Black Hole - How do black holes form? What are they?

"Victoria crater" Image Credit: NASA/JPL/Cornell SPACE

What is aerodynamics?

The word comes from two Greek words: aerios, concerning the air, and dynamis, which means force.
Aerodynamics is the study of forces and the resulting motion of objects through the air. Judging from the
story of Daedalus and Icarus, humans have been interested in aerodynamics and flying for thousands of
years, although flying in a heavier-than-air machine has been possible only in the last hundred years.
Aerodynamics affects the motion of a large airliner, a model rocket, a beach ball thrown near the shore, or a
kite flying high overhead. The curveball thrown by big league baseball pitchers gets its curve from
aerodynamics.

Airplanes are transportation devices which are designed to move people and cargo from one place to
another. Airplanes come in many different shapes and sizes depending on the mission of the aircraft. The
airplane shown on this slide is a turbine-powered airliner which has been chosen as a representative
aircraft.

For any airplane to fly, you must lift the weight of the airplane itself, the fuel, the passengers, and the cargo.
The wings generate most of the lift to hold the plane in the air. To generate lift, the airplane must be pushed
through the air. The jet engines, which are located beneath the wings, provide the thrust to push the
airplane forward through the air. The air resists the motion in the form of aerodynamic drag. Some
airplanes use propellers for the propulsion system instead of jets.

To control and maneuver the aircraft, smaller wings are located at the tail of the plane. The tail usually has a
fixed horizontal piece (called the horizontal stabilizer) and a fixed vertical piece (called the vertical
stabilizer). The stabilizers' job is to provide stability for the aircraft, to keep it flying straight. The vertical
stabilizer keeps the nose of the plane from swinging from side to side, while the horizontal stabilizer
prevents an up-and-down motion of the nose. (On the Wright brother's first aircraft, the horizontal stabilizer
was placed in front of the wings. Such a configuration is called a canard after the French word for "duck").

At the rear of the wings and stabilizers are small moving sections that are attached to the fixed sections by
hinges. In the figure, these moving sections are colored brown. Changing the rear portion of a wing will
change the amount of force that the wing produces. The ability to change forces gives us a means of
controlling and maneuvering the airplane. The hinged part of the vertical stabilizer is called the rudder; it is
used to deflect the tail to the left and right as viewed from the front of the fuselage. The hinged part of the
horizontal stabilizer is called the elevator; it is used to deflect the tail up and down. The outboard hinged part
of the wing is called the aileron; it is used to roll the wings from side to side. Most airliners can also be
rolled from side to side by using the spoilers. Spoilers are small plates that are used to disrupt the flow over
the wing and to change the amount of force by decreasing the lift when the spoiler is deployed.

The wings have additional hinged, rear sections near the body that are called flaps. Flaps are deployed
downward on takeoff and landing to increase the amount of force produced by the wing. On some aircraft,
the front part of the wing will also deflect. Slats are used at takeoff and landing to produce additional force.
The spoilers are also used during landing to slow the plane down and to counteract the flaps when the
aircraft is on the ground. The next time you fly on an airplane, notice how the wing shape changes during
takeoff and landing.

The fuselage or body of the airplane, holds all the pieces together. The pilots sit in the cockpit at the front of
the fuselage. Passengers and cargo are carried in the rear of the fuselage. Some aircraft carry fuel in the
fuselage; others carry the fuel in the wings.

As mentioned above, the aircraft configuration in the figure was chosen only as an example. Individual
aircraft may be configured quite differently from this airliner. The Wright Brothers 1903 Flyer had pusher
propellers and the elevators at the front of the aircraft. Fighter aircraft often have the jet engines buried
inside the fuselage instead of in pods hung beneath the wings. Many fighter aircraft also combine the
horizontal stabilizer and elevator into a single stabilator surface.

Adapted from:

http://www.grc.nasa.gov/WWW/K-12/airplane/airplane.html

All that is necessary to create lift is to turn a flow of air. The airfoil of a wing turns a flow, but so does a
spinning ball. The exact details are fairly complex and are given on a separate slide. Summarizing the
results, the amount of force generated by a spinning ball depends on the amount of spin, the velocity of the
ball, the size of the ball, and the density of the fluid.

The figure shows a view of the flow as if we were moving with the ball looking down from above. The ball
appears stationary, and the flow moves from left to right. As the ball spins, the air near the surface of the
ball moves with the surface of the ball. If there was no free stream flow and the ball was stopped and
spinning, there would be circular flow around the ball which would match the speed of rotation at the
surface and die away to nothing far from the ball. When the free stream flow is added to this circular flow,
the resulting flow has a net turning and produces a force. On the figure the ball spins counterclockwise, so
the free stream flow over the top of the ball is opposed by the circular flow; the free stream flow below the
ball is assisted by the circular flow. In the figure we can see that the streamlines around the ball are
distorted because of the spinning. The net turning of the flow has produced a downward force.

As the force acts on the ball, it is deflected along it's flight path. The mathematical details of the ball's
trajectory are given on a separate slide.

Be particularly aware of the simplifying assumptions that have gone into this analysis. The type of flow field
shown in the figure is called an ideal flow field. We have produced the ideal flow field by superimposing the
flow field from an ideal vortex centered on the ball with a uniform free stream flow. There is no viscosity in
this model, no boundary layer on the ball, even though viscosity is the real origin of the circulating flow! In
reality, the flow around a spinning baseball is very complex. The ball isn't even smooth; the stitches used to
hold the covering together stick up out of the boundary layer. In addition, the flow off the rear of the ball is
separated and can even be unsteady. BUT, the simplified model helps us to determine the important
parameters and the dependence of the lift force on the value of the parameters. To obtain an accurate value
for the force, engineers typically use a lift coefficient that is determined experimentally and accounts for
the details that are too complex to model in the analysis.

Adapted from http://www.grc.nasa.gov/WWW/K-12/airplane/bball.html