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Ducted Fan Thrust

  1. Apr 20, 2008 #1
    :confused:Hi all, I’m new here; my name is Kris aged 53 in Norfolk, UK.
    I’ve been in engineering design all my life so they few theories and ideas I may submit for comment do have some sound basis.

    Anyway, today lesson (for you to teach me) concerns Jet Thrust and ducted fan motors. I understand that thrust measurement is not strictly ‘pressure’ but the ‘equal & opposite reaction’ to the combustion of the fuel in a jet engine?

    Well as I see it the fuel combusts causing massive expansion in the confined space of the engine housing with it’s only direction for expansion being via the outlet. The expansion causes Force in all directions; the engine housing is subject to this force but no reaction can be caused on the lateral periphery but a reaction to the force exiting the outlet can be caused on the front of the engine being axially opposite. Therefore in space where there is no atmosphere to inhibit progress it is the ‘equal & opposite reaction that causes the space craft to move.

    Am I correct so far?

    Ok from here lets go to the aircraft jet. Similar things happen but progress is aided by the engine outlet force coming up against the atmosphere thereby giving this force something to push against. Therefore if this pressure is pushing against the atmosphere, which is under pressure from gravity, it must be causing a greater pressure in the atmosphere locally?

    So, am I right in saying that the engine must cause pressure as a result of the thrust?

    Right now to my question; Radio Control Ducted Fan (jet) Engines. As I see it thrust from this engine is mechanically rather than combustion induced and is more simply ‘pushing’ air out the rear. From this, propulsion is the effect of pressure caused between the propeller and the atmosphere and NOT equal and opposite reaction and is not thrust in the true sense as this system would be totally ineffectual in space? So is it true to say that a Ducted Fan Motor can and does produce pressure as in a ‘Bouncy Castle’ inflation system? How much pressure remains to be measured but is the theory sound?

    I shall await your theory shredding eagerly.
  2. jcsd
  3. Apr 20, 2008 #2


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    Combustion adds energy to the stream. It is a combined effect with the compression prior to the combustion. You are correct in saying that the thrust produced is not simply due to a pressure imbalance.

    Fair enough. I buy that.

    Now you're talking about rockets. Not jet engines. What are you trying to get at?

    Oy. You are really losing me here. There are two things being in an atmosphere provide; a source of combustion air and a source of drag. There is no "pushing" against the atmosphere to produce thrust.

    If you are referring to the pressure at the exhaust exit, that pressure is dependent upon a couple of factors; exhaust geometry and the local atmospheric pressure. Depending on those combinations, there may or may not be a pressure contribution to the thrust. Regardless, the main component of the thrust comes from the change in momentum of the moving fluid within the engine.

    Fans create very little delta P across them. They too provide thrust by changing the momentum of the incoming air stream.

    Do a bit of reading on topics like disk actuator theory and momentum theory to get a better understanding of where the thrust is produced
  4. Apr 20, 2008 #3
    Hi there:

    When dealing with jet generated thrust, here are two URLs that can help you with theoretical background information and related calculations:

    Theoretical background information:

    Online calculator

    I do believe that the above URLs will help you with both engineering theoretical background information and calculations so you can get a good feel for the related input and output values and their trends.


  5. Aug 6, 2010 #4
    links broken.

    Anybody know any current theoretical background sites?

    oh....and bump.
  6. Aug 6, 2010 #5


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    Gold Member

    I think 'bump' gives way to 'necropost' after a year or so...
  7. Aug 6, 2010 #6
    I call it "bi-existential curiosity"
  8. Aug 6, 2010 #7
    oh... and the links were broken...
  9. Aug 6, 2010 #8
    Welcome to the forums krisd.

    For air breathing aircraft propulsion, thrust is based on applying Newton's 2nd law (F=ma) by accelerating a volume of air or gases which generates a reaction force the opposite direction.

    For turbojets and turbofans the main contribution of thrust is not from the compressor blades at the front but from the combustor burning fuel-air mixture resulting in gas expansion and high pressure, high velocity gases out the nozzle. In other words the compressed fuel-air is accelerated mainly by the thermodynamic combustion process.

    For ducted fans it's different. They are essentially propellers operating at high RPM's. Unlike a turbojet or turbofan the main source of thrust is provided by the propeller / ducted fan blade accelerating the air rearwards. This is a "newtonian" way to explain it. Because thrust comes via the blades, we can also use a "bernoullian" method to describe the phenomenom too. A ducted fan blade is nothing more than an airfoil that is producing lift (thrust). This is a result of the difference of lower pressure on the "top" of the blade vs. higher pressure on the "bottom" of the blade created by the motion of the blade through the air.

    In summary, air breathing airplane engines apply F=ma for thrust via accelerating a mass of air. Turbojets/turbofans rely primarily on thermodynamic reactions to do so. Ducted fans/propellers rely on pressure differentials of the blade airfoil to accelerate the air.

    Hope that helps with your understanding.
  10. Aug 12, 2010 #9
    hello sir ,
    how are you , this ahmad new member in the site
    am mechanical engineer , sales ( hvac,plumping systems ) .
    actually i need your help in many things.
    i saw that you are 53 age and has long experince in this feild.
    kindly , confirm my requist if that not bothering for you.
  11. Aug 12, 2010 #10
    the first thing , i would like to know the meaning of thrust for jet fans in car park ventilation system , some fans such axial thrust 50N and centrifugal 100N , whats the different ? and what the positive points for this system compare to duct system?
  12. Aug 12, 2010 #11
    in building a jet engine, it seems the turbines are made from louvred metal- am I right? Could that really give the power transfer to keep the blades spinning and feeding o2?

    If that's the case, then my understanding is that for each unit of gas expelled, there should be a balance of air scooped in, and there should be more scooped in then going out. Then as wide a chord blade makes the most sense (given material strength).

    Then what are you looking for in a turbine to turn a fan without decreasing the velocity of the exit air (velocity of exit air is what provides thrust right? that's what imparts the momentum?)
  13. Aug 13, 2010 #12
    The turbine blades are highly cambered airfoils. I think that's what you mean by the louvred metal. Regarding air mass scooped in & expelled however, actually the mass of air “scooped in” should equal the amount expelled out.

    The turbine blades are highly cambered to capture as much of the kinetic energy of the combusted compressed air. There is a pressure and temperature loss in the process of spinning the turbine to drive the compressor but there’s still enough energy left so that the velocity of the air exiting the engine is greater than the air entering in.
  14. Aug 13, 2010 #13
    In generator applications vs transportation applications, there's got to be a trade-off between how much energy can be sucked out of the exiting jet of hot gases and still be able to do it's job, right?

    I mean, a generator using a jet is built to turn as gnarly a generator as possible while a airplane jet only needs to spin up to it's operating speed with significantly less torque required...is this correct?

    Oooh! I just thought of something: is the entire reason for heating the air to turn the turbo-fans? It would seem that the heated exit gases are what provide the thrust, but I was surprised to hear in low-bypass engines the turbofan provides the bulk of the thrust.

    finally- is there a good resource out there for optimizing turbine-blade design and installation? Because personally, in a model I would just use louvred metal and then file each slate into an airfoil.
  15. Aug 14, 2010 #14
    whats the diffierent between motor CLASS H and MOTOR CLASS F ?
  16. Aug 15, 2010 #15
    1) efficiencies and trade-offs of capturing kinetic energy of exhaust gases:
    My knowledge of gas turbines is limited outside of propulsion applications. However gas turbines use Brayton's Cycle to thermodynamically extract energy to perform useful work. The primary purpose of jets are to accelerate air out the engine and produce thrust. However to do so turbojets & turbofans need a compressor to compress the air. The turbine captures some of the kinetic energy of the exhaust gases to drive the compressor. There is a trade off between the amount of compression desired (capture of kinetic energy) vs. the amount of thrust produced (velocity of exhaust gases) as it varies with turbine inlet temperature and airplane mach number.

    Higher compression ratios increase the thermal efficiency of a turbojet. Reducing the velocity of the exhaust gases by capturing more of the kinetic energy for the compressor to increase the compression also increases the propulsive efficiency. However the trade-off is reduction in the amount of thrust generated. There is a optimum max compression ratio that is bound by the mach number of the airplane (& associated thrust) and the max temperature that the turbine inlet can take.

    As to gas turbines for other applications, I would imagine for power generation for instance you want to capture as much of the kinetic energy of the exhaust gas with the turbine to turn the generator so that the final exhaust gas velocity is low.

    2) Purpose of heating the air:
    Is primarily to thermodynamically accelerate the air. As explained above however some of the energy of the accelerated air is used to turn the turbine and thus the compressor to produce the right level of pressure and temperature as a part of the thermodynamic cycle.

    The difference with turbofans and turbojets relates to the thrust produced vs. the efficiency required. To produce thrust we want to acclerate some volume of mass. We can do this by accelerating a much greater volume by just a little bit, or by accelerating a small volume by a great deal. It's a trade off of efficiency vs. the amount of thrust. Paradoxically higher thrust (greater acceleration of air) means much lower efficiency while the opposite is true. For a turbofan, the turbine captures even more of kinetic energy of the exhaust cases (turbine section tends to be larger) because we're also concerned with not only compressing the air to feed combustion but to also power the turbofan itself. In otherwords the fan is acting like a propeller with a greater volume of air it is accelerating but not nearly as fast as it would through the turbojet core. Turbofans are much more efficient about generating thrust (use much less fuel) but they don't generate nearly the amount of thrust that a pure turbojet would for the equivalent thrust to weight ratio of the engine.
  17. Aug 15, 2010 #16
    @dtango- awesome explanation.

    Is there a good technical resource that you would recommend to learn more about the types of turbines used in jet applications? I have access to the engineering library at the my university, and the math isn't a problem (I'm a math major with an identity crisis- I think I want to get into Aerospace). Or perhaps a good overall technical introduction to turbofans/turbojets?
  18. Aug 15, 2010 #17
    You might try Cumpsty

    Or Flack

    Last edited by a moderator: Apr 25, 2017
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