| Aircraft design | | | | will be needed, in preference to the turbo-prop engines |
| Many light aircraft are braced monoplanes, having a | | | | most efficient for lower speeds. At very high speeds, |
| diagonal bracing strut between the wing and fuselage. | | | | the cross-section of the fuselage and wing together |
| Without this strut, the wing would need to be stiff | | | | are very carefully designed to achieve low drag, and |
| enough to resist all of the bending loads created by | | | | leading to some very complex aircraft shapes. The |
| the lift force on the wings, requiring more structure and | | | | area rule principle considers the cross-sectional area of |
| hence increased weight. The strut takes some of the | | | | the fuselage plus wings - if this area corresponds to |
| lift loads, allowing a lighter structure in the wing, but at | | | | that of the minimum drag body of similar cross section, |
| the expense of extra drag. Because of the low flying | | | | then the transonic or supersonic drag will be minimised. |
| speed of the aircraft, the extra drag caused is small, | | | | Strength-to-weight ratio |
| and therefore acceptable in view of the weight saved. | | | | Because high-speed aircraft need small wings for low |
| Because of the drag penalty of bracing struts, the pure | | | | drag, the loads on these wings will be very high, so the |
| cantilever wing is used for all aircraft of medium and | | | | wings will have to be made much stiffer and stronger |
| high speeds. A cantilever is simply a beam that is | | | | to carry the wing loads. This leads to increased weight, |
| supported at only one end. The cantilever wing | | | | which the designer tries to avoid. Wing loading is |
| arrangement can be categorised as low-wing, midwing | | | | tending to increase over the years, but the designer |
| or high-wing, depending on where it is attached to, or | | | | makes sure that the material is used to best effect, |
| passes through, the fuselage. Typically, the low-wing | | | | and uses the strongest and lightest materials. In this |
| arrangement seems to be preferred for jet aircraft | | | | way, the strength-to-weight ratio of the aircraft |
| and many light aircraft, high wing for turbo-prop | | | | structures is improved. Improved materials can also |
| transport aircraft and both low- and mid-wing | | | | play a part in allowing higher stresses to be used, and |
| (shoulder-wing) for combat aircraft, but there are many | | | | although they may be much more expensive they can |
| exceptions. | | | | save cost by making the design simpler and more |
| A cantilever wing must be strong enough and stiff | | | | efficient. It is important to realise that materials with a |
| enough to carry the whole weight of the aircraft, and | | | | high strength-to-weight ratio do not automatically |
| its aerodynamic loads, without the need for external | | | | produce a structure with the same qualities. What is |
| bracing. For a Boeing 747 weighing 350 tonnes, the | | | | important is that the most suitable material is used, |
| wing will need to be capable of resisting loads of over | | | | together with a simple and effective design. The |
| 1000 tonnes without failure or excess distortion. This is | | | | material must be highly loaded, or it is not being used to |
| because manoeuvres and wind gusts cause loads | | | | best effect, but must not be over-stressed or it will fail |
| that are several times the aircraft weight. It must also | | | | early in service. |
| be able to cope with the highest speeds and | | | | Stiffness-to-weight ratio |
| manoeuvre loads of the aircraft without deflecting too | | | | Another important feature of some aircraft structures |
| much, which can cause aerodynamic flutter and may | | | | design, for instance wings, is the ratio of their stiffness |
| result in collapse or loss of control. | | | | to weight. A wing may be strong enough to withstand |
| Generally, high speeds require a smaller wing span and | | | | the loads upon it, but may lack the stiffness needed to |
| low wing area, hence a high wing loading. Conversely, | | | | keep its shape accurately in flight. This would be a |
| a large span and high wing area, i.e. low wing loading, | | | | major problem, and increasing the stiffness may well |
| are best for low speeds. For take-off and landing, it is | | | | require the use of more material, increasing weight. In |
| possible to change the wing area and wing section to | | | | some applications, particularly small components, the |
| some extent by the use of flaps at the trailing edge. | | | | materials with the highest strength-to-weight ratio may |
| This makes the wing structure more complicated, but | | | | not be the best to use, because the material may |
| is desirable or even essential if the aircraft is to land at | | | | need to be too thin to provide enough stiffness. A |
| a safe speed. The design of the wing for a high-speed | | | | good example of this is model aircraft - they use balsa, |
| aircraft, such as the Tornado, will be principally driven | | | | which is never used structurally in full-size aircraft. If |
| by stiffness requirements, to avoid flutter at high | | | | model makers used aluminium alloys, the components |
| speed. High speeds also require minimum drag, so | | | | would be so thin that they would be very flimsy, and |
| retractable undercarriages and low frontal area are | | | | stiffening them up sufficiently would make the models |
| required. Even with a streamlined aircraft, high speeds | | | | far too heavy to fly. So the stiffness of a structure |
| demand high thrust, and turbo-fan or turbo jet engines | | | | depends on both its design and the materials used. |