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A turbojet/turbofan engine is aimed to accelerate air or combustion gases. Sometimes these engines are named "reaction engines" to emphasize the fact the aircraft is moved forward mostly by Newton's 3rd law of motion.

enter image description here
Source. "For every action in nature there is an equal and opposite reaction"

As far as I understand, this reaction is created where action is created, and greater where action is greater. Air is accelerated backward (action), and logically the reaction (pushing the aircraft) is the greatest where air or gases are the most accelerated, which likely happens to be inside the engine rather than in the exhaust nozzle.

Question

Where is the reaction force (thrust) applied in the engine?

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  • $\begingroup$ I'm interested in why jet engines are special in relation to the 3rd law of motion. Might have to ask a question! $\endgroup$ – Gusdor Nov 9 '16 at 8:31
  • $\begingroup$ @Gusdor you can still ask a new question; perhaps with a link to this one to provide context $\endgroup$ – Manu H Nov 9 '16 at 18:58
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The thrust produced by the engine is due to the net resultant of various forces acting on various surfaces of the engine. The thrust produced is a function of mass flow rate and the change in velocity ($T = \dot{m} (V_{e}-V_{\inf})$). So, both of these have to be taken into account, rather than acceleration alone. The location of maximum thrust generated varies with the type of engine.

  • For high bypass jet engines like those used by the modern commercial airliners, most of the thrust is produced by the bypass fan. Though the acceleration is not the greatest here, mass x acceleration is- so this is where the thrust is maximum. The maximum reaction forces are applied there

  • In case of pure turbojets, the (almost all of the) thrust is produced by the core. For low bypass turbofans, its somewhere in the middle, with the lion's share of thrust produced by the core.

Note that most of the reactive force is applied on the diffuser and the compressor due to the high pressure and the forward facing area (due to cross sectional variation) in this region. This high pressure also acts on the combustion chamber, increasing the reaction force.

In the turbine and nozzle, the variation in the cross section creates a surface area facing the rearward direction, where the gas pressure acts, resulting in an force applied in direction opposite to the compressor. The net result of all these forces gives the thrust.

Thrust distribution

Image from Aircraft Performance and Design by John Anderson; taken from quora.com

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  • $\begingroup$ Is it meaningful to simplify the reaction to a barycenter, like weight can simplified as applying to the center of gravity? If so where is it located in a turbojet and in a turbofan? $\endgroup$ – mins Nov 9 '16 at 6:37
  • $\begingroup$ @ymb1: The explanation for the variable nozzle geometry is simply to balance the flue gas temperature in order to keep a constant pressure level. Speed of sound increases by the root of the gas temperature, while density decreases by the inverse of the gas temperature. Summed up, the mass flow density is roughly proportional to the rooted inverse of the gas temperature. The afterburner shall not affect upstream engine components (air mass flow + pressure need to be constant), thus a larger cross section is required. Switching on the reheat with no surface adjustment causes a pressure increase. $\endgroup$ – Chris Nov 9 '16 at 10:27
  • $\begingroup$ @aeroalias: Nice answer. In the case of the turbojet / low-bypass-ratio turbofan you could add the case of (super-)sonic exhaust stream in which an additional contribution of thrust stems from the pressure gradient between nozzle area and ambience. $\endgroup$ – Chris Nov 9 '16 at 13:28
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    $\begingroup$ This is correct for a turbojet, but totally wrong for a large turbofan, where most of the thrust comes from fan's interaction with the air that does not go through the engine core. By Newton's third law, this produces a forwards force on the fan, which is transmitted along the fan shaft to a thrust bearing, then through the bearing support structure (the rectangular pink piece in the OP's picture) and from there directly into the pylon and the aircraft wing. $\endgroup$ – alephzero Nov 9 '16 at 20:36
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    $\begingroup$ @mins since the only stress applied is a force with no torque, it's application point can lie anywhere on the centreline, it won't matter. In screw theory, it is called a sliding wrench I believe. Though good luck googling these keywords :-\ $\endgroup$ – MrBrushy Nov 16 '16 at 10:26
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Most of the thrust is generated in the combustion chamber, followed by the compressor stages. The exhaust is only contributing a small fraction of the total forward gas load.

enter image description here Source: the jet engine, Rolls Royce (ISBN: 9781119065999)

The picture show the gas load contribution of various parts of a pure jet engine. In turbofan engines, a relatively larger part is contributed by the fan.

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    $\begingroup$ A definition of "gas load" would be a good addition to this answer. $\endgroup$ – pericynthion Nov 9 '16 at 6:42
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    $\begingroup$ @pericynthion The "gas load" is the total axial force applied by the gas to all the engine parts (casings, rotor and stator blades, combustion chamber, etc). In the compressor and turbine sections, most of that force acts on the blades, not directly on the engine casing. $\endgroup$ – alephzero Nov 9 '16 at 20:44
  • $\begingroup$ You are missing a comma in the picture where it says 41091 $\endgroup$ – Ferrybig Nov 10 '16 at 11:38
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The reaction forces are applied, strangely enough, to the fan disk as it produces the lion's share of the thrust by accelerating the largest quantity of air flowing through it; it is the actual object making contact with the airflow and pushing it out the nacelle outlet.

The low and high pressure compressor sections have additional reactive forces imparted upon them as the accelerate an air mass through them.

Reactive force is lost as air flowing through the gas core imputs a reactive force opposing forward thrust on the turbine rotors and hot section stators - the energy is extracted in the form of mechanical work to drive the fan/LPC/HPC.

On turbojets, low bypass turbofans, and to a small extent the gas core of a high bypass turbofan engine, the majority of the reaction forces are being applied to the fwd section of the jetpipe near at the last LPT stage or stator prior to entering the jetpipe due to the imbalance of pressure between this section of the engine and the exhaust nozzle outlet at atmospheric pressure.

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  • $\begingroup$ Interesting answer, thanks. From you answer, the "center of thrust" for a turbojet is near the rear mount point / turbine frame. Correct? $\endgroup$ – mins Nov 11 '16 at 13:08
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For the very special case of engines operating in the supersonic region, a large part of the thrust is applied via the intake.

"The Concorde Air Intake Control System" via http://www.pprune.org/tech-log/426900-concorde-engine-intake-thrust.html :

A huge 75% OF THE TOTAL THRUST is produced by the intake subsonic diffuser section, this being due to the huge rise in static pressure that is occurring in this section. The 'negative thrust' from the forward ramp section this time is 12%, produced by the supersonic compression forces acting on the divergent section of the intake, resulting in an intake thrust component of 63%. So it can be seen that the vast majority of the Mach 2 thrust forces are transmitted to the airframe not via the engine mountings, but via the mountings of the intake, and to a lesser extent the TRA nozzle.

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    $\begingroup$ The wording of that quote is seriously misleading. The force is indeed applied to the diffuser, but it is not generated by it. Because the reason why the thrust exists is the pressure increase in the combustion chamber and the fact that the rear "wall" holding that pressure is the accelerating exhaust stream, so the forward force gets applied to the aircraft while the aft force does not. $\endgroup$ – Jan Hudec Nov 9 '16 at 17:24
  • $\begingroup$ Does that mean for a Concorde engine at Ma 2 that total thrust can be seen as a force originating from a "center of thrust" on the engine axis and rear and close to the intake diffuser? $\endgroup$ – mins Nov 11 '16 at 12:47
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For every action in nature there is an equal and opposite reaction.

That sentence alone is incomplete and misleading, because this short version fails to mention the two-body system it was meant for.

The action and reaction can actually be swapped in position. The difference between a gas generator and a jet engine is the mere (over simplification) addition of the nozzle to accelerate the air out the exhaust.

So, you can confidently reverse the common sentence and write it as: the thrust on the engine (action) causes the mass air flow (reaction). And you won't be wrong.

The answer to your question; all the parts experience a normal force (thrust). Some are forward, some are rearward.

Shameless copy from Wikipedia:

Origin of engine thrust

The familiar explanation for jet thrust only looks at what goes in to the engine, air and fuel, and what comes out, exhaust gas and an unbalanced force. This force, called thrust, is the sum of the momentum difference between entry and exit and any unbalanced pressure force between entry and exit; looking inside shows that the thrust results from all the unbalanced momentum and pressure forces created within the engine itself. These forces, some forwards and some rearwards, are across all the internal parts, both stationary and rotating, such as ducts, compressors, etc., which are in the primary gas flow which flows through the engine from front to rear.

Transferring thrust to the aircraft

The engine thrust acts along the engine centerline. The aircraft "holds" the engine on the outer casing of the engine at some distance from the engine centerline (at the engine mounts). This arrangement causes the engine casing to bend (known as backbone bending) and the round rotor casings to distort (ovalization). Distortion of the engine structure has to be controlled with suitable mount locations to maintain acceptable rotor and seal clearances and prevent rubbing. A well-publicized example of excessive structural deformation occurred with the original Pratt & Whitney JT9D engine installation in the Boeing 747 aircraft. The engine mounting arrangement had to be revised with the addition of an extra thrust frame to reduce the casing deflections to an acceptable amount.

enter image description here

We see in this 3,000 lbf jet engine (image from Flight), each combustor has a net forward force of 50 lbf (difference between flame tube and outer casing). Having 16 combustors yields 800 lbf forward thrust along the centerline.

Other parts are treated the same way. Although I have to admit some are more straight forward than the other.

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Let's take concept of energy for help. All jet engine thrust originate in combustion chamber (energy of burning fuel). Smaller or bigger part of this thrust ( depending on engine construction) is transferred from turbines blades ( high and low pressure) forward via axis to compressor / fan. Let's take example of low pressure turbine/ fan system. Expanding gas in combustion chamber pushes on turbine blade and via axis rotates fan blades. So any forward force created by fan blades is transferred via axis and turbines blades back to its origin i.e. combustion chamber ( backward facing surfaces). So in theory if all rotating parts would be free to move forward during steadily running engine they wouldn't move. So analogically to center of gravity of an object the central point of thrust within the jet engine lies in geometrical center of backward facing surface of combustion chamber.

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