Page 44 - Aerodynamics for Engineering Students

Basic concepts and definitions 27

(a) High speed level flight (b) Climbing flighi

(c) Low speed level flight (d) Banked circling flight
Fig. 1.7 The direction of the lift force

a component perpendicular to the flight direction, the exact direction in which the lift
acts will be obvious, particularly after reference to Fig. 1.7. This may not apply to
certain guided missiles that have no obvious top or bottom, and the exact meaning of
‘up’ must then be defined with care.

Drag, D(-X)
This is the component of force acting in the opposite direction to the line of flight, or
in the same direction as the motion of the undisturbed stream. It is the force that resists
the motion of the aircraft. There is no ambiguity regarding its direction or sense.

Cross-wind force, Y
This is the component of force mutually perpendicular to the lift and the drag, i.e. in
a spanwise direction. It is reckoned positive when acting towards the starboard
(right-hand to the pilot) wing-tip.

Pitching moment, M
This is the moment acting in the plane containing the lift and the drag, i.e. in the
vertical plane when the aircraft is flying horizontally. It is positive when it tends to
increase the incidence, or raise the nose of the aircraft upwards (using this word in the
sense discussed earlier).

Rolling moment, LR
This is the moment tending to make the aircraft roll about the flight direction, i.e.
tending to depress one wing-tip and to raise the other. It is positive when it tends to
depress the starboard wing-tip.

Yawing moment, N
This is the moment that tends to rotate the aircraft about the lift direction, i.e. to
swing the nose to one side or the other of the flight direction. It is positive when it
swings, or tends to swing, the nose to the right (starboard).