Introduction to Aircraft Aeroelasticity and Loads

16

Gust and Turbulence Encounters

It is a well known but unfortunate feature of air travel that aircraft regularly encounter atmospheric turbulence (or ‘rough air’) of varying degrees of severity. Turbulence may be considered as movement of the air through which the aircraft passes. Any component of the velocity of the air (so-called ‘gust velocity’) that is normal to the flight path, as illustrated in Figure 16.1 for the vertical gust case, will change the effective incidence of the aerodynamic surfaces, so causing sudden changes in the lift forces and hence a dynamic response of the aircraft involving flexible deformation; gust inputs are also considered along the flight path. The response will involve both the rigid body and flexible modes, may give rise to passenger and crew discomfort and will introduce internal loads that need to be considered for aircraft safety. Thus it is important for the safe design of the aircraft to calculate the response and internal loads generated under the conditions defined by the Airworthiness Authorities, and to evaluate the effect on the fatigue life. Gust and turbulence loads are significant throughout the aircraft.

In this chapter, gusts and turbulence will be considered in two idealized forms, namely the tuned discrete gust (1-cosine shaped) and continuous (random) turbulence. The process of determining the response of an aircraft to gusts and continuous turbulence will be shown using a progression of models of differing complexity and using ...