The high bypass ratio engine is put into use, and the combat capability of the Y-20 has undergone a qualitative change

In April 2023, many people found that there was something wrong with our Y-20. What was wrong? The engine was thicker. Many friends know that this is called a high bypass ratio turbofan engine. Today we will talk about the difference between a high bypass ratio turbofan engine and a low bypass ratio engine. What performance improvements may the Y-20 have after the engine is replaced?

First of all, what is the bypass ratio? This is a parameter specifically for turbofan engines (abbreviated as "turbofan engines"). The airflow of turbofan engines has two pathways. One is the inner channel that passes through the core engine in the middle. In fact, it has the same principle as the turbojet engine. The other is the one between the core engine and the outer shell, called the outer duct. The ratio of the air flow through the outer duct to the air flow through the inner duct is called the bypass ratio. Therefore, the larger the bypass ratio, the thicker the outer shell, so the thicker the engine.

A high bypass ratio engine will have greater thrust than a low bypass ratio engine and will be more fuel-efficient. Why is this? Simply put, it can throw more air backwards per unit time. We can first understand it from a perceptual point of view. If I am sitting on a boat and rely on throwing things backwards to generate power to run forward, I ask you to choose a few things that can be thrown backwards, ping-pong balls, tennis balls, shot puts, which one do you choose? It must be the shot put, which can generate better thrust. Then we can also analyze it theoretically. Relying on throwing things backwards to generate thrust is actually using the so-called recoil. The momentum of the things I throw out is the momentum I can get, and if this momentum is distributed in the unit time, it is thrust. Momentum is equal to mass multiplied by speed, so either I throw heavier things or I throw them out faster.

Schematic diagram of a turbofan engine, from TuChong.com

So, why do I save energy when I throw heavier things, but it takes more energy when I throw faster? We also need to consider the energy consumed. How much energy should we give to the thing we throw out? In high school, we learned that half the mass is multiplied by the square of the speed. Look at the speed here, which is a square, but there is no square for the mass. Increasing the speed consumes more energy than increasing the mass. Therefore, if an aircraft engine wants to be more fuel-efficient and have greater thrust, it must throw more air backwards. For a high bypass ratio turbofan engine, the gas ejected backwards by the core engine inside it is not the main source of power. The core engine drives the turbine to rotate, the turbine drives the fan to rotate, and the fan rotates to push the air backwards to generate power. How main is it? In the high bypass ratio engine of a civil airliner, the thrust generated by the outer bypass, that is, the fan, accounts for more than 80% of the total thrust.

Although high bypass ratio turbofan engines are more fuel efficient and have greater thrust, they are also large in size, so they generate greater resistance and are therefore not suitable for high-speed flight. Therefore, fighter jets are still equipped with low bypass ratio turbofan engines. Moreover, the fuel efficiency and high thrust mentioned above are only at low altitude and low speed. At high altitude and high speed, they are even worse than turbojets. Therefore, subsonic transport aircraft and passenger aircraft are most suitable for high bypass ratio engines.

What kind of performance improvement can the Y-20 bring after replacing the high bypass ratio turbofan engine? Of course, we don't know what model it is and what the parameters are. According to the general rules mentioned above, its thrust should be improved. The thrust of the Russian D30kp2 engine installed before was about 12 tons. Later, after replacing it with the turbofan 18 that we copied the D30 and improved its performance, the thrust increased a little, probably less than 13 tons, while the thrust of the engine of the US C17 transport aircraft can reach 18.6 tons. Let's be conservative about our new high bypass ratio engine. It should not be so large in one step, but even if it is only increased to about 15 tons, it will greatly enhance the combat capability.

Why? Aircraft design pays attention to a thrust-to-weight ratio, that is, the ratio of total thrust to aircraft weight. This ratio must be large enough for the aircraft to fly. The data says that the maximum take-off weight of the Y-20 before was 220 tons, and the thrust of the previous four engines was about 48 tons, which is about 0.22. For a transport aircraft, this is basically the maximum thrust-to-weight ratio that can fly. Then we use this thrust-to-weight ratio to reversely calculate. If the thrust of a single engine reaches 15 tons, then the four engines are 60 tons, divided by 0.22, more than 270 tons, which means that after the engine is replaced, the Y-20 can take off with 50 tons more things. These 50 tons can be cargo or fuel, that is to say, either load more or fly farther. Combined with the more fuel-efficient characteristics of the large bypass ratio turbofan engine, it is not an exaggeration to say that this comprehensive strength is a qualitative improvement. If we are more conservative, for example, because our technology is not mature enough, the thrust of this engine has not been significantly improved compared to the original, then at least it can be more fuel-efficient, so that at least the Y-20 can fly farther, which is also a major improvement for transport aircraft.

I would say that even if the performance of this high bypass ratio turbofan engine is not significantly improved compared to the original one, it is still a very meaningful replacement, because our previous high bypass ratio engine was blank, and being able to build it is already the first step, and only when someone uses it, someone will study, design, and manufacture it, and our experience in this area will continue to improve. Not only designers and manufacturers, but also users, maintenance and overhaul personnel, we all need to better understand this engine, and become more and more familiar with it, so that it can become more and more advanced. I sincerely hope that in the future, we can see more and more aircraft equipped with our "Chinese Heart", and our "Chinese Heart" will beat stronger and stronger.

This article is a work supported by Science Popularization China Starry Sky Project

Author: Gou Sheng

Reviewer: Shen Haijun (Professor of the School of Aeronautics and Mechanics, Tongji University)

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.