Liu Feng丨Like humans, airplanes also get tired

In the experiment,

The fuselage and wing structures must be twice as long as their actual lifespan.

That is to say, to achieve three times the lifespan,

Only then can it be proved that the design of this aircraft is reasonable and safe.

Liu Feng, Professor, Civil Aviation Flight University of China

Gezhi Lundao Issue 30 | September 20, 2018, Sichuan

Hello everyone, my name is Liu Feng, and I’m from the Civil Aviation Flight University of China. I’m very happy to be here and share my aviation story with you.

For the Chinese, the dream of flying has actually existed since ancient China.

▲ Dunhuang mural "Flying Apsaras"

This is a Dunhuang mural - "Flying Apsaras". Dunhuang murals were created over dozens of dynasties from the Northern Dynasties to the Yuan Dynasty. From here we can see that our ancestors have always had the dream of flying.

▲ Wan Hu statue from the Ming Dynasty

In the Ming Dynasty, there was a man named Wan Hu who tied a rocket to a chair, hoping to fly into the sky with the help of the rocket's power. Of course, due to the technical conditions at the time, he was unable to achieve his goal, and he lost his life.

In 1903, the Wright brothers built the first powered, heavier-than-air aircraft and subsequently completed a sustained flight, marking the beginning of modern aviation history.

▲ The Wright brothers and their airplane

▲ Chinese aviator Feng Ru

What were the Chinese doing at the same time? Did anyone in China also conduct related research? In fact, there were. He was Feng Ru, a Chinese aviator.

At that time, Feng Ru built two models of aircraft, one was Feng Ru No. 1 and the other was Feng Ru No. 2. He returned to China from the United States and conducted test flights in China. Unfortunately, in 1912, Feng Ru lost his life in a plane crash during a test flight.

In that era, when people considered the safety of airplanes, they mainly considered whether the airplane would break down after taking off, whether the wings would break, whether there would be cracks on the fuselage, and other such issues.

▲ Ground experiments use sandbags to simulate distributed loads

Due to the limitations of the technical conditions at the time, they used sandbags when conducting tests on the ground, which are things we can often see now.

At that time, there was no computer assistance and no hydraulic equipment. What should the aviation pioneers do? They used sandbags for loading. What are the benefits of sandbags? When an airplane is flying, the load acting on the airplane is aerodynamic load, which is a distributed load. Sandbags can simulate the effect of distributed load very well.

Here I give the first keyword, which is strength. What is strength? In fact, this concept is very simple. In one sentence, it is the ability of a structure to resist damage. When it comes to structural damage, it must be caused by a load.

The lift of an aircraft is one of the most important loads of an aircraft. How is it generated? Let's take a look.

▲ The mechanism of aircraft lift generation

What you can see in the picture above is a symmetrical airfoil. It is a two-dimensional figure. In fact, you can think of it as a cross-section of an airplane wing.

We will find that the angle of attack of the airfoil is gradually increasing, deflecting upward, and the airflow on the upper wing surface gradually becomes turbulent - at the beginning it is still in a laminar state, flowing very evenly, and when the angle of attack is large enough, the airflow on the upper wing surface is completely separated. Lift is actually generated by the pressure difference between the upper and lower wing surfaces when the wing passes through the air.

Why does the airflow on the upper wing separate? Let me tell you a small example and you will know the answer. Everyone has seen smoke columns. There are chimneys in rural areas. The smoke from the chimney is very stable at first. It moves straight up. When it moves to a certain distance, the air in the smoke column rubs against the air next to it, so it gradually shakes, and after shaking, turbulence will be generated.

The same is true for the upper surface of the wing. The air will rub against the upper surface of the aircraft. Due to the effect of friction resistance, the speed of the airflow on the upper surface will decrease, and turbulence will gradually occur. This is the basic principle of lift generation and the process of the airflow on the upper surface from normal laminar flow to stall.

Considering the load and strength, does the aircraft seem safe? Actually, it is not. So what is the problem?

▲ The aircraft will deform during flight

Aviation pioneers discovered that some of the earliest aircraft only considered strength issues, but in fact other problems would arise during flight, such as excessive deformation of the wings during flight. We now know that when an aircraft is flying, the wings will bend upwards, and in fact, in addition to the upward bending deformation, the wings will also twist.

When flying at high speed, if the wing is too soft, the wing will vibrate. The vibration will include two modes, one is the bending mode, and the other is the torsion mode, and the two modes will be superimposed on each other. If the wing flutters in the air, the aircraft will disintegrate in a very short time.

▲ DC-3 aircraft

After learning this lesson, engineers and technicians began to focus on the issue of stiffness. That is, we need to make the wing have a certain hardness and not allow it to deform too much.

After considering strength and rigidity, these types of aircraft appeared. For example, in the 1940s, the DC-3 aircraft was actually a civilian model at first, but later had problems during flight.

▲ British Comet aircraft

There is another more famous aircraft called Comet, which was produced in the UK. Both models of aircraft have crashed and even suffered structural damage in subsequent operations. Let's take a look at the situation of the Comet aircraft.

▲ The crashed Comet

This is a crashed Comet aircraft.

▲ The Comet aircraft exploded and disintegrated

This picture is even more tragic, the plane has completely disintegrated.

This photo shows that the fuselage has exploded. Why did the fuselage structure suddenly explode while flying in the air?

It's very simple. In fact, when engineers and technicians first studied airplanes, they overlooked a problem. What was that problem? Fatigue. We need materials to make airplanes. There are metal materials and non-metal materials. Almost all materials found on the earth have a problem - fatigue.

Children will get tired after reading for a long time, and materials will also get tired. Therefore, the accidents involving these models of aircraft are mainly caused by fatigue of metal materials and cracks.

When an airplane is flying at high altitude, if the structure of the airplane suddenly bursts, the cabin pressure will drop suddenly, and the gas in the lungs will not be discharged in time, which will cause pulmonary hemorrhage. Therefore, based on the phenomenon at that time, it can be judged that the airplane disintegrated in the air.

▲ Aloha plane crash

This photo is a follow-up to what we call the Aloha crash, when the front half of the fuselage almost completely fell off while the plane was in flight.

You can see that this is the state of emergency evacuation after landing, and we can even see the terrified expressions of the passengers. Fortunately, only one person was killed in the accident, a flight attendant who was standing in the aisle without a seat belt.

From these cases, we can see that fatigue is a problem that we cannot ignore. Engineers and technicians recognize the strength and stiffness problems, as well as the fatigue problem. Does that mean there are no problems at all and it is completely safe?

▲ Computational simulation of crack propagation process

Let's take a look at how we use a testing machine to test the performance of materials when solving fatigue problems. Once we have the performance information, we can use numerical calculation methods to simulate the process from the beginning of crack generation to its continuous expansion.

As you can see, the red area represents the place with greater stress. Since we can analyze it to this extent, why does the aircraft still have problems? In order to solve the fatigue problem well, we even conducted fatigue tests on the entire aircraft.

▲ 787 aircraft fatigue test

This is a fatigue test of the 787 aircraft. Look carefully, the wing is slowly bending up and down under the action of the hydraulic machinery. Isn't this very similar to the wing deformation you see when you are on an airplane?

▲ 787 aircraft fatigue test

In fact, in the experiment, the fuselage and wing structure of the 787 must last twice as long as its actual lifespan, which means it must last three times as long. Only by doing this can it be proven that the design of the aircraft is reasonable and safe.

▲ 707 and Airbus A300

Taking fatigue issues into account, the typical aircraft that appeared during this period were the 707 and the Airbus A300, two very famous commercial aircraft.

▲ China's Y-10 aircraft

Did China have a similar aircraft during the same period? Actually, there was, and that was the Y-10 aircraft. Unfortunately, due to the limited financial resources of our country at that time, the Y-10 aircraft project had to be stopped.

In fact, this aircraft flies quite well. At that time, Tibet was hit by a snowstorm, and this aircraft took off from Chengdu and delivered a lot of relief supplies to the airport in Lhasa.

We can imagine that if the project had continued, the level of our country's civil aircraft would definitely be much higher than it is now. Of course, in recent years we have ARJ21 and C919, and the progress is still very great.

Are there any other problems after considering the fatigue problem? We did a test and were able to achieve a lifespan of three times, which seemed to be very safe. But unfortunately, there were still problems.

▲ First row: F111, F4; Second row: F5A, KC135

The four aircraft in the picture above did not reach our expected lifespan during actual operation. They had only flown for 100 hours when structural damage occurred, but according to our fatigue analysis, the lifespan should be 40,000 hours.

Why did they still break? In fact, we have overlooked a problem. When we do fatigue tests, we always assume that the aircraft is new and all its structural systems should be fine.

But there are problems. For example, aluminum alloys may have pores during the smelting process of raw materials; during the manufacturing process, the machining tools may leave marks on the workpiece, which will cause initial damage to our components.

If these conditions are not noticed, during aircraft operation, under the action of fatigue loads, such as the bending load and torsional load mentioned earlier, the initial damage will slowly expand until fracture occurs.

▲ The fault location of the cargo plane in the Lusaka air crash

How to solve this problem? After the Lusaka crash, our experts proposed the following keyword, damage tolerance. What does damage tolerance mean? It means that we should assume that the aircraft itself has defects when it leaves the factory, and we should take measures to limit the development of defects.

What measures should be taken? First, we can stipulate what amount of damage is acceptable, and when the aircraft leaves the factory, this damage will not cause catastrophic consequences during its entire life cycle.

Second, we can set inspection intervals for the aircraft. When the inspection time comes, we will conduct a certain degree of inspection on the aircraft structure and deal with any problems in a timely manner.

The above are all ways we can solve the problem. The US Air Force proposed such a standard in 1971, and proposed the damage tolerance test flight specification in 1975.

We have reviewed the history of our structural development. Now let's look at the development of structural design ideas. Why did such a change happen? In fact, it has something to do with our aviation technology.

From a design perspective, you can see that with the help of computer technology and computational fluid dynamics, we can now accurately calculate how the velocity field on the surface of an aircraft is distributed as it travels through the air.

▲ Visualization of aircraft velocity field distribution

In this space, what is the speed of each point? In which direction is the speed pointing? What is the pressure at each point? What is the vorticity at each point? Then present it to everyone in a visual way. The air we usually touch is transparent, but through computer technology we can see it.

▲ Using computers to design and assemble aircraft

In addition, we can design the aircraft in detail on the computer, assemble the aircraft on the computer, and even install the equipment together, which was unimaginable before.

In the past, if you designed an aircraft, you had to draw the blueprints by hand. It took about 20 years for a model of aircraft to be delivered to users from the beginning of development. It was considered good for an aircraft designer to participate in the design of 1-2 models of aircraft in his lifetime. But now with the assistance of computer technology, a model of aircraft can be made in about 5-6 years.

▲ Visualization of component stress conditions

We can also show you the stress of the structure in a visual way. This is a common thing on the landing gear of an aircraft, called a torsion arm. The brighter the part, the greater the stress. We can optimize the structure based on the calculation results.

Does the cross section of this structure need to be so large? If the stress or force is relatively low, we can reduce the material a little bit, and the aircraft can be made lighter. We also have to consider environmental issues. When the aircraft flies through the sky, it makes noise. On the one hand, the noise affects the lives of residents, and on the other hand, the noise is also the load of the aircraft.

▲ Visualization of aircraft forces

As we all know, sound is vibration. If the space we are in is a vacuum, you can't hear me talking. Sound is the vibration of particles, and the vibration of particles will produce stress. So sound is also a load, which we need to pay attention to. From the design perspective, we need to consider these issues.

From a manufacturing perspective, I would like to mention a key word, which is composite materials. In addition to the common aluminum alloys, advanced aircraft often use composite materials when manufacturing aircraft.

Composite materials are not new. Our ancestors have been using them. When did they use them? For example, when building walls in ancient China, straw, stalks and soil were used, and even a little glutinous rice paste was added. This is actually a typical composite material, in which straw is the reinforcement, and soil and glutinous rice paste are the matrix.

▲ Composite materials

The composite materials used in aviation today are mainly carbon fiber reinforced composite materials. How strong is carbon fiber? Simply put, it is at least five or six times stronger than ordinary steel, and the best carbon fiber can even reach nine to ten times.

Think about it, if we use high-strength materials to bear the same force, the weight will naturally decrease, and the density of carbon fiber is much lower than that of steel. This will make the aircraft lighter and lighter, carry more and more weight, have a longer range, and better performance.

When you take a plane, the floor you step on when you enter the cabin is a composite material made of honeycomb sandwich. How thick are the floors of A340 and A330? You may not imagine that it is only about five or six millimeters, very thin. But you will not feel much deformation when you step on it, and you will not feel a dent. This is because it is made of carbon fiber and has good rigidity.

▲ Aluminum honeycomb panel

In addition to the polymer honeycombs we use, we also use some metal honeycombs, such as aluminum honeycombs. Aluminum honeycombs can be used as sandwich materials.

▲ Various composite materials on A380

A large amount of composite materials are used in the manufacturing of the A380 aircraft.

After talking about manufacturing, let’s talk about testing. As I just said, we cannot rely solely on analysis in engineering, we must have tests to verify.

▲ 787 aircraft static test

This is a 787 aircraft undergoing static testing. This is a static picture. Let’s take a look at the dynamic picture.

▲ The 787 aircraft is undergoing static testing

As you can see, the wing is bent under the control of hydraulic machinery and computers. How is this different from the wing deformation you usually see? Its variables are greater because we are looking at its extreme state here.

▲ 777 aircraft wing destruction test

This is a video of the 777 wing breaking. We want to see if the load when it breaks is the same as we designed. These are some of our test methods.

▲ A380 landing gear drop test

From a system perspective, in addition to the structure, the aircraft also has many systems such as landing gear, hydraulics, fuel, air conditioning, bleed air, etc. This is the A380 landing gear drop test, simulating the state of the landing gear at the moment the aircraft touches the ground.

▲ A380 brake device

More importantly, the brakes for deceleration are the brakes of the A380. You can see from the video that there is fire coming out of the wheel hub. It also tests an extreme braking state. Only after these tests can we ensure that the aircraft is safe when flying.

In addition to the aircraft itself, what other factors affect safety? Another very important factor is the human factor. Statistics show that 70% to 80% of aviation accidents are caused by human factors.

▲ Simulated aircraft landing

For example, when this A330 aircraft was landing, the captain made an error in judgment and failed to make a go-around in time, so the aircraft was already in the middle of the runway when it landed, and finally it ran off the runway.

How to avoid human error? Let me give you an example from the flight control system of an aircraft. There are two major types of aircraft control systems. One is mechanical, which uses hydraulics for assistance, and the other is fly-by-wire. In the picture, you can see many control surfaces of the aircraft, which can control the aircraft's head up, head down, pitch, roll, and yaw.

▲ Mechanical power control and fly-by-wire control of aircraft

What is the difference? Let's take a look at this picture. Although it looks very professional, it is actually very simple. If a fly-by-wire control system is used, the pilot's signal must first be analyzed and processed by a computer after it is sent out.

For example, when a twin-engine aircraft rolls, the maximum angle is 30 degrees. If the aircraft has rolled to 30 degrees and the pilot is still pressing the stick, the flight control computer will filter out the signal and control it instead.

Below, we see several of the more advanced aircraft.

▲ Su-35 performs Cobra maneuver

This is the Su-35. A friend once asked me why planes have smoke trails during air shows. Many people say it is for the sake of appearance. In fact, I tell you, it has another very important function.

Through the smoke trail, we can see the air flow near the aircraft. When the Su-35 in the video is doing the Cobra maneuver, the airflow on the upper surface of the wing is completely separated.

How can it recover? The Su-35 uses a thrust vectoring nozzle to recover this state.

▲ X-35B test

For example, this is a video of the F-35 aircraft's predecessor, the X-35B, being tested. This is an aircraft that can take off and land vertically.

We can see the entire takeoff process, the tail nozzle deflects downward, the lift fan in front turns on, and there are two jets on the wing to control the lateral balance of the aircraft. The plane begins to take off. We can even see the internal computer adjusting the thrust during takeoff.

In the field of civil aviation, the latest A350 and A380 are both very advanced aircraft. The maximum take-off weight of the A380 is 560 tons. What does 560 tons mean? A family car weighs about 1 to 1.5 tons. You can compare them.

In order to ensure the safety of civil aviation aircraft, we now have several airworthiness systems. For example, China's civil aviation has its own civil aviation regulations, the US Federal Aviation Administration (FAA) also has its airworthiness regulations, and Europe mainly uses the EASA system to ensure safety. With so much work to ensure the safety of aircraft, what problems will the aircraft encounter during operation?

▲ Accidents caused by collisions between birds and airplanes

You can see this picture of a bird hitting a plane. It is a small bird, very weak, why did it hit the plane like this? Because the relative speed is high. Both civil and military aircraft will take these issues into consideration. When designing, these factors have been taken into account and some arrangements have been made in the structure.

▲ Damage to aircraft caused by bad weather

There are also weather factors, such as hail and lightning, and we will also take corresponding measures.

▲ Lightning protection scheme for aircraft

For example, if you connect all the metal parts of the plane to form an equipotential body, the current will only pass through the outer surface of the plane and not into the inside, so you don't have to be afraid when you encounter lightning. In addition, there is also the influence of airflow.

▲ The impact of wind on flight

You can see that the plane encountered headwind when it just took off, encountered downdraft in the middle, and turned into tailwind a little further forward, so the weather conditions also have some impact on the flight of the plane. In addition to wind, there is also thunderstorm.

Our country's current aviation science education is still a little behind that of developed aviation countries. In the past, passengers often argued with the staff at the check-in counter, saying that the weather at the airport was good, so why couldn't they fly?

It's very simple. It's because passengers don't understand that even if the weather at the departure airport is good, there may be thunderstorms on the way. Even if the weather on the way is good, there may be thunderstorms at the destination airport. This is why there are more flight delays in summer.

Why do many people not understand this situation? Because our aviation culture knowledge is not popular enough. In fact, we have entered an era of mass aviation, and every aviation power has a very strong aviation education.

▲ Aviation Museum

Knowledge can be spread through museums, as well as through other activities such as simulated flight.

▲ Flight Simulator

This is a very good channel to learn about aviation knowledge. You can realize simulated flight at home, and can simulate the flight of passenger planes and fighter jets.

▲ Drone

In addition, our drones are developing very rapidly, and there are many drone geeks who design and test-fly drones themselves.

▲ The Civil Aviation Flight University of China made its own unmanned airship

We also have such a group of people in our school. The drones you see are all designed by the students themselves. They also designed some solar-powered airplanes and unmanned airships, which are a bit like jellyfish in the sea and look quite beautiful.

This video was shot at a campus of the Civil Aviation Flight University of China using one of our own unmanned aircraft. Take a look and see if it is very similar to what you see on an airplane. Drones are also a way to popularize aviation culture, and they can change our perspective.

This is a video shot by a netizen. Take a look, this is a very small drone, which we call a flying drone. The images it shoots completely change the way we observe the world. These are all activities that aviation enthusiasts can carry out on their own.

What will future flight look like? Just now, our teacher talked about this issue. My view is similar to theirs. It must be the integration of air and space. We have corresponding aircraft in the atmosphere, such as cargo or passenger transport. The future development should be the era of high-speed aircraft, and we will enter the era of air and space flight.

Taking off from land, entering near-space (the airspace 20 to 100 kilometers above the ground), flying at high Mach speeds, and quickly reaching another destination will greatly change our current understanding of distance.

Think about it, when we didn't have high-speed rail, didn't we feel that the distance was very far? But now that we have high-speed rail, don't we feel that the distance has shortened? So the world will really become a global village in the future. This is the aviation experience I want to share with you today. Thank you.

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The articles and speeches only represent the author’s views and do not represent the position of the Gezhi Lundao Forum.