23 April 2008


Elevators have been around for over 150 years.

In the 1800s, new iron and steel production processes revolutionized the world of construction. With sturdy metal beams as their building blocks, architects and engineers could erect monumental skyscrapers hundreds of feet in the air.

But these towers would have been basically unusable if it weren't for another technological innovation that came along around the same time. Modern elevators are the crucial element that makes it practical to live and work dozens of stories above ground. High-rise cities like New York absolutely depend on elevators. Even in smaller multi-story buildings, elevators are essential for making offices and apartments accessible to handicapped people.

In this article, we'll find out how these ubiquitous machines move you from floor to floor. We'll also look at the control systems that decide where the elevator goes and the safety systems that prevent catastrophes.

Hydraulic Elevators

The concept of an elevator is incredibly simple -- it's just a compartment attached to a lifting system. Tie a piece of rope to a box, and you've got a basic elevator.

Of course, modern passenger and freight elevators are a lot more elaborate than this. They need advanced mechanical systems to handle the substantial weight of the elevator car and its cargo. Additionally, they need control mechanisms so passengers can operate the elevator, and they need safety devices to keep everything running smoothly.

There are two major elevator designs in common use today: hydraulic elevators and roped elevators.

Hydraulic elevator systems lift a car using a hydraulic ram, a fluid-driven piston mounted inside a cylinder. You can see how this system works in the diagram below.

The cylinder is connected to a fluid-pumping system (typically, hydraulic systems like this use oil, but other incompressible fluids would also work). The hydraulic system has three parts:

  • A tank (the fluid reservoir)
  • A pump, powered by an electric motor
  • A valve between the cylinder and the reservoir

The pump forces fluid from the tank into a pipe leading to the cylinder. When the valve is opened, the pressurized fluid will take the path of least resistance and return to the fluid reservoir. But when the valve is closed, the pressurized fluid has nowhere to go except into the cylinder. As the fluid collects in the cylinder, it pushes the piston up, lifting the elevator car.

When the car approaches the correct floor, the control system sends a signal to the electric motor to gradually shut off the pump. With the pump off, there is no more fluid flowing into the cylinder, but the fluid that is already in the cylinder cannot escape (it can't flow backward through the pump, and the valve is still closed). The piston rests on the fluid, and the car stays where it is.

To lower the car, the elevator control system sends a signal to the valve. The valve is operated electrically by a basic solenoid switch (check out How Electromagnets Work for information on solenoids). When the solenoid opens the valve, the fluid that has collected in the cylinder can flow out into the fluid reservoir. The weight of the car and the cargo pushes down on the piston, which drives the fluid into the reservoir. The car gradually descends. To stop the car at a lower floor, the control system closes the valve again.

This system is incredibly simple and highly effective, but it does have some drawbacks. In the next section, we'll look at the main disadvantages of using hydraulics.

Pros and Cons of Hydraulics

The main advantage of hydraulic systems is they can easily multiply the relatively weak force of the pump to generate the stronger force needed to lift the elevator car (see How Hydraulic Machines Work to find out how).

But these systems suffer from two major disadvantages. The main problem is the size of the equipment. In order for the elevator car to be able to reach higher floors, you have to make the piston longer. The cylinder has to be a little bit longer than the piston, of course, since the piston needs to be able to collapse all the way when the car is at the bottom floor. In short, more stories means a longer cylinder.

The problem is that the entire cylinder structure must be buried below the bottom elevator stop. This means you have to dig deeper as you build higher. This is an expensive project with buildings over a few stories tall. To install a hydraulic elevator in a 10-story building, for example, you would need to dig at least nine stories deep! (Some hydraulic elevators don't require quite as much digging. Check out this site to learn about these systems.)

The other disadvantage of hydraulic elevators is that they're fairly inefficient. It takes a lot of energy to raise an elevator car several stories, and in a standard hydraulic elevator, there is no way to store this energy. The energy of position (potential energy) only works to push the fluid back into the reservoir. To raise the elevator car again, the hydraulic system has to generate the energy all over again.

The roped elevator design gets around both of these problems. In the next section, we'll see how this system works.

The Cable System

The most popular elevator design is the roped elevator. In roped elevators, the car is raised and lowered by traction steel ropes rather than pushed from below.

The ropes are attached to the elevator car, and looped around a sheave (3). A sheave is just a pulley with a grooves around the circumference. The sheave grips the hoist ropes, so when you rotate the sheave, the ropes move too.

The sheave is connected to an electric motor (2). When the motor turns one way, the sheave raises the elevator; when the motor turns the other way, the sheave lowers the elevator. In gearless elevators, the motor rotates the sheaves directly. In geared elevators, the motor turns a gear train that rotates the sheave. Typically, the sheave, the motor and the control system (1) are all housed in a machine room above the elevator shaft.

The ropes that lift the car are also connected to a counterweight (4), which hangs on the other side of the sheave. The counterweight weighs about the same as the car filled to 40-percent capacity. In other words, when the car is 40 percent full (an average amount), the counterweight and the car are perfectly balanced.

The purpose of this balance is to conserve energy. With equal loads on each side of the sheave, it only takes a little bit of force to tip the balance one way or the other. Basically, the motor only has to overcome friction -- the weight on the other side does most of the work. To put it another way, the balance maintains a near constant potential energy level in the system as a whole. Using up the potential energy in the elevator car (letting it descend to the ground) builds up the potential energy in the weight (the weight rises to the top of the shaft). The same thing happens in reverse when the elevator goes up. The system is just like a see-saw that has an equally heavy kid on each end.

Both the elevator car and the counterweight ride on guide rails (5) along the sides of the elevator shaft. The rails keep the car and counterweight from swaying back and forth, and they also work with the safety system to stop the car in an emergency.

Roped elevators are much more versatile than hydraulic elevators, as well as more efficient. Typically, they also have more safety systems. In the next section, we'll see how these elements work to keep you from plummeting to the ground if something goes wrong.

Safety Systems

In the world of Hollywood action movies, hoist ropes are never far from snapping in two, sending the car and its passengers hurdling down the shaft. In actuality, there is very little chance of this happening. Elevators are built with several redundant safety systems that keep them in position.

The first line of defense is the rope system itself. Each elevator rope is made from several lengths of steel material wound around one another. With this sturdy structure, one rope can support the weight of the elevator car and the counterweight on its own. But elevators are built with multiple ropes (between four and eight, typically). In the unlikely event that one of the ropes snaps, the rest will hold the elevator up.

Even if all of the ropes were to break, or the sheave system were to release them, it is unlikely that an elevator car would fall to the bottom of the shaft. Roped elevator cars have built-in braking systems, or safeties, that grab onto the rail when the car moves too fast.

In the next section, we'll examine a built-in braking system.

Safety Systems: Safeties

Safeties are activated by a governor when the elevator moves too quickly. Most governor systems are built around a sheave positioned at the top of the elevator shaft. The governor rope is looped around the governor sheave and another weighted sheave at the bottom of the shaft. The rope is also connected to the elevator car, so it moves when the car goes up or down. As the car speeds up, so does the governor. The diagram below shows one representative governor design.

In this governor, the sheave is outfitted with two hooked flyweights (weighted metal arms) that pivot on pins. The flyweights are attached in such a way that they can swing freely back and forth on the governor. But most of the time, they are kept in position by a high-tension spring.

As the rotary movement of the governor builds up, centrifugal force moves the flyweights outward, pushing against the spring. If the elevator car falls fast enough, the centrifugal force will be strong enough to push the ends of the flyweights all the way to the outer edges of the governor. Spinning in this position, the hooked ends of the flyweights catch hold of ratchets mounted to a stationary cylinder surrounding the sheave. This works to stop the governor.

The governor ropes are connected to the elevator car via a movable actuator arm attached to a lever linkage. When the governor ropes can move freely, the arm stays in the same position relative to the elevator car (it is held in place by tension springs). But when the governor sheave locks itself, the governor ropes jerk the actuator arm up. This moves the lever linkage, which operates the brakes.

In this design, the linkage pulls up on a wedge-shaped safety, which sits in a stationary wedge guide. As the wedge moves up, it is pushed into the guide rails by the slanted surface of the guide. This gradually brings the elevator car to a stop.

Safety Systems: More Backups

Elevators also have electromagnetic brakes that engage when the car comes to a stop. The electromagnets actually keep the brakes in the open position, instead of closing them. With this design, the brakes will automatically clamp shut if the elevator loses power.

Elevators also have automatic braking systems near the top and the bottom of the elevator shaft. If the elevator car moves too far in either direction, the brake brings it to a stop.

If all else fails, and the elevator does fall down the shaft, there is one final safety measure that will probably save the passengers. The bottom of the shaft has a heavy-duty shock absorber system -- typically a piston mounted in an oil-filled cylinder. The shock absorber works like a giant cushion to soften the elevator car's landing.

In addition to these elaborate emergency systems, elevators need a lot of machinery just to make their stops. In the next section, we'll find out how an elevator operates under normal conditions.

Making the Rounds

Many modern elevators are controlled by a computer. The computer's job is to process all of the relevant information about the elevator and turn the motor the correct amount to put the elevator car where it needs to be. In order to do this, the computer needs to know at least three things.
  • Where people want to go
  • Where each floor is
  • Where the elevator car is

Finding out where people want to go is very easy. The buttons in the elevator car and the buttons on each floor are all wired to the computer. When you press one of these buttons, the computer logs this request.

There are lots of ways to figure out where the elevator car is. In one common system, a light sensor or magnetic sensor on the side of the car reads a series of holes on a long vertical tape in the shaft. By counting the holes speeding by, the computer knows exactly where the car is in the shaft. The computer varies the motor speed so that the car slows down gradually as it reaches each floor. This keeps the ride smooth for the passengers.

In a building with many floors, the computer has to have some sort of strategy to keep the cars running as efficiently as possible. In older systems, the strategy is to avoid reversing the elevator's direction. That is, an elevator car will keep moving up as long as there are people on the floors above that want to go up. The car will only answer "down calls" after it has taken care of all the "up calls." But once it starts down, it won't pick up anybody who wants to go up until there are no more down calls on lower floors. This program does a pretty good job of getting everybody to their floor as fast as possible, but it is highly inflexible.

More advanced programs take passenger traffic patterns into account. They know which floors have the highest demand, at what time of day, and direct the elevator cars accordingly. In a multiple car system, the elevator will direct individual cars based on the location of other cars.

In one cutting-edge system, the elevator lobby works like a train station. Instead of simply pressing up or down, people waiting for an elevator can enter a request for a specific floor. Based on the location and course of all the cars, the computer tells the passengers which car will get them to their destinations the fastest.

Most systems also have a load sensor in the car floor. The load sensor tells the computer how full the car is. If the car is near capacity, the computer won't make any more pick-up stops until some people have gotten off. Load sensors are also a good safety feature. If the car is overloaded, the computer will not close the doors until some of the weight is removed.

In the next section, we'll look at one of the coolest components in an elevator: the automatic doors.


The automatic doors at grocery stores and office buildings are mainly there for convenience and as an aid for handicapped people. The automatic doors in an elevator, on the other hand, are absolutely essential. They are there to keep people from falling down an open shaft.

Elevators use two different sets of doors: doors on the cars and doors opening into the elevator shaft. The doors on the cars are operated by an electric motor, which is hooked up to the elevator computer. You can see how a typical door-opener system works in the diagram below.

The electric motor turns a wheel, which is attached to a long metal arm. The metal arm is linked to another arm, which is attached to the door. The door can slide back and forth on a metal rail.

When the motor turns the wheel, it rotates the first metal arm, which pulls the second metal arm and the attached door to the left. The door is made of two panels that close in on each other when the door opens and extend out when the door closes. The computer turns the motor to open the doors when the car arrives at a floor and close the doors before the car starts moving again. Many elevators have a motion sensor system that keeps the doors from closing if somebody is between them.

The car doors have a clutch mechanism that unlocks the outer doors at each floor and pulls them open. In this way, the outer doors will only open if there is a car at that floor (or if they are forced open). This keeps the outer doors from opening up into an empty elevator shaft.

In a relatively short period of time, elevators have become an essential machine. As people continue to erect monumental skyscrapers and more small buildings are made handicap-accessible, elevators will become an even more pervasive element in society. It is truly one of the most important machines in the modern era, as well as one of the coolest.

Read More......

22 April 2008

How Caching Works

If you have been shopping for a computer, then you have heard the word "cache." Modern computers have both L1 and L2 caches, and many now also have L3 cache. You may also have gotten advice on the topic from well-meaning friends, perhaps something like "Don't buy that Celeron chip, it doesn't have any cache in it!"

It turns out that caching is an important computer-science process that appears on every computer in a variety of forms. There are memory caches, hardware and software disk caches, page caches and more. Virtual memory is even a form of caching. In this article, we will explore caching so you can understand why it is so important.

A Simple Example: Before Cache

Caching is a technology based on the memory subsystem of your computer. The main purpose of a cache is to accelerate your computer while keeping the price of the computer low. Caching allows you to do your computer tasks more rapidly.

To understand the basic idea behind a cache system, let's start with a super-simple example that uses a librarian to demonstrate caching concepts. Let's imagine a librarian behind his desk. He is there to give you the books you ask for. For the sake of simplicity, let's say you can't get the books yourself -- you have to ask the librarian for any book you want to read, and he fetches it for you from a set of stacks in a storeroom (the library of congress in Washington, D.C., is set up this way). First, let's start with a librarian without cache.

The first customer arrives. He asks for the book Moby Dick. The librarian goes into the storeroom, gets the book, returns to the counter and gives the book to the customer. Later, the client comes back to return the book. The librarian takes the book and returns it to the storeroom. He then returns to his counter waiting for another customer. Let's say the next customer asks for Moby Dick (you saw it coming...). The librarian then has to return to the storeroom to get the book he recently handled and give it to the client. Under this model, the librarian has to make a complete round trip to fetch every book -- even very popular ones that are requested frequently. Is there a way to improve the performance of the librarian?

Yes, there's a way -- we can put a cache on the librarian. In the next section, we'll look at this same example but this time, the librarian will use a caching system.

A Simple Example: After Cache

Let's give the librarian a backpack into which he will be able to store 10 books (in computer terms, the librarian now has a 10-book cache). In this backpack, he will put the books the clients return to him, up to a maximum of 10. Let's use the prior example, but now with our new-and-improved caching librarian.

The day starts. The backpack of the librarian is empty. Our first client arrives and asks for Moby Dick. No magic here -- the librarian has to go to the storeroom to get the book. He gives it to the client. Later, the client returns and gives the book back to the librarian. Instead of returning to the storeroom to return the book, the librarian puts the book in his backpack and stands there (he checks first to see if the bag is full -- more on that later). Another client arrives and asks for Moby Dick. Before going to the storeroom, the librarian checks to see if this title is in his backpack. He finds it! All he has to do is take the book from the backpack and give it to the client. There's no journey into the storeroom, so the client is served more efficiently.

What if the client asked for a title not in the cache (the backpack)? In this case, the librarian is less efficient with a cache than without one, because the librarian takes the time to look for the book in his backpack first. One of the challenges of cache design is to minimize the impact of cache searches, and modern hardware has reduced this time delay to practically zero. Even in our simple librarian example, the latency time (the waiting time) of searching the cache is so small compared to the time to walk back to the storeroom that it is irrelevant. The cache is small (10 books), and the time it takes to notice a miss is only a tiny fraction of the time that a journey to the storeroom takes.

From this example you can see several important facts about caching:

  • Cache technology is the use of a faster but smaller memory type to accelerate a slower but larger memory type.

  • When using a cache, you must check the cache to see if an item is in there. If it is there, it's called a cache hit. If not, it is called a cache miss and the computer must wait for a round trip from the larger, slower memory area.

  • A cache has some maximum size that is much smaller than the larger storage area.

  • It is possible to have multiple layers of cache. With our librarian example, the smaller but faster memory type is the backpack, and the storeroom represents the larger and slower memory type. This is a one-level cache. There might be another layer of cache consisting of a shelf that can hold 100 books behind the counter. The librarian can check the backpack, then the shelf and then the storeroom. This would be a two-level cache.

Computer Caches

A computer is a machine in which we measure time in very small increments. When the microprocessor accesses the main memory (RAM), it does it in about 60 nanoseconds (60 billionths of a second). That's pretty fast, but it is much slower than the typical microprocessor. Microprocessors can have cycle times as short as 2 nanoseconds, so to a microprocessor 60 nanoseconds seems like an eternity.

What if we build a special memory bank in the motherboard, small but very fast (around 30 nanoseconds)? That's already two times faster than the main memory access. That's called a level 2 cache or an L2 cache. What if we build an even smaller but faster memory system directly into the microprocessor's chip? That way, this memory will be accessed at the speed of the microprocessor and not the speed of the memory bus. That's an L1 cache, which on a 233-megahertz (MHz) Pentium is 3.5 times faster than the L2 cache, which is two times faster than the access to main memory.

Some microprocessors have two levels of cache built right into the chip. In this case, the motherboard cache -- the cache that exists between the microprocessor and main system memory -- becomes level 3, or L3 cache.

There are a lot of subsystems in a computer; you can put cache between many of them to improve performance. Here's an example. We have the microprocessor (the fastest thing in the computer). Then there's the L1 cache that caches the L2 cache that caches the main memory which can be used (and is often used) as a cache for even slower peripherals like hard disks and CD-ROMs. The hard disks are also used to cache an even slower medium -- your Internet connection.

Caching Subsystems

Your Internet connection is the slowest link in your computer. So your browser (Internet Explorer, Netscape, Opera, etc.) uses the hard disk to store HTML pages, putting them into a special folder on your disk. The first time you ask for an HTML page, your browser renders it and a copy of it is also stored on your disk. The next time you request access to this page, your browser checks if the date of the file on the Internet is newer than the one cached. If the date is the same, your browser uses the one on your hard disk instead of downloading it from Internet. In this case, the smaller but faster memory system is your hard disk and the larger and slower one is the Internet.

Cache can also be built directly on peripherals. Modern hard disks come with fast memory, around 512 kilobytes, hardwired to the hard disk. The computer doesn't directly use this memory -- the hard-disk controller does. For the computer, these memory chips are the disk itself. When the computer asks for data from the hard disk, the hard-disk controller checks into this memory before moving the mechanical parts of the hard disk (which is very slow compared to memory). If it finds the data that the computer asked for in the cache, it will return the data stored in the cache without actually accessing data on the disk itself, saving a lot of time.

Here's an experiment you can try. Your computer caches your floppy drive with main memory, and you can actually see it happening. Access a large file from your floppy -- for example, open a 300-kilobyte text file in a text editor. The first time, you will see the light on your floppy turning on, and you will wait. The floppy disk is extremely slow, so it will take 20 seconds to load the file. Now, close the editor and open the same file again. The second time (don't wait 30 minutes or do a lot of disk access between the two tries) you won't see the light turning on, and you won't wait. The operating system checked into its memory cache for the floppy disk and found what it was looking for. So instead of waiting 20 seconds, the data was found in a memory subsystem much faster than when you first tried it (one access to the floppy disk takes 120 milliseconds, while one access to the main memory takes around 60 nanoseconds -- that's a lot faster). You could have run the same test on your hard disk, but it's more evident on the floppy drive because it's so slow.

To give you the big picture of it all, here's a list of a normal caching system:

  • L1 cache - Memory accesses at full microprocessor speed (10 nanoseconds, 4 kilobytes to 16 kilobytes in size)
  • L2 cache - Memory access of type SRAM (around 20 to 30 nanoseconds, 128 kilobytes to 512 kilobytes in size)
  • Main memory - Memory access of type RAM (around 60 nanoseconds, 32 megabytes to 128 megabytes in size)
  • Hard disk - Mechanical, slow (around 12 milliseconds, 1 gigabyte to 10 gigabytes in size)
  • Internet - Incredibly slow (between 1 second and 3 days, unlimited size)
As you can see, the L1 cache caches the L2 cache, which caches the main memory, which can be used to cache the disk subsystems, and so on.

Cache Technology

One common question asked at this point is, "Why not make all of the computer's memory run at the same speed as the L1 cache, so no caching would be required?" That would work, but it would be incredibly expensive. The idea behind caching is to use a small amount of expensive memory to speed up a large amount of slower, less-expensive memory.

In designing a computer, the goal is to allow the microprocessor to run at its full speed as inexpensively as possible. A 500-MHz chip goes through 500 million cycles in one second (one cycle every two nanoseconds). Without L1 and L2 caches, an access to the main memory takes 60 nanoseconds, or about 30 wasted cycles accessing memory.

When you think about it, it is kind of incredible that such relatively tiny amounts of memory can maximize the use of much larger amounts of memory. Think about a 256-kilobyte L2 cache that caches 64 megabytes of RAM. In this case, 256,000 bytes efficiently caches 64,000,000 bytes. Why does that work?

In computer science, we have a theoretical concept called locality of reference. It means that in a fairly large program, only small portions are ever used at any one time. As strange as it may seem, locality of reference works for the huge majority of programs. Even if the executable is 10 megabytes in size, only a handful of bytes from that program are in use at any one time, and their rate of repetition is very high. On the next page, you'll learn more about locality of reference.

Locality of Reference

Let's take a look at the following pseudo-code to see why locality of reference works (see How C Programming Works to really get into it):
Output to screen « Enter a number  between 1 and 100 »
Read input from user
Put value from user in variable X
Put value 100 in variable Y
Put value 1 in variable Z
Loop Y number of time
Divide Z by X
If the remainder of the division = 0
then output « Z is a multiple of X »
Add 1 to Z
Return to loop
This small program asks the user to enter a number between 1 and 100. It reads the value entered by the user. Then, the program divides every number between 1 and 100 by the number entered by the user. It checks if the remainder is 0 (modulo division). If so, the program outputs "Z is a multiple of X" (for example, 12 is a multiple of 6), for every number between 1 and 100. Then the program ends.

Even if you don't know much about computer programming, it is easy to understand that in the 11 lines of this program, the loop part (lines 7 to 9) are executed 100 times. All of the other lines are executed only once. Lines 7 to 9 will run significantly faster because of caching.

This program is very small and can easily fit entirely in the smallest of L1 caches, but let's say this program is huge. The result remains the same. When you program, a lot of action takes place inside loops. A word processor spends 95 percent of the time waiting for your input and displaying it on the screen. This part of the word-processor program is in the cache.

This 95%-to-5% ratio (approximately) is what we call the locality of reference, and it's why a cache works so efficiently. This is also why such a small cache can efficiently cache such a large memory system. You can see why it's not worth it to construct a computer with the fastest memory everywhere. We can deliver 95 percent of this effectiveness for a fraction of the cost.

Read More......

Classic Airplanes

In a sense, all airplanes are classic airplanes, because each one represents the very best its designer and builder could do, given the talent, materials, and time available at the moment. No development group ever set out to make a second-best airplane. Instead, every aircraft, and especially every classic aircraft described and pictured within these pages, was the product of the loving care of an intelligent design team.

Classic Airplane Image Gallery

SPAD VII classic airplane
French fighter pilots who were seriously challenged by top-rank German airplanes during World War I welcomed the rugged SPAD VII. See more classic airplane pictures.

The following pages in this article provide links to profiles of classic airplanes built over the last century. You'll begin with classic airplanes of the Early Years, including the Wright Flyers first successful flight. Learn about the military fighter airplanes of World War I and World War II, and explore the aircraft built during the Golden Age of Flight. Then fast-forward to the present day Jet Age and see how much classic airplanes have progressed over the last 100 years.

Lockheed P-38 Lighting, World War II fighter-bomber
Lockheed's immortal P-38 Lightning was a multi-role fighter-bomber that was a scourge of the Axis, particularly the Japanese, during World War II.

Each and every one of these classic airplanes was manufactured by skilled and motivated people who worked long hours -- often at their own expense -- to turn out a world-beater. Each was flown by test pilots who risked their lives to make the designers' dreams come true. Each was flown in war or for commerce by equally dedicated pilots who wanted only to use it in the most effective manner. And each was maintained by loyal workers, often unappreciated, who were responsible for the lives of all who piloted and flew in the craft.

In truth, classic airplanes are no more than mirrors in which we find reflected the human beings who created and used them. See the next page for links to profiles of classic airplanes built between 1903 to 1913.

The Early Years, 1903-1913

The collaborative genius of Orville and Wilbur Wright completely transcended the efforts of all of their predecessors in the field of aviation. Not only had they leaped beyond the most advanced innovator of the day, Otto Lilienthal, they corrected the errors they had found in his mathematical tables.
Bleriot XI classic airplane
In 1909, Louis Bleriot flew his Bleriot XI monoplane above the Notre-Dame de Paris.

On December 17, 1903, the Wright brothers' four flights carried them into the record books as the first to make a controlled powered flight, and established them as being far ahead of all competitors in Europe.

The links below provide details and specifications for early classic airplanes:

The Wright Flyers: 1903, 1905, and 1908
Learn about the Wright brothers' first powered flight, which took place on December 17, 1903. Their first airplane, the Kitty Hawk Flyer, was the world's first military aircraft.

Bleriot XI
The need for speed and the powerful engines of the Bleriot XI led to the tragic deaths of many famous pilots, including America's first licensed female pilot. Get the details here.

Curtiss Golden Flyer
Sometimes called the Gold Bug, this classic airplane was built by Glenn Hammond Curtiss and was inspired by the Wright Flyer. Spot the differences and similarities between the Golden Flyer and the Wright Flyer in this article.

The Wright brothers would demonstrate their leadership in the next five years, only to lose it suddenly and dramatically to new ideas from abroad. Follow links to classic European airplanes of World War I on the next page.

World War I, 1914-1918

World War I had a tremendous effect upon the development of aviation, creating an entirely new military arm, one only dimly foreseen in the past, but immediately important. Huge industries sprang up to produce the aircraft, engines, and compo­nents required for the new and extremely expensive service. Because the stalemated war in the trenches was so horrible, the war in the air was given a sense of chivalry and honor by the press and public.
Red Baron, Sopwith Camel, classic airplane
Above, the Red Baron (red plane) is shown closing on the Sopwith Camel of Canadian Lt. Wilfred May, unaware that Capt. Roy Brown has slid into position behind him.

Each new aircraft was carefully examined and the men who won five victories to become aces were regarded as popular heroes.

Below are links to some of the classic airplanes that these brave soldiers flew:

Curtiss JN-4
Nicknamed "the Jenny," this classic airplane was originally mass-produced for the American effort in World War I but was also used as a crop duster, a stunt plane, and an entertainment aircraft after the war. Learn more here.

Nieuport 17
Learn about this extremely influential classic airplane, also known as "Bebe." Copies of the Nieuport 17 were eventually produced at other companies around the world -- some with fatal results.

Gotha G.V
Because of the Gothas' weight and balance problems, more of these German fighter planes were lost due to pilot mishap and accidents than enemy fighters. Learn about this long-range heavy bomber.

Albatros D. Va
Even though the German Albatros was handsome and well built, it didn't prove successful in the German air effort during World War I. Read about this classic fighter plane.

Sopwith Camel F.1
The most famous of all fighter planes, the British Camel brought down more enemies than any other Allied plane during World War I. Find out how this classic airplane was also lethal to its own pilots.

Fokker Dr I Triplane
In an effort to remedy the problems of the Albatros, the Germans built the Fokker Dr I Triplane, which showed promise until a flawed wing design caused a series of fatal crashes.

This classic airplane's name is an acronym for Societe pour Aaviation et ses Derives, which was a firm under the control of Louis Bleriot. Get more details here.

De Havilland DH-4
When the U.S. entered World War I, the Air Service decided to produce the British D. H.4s with American Liberty engines. Learn about this classic airplane's role in World War I.

The years between World War I and World War II were known as the Golden Age of Flight. Discover the aircraft built across the globe on the next page.

The Golden Age of Flight, 1919-1938

The first two decades after World War I, known as the Golden Age of Flight, saw the appearance of some of the most beautiful and most efficient aircraft in history. All over the world, regardless of their country's size or its relative wealth, aircraft designers were busy producing the very best aircraft they could conceive.
Ryan NYP Spirit of St. Louis, Golden Age of Flight
The excitement and infinite promise of flight during the Golden Age was exemplified by the Ryan NYP Spirit of St. Louis, piloted famously in 1927 by Charles Lindbergh.

The Golden Age of Flight was a time when much could be done with relatively small resources, as when the Granville brothers designed an aircraft in an aban­doned dance hall, and witnessed it suddenly become a world-beater.

Follow these links to classic airplanes of the Golden Age:

Polikarpov I-16
Find out how this classic airplane, considered the Rodney Dangerfield of fighter planes, surprised the German Luftwaffes and the rest of the world during World War II.

Martin B-10
The American Martin B-10 bomber plane is held as the most important and beautiful contributions to the Golden Age of Flight. Learn why in this article.

Ford Tri-Motor
Henry Ford, a name commonly associated with classic cars, turned his talents toward classic airplanes in 1924. Read all about the Ford "Tin Goose" Tri-Motor.

Ryan NYP Spirit of St. Louis
In 1927, Charles Lindbergh and the Spirit of St. Louis successfully flew the first non-stop solo flight across the Atlantic. Learn about the legend here.

Lockheed Vega

"Aviatrix" Amelia Earhart shared many firsts with the Lockheed Vega, including the first successful flight from Hawaii to California. Find out why she chose this classic airplane for her flying adventures.

Hawker Hart
An effective light bomber, a trainer and a seaplane, the Hawker Hart was a classic airplane with many accomplishments. Explore the many sides of the Hawker Hart.

Piper J-3 Cub
With its economical price, the Piper J-3 Cub helped democratize civil aviation. Read about the classic airplane that earned its place as "an American classic in peace and war."

Beech Staggerwing
See specifications for the Beech Model 17, a classic airplane launched at the very depth of the Depression and which got its nickname from the reverse stagger of its wings.

Granville Brothers Gee Bee Super Sportster R-1
The Gee Bee was built for the 1932 air-racing season and, flown by pilot Jimmy Doolittle, quickly became America's sweetheart. Get a closer look into the adventures of this classic airplane.

Martin Model 130 China Clipper
With the ability to make saltwater landings in exotic places, the "flying boat" took passengers throughout the Pacific for attractive scenery. Discover the glamor of this classic airplane.

By the time World War II began, the progress of aviation had taken many great leaps forward. Move on to the next page for links to classic World War II fighter airplanes.

World War II, 1939-1945

World War II accelerated the pace of aviation at an even faster rate than World War I, and on a far greater scale. Aviation was in its infancy prior to the First World War, and almost all of the pro­g­ress in establishing an aviation industry and in determining the uses of air power took place as the war was going on.
F4U Corsair, World War II classic fighter airplane
A carrier-based F4U Corsair bears down relentlessly on a pair of Japanese Zeros in a heated moment during the Pacific air war.

Aviation progress had a running start beginning in 1939, so it was possible for aviation to make great strides not only in aircraft performance but also in many new disciplines required to fight an air war. The field saw advancements in navigation, radar, communications and improved ordnance.

Follow the links below for more details of classic World War II fighter planes:

Douglas C-47
This classic airplane started life as the best-selling airliner of its day, placing the U.S. in the lead in commercial aviation. Learn how the C-47 became one of the most effective warplanes in history.

Lockheed P-38 Lightning
The P-38 was the only U.S. fighter plane that was produced prewar and continued to be produced when the war ended. Find out what made this classic plane so intriguing.

Focke Wulf Fw 190

Read about a classic German airplane that was beautiful, versatile, and considered one of the best fighter airplanes of World War II.

Boeing B-29 Superfortress
The most famous B-29 was the Enola Gay, which dropped an atomic bomb on Hiroshima and forced Japan to surrender, bringing World War II to an end.

Junkers Ju 87 Stuka
Although slow, the Stuka proved incredibly effective as a German dive-bomber during World War II. Discover how this classic airplane fought bravely until the very end.

Supermarine Spitfire
The Spitfire was a superior plane built by the Supermarine Company for the British Royal Air Force. Get details on this handsome hero of World War II.

Boeing B-17
Learn about the four-engine "Flying Fortress," aptly named for its ability to survive damage during bombing missions and still return safely.

Grumman F4F Wildcat
The Wildcat started as an underdog and ended a champion, successfully taking down inferior Japanese bombers. See photos and specs in this article.

Messerschmitt Bf 109
German engineers succeeded with their attempt to fit an enormous engine in a small airframe, resulting in the Bf 109 and 100 more variants of the original design.

Douglas SBD Dauntless

A supreme dive-bomber of the Pacific War, the Dauntless is known for its efforts during the Battle of Midway -- sinking Japanese carriers and reversing the course of World War II.

North American B-25 Mitchell
Explore the details of the classic airplane flown by Lt. Col. Jimmy Doolittle during the first bombing raid against Tokyo in April 1942.

Curtiss P-40 Warhawk
The P-40 served in all theaters of World War II, including the Pacific, Alaska, Africa, and Russia, and is famous for its performance with the AVG Flying Tigers.

Consolidated B-24 Liberator
The Liberator was vital to the American effort in World War II -- its many roles included bombing, special operations, and carrying cargo and passengers. Learn more here.

Mitsubishi A6M Zero
Read about the Japanese Zero, a symbol of Imperial Japan that succeeded early on in the war only to be surpassed by continually improving American aircrafts and pilots.

Grumman F6F Hellcat
The Hellcat's first appearance in World War II battle was during the U.S. attack against the Japanese on Marcus Island in September 1943.

Yakovlev Yak-9

Yak-9s were flown bravely by the Soviets against German fighters. Learn why this classic plane proved faster and more maneuverable than German Bf 109s.

Chance Vought F4U Corsair
Also called the "Bent-wing Bird," the F4U Corsair dominated Japanese enemies in World War II and went on to serve in the Korean War.

North American P-51 Mustang
The Mustang classic airplane had the ability to defeat every Axis fighter it encountered. Learn about it's history and the many restored models that still fly today.

Messerschmitt Me 262
If the production of the German Me 262 had not been delayed, the outcome of World War II might have changed drastically. Learn how this classic airplane could have altered history.

The classic fighter planes of World War II will always be remembered as brave warriors, but the arrival of the Jet Age changed the way we see aircraft. Follow the links on the next page for more details.

The Jet Age, 1946-Present

Douglas AE-4 Skyhawks, classic airplanes of the Jet Age
Jet power in the raw: A double threat of well-armed Douglas AE-4 Skyhawks streaks from the clouds, superbly piloted and ready to take on all challengers.

The end of World War II found the victorious Allies with thousands of fighter planes, all but a few of which were obsolete because the Jet Age had arrived. Although peacetime budgets were cut to a minimum, military and civil leaders poured as much money as possible into research and development.

The result was a series of ever more powerful and reliable jet engines, as well as new and increasingly radical airframes. The growth in jet power was paralleled by a revolution in electronics.

This, plus the combination of new engines, new airframes, and advanced avionics resulted in aircraft of superb performance and amazing longevity.

Below you'll find links to profiles, specifications and photos of these classic airplanes of the Jet Age:

Grumman EA-6B Prowler
The Prowler serves the U.S. Navy, Marines and Air Force with its high-performance electronic capabilities. Learn more here.

Boeing 747
The Boeing 747's size amazed the public and had many people wondering if it would even be able to fly. Read about the largest airliner in history.

Mcdonnell Douglas F-4 Phantom II
Get details on the classic airplane that served as the United States' principal fighter in Vietnam and dominated combat aviation with its versatility and speed.

Lockheed F-117A Nighthawk Stealth Fighter
The sharp, angular frame of the F-117A Stealth gave the classic airplane a modern look, but its main feature made it almost "invisible." Learn more about the F-117A's ability to deflect incoming radar beams here.

Mikoyan-Guryevich MiG-15
The Soviet-built MiG-15 fighter appeared in combat during the Korean War and, by 1953, was abandoned by the Soviets as a front-line fighter.

North American F-86 Sabre
Many pilots view the Sabre as the last "pure" fighter plane. Find out what made this classic airplane so special and versatile.

Douglas A-4 Skyhawk
With more than 29 variants used by the U.S. Navy and Marines, the lightweight Skyhawk was truly a remarkable classic combat airplane, remaining in production for 25 years.

Boeing B-52 Stratofortress
The B-52, also known as the Buff, has proven successful in Vietnam, the Persian Gulf War and other important missions. Learn the Buff's many roles.

Mikoyan-Guryevich MiG-21
This Soviet aircraft was one of the greatest jets of its time with different versions serving in nearly 40 air forces across the globe. Get the details here.

Boeing 707
The creation of the 707 was a huge step for globalization, making international travel easier for the public. Read more about this landmark in aviation history.

Lockheed SR-71 Blackbird
The Blackbird was named for its heat-resistant black paint and remains one of the world's fastest, highest-flying aircraft since its first flight in 1964.

McDonnell Douglas F-15 Eagle
The F-15 Eagle was created after the Vietnam War when pilots felt the need for a new fighter plane that would dominate the sky. Take a closer look at this classic airplane in this article.

General Dynamics F-16 Fighting Falcon
Often referred to as the "Electric Jet" rather than the "Fighting Falcon," the F-16 is fully equipped with the latest radars, night vision equipment and missiles. See specifications and photos here.

Northrop Grumman B-2 Spirit
The B-2's unusual appearance and high cost -- approximately $2.2 billion per copy -- prove just how much classic airplanes have evolved over the century.

Read More......

do you know SMS Works ?

Composing a text message on a cell phone

Just when we're finally used to seeing everybody constantly talking on their cell phones, it suddenly seems like no one is talking at all. Instead, they're typing away on tiny numerical pads, using their cell phones to send quick messages. SMS, or text messaging, has replaced talking on the phone for a new "thumb generation" of texters.

In this article, we'll find out how text messaging works, explore its uses and learn why it sometimes takes a while for your text message to get to its recipient.

SMS stands for short message service. Simply put, it is a method of communication that sends text between cell phones, or from a PC or handheld to a cell phone. The "short" part refers to the maximum size of the text messages: 160 characters (letters, numbers or symbols in the Latin alphabet). For other alphabets, such as Chinese, the maximum SMS size is 70 characters.

SMS Attacks
Recently it has been suggested that SMS messages could be used to attack a cell phone system. The basic idea is very simple. If a large number of SMS messages were sent by computers to phones in a small geographical area (like a city), these messages would overwhelm the control channels and make it impossible for the cell phone system to set up calls. Now that cell phone providers know about the possibility of this threat, they can design systems to throttle messages coming from the SMSC onto the network.

But how do SMS messages actually get to your phone? If you have read How Cell Phones Work, you can actually see what is happening.

Even if you are not talking on your cell phone, your phone is constantly sending and receiving information. It is talking to its cell phone tower over a pathway called a control channel. The reason for this chatter is so that the cell phone system knows which cell your phone is in, and so that your phone can change cells as you move around. Every so often, your phone and the tower will exchange a packet of data that lets both of them know that everything is OK.

Your phone also uses the control channel for call setup. When someone tries to call you, the tower sends your phone a message over the control channel that tells your phone to play its ringtone. The tower also gives your phone a pair of voice channel frequencies to use for the call.

The control channel also provides the pathway for SMS messages. When a friend sends you an SMS message, the message flows through the SMSC, then to the tower, and the tower sends the message to your phone as a little packet of data on the control channel. In the same way, when you send a message, your phone sends it to the tower on the control channel and it goes from the tower to the SMSC and from there to its destination.

Diagram of the path of an text message

The actual data format for the message includes things like the length of the message, a time stamp, the destination phone number, the format, etc. For a complete byte-by-byte breakdown of the message format, see this page.

Why 160 Characters?
SMS was designed to deliver short bursts of data such as numerical pages. To avoid overloading the system with more than the standard forward-and-response operation, the inventors of SMS agreed on a 160-character maximum message size.

But the 160-character limit is not absolute. Length limitations may vary depending on the network, phone model and wireless carrier. Some phones don't allow you to keep typing once the 160-character limit is reached. You must send your message before continuing. However, some services will automatically break any message you send into chunks of 160 characters or less. So, you can type and send a long message, but it will be delivered as several messages.

SMS Criticism and Alternatives

SMS in the News
Because of the impersonal nature of SMS, it raises certain questions of etiquette -- namely, what kind of information is OK to send in a text instead of delivering it in person? Recently, several people have sought legal action after they were fired or notified of divorce proceedings via SMS.

Broadcast text messages have been used to rally political activists in Beijing and to mobilize young people for riots in Belfast. Recently, a contest pitted the efficiency of SMS against Morse code (the Morse coders won).

Despite their popularity, short messaging services have received some criticism. Here are a few of the disadvantages of SMS:

  • You have to pay for it. Most wireless plans charge for a certain number of text messages a month. Some only charge for user-originated messages, while others charge for incoming messages as well. If you exceed your message allowance, you may be charged 10 cents per message, and those little charges can add up.

  • Speedy message delivery is not guaranteed. During periods of high traffic, it might be minutes or even hours before a message gets through.

  • It's strictly for sending text messages. SMS does not support sending pictures, video or music files.

Alternatives to SMS
Alternative messaging services allow for more elaborate types of messages. With EMS (Enhanced Messaging Service), you can send formatted text, sound effects, small pictures and icons. MMS (Multimedia Messaging Service) allows you to send animations, audio and video files in addition to text. If your mobile phone is EMS- or MMS-enabled, you can use these standards just as you would SMS. However, the cost per message will be higher.

Another alternative to using SMS is using an instant messaging program, such as AOL IM, on your cell phone. This can be in the form of software that's pre-installed on your phone, or you can use WAP (Wireless Application Protocol) to access the Internet and sign into your IM account. WAP is a protocol that gives you small, simplified versions of web pages that are easily navigable on your mobile phone or PDA (check out How WAP Works for more information). You can use it to send instant messages or actual e-mails from your phone.

A common complaint about SMS is its inefficient delivery structure -- when the message center is backed up, messages take longer to reach their destination. To make message delivery faster, networks are using more new next-generation technologies such as GPRS (General Packet Radio Service).

To learn more about SMS and other forms of mobile communication, check out the links on the following page.

Read More......

20 April 2008

Why is the Google algorithm so important?

Finding useful information on the World Wide Web is something many of us take for granted. According to the Internet research firm Netcraft, there are nearly 150,000,000 active Web sites on the Internet today [source: Netcraft]. The task of sifting through all those sites to find helpful information is monumental. That's why search engines use complex algorithms -- mathematical instructions that tell computers how to complete assigned tasks.

Google Home Page
Google's search engine gets more traffic than any
other Web site. So what's the company's secret
algorithm? No one can be sure.

Google's algorithm does the work for you by searching out Web pages that contain the keywords you used to search, then assigning a rank to each page based several factors, including how many times the keywords appear on the page. Higher ranked pages appear further up in Google's search engine results page (SERP), meaning that the best links relating to your search query are theoretically the first ones Google lists.

For Web page administrators, being listed prominently on Google can result in a big boost in site traffic and visibility. In 2007, Google surpassed Microsoft as the most visited site on the Web [source: The San Francisco Chronicle]. With that much traffic, getting a good spot on a Google SERP could mean a huge boost in the number of site visitors.

Are You Down with ODP?
The Open Directory Project (ODP) is a Web directory maintained by a large staff of volunteers. Each volunteer oversees a category, and together volunteers list and categorize Web sites into a huge, comprehensive directory. Because a real person evaluates and categorizes each page within the directory, search engines like Google use the ODP as a database for search results. Getting a site listed on the ODP often means it will show up on Google.

Google's keyword search function is similar to other search engines. Automated programs called spiders or crawlers travel the Web, moving from link to link and building up an index page that includes certain keywords. Google references this index when a user enters a search query. The search engine lists the pages that contain the same keywords that were in the user's search terms. Google's spiders may also have some more advanced functions, such as being able to determine the difference between Web pages with actual content and redirect sites -- pages that exist only to redirect traffic to a different Web page.

Keyword placement plays a part in how Google finds sites. Google looks for keywords throughout each Web page, but some sections are more important than others. Including the keyword in the Web page's title is a good idea, for example. Google also searches for keywords in headings. Headings come in a range of sizes, and keywords in larger headings are more valuable than if they are in smaller headings. Keyword dispersal is also important. Webmasters should avoid overusing keywords, but many people recommend using them regularly throughout a page.

Video Gallery: Algorithms
Haile is a robotic percussionist that analyzes live players' music in real time and plays back an improvised beat. It is designed to combine the computational power and algorithmic music with the richness and expression of acoustic performance. Watch the robotic percussionist in concert in this video from Georgia Tech.

In the next section, we'll learn about Google's patented PageRank system.

Google's PageRank System

The Google algorithm's most important feature is arguably the PageRank system, a patented automated process that determines where each search result appears on Google's search engine return page. Most users tend to concentrate on the first few search results, so getting a spot at the top of the list usually means more user traffic.

Google SERP
This Google SERP shows the results of a search
for HowStuffWorks.

So how does Google determine search results standings? Many people have taken a stab at figuring out the exact formula, but Google keeps the official algorithm a secret. What we do know is this:

  • PageRank assigns a rank or score to every search result. The higher the page's score, the further up the search results list it will appear.
  • Scores are partially determined by the number of other Web pages that link to the target page. Each link is counted as a vote for the target. The logic behind this is that pages with high quality content will be linked to more often than mediocre pages.
  • Not all votes are equal. Votes from a high-ranking Web page count more than votes from low-ranking sites. You can't really boost one Web page's rank by making a bunch of empty Web sites linking back to the target page.
  • The more links a Web page sends out, the more diluted its voting power becomes. In other words, if a high-ranking page links to hundreds of other pages, each individual vote won't count as much as it would if the page only linked to a few sites.
  • Other factors that might affect scoring include the how long the site has been around, the strength of the domain name, how and where the keywords appear on the site and the age of the links going to and from the site. Google tends to place more value on sites that have been around for a while.
  • Some people claim that Google uses a group of human testers to evaluate search returns, manually sorting through results to hand pick the best links. Google denies this and says that while it does employ a network of people to test updated search formulas, it doesn't rely on human beings to sort and rank search results.

Google's strategy works well. By focusing on the links going to and from a Web page, the search engine can organize results in a useful way. While there are a few tricks webmasters can use to improve Google standings, the best way to get a top spot is to consistently provide top quality content, which gives other people the incentive to link back to their pages.

Can't Buy Me (Google) Love
Google says that it won't sell prime search results spots. Every site you see in the results section on a Google SERP is there because of the PageRank system. Google does sell space for sponsored links above and next to the search results, but it uses shaded boxes and borders to alert users to the difference between normal search results and sponsored links.

To learn more about search engines and related topics, follow the links on the next page.

Read More......

19 April 2008

the Node Explorer Works

Imagine taking a trip to a battlefield memorial and spending the day walking from monument to monument, reading signs about historical events. To a lot of people, this sounds educational, but not exciting. But suppose that instead of reading signs, you watch reenactments and interviews on a portable media player. As part of an interactive tour, the player also shows you maps and timelines. Also, what you see and hear changes depending on where you are within the park.

Photo courtesy Node

This location-based media player, called the Explorer, changes your walk in the park into an interactive learning experience. Node, a British media company, created the Explorer for use in museums, historical sites and other cultural centers. People have compared it to the "Hitchhiker's Guide to the Galaxy" and the Marauder's Map from the "Harry Potter" series. It uses global positioning system (GPS) technology to determine where someone is within a site and presents interactive information based on that location.

The Explorer's presentations are interactive, and they can include guided tours, images, maps, videos and sound clips. In this article, we'll look at the Explorer's hardware and software and see how its use can affect the tourism industry.


Like a portable media center, the Explorer is essentially a handheld computer. It uses a Linux operating system, and it processes and stores interactive presentations using:

It then plays them using a trans-reflective, high resolution touch screen and 3-D stereo headphones. Its most remarkable feature it is that it uses GPS "Fast Fix" technology to choose which items to play based the visitor's location within the site. It can also mark the visitor's location on an on-screen map.

The Explorer unit is just one part of the wireless Node network, which also includes:

  • Recharging and data collection docks
  • A central server
  • Web-enabled computers, which staff members use to access Node software

In the next section, we'll look at the Node software in more detail.

Location-Based Services
Companies are using location-based services to deliver traffic reports, driving directions, coupons, movie listings and other information to people depending on where they are. These services are a blend of three types of technology:
  • Mobile devices, like PDAs, cell phones or laptop computers
  • Wireless communication systems, like cell phone networks or wireless network connections
  • Positioning technology, like GPS receivers

Read More......