Page 87 - Aeronautical Engineer Data Book

Section 4

Aeronautical definitions

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.