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Similarities between propeller aircraft and jet aircraft?

  1. Nov 19, 2014 #1
    I was thinking about the working principle of propeller aircraft (piston engined) during some dark night hours with nothing to do... Mostly because of flight simulator video games hahaha...

    Basically for the piston engined propeller aircraft (P-51 Mustang) to move forward, the propeller must be engaged correctly with regards to the powerplant. The propeller is the actuator of flying at the end of the day. The blades must move the air in a certain way, in order to create thrust. Is this last sentence correct? The propeller blades must be angled properly. And there must exist enough the necessary energy (petrol), to fuel the powerplant and actuate the propeller blades. Propeller blades cause the thrust to occur due to Newton's third law and atmosphere filled with nice chunky gaseous substances.

    Now if we think about something like an F-15 aircraft with turbofan engine... The principle of flying never actually changes. The fanblades become the same as propeller blades in ancient aircraft. Basically fanblades assume the role of previous propellers.

    Isn't that the true reason why something like turbofan engine works? There must be logical reason what causes air to flow, into the turbofan engine, in the first place. Could somebody elaborate.

    Imagine that an F-15 aircraft is sitting still on the ground not moving anywhere. Aircraft rests on its wheel on the runway. Engines are not running. Air is NOT flowing into the engine when engine is not running.

    OK, now the F-15 pilot engages the engine starter. Engine starts spooling up the fanblades (somehow). Now airflow begins to flow inside the engine?
     
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  3. Nov 19, 2014 #2

    NTW

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    There is no essential difference. A turbofan is a ducted propeller with many blades, moved by a gas turbine.
     
  4. Nov 19, 2014 #3

    A.T.

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    There is even an intermediate type, the propfan:
    http://en.wikipedia.org/wiki/Propfan
     
  5. Nov 19, 2014 #4

    russ_watters

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    Well, except that the engine exhaust also creates some of the thrust.
     
  6. Nov 19, 2014 #5

    SteamKing

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    The basic principle of powered flight is this: thrust is applied to the aircraft which causes the aircraft to move forward at some velocity, which in turn generates lift from the wings and allows the plane to assume a position above the surface of the earth.

    For an aircraft with a reciprocating engine, a propeller is used to convert the torque generated by the engine into the thrust required to move the aircraft and generate lift. Essentially, the blades of the propeller act as individual wings which spin around the propeller hub. As the blades spin, a lifting force is generated, which, because of the orientation of the propeller with respect to the rest of the aircraft, acts as thrust.

    A jet engine operates on a slightly different principle than a reciprocating engine. The jet engine can generate thrust directly, without having to use propeller blades as an intermediate mechanism. A modern jet engine can do this in a couple of ways:

    1. The combustion of fuel in the jet raises the temperature of the combustion products of the jet fuel and the air several hundred degrees. By directing the exhaust gases thru a nozzle at the aft end of the engine, the expanding gases accelerate, which creates thrust, driving the engine and the aircraft forward.
    2. The jet engine requires that the incoming air be compressed before fuel is added and ignited. In order to compress the incoming air, a compressor is used, which is driven using part of the energy from the exhaust gases coming out the back of the engine.

    In pure turbojet engines, all of the incoming air is sent into the engine to burn the fuel, the exhaust from which is converted into thrust.

    In a modern turbofan engine, the blades in the compressor section at the inlet of the engine are made longer and only part of the air is used to burn the fuel. The rest of the air from the fan section is exhausted over the engine casing, but because of the action of the fan section, the flow is accelerated and thrust is generated, and additional thrust of course is generated by having the exhaust from the burned fuel directed thru an exhaust nozzle.

    In a turboprop engine, the exhaust gases from a jet engine are used to drive a turbine, which in turn, is connected to a propeller. The spinning propeller converts the torque generated by the exhaust turbine into thrust, in a manner similar to a propeller driven by a reciprocating engine.

    http://en.wikipedia.org/wiki/Jet_engine
     
  7. Nov 20, 2014 #6
    Bolded part
    What causes air to come inside turbofan engine? What part of an F-15 C engine causes the airmass to start moving into the inside parts of the engine?

    What moves the compressor blades, of an F-15 engine, when the engine is started? From the state of being where the compressor is basically staying still. Nothing is revolving, nothing is moving when the engine is not in the state of being of operation.

    Imagine that the engine is first turned off, F-15 aircraft is sitting on its wheel on tarmac runway. Then the pilot engages engine starter button. The airmass is not moving because of magic, I trust? I'm interested for example in a low-by-pass turbofan engine. The problem is tricky to imagine in my mind, without much of physics knowledge, behind the principles of starting a jet engine (engine starting).

    Fuel flow starts first, I guess, and ignition reaction occurs. I just have a hard time seeing how the airmass is able to move. The compressor blades must move, for the airmass to move at all. Is this correct? Compressor blades are the airfoils according to NASA webpage. ( http://www.grc.nasa.gov/WWW/k-12/airplane/caxial.html )

    With propeller piston engined aircraft, it's easy to imagine how the crankshaft actually causes, the propeller to revolve around the propeller's hub. Crankshaft is the transmitter of the reciprocating motion into propeller's revolving motion basically. Crankshaft is seeing as practical example in hand-operated drills, used to drill holes into wood...I think most people have used these hand drills in woodworking class in school. At least the operating principle looks quite similar to this kind of hand-drill when you think about it... (I was looking at wikipedia page about crankshaft, and its animated image) :w
     
  8. Nov 20, 2014 #7

    A.T.

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    Why not use Google instead of guessing?

    http://science.howstuffworks.com/transport/flight/modern/question411.htm

    "This starting process normally uses an electric motor to spin the main turbine shaft. ... Some jet airplanes have a separate turbine (sometimes in the tail cone of the plane) that does nothing but generate auxiliary power."

    http://engineering.mit.edu/ask/how-do-blades-jet-engine-start-turning

    "The APU also provides the first step in starting the jet’s main engines and causing its blades to rotate at the tens of thousands of RPMs "
     
  9. Nov 20, 2014 #8

    NTW

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    The first Junkers and BMW turbojets were started with the so-called 'Riedelanlasser', a small, two-stroke, two-cylinder engine installed in the front of the turbojet. It was in its turn started by pulling a string...

    Gas turbines, and internal combustion engines, must be started somehow. Depending on its size and power, they may be started by hand, with a crank (Renault cars included that, as an option, till the 1970s...), with a high-torque electric motor, or with compressed air, as in large Diesels...
     
  10. Nov 20, 2014 #9

    A.T.

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    This is true in practice. But I wonder if, in principle, you could start a jet engine just by starting the combustion, based alone on the mechanical advantage of the turbine vs. compressor pitch. See this related explanation:

    http://www.av8n.com/physics/jet-answer.htm
     
  11. Nov 20, 2014 #10

    NTW

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    Interesting... The 'Coffman starter', that was in essence a large blank shotgun cartridge working as gas generator, was also used to start some early turbojets, I assume that by feeding the gas directly to the combustion chamber stage.
     
  12. Nov 20, 2014 #11

    SteamKing

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    There's an entire article about starting aircraft engines of all types here:

    http://en.wikipedia.org/wiki/Jet_engine

    When a jet engine is turning after starting, and the aircraft is still sitting on the airstrip, once the compressor starts to compress air, it moves into the combustion chambers where the fuel is injected and burned. Now, the airflow in the jet engine is in this one direction: from the compressor aft into the combustion chamber. By moving in this one direction, a small pressure difference is created across the inlet opening of the engine, and more air flows into the compressor to replace the air which has already started to move thru the engine. Once the aircraft has started to move and takes off, the speed of the aircraft relative to the still air around it will increase the air flow at the inlet to the jet engine.

    You could also ask why air is drawn into a reciprocating engine when it starts to turn. A reciprocating engine acts as a pump: when the intake valve in the cylinder opens as the piston moves down on the intake stroke, the movement of the piston creates a lower pressure in the cylinder than the ambient pressure. The air outside the engine naturally wants to flow into the engine and fill this space which has lower pressure. As the altitude of the aircraft increases, the ambient pressure outside starts to drop, and the flow of air into the engine is reduced, which lowers the ability of the engine to produce power. To compensate for this, a supercharger, driven by the engine, is used to force air into the cylinders and restore the power which would otherwise be lost at altitude. The supercharger, in effect, acts as a air compressor. Most superchargers fitted to aircraft engines are driven using the exhaust gases from the engine to drive a small turbine, which in turn is connected to the supercharger. In this case, the supercharger is also called a turbocharger.
     
  13. Nov 20, 2014 #12

    rcgldr

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  14. Nov 20, 2014 #13

    CWatters

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    You could say they do now but many early jet engines had a centrifugal compressor on the front not a "propeller like" turbine. The main purpose of the front end was to compress the air.

    These days they have a dual purpose and most of the thrust comes from the air that bypasses the engine core...

    http://en.wikipedia.org/wiki/Bypass_ratio
     
  15. Nov 20, 2014 #14

    SteamKing

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    This was the one great innovation pioneered by the German jet engine maker Junkers Motoren, which developed the Jumo 004 engine used on the Me-262 jet fighter and other jet powered aircraft built by Germany late in WWII: the axial compressor.

    http://en.wikipedia.org/wiki/Junkers_Jumo_004

    The axial compressor made it possible to mount the jet engines under the wings of the Me-262, instead of inside the fuselage as a centrifugal compressor design would have required. The flow of air thru the engine was also greatly simplified by the axial design. Instead of having to change direction in a centrifugal compressor, the air could flow in, be compressed, pass into the combustion cans where the fuel was injected and burned, and then pass out the exhaust, while driving the turbine stages which powered the compressor.
     
  16. Nov 20, 2014 #15

    A.T.

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