The Weight Drivers
The factors that drive aircraft weight are listed herein. References  through  discuss more detail on aircraft material, stress, and structures. Aircraft material properties given herein are typical for comparing relative merits. Material elasticity, E, and density, p, provide the strength-to-weight ratio. In the alloys and material categories, there is variation.
1. Weight is proportionate to size, indicated by geometry (i. e., length, area, and volume).
2. Weight depends on internal structural-member density – that is, the denser, the heavier.
3. Weight depends on a specified limit-load factor n (see Chapter 5) for structural integrity.
4. Fuselage weight depends on pressurization, engine and undercarriage mounts, doors, and so forth.
5. Lifting-surface weight depends on the loading, fuel carried, engine and undercarriage mounts, and so forth.
6. Weight depends on the choice of material. There are seven primary types used in aircraft, as follows:
(a) Aluminum alloy (a wide variety is available – in general, the least expensive)
typical E = 11 x 106 lb/in2; typical density = 0.1 lb/in3
(b) Aluminum-lithium alloy (fewer types available – relatively more expensive)
typical E = 12 x 106 lb/in2; typical density = 0.09 lb/in3
(c) Stainless-steel alloy (hot components around engine – relatively inexpensive)
typical E = 30 x 106 lb/in2; typical density = 0.29 lb/in3
(d) Titanium alloy (hot components around engine – medium-priced but lighter)
typical E = 16 x 106 lb/in2; typical density = 0.16 lb/in3
(e) Composite type varies (e. g., fiberglass, carbon fiber, and Kevlar); therefore, there is a wide variety in elasticity and density (price relatively inexpensive to expensive). (For details, refer to  and .)
(f) Hybrid (metal and composite “sandwich” – very expensive; e. g., Glare).
(g) Wood (rarely used except for homebuilt aircraft; is not discussed in this book – price increasing).
In this book, the primary load-bearing structures are constructed of metal; secondary structures (e. g., floorboard and flaps) could be made from composites. On the conservative side, it generally is assumed that composites and/or new alloys
comprise about 10 to 15% of the MEM for civil aircraft and about 15 to 25% of the MEM for military aircraft. The use of composites is increasing, as evidenced in current designs. Although composites are used in higher percentages, this book remains conservative in approach. All-composite aircraft have been manufactured, although only few in number (except small aircraft). The metal-composite sandwich is used in the Airbus 380 and Russia has used aluminum-lithium alloys. In this book, the consequences of using newer material is addressed by applying factors.