Aeronautical dictionary - www.asec.ir U www.ASEC.ir U Section 4 Aeronautical definitions 4.1 Forces and moments Forces and moments play an important

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4.1 Forces and moments Forces and moments play an important part in the science of aeronautics. The basic definitions are: Weight force ( W ) Weight of aircraft acting vertically downwards. Aerodynamic force Force exerted (on an aircraft) by virtue of the diversion of an airstream from its origi­ nal path. It is divided into three compo­ nents: lift, drag and lateral. Lift force ( L ) Force component perpendicularly ‘upwards’ to the flight direction. Drag force ( D ) Force component in the opposite direction to flight. Total drag is subdivided into pressure drag and surface friction drag. Pressure drag Force arising from resolved components of normal pressure. Pressure drag is sub­ divided into boundary layer pressure or form drag, vortex or induced drag, and wave drag. Surface friction drag Force arising from surface or skin friction between a surface and a fluid. Pitching moment ( M ) Moment tending to raise the nose of an aircraft up or down. It acts in the plane defined by the lift force and drag force.
67 Aeronautical definitions Lift force ( L ) D ) Pitching moment (+ M ) W Drag force ( Aircraft climbing L Rolling moment ( L R ) moment ( N ) W D Yawing Lift (+ L ) x Nose yaws to right (+ N ) Drag (+ D Lateral (+ Y y Left wing Left wing up (+ L R ) up (+ L R ) Nose pitches up (+ M ) Fig. 4.1 Forces, moments and motions
Aeronautical Engineer’s Data Book Rolling moment ( L R ) Moment tending to roll an aircraft about its nose-to-tail axis (i.e. to raise or lower the wing tips). Yawing moment ( N ) Moment tending to swing the nose of an aircraft to the left or right of its direction of flight. Figure 4.1 shows the basic sign conventions that are used. Motions are often also referred to by their relation to x -, y -, z -axes: See Table 4.1. Table 4.1 The general axis system Axis Moment Moment of Angular inertia displacement x L R (roll) I x y M (pitch) I y z N (yaw) I z
Aeronautical definitions 69 Vertical tail arm Overall height Wheelbase Tail span Wing span Wing leading edge Wing trailing edge Tip chord LE sweep angle 1 / 4 chord sweep angle Mean aerodynamic chord (MAC) LE sweep angle Front fuselage Mean aerodynamic tail chord (MAC) Root chord Centre of gravity Overall height Tail span Wing dihedral Γ Wheel track Upper surface Chord line Camber line Lower surface Leading edge Trailing edge Fig. 4.2 Basic aircraft terminology
70 Aeronautical Engineer’s Data Book 4.2 Basic aircraft terminology Table 4.2 Basic aircraft terminology (see also Figure 4.2) Aspect ratio ( A ) A measurement of the ‘narrowness’ of the wing form.

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