To design any aircraft, the simple rule that is followed is that the length is always larger than width or height or thickness in the model. We can take many parts as example. If we see the wing, we can see that the width and depth of it is much lesser than its length. The tail, the fuselage and the propeller all have a larger length compared to their width or thickness.
This is the main reason why nowadays people choose unconditional materials, like steel, aluminum and its alloys and some composite materials as they have unconditional shape. The reason behind using these materials, like 2024 aluminum plate, is that these are strong, flexible and thin. Aluminum, like 2024 t3 aluminum, blended with little quantity of other materials, is widely used for designing aircrafts. This is strong, light and is good with tension. Not only that as aluminum is manufactured, so we can even increase its strength with ease. Steel, another material that is used in aircrafts, is four times stronger and three times stiffer than aluminum. But at the same time, it is very heavy. In certain parts of the plane it can be used where more strength and hardness is required. Steel is mainly used in landing gear.
Composite materials, such as graphite-epoxy, are becoming popular nowadays. It is as strong as aluminum but has a very less weight. One disadvantage for these composite materials is the limited shelf life that they have.Titanium is as strong as steel and it weighs less. It acts well in high temperature. That is why, although being expensive, it is now becoming the preferred material in the industry.
The aircraft is basically made in a symmetric structure. The up and down load is almost same in it and because of this reason, while designing an aircraft we have to see that the structure of the material that is used in it can withstand tension and compression equally.
The unidirectional materials serve this purpose. They act well in tension but they escape the load by bucking away. These thin fibers are embedded with the help of glue. This helps in taking the benefits of tension strength. They are no longer penalized by their own compression weakness because they become compression resistant.
In earlier days, when we started making aircraft, wood was our first choice. The wood was very light, but not as strong as we wanted. But there were a few factors to be considered while using the wood for aircraft designing. One of the main problems with wood was that it was used to splinter. The wood also needed a lot of maintenance. Another problem with wood was the unavailability of the same. So, it is no more used in aircrafts now.