5.2 Introduction

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