Aircraft Structural Loads

Introduction

Before the structure can be designed, we need to determine the loads that will be imposed on the aircraft. This section deals with the general issue of aircraft loads and how they are predicted at the early stages of the design process.

Each part of the aircraft is subject to many different loads. In the final design of an aircraft structure, one might examine tens of thousands of loading conditions of which several hundred may be critical for some part of the airplane. In addition to the obvious loads such as wing bending moments due to aerodynamic lift, many other loads must be considered. These include items such as inertia relief, the weight and inertial forces that tend to reduce wing bending moments, landing loads and taxi-bump loads, pressurization cycles on the fuselage, local high pressures on floors due to high-heeled shoes, and many others.

These loads are predicted using Navier-Stokes computations, wind tunnel tests, and other simulations. Static and dynamic load tests on structural components are carried out to assure that the predicted strength can be achieved. The definition of strength requirements for commercial aircraft is specified in FAR Part 25 and this section deals with those requirements in more detail.

Some Definitions

Many of the load requirements on aircraft are defined in terms of the load factor, n. The load factor is defined as the component of aerodynamic force perpendicular to the longitudinal axis divided by the aircraft weight. Assuming the angle of attack is not large, n = L/W. This is the effective perpendicular acceleration of the airplane in units of g, the acceleration due to gravity.

The FAA establishes two kinds of load conditions:

• Limit Loads are the maximum loads expected in service. FAR Part 25 (and most other regulations) specifies that there be no permanent deformation of the structure at limit load.
  
  
• Ultimate loads are defined as the limit loads times a safety factor. In Part 25 the safety factor is specified as 1.5. For some research or military aircraft the safety factor is as low as 1.20, while composite sailplane manufacturers may use 1.75. The structure must be able to withstand the ultimate load for at least 3 seconds without failure.

The remainder of this section deals with the computation of the limit load factor with additional detail on:

• Design Airspeeds and the Placard Diagram
• The V-n Diagram
• Text of FAA gust criteria (FAR 25 App. G)
• Text of FAA structures requirements (FAR 25.301)