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.

5.2.1 Buffet

At the initial development phase of stall (or during extreme maneuvers), airflow over the wing becomes unsteady; the separation line over the wing (or over any other lifting surface) keeps fluctuating. This causes the aircraft to shudder and is a warning to the pilot. The aircraft structure is not affected and is not necessarily at its maximum loading.

5.2.2 Flutter

This is the vibration of the structure – primarily the wing but also any other component depending on its stiffness. At transonic speed, the load on the aircraft is high while the shock–boundary layer interaction could result in an unsteady flow causing vibration over the wing, for example. The interaction between aerodynamic forces and structural stiffness is the source of flutter. A weak structure enters into flutter; in fact, if it is too weak, flutter could happen at any speed because the deformation would initate the unsteady flow. If it is in resonance, then it could be catastrophic – such failures have occurred. Flutter is an aeroelastic phenomenon.

5.3 Flight Maneuvers

Although throttle-dependent linear acceleration would generate flight load in the direction of the flight path, pilot-induced control maneuvers could generate the extreme flight loads that may be aggravated by inadvertent atmospheric conditions. Aircraft weight is primarily determined by the air load generated by maneuvers in the pitch plane. Therefore, the associated V-n diagram described in Section 5.7 is useful information for proposing candidate aircraft configurations. Section 3.6 describes the six deg of freedom for aircraft motions – three linear and three angular. Given herein are the three Cartesian coordinate planes of interest.

5.3.1 Pitch Plane (X-Z) Maneuver (Elevator/Canard-Induced)

The pitch plane is the symmetrical vertical plane (i.e., X-Z plane) in which the elevator/canard-induced motion occurs with angular velocity, q, about the Y-axis, in addition to linear velocities in the X-Z plane. Changes in the pitch angle due to angular velocity q results in changes in CL. The most severe aerodynamic loading occurs in this plane.