LOADS ON AIRCRAFT STRUCTURE AND STRUCTURAL STRESSES

LOADS ON AIRCRAFT STRUCTURE and structural stresses

Loads on aircraft structure: Structural loads on aircraft are from various sources. 

Outstanding loads are:

(i) Flight load

(ii) Ground load

(iii) G- load

(iv) Load due to maneuvering: Landing load, T.O load, taxi load

Structural stresses: A loaded structural member of the aircraft is subject to a force externally applied to it. It does not deform because an internal reactive force develops to oppose the load. This internal force per unit cross section is called the ‘stress’. Stress is measured by measuring external force per x-sectional area.

Stress = Force /Area

Loaded member always develops stress within it. So, loaded member is also called stressed member.

There are five kinds of structural stresses that may be developed in stressed member or structure due to five kinds of loading. (See Figure 1.1)

a) Tensile Stress:  Due to Tensile Load

b) Compressive Stress: Due to Compressive Load

c)  Torsional Stress: Due to Torsional Load

d)  Shear Stress: Due to Shear Load or Shear Force

e) Bending Stress: Due to Bending Force.

Tensile stress (Figure 1.1a) is the stress that resists a force that tends to pull apart. The engine pulls the aircraft forward, but air resistance tries to hold it back. The result is tension, which tries to stretch the aircraft. The tensile strength of a material is measured in p.s.i. (Pounds per square inch.) and is calculated by dividing the load (in pounds) required to pull the material apart by its cross sectional (in square inches)

Compressive stress or the Compression (Figure 1.1b) is the stress that resists a crushing force. The compressive strength of a material is also measured in p.s.i. compression is the stress that tends to shorten or squeeze aircraft parts.

Torsional stress is the stress that produces twisting (Figure 1.1c). While moving the aircraft forward, the engine also tends to twist it to one side, but other aircraft components hold it on course. Thus, torsion is created. The torsional strength of a material is its resistance to twisting or torque.

Figure 1.1: Five structural stresses

Shear stress is the stress that resists the force tending to cause one layer of a material to slide over an adjacent layer. Two riveted plates in tension (Figure 1.1d) subject the rivets to the shearing force. Usually, the shearing strength of a material is either equal to or less than its tensile or compressive strength. Aircraft parts specially screws, bolts, and rivets, are often subject to a shearing force.

Bending stress is a combination of compression and tension. The rod in Figure 1.1e has been shortened (compressed) on the inside of the bend and stretched on the outside of the bend.

The airframe components are constructed from a wide variety of materials and are joined by rivets, bolts, screws, and welding or adhesives. The aircraft components are composed of various structural members. Aircraft structural members are deigned to carry a load or to resist stress.

A single member of the structure may be subjected to a combination of stresses.  In most cases the structural members are designed to carry and loads rather than side loads that is, to be subjected to tension or compression rather than bending.

Strength may be the principal requirement in certain structures, while others need entirely different qualities. For example, cowling, fairing, and similar parts usually are not required to carry the stresses imposed by flight or the landing loads. However, these parts most have such properties as near appearance and streamlined shapes.