Why do all modern (precisely modern) airliners look alike?

The short answer is that physics (and the standards for aircraft certification and safety, as well as economic efficiency, that follow from it) are common on the planet, but this topic can and should be covered in more detail.

1. They are all subsonic, because supersonic eats up too much expensive fuel and is too noisy (the shock wave from the fuselage sometimes knocks out shop windows and house windows), no airline will buy it. There are a dozen more problems, but these are the main ones. This means that all passenger airliners have subsonic aerodynamics (the “shapes” of the fuselage and wing).

2. They all have a stabilizer and a vertical stabilizer in the tail, no canards (the so-called “little wings” in the nose) or other gimmicks. Because such a design provides the best range of center of gravity (less effort is needed to fly level) – and the greater the range, the longer the fuselage (more passenger seats) can be made. Canards reduce this range, and also create problems with the layout (“how to shove it in?”) – that's why no one installs them.
2.1 They are all low-wing aircraft (wing at the bottom of the fuselage), because low-wing aircraft float better during an emergency landing on water. In a high-wing aircraft (wing on the roof), the fuselage sinks instantly – and this, seriously, is an important point in the list of aircraft safety requirements. Also, high-wing aircraft always have worse takeoff and landing characteristics (a high wing, unlike a low one, does not create a screen effect), their engines are harder to service (because they are high), and when they are destroyed, fragments fly straight into the window to the passengers, but the main point is safety.

(Yes, I know that the An-148 exists. But suddenly (no) none of the commercial airlines needed it, and it did not receive international certificates).

3. They all have 2 engines, the era of 3/4 turbines is gone (now more than two is more expensive, less is not allowed for safety reasons). These engines are located… And here is the question.

There are two types of passenger aircraft – conventionally “small” and “bigger than small”. Both the Boeing 737 and the Boeing 777 have two engines under the wing, in this matter the difference in their dimensions and class is not important. But a “small” aircraft (99% of business jets) has engines in the tail, because there is simply no room under the wing.

(No, if you have billions of money and time, you can try to put the engine on the wing, but here you can write a separate article about the list of problems and difficulties, so just take my word for it – the HondaJet is not just one of its kind).

Here someone may remember that earlier engines were installed in the tail on “large” aircraft too, and ask the question – why have they abandoned this now? In short – because such a scheme gives worse flight characteristics and creates more risks. Now in more detail.

Let's talk about the advantages of “engines in the tail” – this is a “clean” wing, which is good for aerodynamics and for landing on water, these are shorter landing gear (which simplifies their placement, reduces weight and makes it safer (not to jump so high) to urgently abandon the plane if the emergency slide fails), if one engine fails, there is less destabilizing moment (less “pulls to the side”). About 40 years ago, the advantages would have included significantly less noise for passengers, but modern engines under the wing no longer create such discomfort in the cabin as they did then. There is also a myth that a high-mounted engine sucks in less debris from the runway, but in reality it is the other way around, low-mounted air intakes are not prone to vortex formation that raises debris, but high ones – yes. That's it, the list of advantages is over, let's move on to the disadvantages.

The first and most basic is weight. The lighter the plane, the higher its performance characteristics and economic indicators. And by placing the engines in the tail of the fuselage, we condemn it to lengthening, because: 1. the space for the engine will “eat” part of the cabin = fewer passengers = less money, 2. You can't place the tail and the engine on the same line. Why? Safety. The engine has a tendency to break and disintegrate – and if the red-hot debris of the turbine, which was just rotating at a speed of several tens of thousands of revolutions per minute, fly into the vertical stabilizer, then the flight will definitely become a dive. But this way there is a chance to last…

So, a longer fuselage means more mass. The increase in mass does not end there, because the fin also becomes heavier. Why? Because now it must create a greater moment, because the shoulder between it and the wing has become smaller – because the wing has moved back (more on that below), and the same stability is needed. And in addition to the fin, there is a stabilizer (horizontal tail). If on a regular aircraft it can be hung on the fuselage power set, then here it cannot be placed directly behind the hot nozzle, it must be carried by the fin. For which the fin must become stronger and thicker – that is, again, heavier. As well as that part of the fuselage on which we put all this wealth…

So, the tail has become much heavier. This means that our center of gravity (CG) has shifted back. This means that we need to shift the wing as well. But our plane is still empty. We remember that the load (and therefore the CG position) can be different and changes in flight (as the fuel is used up), and we start thinking, how the hell can we constantly balance it? Well, the Il-62 or Karavella carried free ballast in the nose for this purpose…

Someone will say that the wing should become lighter, because now it does not have to carry the engine, hold the reverse, parasitic drag and other things. But… No. In this case, the wing must be made stronger. In flight, the lifting force constantly strives to “fold” the wing over the fuselage (imagine that the airplane “clapped its hands” above itself). If there is an engine under the wing, then its weight pulls the center section down and partially counteracts this effect, if there is no engine, then the wing does not unload anything, and you have to increase its strength the old-fashioned way. And try to guess whose wing will be heavier in the end.

But screw it, everything is clear with the weight culture – in GA the number 1 priority is safety. And here too everything is bad.

Let's go back to the tail again, because there “shadowing” entered the chat. If for some reason the plane reached critical angles of attack (raised its nose), then the wing “covers” the nose from the oncoming air flow, and there is a risk that the engines will get surge, the stabilizers will lose their effectiveness… And it is quite possible that amen.

Let's return to the engines themselves. In addition to destruction, they sometimes also burn. And if the fire extinguishing system says “amen”, then the flames are guaranteed to engulf the second engine (since all the fuel lines are nearby) and the tail assembly. And losing it is much worse than losing part of the wing console skin.

But don't worry! To keep the engine from breaking down, it needs to be serviced. And here… Here you need a stepladder, yeah. The engine is high. The ones under the wing are easier to service. And they're bigger. Now in civil aviation, engines with a high bypass ratio (with a large diameter of the first stage of the compressor) rule the roost – they are simply more economical than engines with a low ratio. Shove them in the tail… Well, good luck, it's much easier to place them under the wing.

Actually, for these reasons, engines in the tail are now installed only on business jets. There is nowhere else to put them, and some of the problems listed above are less common to small airplanes. Therefore, all business jets are similar to each other, and all airliners are similar to airliners. Physics is the same everywhere.

Author: Trifon Dubogryzov

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