make him say, “Why is it so small? Not big enough!”

In a sense, if one finds it unbelievable that an airplane can take off, then when faced with the idea of putting the entire Earth on an engine, propelling a 600 trillion-ton celestial body out of the solar system’s orbit, and wandering through space for thousands of years, one can only imagine how jaw-dropping that would be.

The Key Factor: Speed

The main reason probably lies in the fact that you don’t feel the speed of the airplane, after all, there isn’t much of a reference point in the sky.

Lift Formula , where the lift coefficient C is related to the wing’s shape. After decades of development in aerospace engineering, wing shapes have been largely determined, and there isn’t much room for improvement in the coefficient C (which is also related to the angle of attack). Wing area is also straightforward. What’s left is the most crucial factor: speed V, and it’s a squared relationship. This is why stalling is a very serious aviation incident - without enough speed, there won’t be enough lift, and the aircraft’s altitude will continuously decrease.

Shared Sentiments

I totally agree, totally!

I’m a pilot myself, and I’ve always felt that human-made airplanes are just not scientifically sound.

Helicopters, relying on those few rotor blades to lift something as heavy as several tons.

Passenger planes are somewhat better, after all, they have a larger wing area.

But fighter jets, they’re all just not scientifically sound. Those big chunks of metal getting off the ground with such small wing surfaces, it’s hard to believe.

The Dominance of Intuition

For some people, intuition always takes the lead. Even when they have acquired sufficient expertise, they still instinctively rely on intuition. The knowledge they acquire remains just that - knowledge to be used for exams and work, with little relevance to everyday life, and it cannot be transformed into true understanding.

For instance, someone with a background in optoelectronics, despite their professional knowledge, still refers to a laser pointer as an infrared one. Despite numerous corrections, they continue to make the same mistake, much like those who fall through the cracks in education.

It wasn’t until I said, “Do you really want an optics layman who doesn’t even grasp refraction to correct your professional terminology?” that they were finally moved to change.

Beyond Aerodynamics, Let’s Talk Incredibility

Without delving into the principles of aerodynamics, let’s discuss the incredible aspect of this.

The original poster might not be a fan of running, lacking the visceral sensation of their physical body in rapid motion.

During a runner’s high, the feet aren’t simply passively pulled down to the ground by gravity; instead, they momentarily skim over the surface like skipping stones. The sensation at that moment is akin to flying just above the ground.

Understanding the Role of Fluid Dynamics and Air Mass

In my opinion, this feeling might stem from not being familiar enough with the parameters of fluid dynamics and air mass (weight).

If you calculate the weight of air per cubic meter and then consider the aircraft’s surface area and speed, you can understand why such massive aircraft can fly.

Air weighs approximately 1.29 kilograms per cubic meter. Aircraft speed is relative to the speed of the air. By multiplying the aircraft’s speed by its surface area and then by the weight of the air, you can comprehend why large aircraft can take off.

In fact, most people tend to overlook the power of air in their daily lives because air is something they are accustomed to, and it’s invisible.

As shown in the table above, when the wind reaches 30 meters per second, an 8-meter diameter rotor blade passes through about 1942 kilograms of air per second within the rotor’s area. Nearly 2 tons of air per second, and that’s just for an 8-meter diameter rotor blade.

So, when you don’t have access to relevant data, your intuition may not be accurate enough.

Quite Normal

It’s quite normal.

Before a Soviet MiG-25, a product of Soviet science and engineering, defected to the United States, even American experts who were particularly knowledgeable about aerodynamics found it incredible.

Evolution of the Thought World

The development of the world of thoughts is characterized by a continuous escape from wonderment. When I was a child and rode on trains, I used to worry about train collisions. Back then, I couldn’t fathom how trains could travel thousands of miles on the same tracks without colliding. It seemed unimaginable. It was only when I grew up that I realized how silly that idea was.

Living in the atmosphere all our lives, it’s challenging to feel the impact of the air. When you’re on a high-speed road, you can stick your hand out of the car window and feel the power of the wind. Alternatively, you can calculate the impact of air resistance on fuel consumption (more than eighty percent of fuel consumption is used to overcome air resistance). You can even evacuate the air from a glass tube and observe feathers and iron balls falling simultaneously. It’s at moments like these that you can truly appreciate the influence of the air. The air is not empty at all.

In fact, even a vacuum isn’t empty. In the language of quantum physics, a vacuum is filled with energy. When I first started in this field, I used to doubt how basic particles could possibly be observed by such large and cumbersome detectors. However, when I went to the laboratory and witnessed the detector responses, along with the consistency between the data and simulations, I came to understand how it all worked.

This world is truly miraculous. It’s not that intuition is inaccurate; rather, it’s that we need to adjust our intuition. Only when we comprehend something can we truly appreciate its depth.

The Power of Air

The reason you don’t feel the presence of air is that this sensation is something you and, in fact, the almost non-existent entity known as “air” in China share. Air is often associated with emptiness and nothingness, and yet, it undeniably exists, hence the name “air” (空气) for air in Chinese.

In reality, air can be quite viscous (if you’re fast enough). If you have the opportunity to reach out while on certain fast-moving modes of transportation (even just a bicycle would do), you can feel the power of the wind.

The Astonishing Flight of Aircraft

You find it incredible that airplanes can take off, but it’s actually because of their thrust-to-weight ratio being less than 1.

The thrust-to-weight ratio represents the ratio of the engine’s thrust to the aircraft’s weight.

For rockets, this ratio is greater than 1, making it easy to understand how they can take off since their thrust can overcome gravity.

But for airplanes, their thrust-to-weight ratio is less than 1. For example, the Airbus A320 has a thrust-to-weight ratio of about 0.3.

So, why can airplanes fly?

When an airplane takes off, its wings are not horizontal; the flaps are extended, and the air striking the wings generates force that can be broken down into lift and drag.

Diagram of Forces on an Airplane

Lift, Drag. Here, ρ represents air density, V is the airspeed, A is the wing area, and CL and CD are the coefficients of lift and drag, respectively. These coefficients are related to the wing’s shape and the angle of attack (AOA), which is the angle between the airplane’s plane and the horizontal plane (positive when the nose is up).

T-38 Lift Coefficient vs. Angle of Attack (Three Curves Corresponding to Three Flap States)

When the airplane is taxiing, the angle of attack is 0, but the wings are not completely horizontal, and the flaps are down, increasing the frontal area, increasing the angle of attack (equivalent to increasing the AOA), and increasing the gap, allowing high-speed air to flow over the wing’s upper surface, reducing the pressure on the upper wing surface. The slats are extended forward, allowing high-speed air to pass over the upper surface. These actions help increase lift.

When the airplane reaches a certain speed, the lift can exceed gravity, allowing the airplane to climb.

Flaps and slats configuration

During climb, the airplane’s angle of attack is greater than 0. Referring to the lift coefficient vs. angle of attack curve, we can find the angle of attack corresponding to the maximum lift coefficient. By maintaining this angle of attack, the airplane can achieve maximum lift. Beyond this angle of attack, the lift coefficient will decrease.

Air flowing underneath the wing: This is a simplified understanding of lift.

Going deeper, the Kutta–Joukowski theorem tells us that an object experiencing lift will have circulation, which is the lift per unit chord length:

In actual flight, there is high pressure below the wing’s leading edge, and low pressure above it, creating circulation and generating lift.

However, the exact mechanisms behind the formation of high and low-pressure areas are not yet clear and remain to be solved.

Collision of Intuition and Reason

After studying for several years and working for several years, I still find it incredible.

The collision of intuition and reason, it crackles and pops.