| Home | Privacy | Contact |

Pilot's Handbook of Aeronautical Knowledge
Aerodynamics of Flight
Aircraft Design Characteristics

| First | Previous | Next | Last |

Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Dihedral for lateral stability.
Figure 4-25. Dihedral for lateral stability.

Conversely, excessive dihedral has an adverse effect on
lateral maneuvering qualities. The aircraft may be so stable
laterally that it resists an intentional rolling motion. For this
reason, aircraft that require fast roll or banking characteristics
usually have less dihedral than those designed for less

Sweepback is an addition to the dihedral that increases the
lift created when a wing drops from the level position. A
sweptback wing is one in which the leading edge slopes
backward. When a disturbance causes an aircraft 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 aircraft is restored to its original flight attitude.
Sweepback also contributes to directional stability. When
turbulence or rudder application causes the aircraft 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, turning
the aircraft back to its original path.

Keel Effect and Weight Distribution
An aircraft always has the tendency to turn the longitudinal
axis of the aircraft into the relative wind. This "weather vane"
tendency is similar to the keel of a ship and exerts a steadying
influence on the aircraft laterally about the longitudinal
axis. When the aircraft is disturbed and one wing dips, the
fuselage weight acts like a pendulum returning the airplane
to its original attitude.

Laterally stable aircraft are constructed so that the greater
portion of the keel area is above and behind the CG.
[Figure 4-26] Thus, when the aircraft slips to one side, the
combination of the aircraft's weight and the pressure of the
airflow against the upper portion of the keel area (both acting
about the CG) tends to roll the aircraft back to wings-level

Keel area for lateral stability.
Figure 4-26. Keel area for lateral stability.

Vertical Stability (Yawing)
Stability about the aircraft's vertical axis (the sideways
moment) is called yawing or directional stability. Yawing or
directional stability is the most easily achieved stability in
aircraft design. The area of the vertical fin and the sides of
the fuselage aft of the CG are the prime contributors which
make the aircraft act like the well known weather vane or
arrow, pointing its nose into the relative wind.

In examining a weather vane, it can be seen that if exactly the
same amount of surface were exposed to the wind in front
of the pivot point as behind it, the forces fore and aft would
be in balance and little or no directional movement would
result. Consequently, it is necessary to have a greater surface
aft of the pivot point than forward of it.

Similarly, the aircraft designer must ensure positive
directional stability by making the side surface greater aft
than ahead of the CG. [Figure 4-27] To provide additional
positive stability to that provided by the fuselage, a vertical
fin is added. The fin acts similar to the feather on an arrow
in maintaining straight flight Like the weather vane and the
arrow, the farther aft this fin is placed and the larger its size,
the greater the aircraft's directional stability.

If an aircraft is flying in a straight line, and a sideward gust
of air gives the aircraft a slight rotation about its vertical
axis (i.e., the right), the motion is retarded and stopped by
the fin because while the aircraft is rotating to the right, the
air is striking the left side of the fin at an angle. This causes
pressure on the left side of the fin, which resists the turning
motion and slows down the aircraft's yaw. In doing so, it acts somewhat like the weather vane by turning the aircraft
into the relative wind. The initial change in direction of the
aircraft's flightpath is generally slightly behind its change
of heading. Therefore, after a slight yawing of the aircraft
to the right, there is a brief moment when the aircraft is still
moving along its original path, but its longitudinal axis is
pointed slightly to the right.