Parachute Science - Physics of Parachutes
By Prashant Magar
Published: 8/6/2009
Parachutes have long been an object of interest and renewed developments in human history. The idea of a man descending down from a great height without injuring himself, lead to the development of parachute science or the physics of parachutes...
Parachute Science - Physics of Parachutes
The word parachute is a combination of two words, 'para'
which is a Latin word for 'against' and 'chute', which is a French word for
'fall'. The idea of a parachute is to cushion a fall. It is achieved by a
combination of various physics concepts, which gradually developed over the
years. Records of the use of parachute or attempts at parachuting have been
traced as far back as the 9th century Chinese civilization. Significant
developments and sketching of the parachute design was vigorously pursued in
Italy, during the pre-renaissance and the renaissance period. The most notable
among the contributions in this field was that of Leonardo Da Vinci. Many of
the future designs for parachutes were influenced by his work. The first ever
flying trial was conducted in 1595 by Faust Vrancic, a Croatian inventor. Since
that time, to the present day, the design and implementation of parachutes has
gone through innumerable changes due to developments in parachute science.
Demystifying the Physics of Parachutes
So how does a parachute work? Consider a simple example to
understand the working principle of a parachute. If you drop a shut umbrella
and an open umbrella (in the conventional position) from the top of a building,
the closed one would fall quickly to the surface below, while the open one will
fall slower, and with relatively much less force. A parachute works on similar
lines. It cushions a fall due to greater resistance of air on the large surface
area of the parachute fabric. This large surface area, made of a lightweight
and flexible fabric, creates an air drag, which acts in opposition to the fall.
The air molecules covered by the large surface area of the fabric tend to move
upwards applying a reverse force to the force of gravity. The cloth design is
such that it is sufficiently strong to avoid tear and also elastic enough to
get maximum drag effect.
Depending on the application area, there are different types
of parachute designs. The tapered parachutes provide a variable resistance to
the fall at different points on the envelope. This enables better control and
speed adjustment. Same is the case with the rectangular parachutes. These have
dense fabrication of air cells, which provides greater safety. Such parachutes
are usually used for recreational and training activities.
Physics of parachutes further integrated zero porosity and
rip cord technology. The rip cord works to cushion the sudden stresses that
come into play when a parachute is opened and ensures proper deployment. The
ripping effect on opening a parachute can in fact rip a human body. On the
other hand, a firm grip of the ropes on the fabric can cause problems in
opening or timely opening of the parachute. The rip cord setup facilitates a
smoother functionality of this system. Zero porosity science deals with the
nylon fabric. It prevents the air trapped under the surface of the fabric from
escaping through the cloth fabric, ensuring a safe and cushioned parachute
landing .
There are many other physics applications being used in
parachutes, such as the square or cruciform type shapes, specially designed to
reduce turbulence and vigorous swinging during descent. Annular and pull down
type, Rogallo wing design ram-air parachutes have excellent maneuvering while
ribbon ring parachutes are used to fly out at supersonic speeds.
Parachute science has come a long way, since its
conceptualization in history, and continues to make great strides and
advancements, proving to be a blessing to people stranded in the air, among its
various other uses.
How Helicopters Fly
http://science.howstuffworks.com
by Marshall Brain [accessed, 2009]
You can begin to understand
how a helicopter flies by thinking about the abilities displayed in the
previous section. Let's walk through the different abilities and see how they
affect the design and the controls of a helicopter.
Imagine that we would like to
create a machine that can simply fly straight upward. Let's not even worry
about getting back down for the moment -- up is all that matters. If you are
going to provide the upward force with a wing, then the wing has to be in motion
in order to create lift. Wings create lift by deflecting air downward and
benefiting from the equal and opposite reaction that results (see How Airplanes
Work for details -- the article contains a complete explanation of how wings
produce lift).
A rotary motion is the
easiest way to keep a wing in continuous motion. So you can mount two or more
wings on a central shaft and spin the shaft, much like the blades on a ceiling
fan. The rotating wings of a helicopter are shaped just like the airfoils of an
airplane wing, but generally the wings on a helicopter's rotor are narrow and
thin because they must spin so quickly. The helicopter's rotating wing assembly
is normally called the main rotor. If you give the main rotor wings a slight
angle of attack on the shaft and spin the shaft, the wings start to develop
lift.
In order to spin the shaft
with enough force to lift a human being and the vehicle, you need an engine of
some sort. Reciprocating gasoline engines and gas turbine engines are the most
common types. The engine's driveshaft can connect through a transmission to the
main rotor shaft. This arrangement works really well until the moment the
vehicle leaves the ground. At that moment, there is nothing to keep the engine
(and therefore the body of the vehicle) from spinning just like the main rotor
does. So, in the absence of anything to stop it, the body will spin in an
opposite direction to the main rotor. To keep the body from spinning, you need
to apply a force to it.
The usual way to provide a force
to the body of the vehicle is to attach another set of rotating wings to a long
boom. These wings are known as the tail rotor. The tail rotor produces thrust
just like an airplane's propeller does. By producing thrust in a sideways
direction, counteracting the engine's desire to spin the body, the tail rotor
keeps the body of the helicopter from spinning. Normally, the tail rotor is
driven by a long drive shaft that runs from the main rotor's transmission back
through the tail boom to a small transmission at the tail rotor.
What you end up with is a
vehicle that looks something like this:
The helicopter shown in the
previous videos has all of the parts labeled in the diagram above.
In order to actually control
the machine, both the main rotor and the tail rotor need to be adjustable. The
following two sections explain how the adjustability works.
Elephant and Feather - Air
Resistance
http://www.physicsclassroom.com
1996-2009 The Physics
Classroom
Suppose that an elephant and
a feather are dropped off a very tall building from the same height at the same
time. We will assume the realistic situation that both feather and elephant
encounter air resistance. Which object - the elephant or the feather - will hit
the ground first? The animation at the right accurately depicts this situation.
The motion of the elephant and the feather in the presence of air resistance is
shown. Further, the acceleration of each object is represented by a vector
arrow.
Most people are not surprised
by the fact that the elephant strikes the ground before the feather. But why
does the elephant fall faster? This question is the source of much confusion
(as well as a variety of misconceptions). Test your understanding by making an
effort to identify the following statements as being either true or false.
TRUE or FALSE:
1. The elephant encounters a smaller force of air
resistance than the feather and therefore falls faster.
2. The elephant has a greater acceleration of gravity
than the feather and therefore falls faster.
3. Both elephant and feather have the same force of
gravity, yet the acceleration of gravity is greatest for the elephant.
4. Both elephant and feather have the same force of
gravity, yet the feather experiences a greater air resistance.
5. Each object experiences the same amount of air
resistance, yet the elephant experiences the greatest force of gravity.
6. Each object experiences the same amount of air
resistance, yet the feather experiences the greatest force of gravity.
7. The feather weighs more than the elephant, and
therefore will not accelerate as rapidly as the elephant.
8. Both elephant and feather weigh the same amount, yet
the greater mass of the feather leads to a smaller acceleration.
9. The elephant experiences less air resistance and
than the feather and thus reaches a larger terminal velocity.
10. The feather experiences more air resistance than the
elephant and thus reaches a smaller terminal velocity.
11. The elephant and the feather encounter the same amount
of air resistance, yet the elephant has a greater terminal velocity.
If you answered TRUE to any
of the above questions, then perhaps you have some confusion about either the
concepts of weight, force of gravity, acceleration of gravity, air
resistance and terminal velocity. The elephant and the feather are
each being pulled downward due to the force of gravity. When initially dropped,
this force of gravity is an unbalanced force. Thus, both elephant and feather
begin to accelerate (i.e., gain speed). As the elephant and the feather begin
to gain speed, they encounter the upward force of air resistance. Air
resistance is the result of an object plowing through a layer of air and
colliding with air molecules. The more air molecules which an object collides
with, the greater the air resistance force. Subsequently, the amount of air
resistance is dependent upon the speed of the falling object and the surface
area of the falling object. Based on surface area alone, it is safe to assume
that (for the same speed) the elephant would encounter more air resistance than
the feather.
But why then does the
elephant, which encounters more air resistance than the feather, fall faster?
After all doesn't air resistance act to slow an object down? Wouldn't the
object with greater air resistance fall slower?
Answering these questions
demands an understanding of Newton's first and second law and the concept of
terminal velocity. According to Newton's laws, an object will accelerate if the
forces acting upon it are unbalanced; and further, the amount of acceleration
is directly proportional to the amount of net force (unbalanced force) acting
upon it. Falling objects initially accelerate (gain speed) because there is no
force big enough to balance the downward force of gravity. Yet as an object
gains speed, it encounters an increasing amount of upward air resistance force.
In fact, objects will continue to accelerate (gain speed) until the air
resistance force increases to a large enough value to balance the downward
force of gravity. Since the elephant has more mass, it weighs more and experiences
a greater downward force of gravity. The elephant will have to accelerate (gain
speed) for a longer period of time before their is sufficient upward air
resistance to balance the large downward force of gravity.
Once the upward force of air
resistance upon an object is large enough to balance the downward force of
gravity, the object is said to have reached a terminal velocity. The terminal
velocity is the final velocity of the object; the object will continue to fall
to the ground with this terminal velocity. In the case of the elephant and the
feather, the elephant has a much greater terminal velocity than the feather. As
mentioned above, the elephant would have to accelerate for a longer period of
time. The elephant requires a greater speed to accumulate sufficient upward air
resistance force to balance the downward force of gravity. In fact, the
elephant never does reach a terminal velocity; the animation above shows that
there is still an acceleration on the elephant the moment before striking the ground.
If we were to depict the relative magnitude of the two forces acting upon the
elephant and the feather at various times in their fall, perhaps it would
appear as shown below. (NOTE: The magnitude of the force vector is indicated by
the relative size of the arrow.)
Observe from the above
diagrams and the above animation that the feather quickly reaches a balance of
forces and thus a zero acceleration (i.e., terminal velocity). On the other
hand, the elephant never does reach a terminal velocity during its fall; the
forces never do become completely balanced and so there is still an
acceleration. If given enough time, perhaps the elephant would finally
accelerate to high enough speeds to encounter a large enough upward air
resistance force in order to achieve a terminal velocity. If it did reach a
terminal velocity, then that velocity would be extremely large - much larger
than the terminal velocity of the feather.
So in conclusion, the
elephant falls faster than the feather because it never reaches a terminal
velocity; it continues to accelerate as it falls (accumulating more and more
air resistance), approaching a terminal velocity yet never reaching it. On the
other hand, the feather quickly reaches a terminal velocity. Not requiring much
air resistance before it ceases its acceleration, the feather obtains the state
of terminal velocity in an early stage of its fall. The small terminal velocity
of the feather means that the remainder of its fall will occur with a small
terminal velocity.