Page 236 - Aircraft Stuctures for Engineering Student

220 Principles of stressed skin construction

than epoxy resin, has an indefinite shelf life and performs well under impact, are being
developed; fabrication, however, requires much higher temperatures. Metal matrix
composites such as graphite-aluminium and boron-aluminium are light-weight
and retain their strength at higher temperatures than aluminium alloys, but are
expensive to produce.
Generally, the use of composites in aircraft construction appears to have reached a
plateau, particularly in civil subsonic aircraft where the fraction of the structure
comprising composites is approximately 15%. This is due largely to the greater
cost of manufacturing composites compared with aluminium alloy structures since
composites require hand crafting of the materials and manual construction processes.
These increased costs are particularly important in civil aircraft construction and are
becoming increasingly important in military aircraft.

The structure of an aircraft is required to support two distinct classes of load: the first,
termed ground load.7, includes all loads encountered by the aircraft during movement
or transportation on the ground such as taxiing and landing loads, towing and
hoisting loads; while the second, air loads, comprises loads imposed on the structure
during flight by manoeuvres and gusts. In addition, aircraft designed for a particular
role encounter loads peculiar to their sphere of operation. Carrier born aircraft, for
instance, are subjected to catapult take-off and arrested landing loads; most large
civil and practically all military aircraft have pressurized cabins for high altitude
flying; amphibious aircraft must be capable of landing on water and aircraft designed
to fly at high speed at low altitude, e.g. the Tornado, require a structure of above
average strength to withstand the effects of flight in extremely turbulent air.
The two classes of loads may be further divided into surface forces which act upon
the surface of the structure, e.g. aerodynamic and hydrostatic pressure, and hoc{i,
forces which act over the volume of the structure and are produced by gravitational
and inertial effects. Calculation of the distribution of aerodynamic pressure over the
various surfaces of an aircraft’s structure is presented in numerous texts on aero-
dynamics and will therefore not be attempted here. We shall, however, discuss the
types of load induced by these various effects and their action on the different
structural components.
Basically, all air loads are the resultants of the pressure distribution over the sur-
faces of the skin produced by steady flight, manoeuvre or gust conditions. Generally,
these resultants cause direct loads, bending, shear and torsion in all parts of the
structure in addition to local, normal pressure loads imposed on the skin.
Conventional aircraft usually consist of fuselage, wings and tailplane. The fuselage
contains crew and payload, the latter being passengers, cargo, weapons plus fuel,
depending on the type of aircraft and its function; the wings provide the lift and
the tailplane is the main contributor to directional control. In addition, ailerons,
elevators and the rudder enable the pilot to manoeuvre the aircraft and maintain
its stability in flight, while wing flaps provide the necessary increase of lift for take-
off and landing. Figure 7.3 shows typical aerodynamic force resultants experienced
by an aircraft in steady flight.