Over the last hundred years, airplanes have become ubiquitous, and most passenger jets found in airports today have similar design characteristics. A typical airplane has a long body (also known as a fuselage), long wings that stick out on either side, and large tail assemblies at the very back. It turns out that with some basic principles in physics, we can understand why airplanes have their basic shapes.
First comes the question of why wings are so long. The answer here comes by considering drag, a force that opposes the motion of a plane and therefore makes it more difficult for it to fly. The two primary types of drag are parasite drag and induced drag. Parasite drag can be thought of as an intrinsic cost that comes from the shape and materials a plane is made out of. Because of its form, a plane will always experience parasite drag, which will hinder its efficiency. Induced drag, on the other hand, comes as a consequence of producing lift. Experimentation and analysis has shown that the force of induced drag is inversely proportional to the square of the wingspan. This means that if a plane has wings that stretch out twice as far, the induced drag it experiences will be reduced by a factor of four. Therefore, it is advantageous for engineers to build jets with long wings, as this will reduce induced drag, and therefore make planes more economical.
Next comes the question of why so many aircraft have such long fuselages. The answer here lies in stability and control. In order for an airplane to achieve stability, control surfaces have to exert large torques about the plane’s center of gravity. A torque is effectively a force multiplied by the distance from the point at which it is exerted to a point of reference. For aerospace engineers, this reference point is the center of gravity, which is typically near the center of an airplane. In order to maximize the torque exerted by a control surface, that control surface must therefore be placed far away from the center of gravity. To achieve this, an aircraft can have a long fuselage.
The third question to be considered is why the control surfaces themselves are so large. Indeed, an airliner’s tail can be as tall as a two story building. The reason for this comes from the equations for the forces that these control surfaces exert. It turns out that tails with larger surface areas have greater ability to control an airplane in flight. In turn, large control surfaces are necessary to provide sufficient surface area.
There are, of course, many further details to aircraft design to explore in the future. Central to all of these design considerations are principles of mechanics, fluid dynamics, mathematics, and economics. While the equations governing flight can be very complicated, there is a lot that can be learned at a conceptual level and applied whenever looking at a plane in the wild.