FOUR BASIC FORCES
One of the
most often asked questions by people who are not familiar with aviation is,
"How do airplanes fly?" Although a seemingly simple question at
first, the answer has four complex components.
Four forces act
upon the airframe and surfaces of an aircraft to provide it with an ability to
fly. Those forces are lift, drag, thrust, and gravity.
The wings of
the aircraft produce the lift component. The wing of an aircraft is a specially
designed shape with a curved upper surface and a relatively flat underside. As
the wing (or airfoil) moves through the air, the air moving along the upper
surface of the wing must travel a greater distance to reach the end of the wing
than the air traveling along the underside of the wing. As both the sides of the wing are traveling
through the air at same speed, the air on the upper surface must accelerate to
a higher speed and travel a greater distance than does the air traveling along
the underside of the wing. As this air accelerates along the upper side of the
wing the relative pressure of the air is lowered. The difference between the
lower pressure on the upper side of the wing and the higher pressure on the
lower side of the wing produces a lifting moment towards the side of low
pressure, this accounts for the lift component.
You can
demonstrate this for yourself, take a letter-sized sheet of paper, and hold it
between your fingers in front of your face. The paper should be held at the
bottom and allowed to fall away from you. The paper will take the approximate
shape of an airfoil. Now blow gently directly at the curved front edge of the
paper. One would expect the paper to move away from the gentle blow of air
however, the paper will rise due to the lowered pressure on the upper side.
This same
principle applies in the way a carburetor on an engine works in pulling fuel to
the cylinders, which is as simple a thing as pulling liquid through a
straw. About 80 percent of the lift
produced by a wing is due to the lower pressure on the upper side. The higher
pressure on the underside of the wing produces only about 20 percent.
The second
force to act upon the aircraft is that of drag. There are two different types
of drag associated with aircrafts. The first is parasite or form drag. This is
the force holding an aircraft back due to the structure that is being pulled or
pushed through the air and is the reason all modern aircrafts are
streamlined. Naturally a small
streamlined object will travel through the air more easily than will a large
boxy object.
The second
type of drag is known as induced drag. Induced drag is drag formed during the
production of lift. As the wings produce lift a portion of air is displaced
downward and to the rear of the wing. The portion that is displaced downward
aids in the production of lift. The portion that is displaced to the rear is
known as induced drag.
The third
force that acts upon the aircraft is thrust. All aircraft with the exception of
gliders and sailplanes rely upon either piston engines or jet engines to
produce thrust. Piston engine aircrafts turn propellers to provide rearward
thrust thereby moving the aircraft forward. The blades of the propeller are
wings turning very fast and providing lift in the horizontal plane. Propellers
can also be turned from the power of a jet engine as seen on many corporate and
smaller commuter aircraft. Turbojet and Fanjet engines produce thrust through
the continuous burning of jet fuel that is expelled at the rear of the engine,
providing high thrust and capability for higher performance. Jet engines are
very expensive and fuel thirsty, however they do provide greater amounts of
power at higher altitudes than do piston engines.
The last force
that acts upon an aircraft is that of gravity or weight. This force keeps
everything firmly planted upon the ground without the use of aerodynamic
surfaces to help. For the aircraft to fly the amount of lift produced must
equal the weight of the aircraft and cargo.
There are
complex relationships between the forces of lift, drag, thrust, and gravity,
especially when turning forces and changing weather patterns including
temperature are factored in.
Robert Perry
will answer all questions on matters aeronautical. Address your questions to "PLANE
TALK" care of this newspaper.
EAA group
meets Sep. 15
EAA Chapter
491 for Santa Ynez will be holding its monthly meeting at 10 a.m. on Saturday,
Sep. 15 in the group’s hanger on “J” row. Non-members are invited to this
meeting of pilots, enthusiasts and other interested in aviation. Coffee and
snacks will be served and open hangers and hanger stories will follow the
meeting by those involved.