Engineering is the art of organizing and directing men and controlling the forces and materials of nature for the benefit of the human race.
Henry G. Stott, 1907
Directional stability is stability around the
vertical or normal axis. The most important feature that affects
directional stability is the vertical tail surface, that is, the
fin and rudder. Keel effect and sweepback also contribute to
directional stability to some degree.
The Fin
An airplane has the tendency always to fly
head-on into the relative airflow. This tendency which might be
described as weather vaning is directly attributable to the
vertical tail fin and to some extent also the vertical side areas
of the fuselage. If the airplane yaws away from its course, the
airflow strikes the vertical tail surface from the side and
forces it back to its original line of flight. In order for the
tail surfaces to function properly in this weather vaning
capacity, the side area of the airplane aft of the center of
gravity must be greater than the side area of the airplane
forward of the C.G. If it were otherwise, the airplane would tend
to rotate about its vertical axis.
Lateral stability is stability around the
longitudinal axis, or roll stability. Lateral stability is achieved through (1)
dihedral, (2) sweepback, (3) keel effect
Dihedral
The dihedral angle is the angle that each wing
makes with the horizontal. The purpose of dihedral is to improve
lateral stability. If a disturbance causes one wing to drop, the
unbalanced force produces a sideslip in the direction of the
downgoing wing. This will, in effect, cause a flow of air in the
opposite direction to the slip. This flow of air will strike the
lower wing at a greater angle of attack than it strikes the upper
wing. The lower wing will thus receive more lift and the airplane
will roll back into its proper position. Since dihedral inclines the wing to the
horizontal, so too will the lift reaction of the wing be inclined
from the vertical. Hence an excessive amount of dihedral will, in
effect, reduce the lift force opposing weight. Some modern airplanes have a measure of
negative dihedral or anhedral, on the wings and/or
stabilizer. The incorporation of this feature provides some
advantages in overall design in certain type of airplanes.
However, it does have an effect, probably adverse, on lateral
stability.
Keel Effect
Dihedral is more usually a feature on low wing
airplanes although some dihedral may be incorporated in high wing
airplanes as well. Most high wing airplanes are laterally stable
simply because the wings are attached in a high position on the
fuselage and because the weight is therefore low. When the
airplane is disturbed and one wing dips, the weight acts as a
pendulum returning the airplane to its original attitude.
Sweepback
A sweptback wing is one in which the leading
edge slopes backward. When a disturbance causes an airplane with
sweepback to slip or drop a wing, the low wing presents its
leading edge at an angle that is perpendicular to the relative
airflow. As a result, the low wing acquires more lift, rises and
the airplane is restored to its original flight attitude. Sweepback also contributes to directional
stability. When turbulence or rudder application causes the
airplane to yaw to one side, the right wing presents a longer
leading edge perpendicular to the relative airflow. The airspeed
of the right wing increases and it acquires more drag than the
left wing. The additional drag on the right wing pulls it back,
yawing the airplane back to its original path.
Longitudinal stability is pitch stability, or
stability around the lateral axis of the airplane. To obtain longitudinal stability, airplanes are
designed to be nose heavy when correctly loaded. The center of
gravity is ahead of the center of pressure. This design feature
is incorporated so that, in the event of engine failure, the
airplane will assume a normal glide. It is because of this nose
heavy characteristic that the airplane requires a tailplane. Its
function is to resist this diving tendency. The tailplane is set
at an angle of incidence that produces a negative lift and
thereby, in effect, holds the tail down. In level, trimmed
flight, the nose heavy tendency and the negative lift of the
tailplane exactly balance each other. Two principal factors influence longitudinal
stability: (1) size and position of the horizontal stabilizer,
and (2) position of the center of gravity.
The Horizontal Stabilizer
The tail plane, or stabilizer, is placed on the
tail end of a lever arm (the fuselage) to provide longitudinal
stability. It may be quite small. However, being situated at the
end of the lever arm, it has great leverage. When the angle of
attack on the wings is increased by a disturbance, the center of
pressure moves forward, tending to turn the nose of the airplane
up and the tail down. The tailplane, moving down, meets the air
at a greater angle of attack, obtains more lift and tends to
restore the balance. On most airplanes, the stabilizer appears to be
set at an angle of incidence that would produce an upward lift.
It must, however, be remembered that the tailplane is in a
position to be in the downwash from the wings. The air that
strikes the stabilizer has already passed over the wings and been
deflected slightly downward. The angle of the downwash is about
half the angle of attack of the main airfoils. The proper angle
of incidence of the stabilizer therefore is very important in
order for it to be effective in its function.
Center of Gravity
The center of gravity is very important in
achieving longitudinal stability. If the airplane is loaded with
the center of gravity too far aft, the airplane may assume a nose
up rather than a nose down attitude. The inherent stability will
be lacking and, even though down elevator may correct the
situation, control of the airplane in the longitudinal plane will
be difficult and perhaps, in extreme cases, impossible.
