The Dynamics of Airplane Flight

8/9/2019 The Dynamics of Airplane Flight

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The Dynamics of Airplane Flight

Air is a physical substance which has weight. It has molecules which are constantlymoving. Air pressure is created by the molecules moving around. Moving air has a forcethat will lift kites and balloons up and down. Air is a mixture of different gases; oxygen,carbon dioxide and nitrogen. All things that fly need air. Air has power to push and pull onthe birds, balloons, kites and planes. In 1640, Evangelista Torricelli discovered that air hasweight. When experimenting with measuring mercury, he discovered that air putpressure on the mercury.

Francesco Lana used this discovery to begin to plan for an airship in the late 1600s. Hedrew an airship on paper that used the idea that air has weight. The ship was a hollowsphere which would have the air taken out of it. Once the air was removed, the spherewould have less weight and would be able to float up into the air. Each of four sphereswould be attached to a boat-like structure and then the whole machine would float. Theactual design was never tried.

Hot air expands and spreads out and it becomes lighter than cool air. When a balloon isfull of hot air it rises up because the hot air expands inside the balloon. When the hot aircools and is let out of the balloon the balloon comes back down.

How Wings Lift the Plane

Airplane wings are curved on the top which make air move faster over the top of the wing.The air moves faster over the top of a wing. It moves slower underneath the wing. Theslow air pushes up from below while the faster air pushes down from the top. This forcesthe wing to lift up into the air.

Laws of Motion

Sir Isaac Newton proposed three laws of motion in 1665. These Laws of Motion help toexplain how a planes flies.

1. If an object is not moving, it will not start moving by itself. If an object is moving,it will not stop or change direction unless something pushes it. 2. Objects will move farther and faster when they are pushed harder. 3. When an object is pushed in one direction, there is always a resistance of the samesize in the opposite direction.

Forces of Flight
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Four Forces of FlightLift - upward

Drag - down and backward

Weight - downward

Thrust - forward

Controlling the Flight of a Plane

How does a plane fly? Let's pretend that our arms are wings. If we place one wing downand one wing up we can use the roll to change the direction of the plane. We are helping

to turn the plane by yawing toward one side. If we raise our nose, like a pilot can raise thenose of the plane, we are raising the pitch of the plane. All these dimensions togethercombine to control the flight of the plane. A pilot of a plane has special controls that canbe used to fly the plane. There are levers and buttons that the pilot can push to changethe yaw, pitch and roll of the plane.

To roll the plane to the right or left, the ailerons are raised on one wing and lowered onthe other. The wing with the lowered aileron rises while the wing with the raised ailerondrops.

Pitch is to make a plane descend or climb. The pilot adjusts the elevators on the tail tomake a plane descend or climb. Lowering the elevators caused the airplane's nose to drop,sending the plane into a down. Raising the elevators causes the airplane to climb.

Yaw is the turning of a plane. When the rudder is turned to one side, the airplane movesleft or right. The airplane's nose is pointed in the same direction as the direction of therudder. The rudder and the ailerons are used together to make a turn

How does a Pilot Control the Plane?


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To control a plane a pilot uses several instruments...

The pilot controls the engine power using the throttle. Pushing the throttle increasespower, and pulling it decreases power.

Left: Picture of plane in roll

The ailerons raise and lower the wings. The pilotcontrols the roll of the plane by raising one aileron or theother with a control wheel. Turning the control wheelclockwise raises the right aileron and lowers the leftaileron, which rolls the aircraft to the right.

Right: Picture of plane YawThe rudder works to control the yaw of the plane. Thepilot moves rudder left and right, with left and rightpedals. Pressing the right rudder pedal moves therudder to the right. This yaws the aircraft to the right. Usedtogether, the rudder and the ailerons are used to turn theplane.

Left: Picture of Plane Pitch

The elevators which are on the tail section are used tocontrol the pitch of the plane. A pilot uses a controlwheel to raise and lower the elevators, by moving itforward to back ward. Lowering the elevators makes theplane nose go down and allows the plane to go down. Byraising the elevators the pilot can make the plane go up.


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The pilot of the plane pushes the top of the rudder pedals to use the brakes . The brakesare used when the plane is on the ground to slow down the plane and get ready forstopping it. The top of the left rudder controls the left brake and the top of the right pedalcontrols the right brake.

If you look at these motions you can see that each type of motion helps control thedirection and level of the plane when it is flying.

Sound Barrier

Sound is made up of molecules of air that move. They push together and gather togetherto form sound waves . Sound waves travel at the speed of about 750 mph at sea level.When a plane travels the speed of sound the air waves gather together and compressthe air in front of the plane to keep it from moving forward. This compression causes ashock wave to form in front of the plane.

In order to travel faster than the speed of sound the plane needs to be able to breakthrough the shock wave. When the airplane moves through the waves, it is makes thesound waves spread out and this creates a loud noise or sonic boom . The sonic boom iscaused by a sudden change in the air pressure. When the plane travels faster than soundit is traveling at supersonic speed. A plane traveling at the speed of sound is traveling atMach 1 or about 760 MPH. Mach 2 is twice the speed of sound.

Regimes of Flight

Sometimes called speeds of flight , each regime is a different level of flight speed.

Example Regimes of Flight

General Aviation (100-350 MPH).

Most of the early planes were only able to fly at thisspeed level. Early engines were not as powerful asthey are today. However, this regime is still used


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Seaplane today by smaller planes. Examples of this regime arethe small crop dusters used by farmers for their fields,two and four seater passenger planes, and seaplanes

that can land on water.

Boeing 747

Subsonic (350-750 MPH).This category contains most of the commercial jetsthat are used today to move passengers and cargo.The speed is just below the speed of sound. Enginestoday are lighter and more powerful and can travel

quickly with large loads of people or goods.

Concorde

Supersonic (760-3500 MPH - Mach 1 - Mach 5).

760 MPH is the speed of sound. It is also called MACH1. These planes can fly up to 5 times the speed of

sound. Planes in this regime have specially designedhigh performance engines. They are also designed

with lightweight materials to provide less drag. TheConcorde is an example of this regime of flight.

Space Shuttle

Hypersonic (3500-7000 MPH - Mach 5 to Mach 10).

Rockets travel at speeds 5 to 10 times the speed of sound as they go into orbit. An example of a

hypersonic vehicle is the X-15, which is rocketpowered. The space shuttle is also an example of this

regime. New materials and very powerful engines weredeveloped to handle this rate of speed.

How Gas Turbine Engines Work


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Types of TurbinesThere are many different kinds of turbines:

You have probably heard of a steam turbine . Most power plants use coal,natural gas, oil or a nuclear reactor to create steam. The steam runs through ahuge and very carefully designed multi-stage turbine to spin an output shaft thatdrives the plant's generator.

Hydroelectric dams use water turbines in the same way to generate power.The turbines used in a hydroelectric plant look completely different from asteam turbine because water is so much denser (and slower moving) thansteam, but it is the same principle.

Wind turbines , also known as wind mills, use the wind as their motive force. A

wind turbine looks nothing like a steam turbine or a water turbine because windis slow moving and very light, but again, the principle is the same.A gas turbine is an extension of the same concept. In a gas turbine, a pressurized gasspins the turbine. In all modern gas turbine engines, the engine produces its ownpressurized gas, and it does this by burning something like propane, natural gas,kerosene or jet fuel. The heat that comes from burning the fuel expands air, and thehigh-speed rush of this hot air spins the turbine

Advantages and Disadvantages of Jet EnginesSo why does the M-1 tank use a 1,500 horsepower gas turbine engine instead of adiesel engine ? It turns out that there are two big advantages of the turbine over thediesel:

Gas turbine engines have a great power-to-weight ratio compared toreciprocating engines . That is, the amount of power you get out of the enginecompared to the weight of the engine itself is very good.

Gas turbine engines are smaller than their reciprocating counterparts of thesame power.

The main disadvantage of gas turbines is that, compared to a reciprocating engine of the same size, they are expensive . Because they spin at such high speeds andbecause of the high operating temperatures, designing and manufacturing gas
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turbines is a tough problem from both the engineering and materials standpoint. Gasturbines also tend to use more fuel when they are idling, and they prefer a constantrather than a fluctuating load. That makes gas turbines great for things liketranscontinental jet aircraft and power plants, but explains why you don't have oneunder the hood of your car.

The Gas Turbine ProcessGas turbine engines are, theoretically, extremely simple. They have three parts:

Compressor - Compresses the incoming air to high pressure Combustion area - Burns the fuel and produces high-pressure, high-velocity

gas Turbine - Extracts the energy from the high-pressure, high-velocity gas flowing

from the combustion chamber The following figure shows the general layout of an axial-flow gas turbine -- the sortof engine you would find driving the rotor of a helicopter , for example:

JavaScript or it is disabled.In this engine, air is sucked in from the right by the compressor. The compressor isbasically a cone-shaped cylinder with small fan blades attached in rows (eight rows of blades are represented here). Assuming the light blue represents air at normal air pressure, then as the air is forced through the compression stage its pressure risessignificantly. In some engines, the pressure of the air can rise by a factor of 30. Thehigh-pressure air produced by the compressor is shown in dark blue.

Combustion AreaThis high-pressure air then enters the combustion area, where a ring of fuel injectors injects a steady stream of fuel. The fuel is generally kerosene, jet fuel, propane or n-atural gas. If you think about how easy it is to blow a candle out, then you can see the

design problem in the combustion area -- entering this area is high-pressure air moving at hundreds of miles per hour. You want to keep a flame burning continuouslyin that environment. The piece that solves this problem is called a "flame holder," or sometimes a "can." The can is a hollow, perforated piece of heavy metal. Half of thecan in cross-section is shown below:
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The injectors are at the right. Compressed air enters through the perforations.Exhaust gases exit at the left. You can see in the previous figure that a second set of cylinders wraps around the inside and the outside of this perforated can, guiding thecompressed intake air into the perforations.

The TurbineAt the left of the engine is the turbine section. In this figure there are two sets of turbines. The first set directly drives the compressor. The turbines, the shaft and thecompressor all turn as a single unit:

At the far left is a final turbine stage, shown here with a single set of vanes. It drivesthe output shaft. This final turbine stage and the output shaft are a completely stand-alone, freewheeling unit. They spin freely without any connection to the rest of theengine. And that is the amazing part about a gas turbine engine -- there is enough


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energy in the hot gases blowing through the blades of that final output turbine togenerate 1,500 horsepower and drive a 63-ton M-1 Tank ! A gas turbine engine reallyis that simple.In the case of the turbine used in a tank or a power plant , there really is nothing to dowith the exhaust gases but vent them through an exhaust pipe, as shown. Sometimes

the exhaust will run through some sort of heat exchanger either to extract the heat for some other purpose or to preheat air before it enters the combustion chamber.The discussion here is obviously simplified a bit. For example, we have not discussedthe areas of bearings , oiling systems, internal support structures of the engine, stator vanes and so on. All of these areas become major engineering problems because of the tremendous temperatures, pressures and spin rates inside the engine. But thebasic principles described here govern all gas turbine engines and help you tounderstand the basic layout and operation of the engine.

Gas Turbine VariationsLarge jetliners use what are known as turbofan engines, which are nothing more than

gas turbines combined with a large fan at the front of the engine. Here's the basic(highly simplified) layout of a turbofan engine:

You can see that the core of a turbofan is a normal gas turbine engine like the onedescribed in the previous section. The difference is that the final turbine stage drives ashaft that makes its way back to the front of the engine to power the fan (shown in redin this picture). This multiple concentric shaft approach , by the way, is extremelycommon in gas turbines. In many larger turbofans, in fact, there may be twocompletely separate compression stages driven by separate turbines, along with thefan turbine as shown above. All three shafts ride within one another concentrically.The purpose of the fan is to dramatically increase the amount of air moving throughthe engine, and therefore increase the engine's thrust . When you look into the engineof a commercial jet at the airport , what you see is this fan at the front of the engine. Itis huge -- on the order of 10 feet (3 m) in diameter on big jets, so it can move a lot of
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air. The air that the fan moves is called " bypass air " (shown in purple above) becauseit bypasses the turbine portion of the engine and moves straight through to the back of the nacelle at high speed to provide thrust.A turboprop engine is similar to a turbofan, but instead of a fan there is aconventional propeller at the front of the engine. The output shaft connects to a

gearbox to reduce the speed, and the output of the gearbox turns the propeller.Thrust BasicsThe goal of a turbofan engine is to produce thrust to drive the airplane forward. Thrustis generally measured in pounds in the United States (the metric system usesNewtons, where 4.45 Newtons equals 1 pound of thrust). A "pound of thrust" is equalto a force able to accelerate 1 pound of material 32 feet per second per second (32feet per second per second happens to be equivalent to the acceleration provided bygravity ). Therefore, if you have a jet engine capable of producing 1 pound of thrust, itcould hold 1 pound of material suspended in the air if the jet were pointed straightdown. Likewise, a jet engine producing 5,000 pounds of thrust could hold 5,000

pounds of material suspended in the air. And if a rocket engine produced 5,000pounds of thrust applied to a 5,000-pound object floating in space, the 5,000-poundobject would accelerate at a rate of 32 feet per second per second.Thrust is generated under Newton's principle that "every action has an equal andopposite reaction." For example, imagine that you are floating in space and you weigh100 pounds on Earth. In your hand you have a baseball that weighs 1 pound on Earth.If you throw the baseball away from you at a speed of 32 feet per second (21 mph / 34kph), your body will move in the opposite direction (it will react ) at a speed of 0.32 feetper second. If you were to continuously throw baseballs in that way at a rate of oneper second, your baseballs would be generating 1 pound of continuous thrust. Keep inmind that to generate that 1 pound of thrust for an hour you would need to be holding

3,600 pounds of baseballs at the beginning of the hour. If you wanted to do better, thething to do is to throw the baseballs harder. By "throwing" them (with of a gun, say) at3,200 feet per second, you would generate 100 pounds of thrust.

Jet Engine ThrustIn a turbofan engine, the baseballs that the engine is throwing out are air molecules .The air molecules are already there, so the airplane does not have to carry themaround at least. An individual air molecule does not weigh very much, but the engineis throwing a lot of them and it is throwing them at very high speed. Thrust is comingfrom two components in the turbofan:

The gas turbine itself - Generally a nozzle is formed at the exhaust end of thegas turbine (not shown in this figure) to generate a high-speed jet of exhaustgas. A typical speed for air molecules exiting the engine is 1,300 mph (2,092kph).

The bypass air generated by the fan - This bypass air moves at a slower speed than the exhaust from the turbine, but the fan moves a lot of air.

As you can see, gas turbine engines are quite common. They are also quitecomplicated, and they stretch the limits of both fluid dynamics and materials sciences.If you want to learn more, one worthwhile place to go would be the library of a
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university with a good engineering department. Books on the subject tend to beexpensive, but two well-known texts include " Aircraft Gas Turbine Engine Technology "and " Elements of Gas Turbine Propulsion ."There is a surprising amount of activity in the home-built gas-turbine arena, and youcan find other people interested in the same topic by participating in newsgroups or

mailing lists on the subject.For more information on gas turbine engines and related topics, check out the links onthe following page

Jet Engine TypesCan you explain how various jet engines work, including the turbojet,

turbofan, turboprop, and turboshaft? In particular, what is the differencebetween a turbojet and a turbofan and which is more efficient?

-question from Tieo Jing Jin

The term "jet engine" is often used as a generic name for a variety of engines,including the turbojet, turbofan, turboprop, and ramjet. These engines all operate bythe same basic principles, but each has its own distinct advantages anddisadvantages. All jet engines operate by forcing incoming air into a tube where the air is compressed, mixed with fuel, burned, and exhausted at high speed to generatethrust. The key to making a jet engine work is the compression of the incoming air. If uncompressed, the air-fuel mixture won't burn and the engine can't generate anythrust. Most members of the jet family employ a section of compressors, consisting of rotating blades, that slow the incoming air to create a high pressure. This compressedair is then forced into a combustion section where it is mixed with fuel and burned. Asthe high-pressure gases are exhausted, they are passed through a turbine sectionconsisting of more rotating blades. In this region, the exhausting gases turn theturbine blades which are connected by a shaft to the compressor blades at the front of the engine. Thus, the exhaust turns the turbines which turn the compressors to bringin more air and keep the engine going. The combustion gases then continue toexpand out through the nozzle creating a forward thrust. The above explanationdescribes a simple turbojet, as illustrated below.
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Diagram of an axial-flow turbojet The turbojet (and the turbofan) can also be fittedwith an afterburner. An afterburner is simply a long tube placed in between the turbine

and the nozzle in which additional fuel is added and burned to provide a significantboost in thrust. However, afterburners greatly increase fuel consumption, so aircraftcan only use them for brief periods.

Comparison of a turbojet and a turbojet with an afterburner A further variation onthe turbojet is the turbofan. Although most components remain the same, the turbofanintroduces a fan section in front of the compressors. The fan, another rotating series of blades, is also driven by the turbine, but its primary purpose is to force a large volumeof air through outer ducts that go around the engine core. Although this "bypassed" air flow travels at much lower speeds, the large mass of air that is accelerated by the fan


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produces a significant thrust (in addition to that created by the turbojet core) withoutburning any additional fuel. Thus, the turbofan is much more fuel efficient than theturbojet. In addition, the low-speed air helps to cushion the noise of the jet coremaking the engine much quieter.

Comparison of a low-bypass turbofan with long ducts and a high-bypassturbofan with short ducts Turbofans are typically broken into one of two categories--low-bypass ratio and high-bypass ratio--as illustrated above. The bypass ratio refers tothe ratio of incoming air that passes through the fan ducts compared to the incomingair passing through the jet core. In a low-bypass turbofan, only a small amount of air passes through the fan ducts and the fan is of very small diameter. The fan in a high-bypass turbofan is much larger to force a large volume of air through the ducts. Thelow-bypass turbofan is more compact, but the high-bypass turbofan can producemuch greater thrust, is more fuel efficient, and is much quieter.

A concept similar to the turbofan is the turboprop. However, instead of the turbinedriving a ducted fan, it drives a completely external propeller. Turboprops arecommonly used on commuter aircraft and long-range planes that require greatendurance like the P-3 Orion and Tu-95 .
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Schematic of a turboprop engine The turboprop is attractive in these applicationsbecause of its high fuel efficiency, even greater than the turbofan. However, the noiseand vibration produced by the propeller is a significant drawback, and the turboprop islimited to subsonic flight only. In a typical turboprop, the jet core produces about 15%of the thrust while the propeller generates the remaining 85%.

Another noteworthy variation on the turbojet is the ramjet. The idea behind this type of engine is to remove all the rotary components of the engine (i.e. fans, compressors,and turbines) and allow the motion of the engine itself to compress incoming air for combustion.

Simple schematic of a ramjet However, the price of this simplicity is that the ramjetcan only produce thrust when it is already in motion. Instead of using a compressor todraw in air and compress it for combustion, the ramjet relies on the motion of theaircraft to ram air into the engine at high enough speed that it is already sufficientlycompressed for combustion to occur. Since ramjets typically cannot function untilreaching about 300 mph (485 km/h) at sea level, they have been rarely used onmanned aircraft. However, the ramjet is more fuel efficient than turbojets or turbofansstarting at about Mach 3 making them very attractive for use on missiles. Suchmissiles are typically launched using rocket motors that accelerate the vehicle to high-subsonic or low-supersonic speeds where the ramjet is engaged.Finally, let us talk briefly about the turboshaft, a version of the jet engine that powersnearly every helicopter built today. As the below image illustrates, the turboshaftutilizes many of the same components as a turbojet.


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Schematic of a turboshaft engine Air is drawn in through an inlet, compressed bylow- and high-pressure compressor blades, mixed with fuel and burned in acombustion chamber, passed through turbine blades, and exhausted through anozzle. The key difference between the turboshaft and previously discussed engines

is that the turbine not only drives the compressors, but the shaft is also connected to agear box that drives a helicopter's rotor blades. Although the engine shaft rotatesabout the horizontal, the gear box contains a sequence of gears that transform thatmotion to a rotation about the vertical axis as required by a helicopter main rotor.Helicopters also typically operate at much lower altitudes than aircraft where dust,sand, and other debris can easily be sucked into the engine. To address this problem,most turboshaft engines are equipped with a particle separator that filters out andexpels the unwanted dust before the air flow reaches the compressor.


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Schematic of a turboshaft engine particle separator While the turboprop is stillpopular on aircraft where low fuel consumption is vital, nearly all aircraft today employsome version of the turbofan, usually high-bypass turbofans. The high thrust, low fuelconsumption, and low noise levels of these engines make them well-suited to bothmilitary and commercial applications. Today, about the only use for turbojets andramjets is in missiles. Air-breathing, long-range, subsonic missiles like the Tomahawkuse turbojets since these are small, relatively low-cost systems that provide muchgreater range than is possible with a rocket of comparable size. Ramjets findapplications on air-breathing, long-range, supersonic missiles for similar reasons.Turboshafts, of course, have displaced the piston engine as the primary powerplantused on helicopters. To continue learning more about aircraft propulsion, be sure tocheck out NASA's Learning Guide on Propulsion for a wealth of information,animations, and interactive applets about rockets, propellers, ramjets, and gas turbineengines

Jets and RocketsWhat is the difference between a jet engine and a rocket engine?

-question from name withheld

In order to understand how an engine works, we first need to understand the processof combustion. Combustion is defined as the burning, or oxidation, of matter toproduce energy. Two substances are necessary for combustion to occur--a fuel andan oxidizer. A fuel can be anything from the wood, coal, or natural gas used toproduce heat in a furnace to gasoline or hydrogen used in an internal combustionengine. An oxidizer, as its name implies, is a substance that contains oxygen.
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Simple representation of the combustion process The purpose of both the jet engine and the rocket engine is to combust a mixture of fuel and oxidizer. This combustion process generates a high-pressure exhaust thatcreates thrust to push a vehicle forward. The fundamental difference between the twotypes of engines, however, is where the oxidizer comes from.A jet engine obtains its oxidizer from the external atmosphere, as illustrated in thediagram below. Air enters the engine through an inlet and is then slowed down andcompressed by a series of compressor blades. The compressed air is then mixed withfuel, typically a petroleum-based liquid similar to kerosene, and burned. The high-

pressure gas is exhausted through a nozzle to generate thrust.

Schematic of a simple jet engine
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A rocket engine differs from a jet engine primarily in one key way. Whereas the jetpulls in oxidizer from the atmosphere, a rocket carries its own supply of oxygenaboard the vehicle. An example shown below is the liquid rocket engine. This class of rocket carries a liquid fuel and a liquid oxidizer in two separate tanks. The two liquidsare pumped into a combustion chamber at some rate, called the mass flow rate,

where they are mixed and burned. Just as in the jet engine described earlier, thiscombustion process generates a high-pressure gas that is exhausted through anozzle to generate thrust.

Schematic of a liquid rocket engine There are many different combinations of liquids that can be combusted in liquidrockets. One of the more common combinations, however, is liquid hydrogen as thefuel and liquid oxygen as the oxidizer. Other common fuels include kerosene andhydrazine while a frequently used oxidizer is nitrogen tetroxide. Liquid rockets are

most commonly used on large vehicles that launch payloads into space, like theAmerican Delta and Titan, Russian Soyuz and Proton, and European Ariane rockets.Another major form of the rocket is the solid rocket motor, like that illustrated below. Asolid rocket also carries both the fuel and oxidizer aboard the vehicle. The differencebetween a solid rocket and a liquid rocket, however, is that the fuel and oxidizer aremixed together and cast into a solid mass. This mixture is inert and does not burnunder normal conditions. When exposed to a heat source, like an igniter, however, aflame travels along the surface of the solid and combusts the mixed fuel and oxidizer.Once started, this reaction cannot be stopped, and the flame front will continuecombusting the solid fuel until none remains.


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Schematic of a solid rocket motor Because they are so much easier to handle and store for long periods of time, solidrockets are commonly used on military missiles like Minuteman, AMRAAM, and

HARM. The rocket motors that you and I can purchase in a store and use to launchmodel rockets are also solid rockets.However, while solid rockets are typically safer, they are usually not as powerful or efficient as their liquid cousins. Another advantage of liquid rockets is that they canalso be throttled by slowing or increasing the rate at which fuel is combusted. A solidrocket, by comparison, cannot be stopped once ignited. For these reasons, a number of hybrid classes of rockets have been developed to take advantage of the strengthsof each type.The aptly named hybrid rocket is a cross between a solid rocket and a liquid rocket.This type of rocket combusts a solid fuel using a liquid or gaseous oxidizer stored in atank aboard the vehicle. The chief advantage of the hybrid rocket is the relative safety

of the solid rocket, but the rocket can be throttled by adjusting the flow rate of theoxidizer.A similar device is the ducted rocket, which is a cross between a jet engine a solidrocket. The ducted rocket works in the same way as the hybrid rocket except that theoxygen is taken from the external atmosphere, like a jet, instead of carried aboard thevehicle. Calling such a device a "rocket" is a little confusing since we have alreadysaid that a rocket carries its own supply of oxidizer, but that is the name this class of propulsion system has been given. The ducted rocket is attractive as a potentialbooster for high-speed military missiles, but few such designs have entered service sofar.To summarize, the primary difference between a jet and a rocket is that a rocket

carries its own supply of oxygen internally while a jet must obtain oxygen from theexternal atmosphere. Another more technical way to explain this difference is that thefluid a jet engine accelerates to produce thrust is air from the atmosphere whereas thefluid a rocket accelerates to produce thrust is the exhaust from its own combustionprocess. It is for this reason that a rocket works in the vacuum of space, where thereis no atmosphere, while a jet engine will not.


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Turboprops and Jet Engines

I saw a comment on your site saying that the C-130 is powered by jetengines. Correct me if I am wrong, but I flew on a lot of C-130s and none had jet powered engines, they all had propellers.

-question from Tim

I understand your confusion. However, the C-130 actually is powered by jet engines.They are a class of jet engines called turboprops in which a small jet engine is used toturn a large propeller. You can learn more about how these engines work in another

question we answered about the different kinds of jet engines.It is a common misconception that if a plane has propellers, then it cannot be a "jet."However, this is not so. The turboprops used on planes like the C-130, P-3 Orion , andmany commuter planes such as the ATR-42 do indeed use propellers to generatethrust, but the basic technology that turns those propellers is based on the turbojet. Inmore general terms, a turbojet is a gas turbine engine, and that same gas turbinecycle is used to power a turboprop as well as a turbofan. A turbofan is just like aturboprop except that some of the power generated by the gas turbine core is used to

turn a series of ducted fan blades that generate additional thrust.

Fundamental gas turbine core used in a turbojet, turboprop, and turbofanA similar class of engines that is just starting to be used on production aircraft is thepropfan . Although the propfan also utilizes a gas turbine core, the power generated by
http://www.aerospaceweb.org/aircraft/transport-m/c130/http://www.aerospaceweb.org/question/propulsion/q0033.shtmlhttp://www.aerospaceweb.org/question/propulsion/q0033.shtmlhttp://www.aerospaceweb.org/aircraft/maritime/p3/http://www.aerospaceweb.org/aircraft/commuter/atr42/http://www.aerospaceweb.org/question/propulsion/q0067.shtmlhttp://www.aerospaceweb.org/aircraft/transport-m/c130/http://www.aerospaceweb.org/question/propulsion/q0033.shtmlhttp://www.aerospaceweb.org/question/propulsion/q0033.shtmlhttp://www.aerospaceweb.org/aircraft/maritime/p3/http://www.aerospaceweb.org/aircraft/commuter/atr42/http://www.aerospaceweb.org/question/propulsion/q0067.shtmlhttp://www.aerospaceweb.org/question/propulsion/q0067.shtml


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the core is used to turn a series of unducted fan blades rather than propellers. Thesefan blades convert the power generated by the engine into thrust to push the aircraft

forward.The key point to remember is that both the turboprop and the propfan are classified as"jets" since a jet engine is at the heart of the system. This is not true in the case of

other propeller-driven aircraft, such as those of WWII or most modern-day lightgeneral aviation planes. These craft are powered by piston engines, a much differenttechnology that is more akin to how an automobile engine works.

-answer by Joe Yoon , 3 August 2003UPDATE!

To further illustrate the point about turboprop engines being jet-powered, I recentlycame across the following quote in a copy of Skywest Airlines Magazine. The articlewas about the EMBRAER EMB-120 Brasilia commuter plane operated by the airline,and included the remark

..."Don't be fooled by the propellers you see: the same technology that powers jetaircraft actually powers the EMB-120 as well. Like jet engines, the EMB-120 is

powered by a gas turbine design, allowing for the superior reliability and power that jetengines enjoy"...While the article does not mention the turboprop by name, it does a good job of summarizing the key points of our answer about the gas turbine core used in turbojetand turboprop engines.

Turboshaft and TurbopropI read on your site that a turboshaft uses a series of gear connections to turn a rotor

blade. A lot of turboprop engines don't have a direct connection to the propeller but a connecionvia a series of gears like a turboshaft. Would it be more appropriate to call these turboprops

"turboshafts" then?-question from name withheld

This question makes reference to two past articles discussing the types of jet engines and theturboprop . We have seen that both the turboprop and the turboshaft are types of jet engines becausethey use the gas turbine cycle. A gas turbine engine works by combusting a mixture of air and fuel togenerate a high-speed exhaust. As this exhaust escapes, it passes through a series of blades called aturbine causing the blades to rotate. This rotation converts the thermal energy of the exhaust gases intomechanical energy.
http://www.aerospaceweb.org/about/bios/joeyoon.shtmlhttp://www.aerospaceweb.org/question/propulsion/q0033.shtmlhttp://www.aerospaceweb.org/question/propulsion/q0033.shtmlhttp://www.aerospaceweb.org/question/propulsion/q0135b.shtmlhttp://www.aerospaceweb.org/about/bios/joeyoon.shtmlhttp://www.aerospaceweb.org/question/propulsion/q0033.shtmlhttp://www.aerospaceweb.org/question/propulsion/q0135b.shtml


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Schematic of a turboshaft engine The turboshaft engine receives its name from the fact that this rotation is used to turn a shaft of somekind. In the aerospace field, the turboshaft engine is used on helicopters. The rotating shaft of theengine is used to turn the rotor blades that provide lift and forward motion of the vehicle. However, thisapplication is only one of many to which the turboshaft has been applied. The power industry alsomakes use of turboshaft engines to generate electricity. In this case, the rotating shaft is used to rotatea coil through a magnetic field to generate electrical current. Turboshafts are also used in ships andboats where the rotating shaft turns the vehicle's propeller. Turboshafts can even be used on groundvehicles, particularly military tanks and some racecars.

Schematic of an electrical generator You are correct that there is a strong relationship between the turboshaft and another type of jet enginecalled the turboprop. The turboprop also uses a gas turbine core to turn a shaft. However, the turbopropgets its name from the fact that the shaft is connected to an aircraft propeller causing it to turn.Nevertheless, you are incorrect in saying that a turboprop must have a direct connection to thepropeller. Perhaps your confusion is based on the following example provided in one of our previous


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explanations about this subject. This particular design does indeed illustrate a propeller that is directlyconnected to the engine shaft.

Schematic of a turboprop engine However, most turboprops, like that below, are not directly connected. The engine shaft is insteadattached to a gearbox that is connected to the propeller. This system includes reduction gears that

allow the engine shaft and propeller to spin at different rates.

Schematic of a turboprop engine with a gearbox It would be incorrect to call these engines turboshafts just because of how the engine shaft andpropeller are connected. Any jet engine that turns a propeller is called a turboprop for that reason.However, it is correct to say that the turboprop is a subcategory or variant of the more generic class of engines known as turboshafts. In fact, a turboshaft is defined as any type of gas turbine engine thatturns a shaft connected to something other than an aircraft propeller.Additional information on how a turboprop engine works, including several animated images, can befound at NASA's Learning
http://www.grc.nasa.gov/WWW/K-12/airplane/aturbp.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/aturbp.html

Documents

The Dynamics of Airplane Flight