Why are some airplane engines hung on the wings while others are inserted next to the buttocks?

Students who often take planes must have the impression that some airplane engines are hung on the wings, and some are installed on the butt. Can they be arranged randomly?

In fact, aircraft engines not only differ greatly in performance, but also in the specific locations where they are installed. Generally speaking, jet aircraft engines have wing-root, tail-mounted, and wing-tail-mounted layouts in addition to the common wing-mounted layout.

Wing-mounted layout

Let's start with the most common wing-mounted layout. This layout first appeared on bombers and was later adopted on the 707 aircraft that helped Boeing lay the foundation for its industry giant status. Once adopted, this layout has become the mainstream. The reason why it is so popular is that it has many advantages. First of all, this layout is conducive to using the weight of the engine to offset part of the torque at the connection between the aircraft wing and the fuselage, playing an important "unloading" role.

During flight, the aircraft's wings generate upward lift, while the heavier fuselage generates a large downward gravity. As a result, at the junction of the wing and the fuselage, the lift generated by the wing and the gravity generated by the fuselage form a large torque, making the wing root the heaviest part of the aircraft structure. If a very heavy engine is hung under the wing, part of the weight can be distributed to the wing, which helps to balance the torque at the wing root.

Hanging the engine under the wing can also reduce the discomfort caused by noise. When the plane is flying, the engine makes a very loud noise, which will make passengers near the engine area feel very noisy. A quiet cabin environment is an important criterion for measuring the comfort of an aircraft. For this reason, aircraft designers imagine that if the engine is hung under the wing, not only can the wing form a shielding barrier to the engine noise, but also the engine can be as far away from the fuselage as possible, thereby reducing the impact of noise.

In addition, hanging the engine under the wing has the following advantages: the engine is closer to the ground, making it easier to repair and maintain; the engine installed under the wing is close to the center of gravity of the aircraft, making the aircraft easier to control; due to the large area of ​​the wing, it is relatively easy to increase or decrease the number of engines suspended under the wing.

Hanging the engine under the wing is not a casual matter. It requires comprehensive consideration of factors such as resistance and airflow during flight. Civil aircraft are basically single-wing aircraft. In this case, aircraft designers must ensure that the engine hung under the wing is not too close to the ground to prevent it from sucking in ground debris and damaging the engine.

What we usually call wing-mounted engines are engines suspended under the wings, but there are also reverse operations, where the engines are placed above the wings. In fact, from the perspective that the wing-mounted engines are not restricted by the height above the ground under the wings and have the same unloading function as the underwing engines, the wing-mounted engine layout is very scientific. However, once the engine is placed above the wing, the wing cannot block the engine noise. In addition, the wing-mounted engine layout makes engine maintenance difficult because the position of the engine is raised. After taking various factors into consideration, there are not many aircraft that currently use the wing-mounted engine layout.

Wing root layout

The wing-root layout is an engine layout with a long history. Starting from the early jet airliners "Comet" and Tu-104, designers installed the engines at the root of the wings. The reason why designers chose this layout is that it allows the aircraft to maintain the shape formed by the three main aerodynamic components of the wing, fuselage, and tail, with minimal wind resistance. Moreover, since the engine is relatively close to the center axis of the fuselage and the center of gravity of the fuselage, once one side of the engine fails, the thrust imbalance generated is relatively small, and flight control is relatively simple. Therefore, most early jet airliners adopted this engine layout.

Of course, this layout also has obvious limitations. The wing-root engine layout places the engine close to the fuselage, which will make the cabin very noisy and the hot airflow ejected by the engine will easily damage the fuselage. In addition, this layout allows the wing to pass through the engine compartment and connect to the fuselage, which complicates the design of the force system and increases the structural weight of the wing root. In addition, since the engine is installed inside the wing structure, it is more difficult to repair the engine.

Tail-mounted layout

The tail-mounted layout is to place the engine at the tail of the aircraft. The first jet airliner to adopt this engine layout was the "Clipper", and the later Boeing 727 also adopted this layout.

The advantages of this layout are obvious: first, there are no extra protrusions under the wing, which reduces the impact of the engine pod on lift and drag; second, there is no rigid requirement for the underwing space, and designers can shorten the landing gear height and save structural weight; third, it can provide a quieter and more comfortable environment for the first-class cabin, business class, and even high-end economy class located at the front of the fuselage; fourth, the diameter of the engine is not limited by space, and engines with very large bypass ratios can be used; fifth, because the engines are close, once a single engine fails, the impact on the aircraft yaw is far less than that of the underwing suspension layout.

This layout also has disadvantages. For example, it eliminates the unloading effect of the engine on the wing. The tail engine layout requires the use of a high horizontal tail, which requires strengthening the vertical structure when designing the aircraft. The number of engines cannot be increased or decreased at will.

"Wing-mounted + tail-mounted" layout

The "wing-mounted + tail-mounted" engine layout can be seen on some aircraft that use three engines. Among them, the most representative are the DC-1 and MD-11 long-range wide-body aircraft produced by McDonnell Douglas of the United States. At the beginning, the guiding ideology of adopting this engine layout method was to install an engine on the tail wing to increase the power of the aircraft and extend the flight distance. This "wing-mounted + tail-mounted" engine layout method has gradually been eliminated with the increasing popularity of second engine hoisting on civil aircraft.