PLANE TALK
The
four forces
One of
the most often asked questions by those 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 give it flight.
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 does 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 toward 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 breeze; however
the paper will rise due to the lowered pressure on the upper side.
This same
principle applies in the way the carburetor of an engine works in providing
fuel to the cylinders and in 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 twenty percent.
The
second force to act upon the aircraft is drag. There are two different types of
drag associated with aircraft. 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 aircraft 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 formed during the
production of lift. As the wings produce lift, a portion of the 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 aircraft 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
also can be turned from the power of a jet engine, as is the case seen on many
corporate and smaller commuter “turboprop” aircraft.
Pure jet
and fan jet engines rely on the thrust of the jet only or on the jet and an
attached fan to provide thrust. Jet engines are very expensive and fuel
thirsty, however they do provide much greater amounts of power at higher
altitudes than do piston engines.
The last
force that acts upon an aircraft is that of gravity. This force keeps humans
firmly planted upon the ground without the use of aerodynamic surfaces to help
them rise above it.
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;
however this explanation suffices to explain the forces that act upon an
aircraft in flight.