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Issue No 59, 8 April 2024
By: Anthony O. Ives
The aircraft structure is obviously an important part of the aircraft without it the aircraft could not fly. Aerodynmics and Aircraft Structural Design is the two core topics in aeronautical engineering with aeronautical engineers specialising in one of these topics. Aeronautical engineers specialising in aircraft structures are usually called stress engineers. Unlike other structures an aircraft most be designed to be lightweight but also withstand all the forces applied to the structure without failing. Aircraft structural failure in most cases can have fatal consequences.
Out of the two core topics that aeronautical engineers study, I specialised in aerodynamics. So my knowledge of aircraft structures is purely mathematical and theoretical. Any practical aircraft structural experience I have would be mainly from building and flying RC model aircraft. So therefore treat this article as a basic introduction to aircraft structural design. However, you can also refer to references [1] and [2] for further reading. So we will start from basics such as what stress and strain, then explain the functions of common aircraft structural components.
\[\sigma=\frac{F}{A}\]
\[\epsilon=\frac{e}{L}\]
The above equations mathematical define stress σ and strain ε, where F is force applied, A is the cross sectional area of which the force is applied to, L is the original length of the string, bar, etc that the force is applied to and e is amount of stretch of the string, bar, etc on applying the force. If that does not make much sense to you maybe the diagrams later may help.
However, if you think of string or bar supporting a mass or someone trying to stretch it. The force, F would be the weight of the mass or force by which someone is trying to stretch it. The area would be the cross sectional of the string or bar, essentially the area based on diameter of string or bar [3]. The length of string before a force is applied is L, the distance the string is extended by the force is e. Think of a elastic band all structures stretch when force is applied similarly to an elastic band but maybe just not as obviously.
The type of stress explained in the previous paragraphs is tensile stress. Compression stress is also defined in a simlar way to tensile stress except the forces are applied in the opposite direction. Tensile and compressive stresses are due to force being applied perpendicular to the cross sectional area however, if the forces are applied parallel to the cross sectional area then this is known as shear stress.
\[\tau=\frac{S}{A}\]
\[\tau=\frac{q}{t}\]
The above equations define shear stress, where τ is shear stress and S is shear force. Aircraft structures are often described as thin walled structures. The thin walls, often referred to as skins usually carry a lot of shear stresses so a term known as shear flow is often referred to when calculating shear stresses in aircraft structures. Shear flow is defined by the equation above where q is stress flow and t is the thickness of the thin wall or skin. Torison induces a shear stress sometimes referred to as torison stress. Torison is applied to bar for example by twisting which if breaks it will shear at angle and not straight across as previously described. Aircraft structures often have various shear stresses applied to them from bending, torison so shear flow can be useful for working out different combinations on the aircraft structure. The diagram below tries to explain all the different types of stress including tensile, compressive, shear and torison stresses.
Structures can have limited lifespans especially when exposed to cyclical stress, that is changing amount of force applied to structure over a period of time such increasing and decreasing it repeatedly. If you have ever debilitatly broke something by twisting or bending it then you were using structural fatigue. Structural fatigue is really related to cracks in the structural material which grow with the longer the cyclical stresses are applied until they reach a critical length which causes a catastrophic structural failure.
Structural fatigue is the reason why certain parts of an aircraft are replaced or overhauled after a number of specific flight hours, a good example of this is helicopter rotor blades which are replaced after a specified number of flight hours. Structural fatigue is something that needs to be taken an account of in helicopters due to vibrations and other cyclical loading typical of helicopters.
Traditional aircraft structures were manufactured from metals however, composites are often used in modern aircraft structures as they give high strength for a low weight exactly what you want for an aircraft structure. Composites consist of two things a matrix and a reinforcememt defined as follows:
Reinforcememt is fibre like material which gives a composite material most of it strength there typical threes types of fibre used, glass fibre, carbon fibre and aramid fibre.
Matrix the reinforcement generally has no stiffness hence the purpose of the matix is to give it stiffness, a matrix could very basically described as a glue type material which the reinforcement absorded in and allowed to form into a specific desired shape. Epoxy resin is a typical example of a glue that could be used as a matrix.
Composites vary from very basic low cost types used by hobbyists to very expensive highly sophisticated types used by most advanced miltary aircraft. The process for manufacturing composites also varies in cost and complexity. Glass fibre reinforced composites are commonly low cost composite type materials used in model aircraft as well as for boat building. A finished composite material could be mistaken as plastic hence sometimes they are referred to as reinforced plastic. If you want to know more about composites or even trying making things out of composite trying visiting the website in reference [5]
Aircraft wings, tail surface and fuselaged all behave like a beam. Beams bend under load hence so does the whole aircraft structure. In a later article beam theory will be explained in more detail but this article will finish looking at sub components of the aircraft. These sub components were sort of introduced in an earlier article [5].
Ribs are used primarily to give the wing an airfoil shape but can be used to transfer loads into the wing structure or more simply support the undercarriage, the engine pod or to attach the wing to the fuselage. The Ribs are also necessary to stop the wing structure buckling. Buckling is a complicated topic which I might do separate article on, but brief description is when structure is unable to support a compressive load due deflecting out shape due to the compressive load.
Fuselage Frames perform the simliar task as ribs only of course they are in the fuselage. In RC model aircraft they are referred to as formers as they form the shape of fuselage.
Wing Spar The main structural component in the wing which takes most of the wing bending stresses that is tensile and compressive stresses.
Stringers these are smallest beam like structure that can be added to the wing or fuselage to take tensile or compressive stresses and also maybe stop the skin buckling.
Longerons these are longer stringers mainly in the fuselage to primarily take tensile and compressive bending stresses. The fuselage does not have a spar like the wing so Longerons are important for taking the loads in a fuselage however, you can get Longerons in the wing structure as well.
Fuselage/Wing Skins The skin closes the structure as primarily purpose, being very important as they are needed to produce lift in wings. However, they also carry stress loads mainly shear stresses from bending or torison. They often referred as stressed skin when they take loads.
These components are typical of traditional metallic aircraft structures with large helicopter fuselages being constructed in a similar way. RC model aircraft are constructed in a similar way only they use balsa instead of metal. Helicopter rotor blades were tyically constructed using a D spar at the leading edge and metal honeycomb structure towards the trailing edge covered by metal skins. With the development of composite materials there is the possibility of manufacturing one piece structures, such as one piece fuselages for fixed wing aircraft and helicopters with the no need for stringers, longerons, frames, etc. Helicopter rotor blades can also be made as one piece using composite materials. Composite materials have revolutionised rotor blade design making hingeless and bearingless rotor systems possible.
Please leave a comment on my facebook page or via email and let me know if you found this blog article useful and if you would like to see more on this topic. Most of my blog articles are on:
Mathematics
Helicopters
VTOL UAVs (RC Helicopters)
Sailing and Sailboat Design
If there is one or more of these topics that you are specifically interested in please also let me know in your comments this will help me to write blog articles that are more helpful.
References:
[1] Aircraft Structires for Engineering Students (Aerospace Engineering), T.H.G Megson, 3rd Edition, 1999, Butterworth Heinemann
[2] Mechanics of Engineering Materials, P.P.Benham, R.J.Crawford,C.G.Armstrong, 2nd Edition, 1996, Prentice Hall
[3] http://www.eiteog.com/EiteogBLOG/No10EiteogBlogCircle.html
[4] https://www.easycomposites.co.uk/
[5] http://www.eiteog.com/EiteogBLOG/No27EiteogBlogPlans.html
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