Flight mechanics

Pagina 1

FLIGHT DYNAMICS, TOPIC 2 FLIGHT MECHANICS 2.1 Longitudinal static stability: isolated wing and tailless aircraft In order to analyze the longitudinal equilibrium and static stability of an isolated wing, some assumptions might be taken. The aircraft is assumed to be a rigid body (no aero-elastic effects are taken into account) and its configuration features a longitudinal plane of symmetry in terms of geometry, aerodynamics, inertia and propulsion; it is assumed to fly in cruise flight, hence the center of gravity moves following a straight trajectory, the velocity lies on the longitudinal plane and the climb angle is null. Moreover, the thrust do not provide any normal forces. Considering the atmosphere to be not disturbing, the only present forces are: aerodynamic linear forces (lift, drag, pitch moment), mass forces (weight) and thrust forces. It should be noted that inertia forces are neglected due to the static analysis. Applying these hypothesis, the equilibrium system can be written as:  It shall be noted that the pitch moment computed from the center of gravity is only affected by aerodynamic forces. The pitch moment and, as a consequence, the pitch moment coefficient changes when the angle of attack changes. For instance, either the possible shift of the aerodynamic resultant and the motion of the center of gravity due to fuel consumption affects the value of  , but this causes are neglected for a first approximation order. By looking at the  tendency with respect to the angle of attack, it is possible to infer that the longitudinal equilibrium condition is reached for one and only one value of the angle of attack  (up to this point stick are fixed and no control surfaces are present). If aerodynamics is linear (low angle of attack flight), the lift coefficient…