Parachute Science - Physics of Parachutes

http://www.buzzle.com

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.