Aerospace Design

Aerospace Design

Many light aircraft are braced monoplanes, having a diagonal bracing strut between the wing and fuselage. Without this strut, the wing would need to be stiff enough to resist all of the bending loads created by the lift force on the wings, requiring more structure and hence increased weight. The strut takes some of the lift loads, allowing a lighter structure in the wing, but at the expense of extra drag. Because of the low flying speed of the aircraft, the extra drag caused is small, and therefore acceptable in view of the weight saved.

Because of the drag penalty of bracing struts, the pure cantilever wing is used for all aircraft of medium and high speeds. A cantilever is simply a beam that is supported at only one end. The cantilever wing arrangement can be categorised as low-wing, midwing or high-wing, depending on where it is attached to, or passes through, the fuselage. Typically, the low-wing arrangement seems to be preferred for jet aircraft and many light aircraft, high wing for turbo-prop transport aircraft and both low- and mid-wing (shoulder-wing) for combat aircraft, but there are many exceptions.

A cantilever wing must be strong enough and stiff enough to carry the whole weight of the aircraft, and its aerodynamic loads, without the need for external bracing. For a Boeing 747 weighing 350 tonnes, the wing will need to be capable of resisting loads of over 1000 tonnes without failure or excess distortion. This is because manoeuvres and wind gusts cause loads that are several times the aircraft weight. It must also be able to cope with the highest speeds and manoeuvre loads of the aircraft without deflecting too much, which can cause aerodynamic flutter and may result in collapse or loss of control.

Generally, high speeds require a smaller wing span and low wing area, hence a high wing loading. Conversely, a large span and high wing area, i.e. low wing loading, are best for low speeds. For take-off and landing, it is possible to change the wing area and wing section to some extent by the use of flaps at the trailing edge. This makes the wing structure more complicated, but is desirable or even essential if the aircraft is to land at a safe speed. The design of the wing for a high-speed aircraft, such as the Tornado, will be principally driven by stiffness requirements, to avoid flutter at high speed. High speeds also require minimum drag, so retractable undercarriages and low frontal area are required. Even with a streamlined aircraft, high speeds demand high thrust, and turbo-fan or turbo jet engines will be needed, in preference to the turbo-prop engines most efficient for lower speeds. At very high speeds, the cross-section of the fuselage and wing together are very carefully designed to achieve low drag, and leading to some very complex aircraft shapes. The area rule principle considers the cross-sectional area of the fuselage plus wings – if this area corresponds to that of the minimum drag body of similar cross section, then the transonic or supersonic drag will be minimised.

Strength-to-weight ratio

Because high-speed aircraft need small wings for low drag, the loads on these wings will be very high, so the wings will have to be made much stiffer and stronger to carry the wing loads. This leads to increased weight, which the designer tries to avoid. Wing loading is tending to increase over the years, but the designer makes sure that the material is used to best effect, and uses the strongest and lightest materials. In this way, the strength-to-weight ratio of the aircraft structures is improved. Improved materials can also play a part in allowing higher stresses to be used, and although they may be much more expensive they can save cost by making the design simpler and more efficient. It is important to realise that materials with a high strength-to-weight ratio do not automatically produce a structure with the same qualities. What is important is that the most suitable material is used, together with a simple and effective design. The material must be highly loaded, or it is not being used to best effect, but must not be over-stressed or it will fail early in service.

Stiffness-to-weight ratio

Another important feature of some aircraft structures design, for instance wings, is the ratio of their stiffness to weight. A wing may be strong enough to withstand the loads upon it, but may lack the stiffness needed to keep its shape accurately in flight. This would be a major problem, and increasing the stiffness may well require the use of more material, increasing weight. In some applications, particularly small components, the materials with the highest strength-to-weight ratio may not be the best to use, because the material may need to be too thin to provide enough stiffness. A good example of this is model aircraft – they use balsa, which is never used structurally in full-size aircraft. If model makers used aluminium alloys, the components would be so thin that they would be very flimsy, and stiffening them up sufficiently would make the models far too heavy to fly. So the stiffness of a structure depends on both its design and the materials used.

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