be located on the bottom centerline of the fuselage or on the wings' upper surfaces. The purpose of
speed brakes is to retard an aircraft's speed. Such brakes are generally used on aerodynamically clean
aircraft such as jets.
The tail section of an aircraft consists of the horizontal stabilizer and elevator, vertical stabilizer, and
rudder as shown in figure 2.8.
The horizontal stabilizer gives the pilot control about the aircraft's lateral (pitch) axis. Some aircraft
have a flying tail in which there is no elevator, only one large movable surface called a stabilizer.
The vertical stabilizer acts the same as a keel surface on a boat, and it is needed for adequate
directional stability. The rudder is also used for the same purpose as a rudder on a boat and it controls
the aircraft around its vertical (yaw) axis.
Daniel Bernoulli, born 1700, discovered the principle bearing his name. Giovanni Venturi, in 1822,
noted the effects of constricting a passage through which fluid flowed. The Venturi tube was invented
by Clemens Hershel, an American engineer. He named it in honor of G. B. Venturi. Newton's three
laws on force and motion are applicable to aerodynamics.
The four forces acting on an aircraft are weight, lift, thrust, and drag. Flight becomes possible when
lift overcomes weight and thrust overcomes drag. The two kinds of airfoils used on Army aircraft are
symmetrical and asymmetrical. An airfoil uses low-pressure air on top of the wing and high-pressure air
under the wing to produce lift. When the wing's angle of attack is increased, the deflection of the
airstream over the wing's upper surface creates more lift. However, if the angle of attack is too great the
airstream breaks away from the upper surface and burbles. At this point, the aircraft can stall.
Ailerons control the aircraft about its bank or roll (longitudinal) axis. Flaps are used to increase the
lift capability of a wing and are used mostly in landing and takeoff.
A true cantilever wing derives its strength from internal wing-design. The boundary layer is that air
closest to the surface of an aircraft's wings, and flows in layers called laminae.