In light aircraft the covering is often of fabric, doped to proof it against air, weather and sunlight. The covering is required to take the pressure of air flowing over it, but makes only a small contribution to the strength. Fig 2-10 Cantilever Structure Fig 2-11 Shear Forces
In faster aircraft, sheet metal, normally aluminium alloy, is used. Because of its extra strength, a large part of the load can be borne by this skin, and the underlying structure can be reduced to save weight. This is called stressed skin construction. It also has the advantage of providing a very smooth surface, because the skin is stiff enough not to sag, which is important for high-speed aircraft. It is possible to make the skin strong enough to carry all of the load, without the need for any supporting framework, a method called monocoque construction. In practice, the size of most aircraft would mean that the skin would need to be very thick to avoid buckling, and a compromise, called semi-monocoque , is usually used (Figure 2-12). In modern aircraft, it is common for the stressed skin to carry about half of the total load carried by the skin/stringer combination. The figure shows a fuselage, consisting of a series of hoops, or frames , at intervals along the skin, which give the fuselage its cross-sectional shape, connected by stringers which run the length of the fuselage. Stringers are really stiffeners to prevent the skin buckling. Semi-monocoque is an ideal form of construction for fuselages, since it leaves most of the interior free of obstruction, and is used in most aircraft. 33.4.2-7 CHAPTER 2 Stressed Skin Construction Fig 2-12 Semi- Monocoque Fuselage Monocoque Construction Stringers
AIRFRAME DESIGN STRUCTURES 33.4.2-8 Self Assessment Questions 1. How many main types of structural pieces in an airframe component? a. 2 b. 3 c. 5 d. 4 2. What structure is widely used in aircraft design? 3. What are stringers? Do not mark the paper in any way - write your answers on a separate piece of paper, in the form of a sentence.
33.4.3-1 CHAPTER 3 AIRFRAME DESIGN FEA TURES - SHAPE Introduction 1. The shape of an aircraft is extremely important, because it dictates how well an aircraft can do a particular job. For a slow-flying aircraft which needs to lift heavy loads, a large wing is needed, together with a fairly light structure. For fast jets, a much smaller wing is required, and the aircraft will be more streamlined. The structure of the aircraft will be much stronger and stiffer, to resist the high loads arising from high speed flight, and tight turns. In this chapter, we will look at how aircraft shapes are decided, and other factors which influence the design. WING LOADING 2. One of the most important factors in an aircraft design is its wing loading , which is simply its weight divided by its wing area. The weight of the aircraft can vary, both with the load it is carrying and as a result of manoeuvres (flying at 4g in a turn increases an aircraft’s effective weight to four times its normal weight), so its wing loading will change. A useful guide is to use the maximum take-off weight
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