Daily popular science of Hang Xiaobei/text
For the aviation industry in the past two years, it has been bumpy. The day before yesterday, a serious accident occurred in the 777 passenger plane produced by Boeing.
On the afternoon of February 20th local time, shortly after a United Airlines Boeing 777 passenger plane took off, the engine shell on the right side of the plane shattered and began to catch fire. The plane returned immediately. Fortunately, no major accident occurred. Soon United Airlines and Japan decided to ban all 777s (News review: ↑ Boeing 777 passenger plane with the engine burning↑
A detail in the accident
Of course, this accident was not Boeing's fault, but the focus should be on this PW4000 engine. The cause of the accident is still under investigation. Let's talk about one of the "unobtrusive" technical details today. I don't know if you are paying attention. The picture below is a video of the accident engine taken by the passengers at that time. In addition to the unrecognizable engine and the burning flames, a striking point in the video is that the rotor in this engine is actually rotating.
↑The burning engine is still rotating↑
And it is not just this engine accident. For example, the Delta Airlines flight DL1425 accident in July 2019 also caused a serious engine failure. According to the video taken by the passengers on the plane, you can also see that the rotor in the engine is still rotating.
↑The engine is still rotating in the 2019 Delta Airlines accident ↑
Is the pilot so brave? Such a serious accident has occurred, how can the engine still be rotated there? Is this afraid that the engine is not thorough enough? Don’t say that this makes the engine "turn" on purpose, and it is not possible to turn it. It must be turned.
The windmill status of the engine
Of course, this The "turning" of the engine is not to give the engine thrust and keep it rotating, but to let it enter a mode called "windmill state".
We know that when the aircraft is flying at a high speed, the airflow flows from front to back at a high speed compared to the aircraft. The big fan at the front of the engine is like a windmill. When the stationary fan blows by this airflow, the engine's rotor will rotate, and this state is called the windmill state.
↑ Windmill that rotates under the wind blowing↑
And in the engine accident mentioned above, the engine entered the windmill state. At this time, the engine has completely lost its autonomous power and only rotated under the driving of air.
And the reason why the engine needs to enter the windmill state after an accident is very simple: because only when the rotor rotates and enters the windmill state, the engine that loses power can control the resistance of the engine after the accident is very simple: because only when the rotor rotates and enters the windmill state, the engine that loses power can control the resistance Reduce to the lowest level.
You should know that once the engine of the aircraft fails, the effect it produces is not only as simple as there is no thrust, but more importantly, not only does there be no thrust, but engines with such a large diameter have great resistance.
Passenger aircraft like Boeing 777, which has only two engines, once one engine loses power, one engine has thrust and the other engine has resistance, it will inevitably cause the aircraft to suffer With a large torque, the aircraft will become difficult to operate, reducing the possibility of a safe landing by the aircraft.
↑Boeing 777 Technology after a single jet failure↑
Therefore, after the engine fails, the engine must be put into the windmill state, because the rotor speed will be much lower than the normal working speed, and will not cause further damage to the engine, and the resistance generated by the engine can be reduced as much as possible, so that the aircraft can land normally.
Engine safety design
You must not underestimate this "windmill status". In fact, it involves a very new aero engine design technology called "safety" design. Simply put, this design requires that when the engine encounters a very rare accident, it must not hinder the normal flight of the aircraft. Therefore, this design does not mean that the aircraft or engine is not bad, but is bad in the way we design it.
For example, the famous GE90 engine has two bearings behind the huge fan to restrict the rotation of the fan. So, is the support structure of these two places more difficult to break? Or is it easier to get bad?
↑Two bearings behind the GE90 engine fan↑
You will definitely think: Then it goes without saying that of course, the less likely it is to break, the better. It is best to make it very strong. No matter how you make it, it is absolutely not broken.
But the security design is not what I think about.
You should know that once the engine fails, the first problem is generally the bearing. Once the bearing is locked, it means that the rotor cannot rotate anymore. However, at this time, the engine rotation still has huge inertia, so the shaft may be directly broken, so the turbine will fly, which will easily cause more serious accidents. In addition, after the bearing is locked, the engine cannot enter the windmill state, which will also pose a serious threat to the plane's forced landing.
Therefore, the safety design requires that the support structures of these two must be relatively easy to break. In this way, once an accident occurs, as long as the bearing is locked, the huge rotational inertia will "break" the support structure behind the fan, so that the rotor can be "free" and will not cause more serious accidents.
Don’t look at this little detail. The considerations behind it are actually much more complicated than you think, and I feel it is more.
