5.1 Overview

Aircraft structures must withstand the imposed load during operations; the extent depends on what is expected from the intended mission role. The bulkiness of the aircraft depends on its structural integrity to withstand the design load level. The heavier the load, the heavier is the structure; hence, the MTOW affecting aircraft performance. Aircraft designers must comply with mandatory certification regula­tions to meet the minimum safety standards.

This book does not address load estimation in detail but rather continues with design information on load experienced by aircraft. Although the information pro­vided herein is not directly used in configuring aircraft, the knowledge and data are essential for understanding design considerations that affect aircraft mass (i. e., weight). Only the loads and associated V-n diagram in symmetrical flight are dis­cussed herein. It is assumed that designers are supplied with aircraft V-n diagrams by the aerodynamics and structures groups. Estimation of load is a specialized sub­ject covered in focused courses and textbooks. However, this chapter does outline the key elements of aircraft loads. Aircraft shaping dictates the pattern of pressure distribution over the wetted surface that directly affects load distribution. There­fore, aircraft loads must be known early enough to make a design “right the first time.”

5.1.1 What Is to Be Learned?

This chapter covers the following topics:

Theory and definitions (limit and ultimate load) Limits (load limit and speed limit)

V-n diagram (the safe flight envelope)

Gust envelope

5.1.2 Coursework Content

This chapter provides the basic information required to generate conceptual air­craft configurations. To continue, it is recommended that readers peruse this chapter even though there is no coursework involved yet. The chapter can be skipped if the subject has been learned in other coursework. However, readers should be able to draw schematically a representative V-n diagram of their aircraft (explained in Section 5.8).

5.2 Introduction

Loads are the external forces applied to an aircraft – whether static or dynamic, in flight or on the ground. In-flight loads are due to symmetrical flight, unsymmet­rical flight, or atmospheric gusts from any direction; on-ground loads result from ground handling and field performance (e. g., takeoff and landing). Aircraft design­ers must be aware of aircraft loads given that configurations must be capable of with­standing them. During the design study phase, aerodynamicists compute in-flight aerodynamic loads and relate the information to stress engineers, who ensure struc­tural integrity. Computation of aerodynamic load is involved, currently undertaken using computers. The subject matter concerns interaction between aerodynamics and structural dynamics (i. e., deformation occurring under load), a subject that is classified as aeroelasticity. Even the simplified assumption of an aircraft as an elastic body requires study beyond the scope of this book. Generally, conceptual design addresses rigid aircraft.

User specifications define the maneuver types and speeds that influence aircraft weight (i. e., MTOM), which then dictates aircraft-lifting and control surface design. In addition, enough margin must be allocated to cover inadvertent excessive load encountered through pilot induced maneuvers (i. e., inadvertent internal input in excess of the specifications), or sudden severe atmospheric disturbances (i. e., exter­nal input), or a combination of the two scenarios. The limits of these inadvertent sit­uations are derived from historical statistical data and pilots must avoid exceeding the margins. To ensure safety, governmental regulatory agencies have intervened with mandatory requirements for structural integrity. Load factor (not to be con­fused with the passenger load factor, as described in Section 4.4.1) is a term that expresses structural-strength requirements. The structural regulatory requirements are associated with V-n diagrams, which are explained in Section 5.7. Limits of the margins are set by the regulatory agencies. In fact, they not only stipulate the load limits, they also require mandatory strength tests to determine ultimate loads. The ultimate load tests must be completed before the first flight, with the exceptions of homebuilt and experimental categories of aircraft.

Civil aircraft designs have conservative limits; there are special considerations for the aerobatic category aircraft. Military aircraft have higher limits for hard maneuvers, and there is no guarantee that under threat, a pilot would be able to adhere to the regulations. Survivability requires widening the design limits and strict maintenance routines to ensure structural integrity. Typical human limits are currently taken at 9 g in sustained maneuvers and can reach 12 g for instanta­neous loading. Continuous monitoring of the statistical database retrieved from aircraft-mounted “black boxes” provides feedback to the next generation of aircraft design or at midlife modifications. A g-meter in the flight deck records the g-force and a second needle remains at the maximum g reached in the sortie. If the pre­scribed limit is exceeded, then the aircraft must be grounded for a major inspection and repaired, if required.

An important aspect of design is to know what could happen at the extreme points of the flight envelope (i. e., the V-n diagram). In the following sections, buffet and flutter are introduced.