Gas turbine engines come in many shapes and sizes. One type discussed in How turbine engines work includes a normal "jet" engine on an airplane. The hot gases produced by the burning fuel drive vanes in exactly the same way that wind turns a windmill. The vanes connect to a shaft that also spins the turbine's compressor. Another type of gas turbine engine, popular in tanks and helicopters, has one set of vanes for driving the compressor, as well as a separate set of vanes that drive the output shaft. In both of these types of engines, you need to get the main shaft spinning to start the engine.
This starting process normally uses an electric motor to spin the main turbine shaft. The motor is bolted to the outside of the engine and uses a shaft and gears to connect to the main shaft. The electric motor spins the main shaft until there is enough air blowing through the compressor and the combustion chamber to light the engine. Fuel starts flowing and an igniter similar to a spark plug ignites the fuel. Then fuel flow is increased to spin the engine up to its operating speed. If you have ever been at the airport and watched a big jet engine start up, you know that the blades start rotating slowly. The electric starter motor does that. Then you (sometimes) hear a pop and see smoke come out of the back of the engine. Then the engine spins up and starts producing thrust.
On smaller turbine engines (especially home-built models), another way to start the engine is to simply blow air through the air intake with a hair dryer or leaf blower. This technique has the same effect of getting air moving through the combustion chamber, but does not require the complexity or weight of an attached starter motor.
Besides the starter shaft, most big jet engines include another output shaft for driving things like electrical generators, air conditioning compressors, etc. needed to operate the plane and keep it comfortable. This shaft can connect to the main turbine shaft at the same point the starter does or elsewhere. Some jet airplanes have a separate turbine (sometimes in the tail cone of the plane) that does nothing but generate auxiliary power. It is more efficient to run this smaller turbine when the plane is sitting on the tarmac.
When we think of stored oxygen, what we usually think about is large metal tanks holding pressurized oxygen gas. This is the way we see oxygen in hospitals and on welding rigs. We also see SCUBA divers taking their oxygen with them in the form of compressed air in SCUBA tanks. Because they are so common, we tend to think that heavy metal tanks are the only way to store oxygen.
It turns out that there is also a chemical way to store oxygen. Many chemicals, including potassium chlorate (KCl03) and sodium chlorate (NaCl03) are rich in oxygen and are willing to give it up as a nearly pure gas when heated. The SCUBA tanks that divers wear might weigh up to 80 pounds (36 kg) but can store only a few hours of air. An oxygen canister weighing half that can provide about four days worth of oxygen. The sodium chlorate is acting something like an oxygen sponge, and you squeeze the oxygen out with heat.
Modern oxygen canisters are an extremely lightweight way to store oxygen. You find oxygen canisters (also known as chemical oxygen generators) on airplanes, submarines and space stations -- places where oxygen can run out unexpectedly. Typically, an oxygen canister contains a sodium chlorate pellet or cylinder and an igniter. The igniter can be triggered by friction or impact. It generates enough heat to start the sodium chlorate reaction, and then the heat of the reaction sustains itself. The sodium chlorate does not burn -- its decomposition just happens to give off lots of heat and lots of oxygen.
The reason why oxygen canisters can cause fires is because they are hot and they generate oxygen. Anything nearby that happens to ignite will burn intensely because of the rich oxygen supply.
A jet engine, like a rocket engine, is a reaction engine. It works by throwing mass in one direction and taking advantage of the reaction in the opposite direction. In the case of a jet engine, the engine burns fuel (like kerosene) with air from the atmosphere. The burning fuel heats and expands the air, and this hot air shoots out of the exhaust-end of the engine to create thrust.
Most modern jet engines use a turbine to improve the efficiency of the engine and allow the engine to work at low speeds. One part of the turbine sucks in air and compresses it before the fuel is injected. The back portion of the turbine acts like a windmill, extracting energy from the exhaust gases and using the energy to spin the compressor portion. See How Jet Engines Work for details.
A modern turbine engine is extremely efficient, and there is still a lot of oxygen available in the exhaust stream. The idea behind an afterburner is to inject fuel directly into the exhaust stream and burn it using this remaining oxygen. This heats and expands the exhaust gases further, and can increase the thrust of a jet engine by 50% or more.
The big advantage of an afterburner is that you can significantly increase the thrust of the engine without adding much weight or complexity to the engine. An afterburner is nothing but a set of fuel injectors, a tube and flame holder that the fuel burns in, and an adjustable nozzle. A jet engine with an afterburner needs an adjustable nozzle so that it can work both with the afterburners on and off.
The disadvantage of an afterburner is that it uses a lot of fuel for the power it generates. Therefore most planes use afterburners sparingly. For example, a military jet would use its afterburners when taking off from the short runway on an aircraft carrier, or during a high-speed maneuver in a dogfight.
The following pictures, taken at the Virginia Air and Space Museum, show you some of the details of an afterburner-equipped engine. This particular engine comes from an F-4. Here is the main part of the engine:
This includes the compressor, combustion chamber and exhaust turbine. At the exhaust end of the engine, you can see a ring of injectors for the afterburner, as shown here:
Here's a close-up of one of the injectors:
Attached to the end of the engine would be a tube and adjustable nozzle, as shown here:
This tube is about 8 feet long (2.7 meters). The engine itself is about 12 feet long (4 meters).
Gravity is one of those amazing things -- because it is constant, we take it completely for granted. For example, the fuel system in any normal car or airplane depends on gravity to position and move the fuel in the fuel tank. A high-wing monoplane like the one shown in How Airplanes Work would stall almost immediately if you tried to fly it upside down. Gravity draws the fuel from the fuel tanks (located inside the wings) down to the engine.
So how does an aerobatic plane that flies upside down and does loops get fuel from the gas tank to the engine? To answer this question, I spoke to Randy Henson, pilot of the aerobatic biplane shown below:
According to Randy, there are two techniques:
"The first is the flop tube design used in my airplane, a Pitts S-1T. The fuel tank is located in the fuselage in front of the pilot's knees, and inside of the tank is a flexible hose with a weight attached to the free end. When the plane is right side up, this hose, or flop tube, 'flops' to the bottom of the tank because of the weight and draws fuel from the bottom of the tank. When the plane is rolled to inverted, the weight causes to hose to flop to the top of the tank (which is really the bottom now) and draw fuel from there. This is really a cool design because it uses only one tank, and you have access to all the fuel in the tank whether you are right side up or inverted. This design is used on all the high-performance aerobatic airplanes with which I am familiar -- these planes all have a fuel tank in the fuselage.
"The second solution to the problem is the header tank. This is used in airplanes such as the Super Decathlon, a high-wing monoplane. In this type of plane the main fuel tanks are located in the wings, which are higher than the engine. In upright flight, the fuel has a gravity head to the suction of the engine-driven fuel pump (in planes like the Cessna 150, which does not have an inverted fuel system, you don't need a fuel pump -- the fuel is gravity-fed to the carb). For inverted flight, there is a small header tank near the pilot's feet. The header tank is connected to the main tanks in the wings; during upright flight, fuel from the wing tanks flows by gravity into the header tank until it is full. The header tank is connected to the suction side of the fuel pump -- when the plane is rolled inverted, the header tank is above the engine, and the fuel gravity flows from the header tank to the fuel pump. There is a check valve in the line connecting the main tank to the header tank; this prevents fuel from the header tank from draining back into the main tank when the plane is inverted. In the Decathlon, the header tank holds enough fuel for about two minutes of inverted flight.
"My plane and all of the more modern aerobatic planes I have seen are fuel injected. However, some of the older Pitts I have seen have a pressure carb, and it works in inverted flight. "
The typical home toilet uses a bowl filled with water. When you flush the toilet, it starts a siphon that drains the bowl. Gravity then carries the water into the septic tank or the sewer system.
The problem with this approach on an airplane (or a train, bus, boat, etc.) is that the motion of the vehicle means you cannot use a bowl filled with water -- it would splash out every time a little turbulence came along. Since there is no bowl of water, you cannot use a siphon or gravity to empty the bowl.
Airplane toilets use an active vacuum instead of a passive siphon, and they are therefore called vacuum toilets. When you flush, it opens a valve in the sewer line, and the vacuum in the line sucks the contents out of the bowl and into a tank. Because the vacuum does all the work, it takes very little water (or the blue sanitizing liquid used in airplanes) to clean the bowl for the next person. Most vacuum systems flush with just half a gallon (2 liters) of fluid or less, compared to 1.6 gallons (6 liters) for a water-saving toilet and up to 5 gallons (19 liters) for an older toilet.
It turns out that vacuum toilets have lots of advantages, even for normal installations:
They use very little water.
They can use much smaller diameter sewer pipes.
They can flush in any direction, including upward. Since a vacuum system does not use gravity to move the water, there is nothing to stop the sewer pipe from going straight up.
That the pipe does not have to go downward also means you can avoid cutting into the floor to put in new toilets.
I happen to fly a lot on business. For me, personally, airplanes are one of the most amazing things that I see on a daily basis. When I get on a 747, I am boarding a gigantic vehicle capable of carrying 500 or 600 people. A 747 weighs up to 870,000 pounds at takeoff. Yet it rolls down the runway and, as though by magic, lifts itself into the air and can fly up to 7,000 nautical miles without stopping. It is truly incredible when you think about it!
If you have ever wondered what allows a 747 -- or any airplane for that matter -- to fly, then read on. In this article, we will walk through the theory of flight and talk about the different parts of a standard airplane
Aerodynamic Forces
Before we dive into how wings keep airplanes up in the air, it's important that we take a look at four basic aerodynamic forces: lift, weight, thrust and drag.
Straight and Level Flight In order for an airplane to fly straight and level, the following relationships must be true:
Thrust = Drag
Lift = Weight
If, for any reason, the amount of drag becomes larger than the amount of thrust, the plane will slow down. If the thrust is increased so that it is greater than the drag, the plane will speed up.
Similarly, if the amount of lift drops below the weight of the airplane, the plane will descend. By increasing the lift, the pilot can make the airplane climb.
Thrust Thrust is an aerodynamic force that must be created by an airplane in order to overcome the drag (notice that thrust and drag act in opposite directions in the figure above). Airplanes create thrust using propellers, jet engines or rockets. In the figure above, the thrust is being created with a propeller, which acts like a very powerful version of a household fan, pulling air past the blades.
Now, let's look at drag.
Drag
Drag is an aerodynamic force that resists the motion of an object moving through a fluid (air and water are both fluids). If you stick your hand out of a car window while moving, you will experience a very simple demonstration of this effect. The amount of drag that your hand creates depends on a few factors, such as the size of your hand, the speed of the car and the density of the air. If you were to slow down, you would notice that the drag on your hand would decrease.
We see another example of drag reduction when we watch downhill skiers in the Olympics. You'll notice that, whenever they get the chance, they will squeeze down into a tight crouch. By making themselves "smaller," they decrease the drag they create, which allows them to move faster down the hill.
If you've ever wondered why, after takeoff, a passenger jet always retracts its landing gear (wheels) into the body of the airplane, the answer (as you may have already guessed) is to reduce drag. Just like the downhill skier, the pilot wants to make the aircraft as small as possible to reduce drag. The amount of drag produced by the landing gear of a jet is so great that, at cruising speeds, the gear would be ripped right off of the plane.
But what about the other two aerodynamic forces, weight and lift?
Max. landing weight: 630,000 pounds (~ 285,763 kilograms) (explains why planes may need to dump fuel for emergency landings)
Engines: four turbofan engines, 57,000 pounds of thrust each
Fuel capacity: up to 57,000 gallons (~ 215,768 liters)
Max. range: 7,200 nautical miles
Cruising speed: 490 knots
Takeoff distance: 10,500 feet (~ 3,200 meters)
Weight This one is the easiest. Every object on earth has weight (including air). A 747 can weigh up to 870,000 pounds (that's 435 tons!) and still manage to get off the runway. (See the table below for more 747 specs.)
Lift Lift is the aerodynamic force that holds an airplane in the air, and is probably the trickiest of the four aerodynamic forces to explain without using a lot of math. On airplanes, most of the lift required to keep the plane aloft is created by the wings (although some is created by other parts of the structure).
A principal concept in aerodynamics is the idea that air is a fluid. Let's investigate that concept more closely.
A Few Words About Fluid
As we mentioned, a principal concept in aerodynamics is the idea that air is a fluid. Like all gases, air flows and behaves in a similar manner to water and other liquids. Even though air, water and pancake syrup may seem like very different substances, they all conform to the same set of mathematical relationships. In fact, basic aerodynamic tests are sometimes performed underwater.
Another important concept is the fact that lift can exist only in the presence of a moving fluid. This is also true for drag. It doesn't matter if the object is stationary and the fluid is moving, or if the fluid is still and the object is moving through it. What really matters is the relative difference in speeds between the object and the fluid.
Consequently, neither lift nor drag can be created in space (where there is no fluid). This explains why spacecraft don't have wings unless the spaceship spends at least some of its time in air. The space shuttle is a good example of a spacecraft that spends most of its time in space, where there is no air that can be used to create lift. However, when the shuttle re-enters the earth's atmosphere, its stubby wings produce enough lift to allow the shuttle to glide to a graceful landing.
The Longer Path Explanation
What is it? The Longer Path explanation holds that the top surface of a wing is more curved than the bottom surface. Air particles that approach the leading edge of the wing must travel either over or under the wing. Let's assume that two nearby particles split up at the leading edge, and then come back together at the trailing edge of the wing. Since the particle traveling over the top goes a longer distance in the same amount of time, it must be traveling faster.
Bernoulli's equation, a fundamental of fluid dynamics, states that as the speed of a fluid flow increases, its pressure decreases. The Longer Path explanation deduces that this faster moving air develops a lower pressure on the top surface, while the slower moving air maintains a higher pressure on the bottom surface. This pressure difference essentially "sucks" the wing upward (or pushes the wing upward, depending on your point of view).
Why is it not entirely correct? There are several flaws in this theory, although this is a very common explanation found in high school textbooks and even encyclopedias:
The assumption that the two air particles described above rejoin each other at the trailing edge of the wing is groundless. In fact, these two air particles have no "knowledge" of each other's presence at all, and there is no logical reason why these particles should end up at the rear of the wing at the same moment in time.
For many types of wings, the top surface is longer than the bottom. However, many wings are symmetric (shaped identically on the top and bottom surfaces). This explanation also predicts that planes should not be able to fly upside down, although we know that many planes have this ability.
Why is it not entirely wrong? The Longer Path explanation is correct in more than one way. First, the air on the top surface of the wing actually does move faster than the air on the bottom -- in fact, it is moving faster than the speed required for the top and bottom air particles to reunite, as many people suggest. Second, the overall pressure on the top of a lift-producing wing is lower than that on the bottom of the wing, and it is this net pressure difference that creates the lifting force.
The Newtonian Explanation
What is it? Isaac Newton stated that for every action there is an equal, and opposite, reaction (Newton's Third Law). You can see a good example of this by watching two skaters at an ice rink. If one pushes on the other, both move -- one due to the action force and the other due to the reaction force.
In the late 1600s, Isaac Newton theorized that air molecules behave like individual particles, and that the air hitting the bottom surface of a wing behaves like shotgun pellets bouncing off a metal plate. Each individual particle bounces off the bottom surface of the wing and is deflected downward. As the particles strike the bottom surface of the wing, they impart some of their momentum to the wing, thus incrementally nudging the wing upward with every molecular impact.
Note: Actually, Newton's theories on fluids were developed for naval warfare, in order to help decrease the resistance that ships encounter in the water -- the goal was to build a faster boat, not a better airplane. Still, the theories are applicable, since water and air are both fluids.
Why is it not entirely correct? The Newtonian explanation provides a pretty intuitive picture of how the wing turns the air flowing past it, with a couple of exceptions:
The top surface of the wing is left completely out of the picture. The top surface of a wing contributes greatly to turning the fluid flow. When only the bottom surface of the wing is considered, the resulting lift calculations are very inaccurate.
Almost a hundred years after Newton's theory of ship hulls, a man named Leonhard Euler noticed that fluid moving toward an object will actually deflect before it even hits the surface, so it doesn't get a chance to bounce off the surface at all. It seemed that air did not behave like individual shotgun pellets after all. Instead, air molecules interact and influence each other in a way that is difficult to predict using simplified methods. This influence also extends far beyond the air immediately surrounding the wing.
Why is it not entirely wrong? While a pure Newtonian explanation does not produce accurate estimates of lift values in normal flight conditions (for example, a passenger jet's flight), it predicts lift for certain flight regimes very well. For hypersonic flight conditions (speeds exceeding five times the speed of sound), the Newtonian theory holds true. At high speeds and very low air densities, air molecules behave much more like the pellets that Newton spoke of. The space shuttle operates under these conditions during its re-entry phase.
Unlike the Longer Path explanation, the Newtonian approach predicts that the air is deflected downward as it passes the wing. While this may not be due to molecules bouncing off the bottom of the wing, the air is certainly deflected downward, resulting in a phenomenon called downwash. (See NASA: Glenn Research Center for more on downwash.)
How Lift is Created
Pressure Variations Caused By Turning a Moving Fluid Lift is a force on a wing (or any other solid object) immersed in a moving fluid, and it acts perpendicular to the flow of the fluid. (Drag is the same thing, but acts parallel to the direction of the fluid flow). The net force is created by pressure differences brought about by variations in speed of the air at all points around the wing. These velocity variations are caused by the disruption and turning of the air flowing past the wing. The measured pressure distribution on a typical wing looks like the following diagram:
A. Air approaching the top surface of the wing is compressed into the air above it as it moves upward. Then, as the top surface curves downward and away from the airstream, a low-pressure area is developed and the air above is pulled downward toward the back of the wing.
B. Air approaching the bottom surface of the wing is slowed, compressed and redirected in a downward path. As the air nears the rear of the wing, its speed and pressure gradually match that of the air coming over the top. The overall pressure effects encountered on the bottom of the wing are generally less pronounced than those on the top of the wing.
C. Lift component
D. Net force
E. Drag component
When you sum up all the pressures acting on the wing (all the way around), you end up with a net force on the wing. A portion of this lift goes into lifting the wing (lift component), and the rest goes into slowing the wing down (drag component). As the amount of airflow turned by a given wing is increased, the speed and pressure differences between the top and bottom surfaces become more pronounced, and this increases the lift. There are many ways to increase the lift of a wing, such as increasing the angle of attack or increasing the speed of the airflow. These methods and others are discussed in more detail later in this article.
Consider This
It is important to realize that, unlike in the two popular explanations described earlier, lift depends on significant contributions from both the top and bottom wing surfaces. While neither of these explanations is perfect, they both hold some nuggets of validity. Other explanations hold that the unequal pressure distributions cause the flow deflection, and still others state that the exact opposite is true. In either case, it is clear that this is not a subject that can be explained easily using simplified theories.
Likewise, predicting the amount of lift created by wings has been an equally challenging task for engineers and designers in the past. In fact, for years, we have relied heavily on experimental data collected 70 to 80 years ago to aid in our initial designs of wings.
Calculating Lift Based on Experimental Test Results
In 1915, the U.S. Congress created the National Advisory Committee on Aeronautics (NACA -- a precursor of NASA). During the 1920s and 1930s, NACA conducted extensive wind tunnel tests on hundreds of airfoil shapes (wing cross-sectional shapes). The data collected allows engineers to predictably calculate the amount of lift and drag that airfoils can develop in various flight conditions.
The lift coefficient of an airfoil is a number that relates its lift-producing capability to air speed, air density, wing area and angle of attack -- the angle at which the airfoil is oriented with respect to the oncoming air flow (we'll discuss this in greater detail later in the article). The lift coefficient of a given airfoil depends upon the angle of attack.
Image courtesy NASA The lift-curve slope of a NACA airfoil
Here is the standard equation for calculating lift using a lift coefficient:
L = lift Cl = lift coefficient (rho) = air density V = air velocity A = wing area
As an example, let's calculate the lift of an airplane with a wingspan of 40 feet and a chord length of 4 feet (wing area = 160 sq. ft.), moving at a speed of 100 mph (161 kph) at sea level (that's 147 feet, or 45 meters, per second!). Let's assume that the wing has a constant cross-section using an NACA 1408 airfoil shape, and that the plane is flying so that the angle of attack of the wing is 4 degrees.
We know that:
A = 160 square feet
(rho) = 0.0023769 slugs / cubic foot (at sea level on a standard day)
V = 147 feet per second
Cl = 0.55 (lift coefficient for NACA 1408 airfoil at 4 degrees AOA)
So let's calculate the lift:
Lift = 0.55 x .5 x .0023769 x 147 x 147 x 160
Lift = 2,260 lbs
Try your hand at airfoil design on NASA's Web site using a virtual wind tunnel.
Calculating Lift Using Computer Simulations
In the years since NACA's experimental data was collected, engineers have used this information to calculate the lift (and other aerodynamic forces) produced by wings and other objects in fluid flows. In recent years, however, computing power has increased such that wind tunnel experiments can now be simulated on an average personal computer.
Software packages, such as FLUENT, have been developed to create simulated fluid flows in which solid objects can be virtually immersed. The applications of this type of software range from simulating the air flowing over a wing, to mapping the airflow through a computer case to ensure that there is enough cool air passing over the CPU to prevent the computer from overheating.
Interesting Things about Wings
There are several interesting facts about wings that are useful in developing a more detailed understanding of how they work. Wing shape, the angle of attack, flaps, slats, rotating surfaces and blown surfaces are all important elements to consider.
Let's start with wing shape.
Wing Shape The "standard" airfoil shape that we examined above is not the only shape for a wing. For example, both stunt planes (the kind that fly upside down for extended periods of time at air shows) and supersonic aircraft have wing profiles that are somewhat different than you would expect:
The upper airfoil is typical for a stunt plane, and the lower airfoil is typical for supersonic fighters. Note that both are symmetric on the top and bottom. Stunt planes and supersonic jets get their lift totally from the angle of attack of the wing.
Angle of Attack
The angle of attack is the angle that the wing presents to oncoming air, and it controls the thickness of the slice of air the wing is cutting off. Because it controls the slice, the angle of attack also controls the amount of lift that the wing generates (although it is not the only factor).
Zero angle of attack
Shallow angle of attack
Steep angle of attack
Flaps
In general, the wings on most planes are designed to provide an appropriate amount of lift (along with minimal drag) while the plane is operating in its cruising mode (about 560 miles per hour, or 901 km per hour, for the Boeing 747-400). However, when these airplanes are taking off or landing, their speeds can be reduced to less than 200 miles per hour (322 kph). This dramatic change in the wing's working conditions means that a different airfoil shape would probably better serve the aircraft.
To accommodate both flight regimes (fast and high as well as slow and low), airplane wings have moveable sections called flaps. During takeoff and landing, the flaps are extended rearward and downward from the trailing edge of the wings. This effectively alters the shape of the wing, allowing the wing to turn more air, and thus create more lift. The downside of this alteration is that the drag on the wings also increases, so the flaps are put away for the rest of the flight.
Slats
Slats perform the same function as flaps (that is, they temporarily alter the shape of the wing to increase lift), but they are attached to the front of the wing instead of the rear. They are also deployed on takeoff and landing.
Rotating Surfaces
Given what we know so far about wings and lift, it seems logical that a simple cylinder would not produce any lift when immersed in a moving fluid (imagine a plane with wings shaped like cardboard paper-towel tubes). In a simplified world, the air would just flow around the cylinder evenly on both sides, and keep right on going. In reality, the downstream air would be a little turbulent and chaotic, but there still would be no lift created.
However, if we were to begin rotating the cylinder, as in the figure below, the surface of the cylinder would actually drag the surrounding layer of air around with it. The net result would be a pressure difference between the top and bottom surfaces, which deflects the airflow downward. Newton's Third Law states that if the air is being redirected downward, the cylinder must be deflected upward (sounds like lift to me!). This is an example of the Magnus Effect (also known as the Robbins Effect), which holds true for rotating spheres as well as cylinders (see any similarities to curveballs here?)
Believe it or not, in 1926, Anton Flettner built a ship named the Bruckau that used huge spinning cylinders instead of sails to power itself across the ocean.
Blown Surfaces
Let's take our cylindrical wing from the above examples and find another way to create lift with it. If you've ever held the back of your hand vertically under the faucet, you may have noticed that the water did not simply run down to the bottom of your hand and then drip off. Instead, the water actually runs back up and around the side of your hand (for a few millimeters) before falling into the sink. This is known as the Coanda Effect (after Henri Coanda), which states that a fluid will tend to follow the contour of a curved surface that it contacts.
In our cylinder example, if air is forced out of a long slot just behind the top of the cylinder, it will wrap around the backside and pull some surrounding air with it. This is a very similar situation to the Magnus Effect, except that the cylinder doesn't have to spin.
The Coanda Effect is used in specialized applications to increase the amount of additional lift provided by the flaps. Instead of just altering the shape of the wing, compressed air can be forced through long slots on the top of the wing or the flaps to produce extra lift.
Believe it or not, in 1990, McDonnell Douglas Helicopter Co. (now known as MD Helicopters, Inc.) removed the tail rotors from some of its helicopters and replaced them with cylinders! Instead of using a conventional tail rotor to steer the aircraft, the tail boom is pressurized and air is blown out through long slots exactly like the figure above.
More Airplane Parts
The wing is obviously the most important part of an airplane -- it's what gets the airplane in the air. But airplanes have a lot of other characteristic parts designed to control the plane or get it moving. Let's examine the parts you find in a typical airplane by looking at a Cessna 152.
The landing gear is essential during take-off and landing.
Front landing gear
Rear landing gear
The Cessna 152 has fixed landing gear, but most planes have retractable landing gear to reduce drag while in flight.
Now, let's check out the propeller.
The Propeller
Probably the most important parts of an airplane, after the wing, are the propeller and engine. The propeller (or, on jet aircraft, the jets) provides the thrust that moves the plane forward. (Check out How Gas Turbine Engines Work to learn about jet engines.)
A propeller is really just a special, spinning wing. If you looked at the cross section of a propeller, you'd find that a propeller has an airfoil shape and an angle of attack. Just by looking at the propeller pictured above, you can see that the angle of attack changes along the length of the propeller -- the angle is greater toward the center because the speed of the propeller through the air is slower close to the hub. Many larger propeller aircraft have more elaborate three-blade or four-blade props with adjustable pitch mechanisms. These mechanisms let the pilot adjust the propeller's angle of attack depending on air speed and altitude.
Horizontal and Vertical Stabilizers
The tail of the airplane has two small wings, called the horizontal and vertical stabilizers, that the pilot uses to control the direction of the plane. Both are symmetrical airfoils, and both have large flaps on them that the pilot controls with the control stick to change their lift characteristics.
Horizontal tail wing
Vertical tail wing
With the horizontal tail wing, the pilot can change the plane's angle of attack, and therefore control whether the plane goes up or down. With the vertical tail wing, the pilot can turn the plane left or right.
Controlling the Direction
The Main Wing and Flaps The plane's main wing is 40 feet (~ 12 m) long from end to end, and about 4 feet (~ 1.2 m) wide. On the inner portion of the wing, there are flaps used during takeoff, landing and other low-speed situations. On the outer ends, there are ailerons used to turn the plane and keep it level.
Main wing
Flaps
The flaps are actuated by electric motors in the wing. Also enclosed in the wings are two fuel tanks, each of which holds about 20 gallons of gas.
Airplane Sensors
From this description you can see that a plane has four different moveable control surfaces, as shown here:
The plane also has two different sensors mounted on the wing:
The L-shaped tube is called a pitot tube. Air that rams into this tube during flight creates pressure, and that pressure moves the needle on the air-speed indicator in the cockpit. The small opening on the right is a whistle that sounds as the wing nears a stall. The larger opening visible near the cockpit is used for ventilation.
Helicopters are the most versatile flying machines in existence today. This versatility gives the pilot complete access to three-dimensional space in a way that no airplane can. If you have ever flown in a helicopter you know that its abilities are exhilarating.
The amazing flexibility of helicopters means that they can fly almost anywhere. However, it also means that flying the machines is complicated. The pilot has to think in three dimensions and must use both arms and both legs constantly to keep a helicopter in the air. Piloting a helicopter requires a great deal of training and skill, as well as continuous attention to the machine.
In this article, you will learn about all of a helicopter's different capabilities and how it's able to do such incredible things.
Comparing Modes of Transport
To understand how helicopters work and also why they are so complicated to fly, it is helpful to compare the abilities of a helicopter with those of trains, cars and airplanes. By looking at these different modes of transportation, you can come to understand why helicopters are so versatile!
If you have ever been inside of the cab of a locomotive, you know that trains are fairly simple to drive. After all, there are only two directions that a train can travel in -- forward and reverse. There is a brake to stop the train's travel in either direction, but there is no steering mechanism of any kind on a train. The tracks take the train where it needs to go.
Because a train has only two directions in which it can travel, you can drive a train with one hand.
A car, of course, can go forward and backward like a train. While you are traveling in either direction you can also turn left or right:
To handle the steering, a car uses a steering wheel that the driver can turn clockwise or counterclockwise. It is possible to drive a car with one hand and one foot.
Anyone who has taken pilot lessons or looked inside the cockpit while boarding a jumbo jet knows that planes are a lot more complicated to fly than a car is to drive. However, a plane is really only one step away from a car:
A plane can move forward and turn left or right. It also adds the ability to go up and down. However, it loses the ability to reverse. So a plane can move in five different directions instead of a car's four directions. The ability to go up and down adds a whole new dimension to a plane, and this dimension is one of the things that makes airplanes different from a car. To control the upward and downward motion of the plane, either a joystick replaces the steering wheel or the steering wheel gains the ability to move in and out (in addition to turning clockwise and counterclockwise). In most planes (but not all), the pilot also has access to two pedals to control the rudder. Therefore, a pilot could fly a plane with one hand and two feet.
A helicopter can do three things that an airplane cannot:
A helicopter can fly backwards.
The entire aircraft can rotate in the air.
A helicopter can hover motionless in the air.
In a car or a plane, the vehicle must be moving in order to turn. In a helicopter, you can move laterally in any direction or you can rotate 360 degrees. These extra degrees of freedom and the skill you must have to master them is what makes helicopters so exciting, but it also makes them complex.
To control a helicopter, one hand grasps a control called the cyclic, which controls the lateral direction of the helicopter (including forward, backward, left and right). The other hand grasps a control called the collective, which controls the up and down motion of the helicopter (and also controls engine speed). The pilot's feet rest on pedals that control the tail rotor, which allows the helicopter to rotate in either direction on its axis. It takes both hands and both feet to fly a helicopter!
Special Thanks
Special thanks to the staff of Air Atlanta Helicopters for their help with this article and the associated video. Additional thanks goes to Blake Moore, president, who is the helicopter pilot appearing in these videos.
Thanks also to Glenn Brown, the president of Raleigh Helicopters, and Ellen Turcio, pilot, for their assistance.
Special Capabilities of Helicopters
Helicopters have a number of unique abilities that airplanes do not have. Several of these capabilities are shown in the following videos (if you have a high-speed Internet connection, these videos are quick and fun to watch!).
The signature of a helicopter is its ability to hover over one point on the ground. While hovering, a helicopter can also spin on its axis so that the pilot can look in any direction.
Another unique feature of a helicopter is its ability to fly backwards. A helicopter can also fly sideways just as easily.
Since a helicopter can fly backwards and sideways, it can do a number of interesting tricks. The following video shows a helicopter performing a pirouette, in which it rotates 360 degrees while it travels down a straight line relative to the ground:
A helicopter that is flying forward can also stop in mid-air and begin hovering very quickly, as demonstrated in this video:
All of these maneuvers are impossible in an airplane, which must fly forward at all times in order to develop lift from its wings
How Helicopters Fly
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.
The Tail Rotor
The adjustability of the tail rotor is straightforward -- what you want is the ability to change the angle of attack on the tail rotor wings so that you can use the tail rotor to rotate the helicopter on the drive shaft's axis.
The pilot has two foot pedals that control the angle of attack. These two videos let you take a look at the pedals and see how they affect the tail rotor:
Click here to download the 15-second video showing the helicopter's pedals. (1.1 MB)
You can see the pedals in this shot of the cockpit
The blades of the tail rotor are only about 2 feet (61 cm) long.
The tail rotor's hub allows the pilot to change the angle of attack of the rotor's wings.
The Main Rotor
A helicopter's main rotor is the most important part of the vehicle. It provides the lift that allows the helicopter to fly, as well as the control that allows the helicopter to move laterally, make turns and change altitude.
To handle all of these tasks, the rotor must first be incredibly strong. It must also be able to adjust the angle of the rotor blades with each revolution of the hub. The adjustability is provided by a device called the swash plate assembly, as shown in this photograph:
The main rotor hub, where the rotor's drive shaft and blades connect, has to be extremely strong as well as highly adjustable. The swash plate assembly is the component that provides the adjustability.
The swash plate assembly has two primary roles:
Under the direction of the collective control, the swash plate assembly can change the angle of both blades simultaneously. Doing this increases or decreases the lift that the main rotor supplies to the vehicle, allowing the helicopter to gain or lose altitude.
Under the direction of the cyclic control, the swash plate assembly can change the angle of the blades individually as they revolve. This allows the helicopter to move in any direction around a 360-degree circle, including forward, backward, left and right.
The swash plate assembly consists of two plates -- the fixed and the rotating swash plates -- shown above in blue and red, respectively.
The rotating swash plate rotates with the drive shaft (green) and the rotor's blades (gray) because of the links (purple) that connect the rotating plate to the drive shaft.
The pitch control rods (orange) allow the rotating swash plate to change the pitch of the rotor blades.
The angle of the fixed swash plate is changed by the control rods (yellow) attached to the fixed swash plate.
The fixed plate's control rods are affected by the pilot's input to the cyclic and collective controls.
The fixed and rotating swash plates are connected with a set of bearings between the two plates. These bearings allow the rotating swash plate to spin on top of the fixed swash plate.
The swash plate assembly changes the angle of attack of the main rotor's wings as the wings revolve. A steep angle of attack provides more lift than a shallow angle of attack.
The collective control changes the angle of attack on both blades simultaneously:
The collective lets you change the angle of attack of the main rotor simultaneously on both blades.
The cyclic control tilts the swash plate assembly so that the angle of attack on one side of the helicopter is greater than it is on the other, like this:
The cyclic changes the angle of attack of the main rotor's wings unevenly by tilting the swash plate assembly. On one side of the helicopter, the angle of attack (and therefore the lift) is greater.
Hovering in a helicopter requires experience and skill. The pilot adjusts the cyclic to maintain the helicopter's position over a point on the ground. The pilot adjusts the collective to maintain a fixed altitude (especially important when close to the ground, as shown in the videos). The pilot adjusts the foot pedals to maintain the direction that the helicopter is pointing. You can imagine that windy conditions can make hovering a real challenge!
Relating the Controls and the Swash Plate
The following videos help you understand the relationship between the cyclic and collective controls and the swash plate assembly. In general:
The collective control raises the entire swash plate assembly as a unit. This has the effect of changing the pitch of both blades simultaneously.
The cyclic control pushes one side of the swash plate assembly upward or downward. This has the effect of changing the pitch of the blades unevenly depending on where they are in the rotation. The result of the cyclic control is that the rotor's wings have a greater angle of attack (and therefore more lift) on one side of the helicopter and a lesser angle of attack (and less lift) on the opposite side. The unbalanced lift causes the helicopter to tip and move laterally.
Other Important Components
Click here to download the 150-second video explaining the main instrument panel. (10.2 MB)
Click here to download the 25-second video explaining the engine instrument panel. (1.6 MB)
Click here to download the 30-second video explaining the engine main drive pulley system. (2.0 MB)
Click here to download the 50-second video showing the helicopter's engine and cooling system. (3.4 MB)
