Anthony B. Wight
Studies by da Vinci, Newton, Cayley and others produced a clear picture of the basic forces which act upon objects in flight, whether kite, bird or plane. It will be helpful for students to understand these forces—gravity, lift, drag, and thrust—and their relationships to each other as they trace the Daedalus odyssey.
Weight
is the force produced by earth’s gravity which tends to move all mass toward its center. Gravity acts on an object in flight just as it does on the object on the ground—producing a downward pull or force. (Weight = mass of the object x acceleration due to gravity)
Lift
is a force which acts against the weight to push it upward. When lift is greater than weight, the object rises; when weight is greater than lift, it sinks. Lift (leaving aside considerations of buoyancy) acts only on objects in motion and it is the shape and tilt of the object which influences the amount of the lift.
Thrust
is the force produced by flapping wings, a propeller, jet engine or rocket. For a kite, thrust is provided by the pull of the string (see diagram in figure 2). Thrust, the forward push or pull, acts only on an object in motion.
Drag
if the force produced by the resistance of the air against the moving object. It, too, acts only on an object in motion and can be represented in a diagram as a backward force or pull.
Figure 2 provides simple sketches of a bird, kite and airplane with the four forces drawn as arrows (or “vectors’’). The most fundamental principle of true flight can be clearly stated in terms of the forces: A kite, bird or airplane is in level, steady flight when its
lift equals its weight
and its
drag equals its thrust
.
Air is the medium through which birds and planes fly. In order to fly a plane is dependent upon surfaces with which to support itself in the air. Such surfaces are called
airfoils
. Airfoils are surfaces designed to produce efficient lifting force—wings, tails and propellers are examples. Sir George Cayley, by building many models, learned a key lesson which most students have learned at one time or another with folded paper airplanes—some bodies projected through air give a large amount of lift. A properly designed body projected through the air in a horizontal direction will produce a net lifting force perpendicular to the horizontal direction.
(figure available in print form)
Figure 2: The four forces affecting flight of Birds, Kites, Planes.
