7 ms·

This is a very nice demonstration of turbojet. While it has not been claimed as such in the video, this is how most of the modern airplanes DO NOT work. Modern

by subhro 4y ago

This is a very nice demonstration of turbojet. While it has not been claimed as such in the video, this is how most of the modern airplanes DO NOT work. Modern airplanes use turbofans which has an extremely high bypass ratio. In a turbofan, the turbine mostly rotates the various stages and creates very little thrust.

gorgoiler 4y ago
What’s the deal with high bypass turbofans vs turboprops? Turbofans look a lot like the jet engine in this video but, as you point out, only the core is the turbine and most of what’s inside the cowling is a fan for pulling air past the engine, which pulls the plane forwards. Turboprops do this but, to a lay person, in a different way. Four propeller blades instead of hundreds, and no cowling. Is it just cheaper to build a turboprop, and turbofans are the ultimate in terms of performance?
- mlsu 4y ago
  I think turboprops are less efficient, because at high airspeeds, drag is created by vortices generated at the tips of the blades, which are exposed to the free air. By contrast, the cowling around a turbofan prevents such vortices from being created. Ducted fans are, in general, a more efficient design because of this.
  - subhro 4y ago
    Turboprops indeed fight wing tip vortices. But they also have a problem of not being able to shape air properly like Turbofans can do with stator vanes and such. As you stated, ducts are awesome.
- subhro 4y ago
  Great question. Turbo props and turbo fans, both produce thrust, but very differently. In a turbofan, the turbine spends most of its effort turning the compressor blades. In a modern turbofan (TF) engine like GE 9x, you are looking at a 80-20 bypass ratio. This means, 80% of the air drawn from the front just flows through. Now, as the "FAN" (duh!) rotates, a huge volume of air is drawn in and compressed. As the volume moves deeper into the engine, the air pressure increases because the engine tapers down, as well as the compressor works in "compressing" the voume of air. As the air pressure increases, so does the speed and temperature. Now, as this compressed air exits from the back of the engine, it provides a forward thrust. The 20% air that DID get into the core, carrys on the combustion in the engine that provides the energy required to rotate the compressor blades. Now this is definitely simplified as there are many additional pieces, like stator vanes, equipment to shape and correct the velocity of the air, fuel injectors, igniters, afterburners (not in civilian airplanes of course) and many more contraptions. While not completely comparable, a turbo prop (TP) behaves a lot like its piston counterpart. The turbine is simply replacing a traditional 4 stroke IC engine in this case and just tasked with rotating the blades. Each of the blade is a mini (well not so mini) wing that has the shape of an airfoil. As these rotate, they generate lift but since the rotational axis is parallel to the fuselage, it is actually thrust. > Is it just cheaper to build a turboprop, and turbofans are the ultimate in terms of performance? It is definitely cheaper to build a TP. And TPs can take a lot of abuse and needs miniscule maintanance compared to TFs. But if you are talking of raw performance, turbojets (TJ) actually run (fly?) around in circles around TFs. TJs are just incredibly wasteful, inefficient and needs a lot of maintenance.
  - Toutouxc 4y ago
    It sounds like you know your way around a turbofan, but you wrote some confusing stuff, mainly this: > Now, as this compressed air exits from the back of the engine, it provides a forward thrust. This is something you generally don't want to happen in a reaction engine. If you eject above-atmospheric-pressure out of your engine, the expansion takes place downstream, in the wake, and that energy is mostly wasted (some pressure thrust is still created, but it's minuscule). You always want all the expansion to happen inside your engine, in a properly designed propelling nozzle, which should always result in a flow of low-pressure high-speed air leaving the engine. If the nozzle fails to expand the air (choked flow, nozzle too short, wrong shape), you're losing thrust (and efficiency).
    - subhro 4y ago
      You are right. I was mostly simplifying the description for NOT aeronautical engineers. But you are absolutely right.
  - the__alchemist 4y ago
    So, to confirm for the comparison, in a nutshell (Some of the upstream posts in this sub-thread imply the bypass section of a TF provides its thrust directly, which I believe your post is showing is incorrect): - Treat TJ as base model - TF uses the bypass section to provide additional power to the compressor - TP generates its thrust from a prop on the shaft, vice the core's output (is this called exhaust?). TJ and TF both use the core's output for thrust. Is this right?
- credit_guy 4y ago
  Energy is proportional with the speed squared, momentum with the speed. You have a fixed budget of energy, initially stored as chemical energy in jet fuel. The engines have a more or less fixed thermodynamic efficiency, so you transform the chemical energy in a fixed amount of kinetic energy of the air being pushed out of the engine. You want that air to have as much momentum as possible (Newton’s action-reaction principle: the momentum the plane gives to the air is the momentum the air gives to the plane). Now, for a given energy, you can push some mass of air with speed v or 4 times as much with speed v/2. The momentum of the air in the latter case is double the first case: 4 m v/2 = 2mv. You always want to move more air at lower speed. You achieve that with higher and higher bypass ratios. Of course, with a higher bypass ratio the engine becomes larger and heavier (and draggier), so there are limits. But if you could increase the bypass ratio without increasing the engine weight and drag, you would always do it. As a bonus, with lower air speed come lower vibrations and noise.
  - mlsu 4y ago
    This is also why planes with large wings (think of a glider vs. a fighter jet) are more efficient; they are pushing a larger volume of air slower.
    - nautilius 4y ago
      It’s a bit more complicated than that: glider wing area ~10m2, F16 ~30m2, F22 ~80m2
  - TylerE 4y ago
    Also a limiting factor on turboprops (due to their larger diameter) run at a much lower rpm… they’re geared down quite a bit m. This is a complicated area, but in general basically: Supersonic flow is bad. At best you get a ton of noise (like the Russian Tu-95). Efficiency also goes to hell. This is also why the SR-71 had those crazy moving brake cones… they manipulated the shockwave so the air actually entering the engine was subsonic.
    - adgjlsfhk1 4y ago
- yread 4y ago
  The other comments have some good explanations, I just want to to confuse you further by pointing out the "open rotor engines" https://en.m.wikipedia.org/wiki/Propfan https://en.m.wikipedia.org/wiki/Propfan exist and are different from both
- Gibbon1 4y ago
  They're both exploiting the same principal. It's more efficient to generate thrust by accelerating a large amount of air a little than a little amount of a air a lot. Figure of merit is the bypass ratio. Props are 50-100, fans are 3-10. So big advantage of a turboprop is a much higher bypass ratio than fans. Downside is props can't spin as fast and so need a high speed gearbox to drive them. And props loose efficiency at high speeds. I think for large fast aircraft turbofans are a win. For medium sized slower aircraft turboprops are a win.
  - sien 4y ago
    The turbofan wikipedia page has a very nice graph that shows the propulsive efficiency of turbofans, turboprops and turbojets at different speeds. https://en.wikipedia.org/wiki/Turbofan#Efficiency https://en.wikipedia.org/wiki/Turbofan#Efficiency
- salty_biscuits 4y ago
  Basically this plot is why. Efficiency versus speed characteristics due to aerodynamics of propellors versus cowled rotors. Turboprop for short low flights, high bypass turbofan for long and high flights. https://en.m.wikipedia.org/wiki/Propulsive_efficiency#/media/File%3APropulsive_efficiency_for_different_engine_types_and_Mach_numbers.png https://en.m.wikipedia.org/wiki/Propulsive_efficiency#/media...
- mnw21cam 4y ago
  The main difference between a turboprop and a turbofan is the speed of sound. A propeller needs to avoid breaking the speed of sound, otherwise you get obscene noise, a reduction in efficiency, and potentially damage to the propeller (although there are some rare propellers that do regularly break the sound barrier). This means that propeller-driven planes must have a speed limit that is quite a lot lower than the speed of sound, to avoid the propeller tips breaking that speed barrier. A turbofan however is typically installed on planes that want to fly around 80% of the speed of sound. That's really close to the limit, and there isn't enough leeway there for a propeller to get away with it. A turbofan manages by using lots of blades to more gently push the air back, and then the cowling helps by preventing blade-tip vortices, and by constricting the airflow slightly, which increases the air's backwards speed. So a propeller and a turbofan are suitable for different plane speed regimes. When you're travelling slowly, it is most efficient to push a very large amount of air back just a little bit faster than the plane is moving, and a propeller is great at doing that. As you approach the speed of sound, that becomes infeasible, and a turbofan becomes more efficient. If you're trying to break the speed barrier, then you need a lower bypass ratio, or you may even need to switch to a turbojet.
  - deleted 4y ago
    [deleted]
  - kloch 4y ago
    Virtually all turbofan engine blades exceed the speed of sound at high N1% even when moving slow (like during takeoff). This is part of what generates the distinct "buzzsaw" sound at or near max thrust when seated forward of the engines. The pitch/tone of the buzzsaw sound depends on the number of fan blades and rpm of the main fan (and likely stator design). In this video of an Iran Air 747SP the buzzsaw effect is particularly isolated/exaggerated due to the camera/mic placement, the use of 1970's design (louder) JT9D engines, and the shorter length of the 747SP. https://youtu.be/3lhHwKK-6ms https://youtu.be/3lhHwKK-6ms On a 737 the pitch is higher due to the smaller diameter fan (higher max RPM), while a 777 has the lowest buzzsaw pitch due to having the largest fan diameter. The most recent jet engines (like on the 747-8) have highly suppressed this sound with contoured fan blades.
  - 4y ago
neilpanchal 4y ago
> In a turbofan, the turbine mostly rotates the various stages and creates very little thrust. That undersells the turbine. Turbine powers not only the various compressor stages, but also the turbofan itself. All power comes from the turbine and it is is arguably the most challenging part of a jet engine. Turbine blades are tiny and fit in the palm of your hand with fingers closed [1]. Temperatures through the roof, it needs compressor bleed air to keep the turbine blades cool or the blades would melt. I was part of the team that did mechanical design of turbine blades and one of interesting challenges I remember was the trade off between the pressure-side bleed vs. trailing-edge dump techniques of cooling turbine blades. Aero engineers prefer razor thin trailing edge for better efficiency (pressure-side bleed as you can see in the picture). These are tiny holes/slots on the edge of the blade that bleed compressor air from various stages which is at 800F and trying to cool 2200F blade edge. So mechanical engineers (us) would prefer a trailing-edge-dump technique of cooling where we would make the edge thicker and drill the holes at the apex of the edge for better cooling performance. Pressure-side bleed usually won because higher fuel efficiency requirements and pushed us to do other things to cool the blade, even if that requires pulling air from a cooler compressor stage, which impacts efficiency too. Another fun fact, the whole thing leaks hot gas like a sieve until engine reaches temperature and the tip of the turbine blades seal the hot gas path through elongation along the radial direction. Reminds me of the famous SR-71 fuel tank seals. Jet engines are insane. So much complexity. [1] https://www.theengineer.co.uk/media/4kgoz1mg/rr-turbine-blade.jpg https://www.theengineer.co.uk/media/4kgoz1mg/rr-turbine-blad...
- subhro 4y ago
  Very true. I never said the turbine is not a marvelous piece of engineering. But it DOES provide very little thrust.
  - Toutouxc 4y ago
    A turbine, by definition, provides zero thrust, because it works the other way (it extracts energy from the flow). What you meant to say was that the air flowing through the engine core provides very little thrust, because most of its energy is being extracted by the turbine almost immediately after combustion.
globular-toast 4y ago
I think what you mean is very little thrust exits the turbine. In other words, the air expansion from burning the fuel is used almost entirely by the turbine which, in turn, powers both the compressor and a huge fan which provides the actual thrust used for flight.

by subhro 4y ago

gorgoiler 4y ago

What’s the deal with high bypass turbofans vs turboprops? Turbofans look a lot like the jet engine in this video but, as you point out, only the core is the turbine and most of what’s inside the cowling is a fan for pulling air past the engine, which pulls the plane forwards. Turboprops do this but, to a lay person, in a different way. Four propeller blades instead of hundreds, and no cowling. Is it just cheaper to build a turboprop, and turbofans are the ultimate in terms of performance?

mlsu 4y ago
I think turboprops are less efficient, because at high airspeeds, drag is created by vortices generated at the tips of the blades, which are exposed to the free air. By contrast, the cowling around a turbofan prevents such vortices from being created. Ducted fans are, in general, a more efficient design because of this.
- subhro 4y ago
  Turboprops indeed fight wing tip vortices. But they also have a problem of not being able to shape air properly like Turbofans can do with stator vanes and such. As you stated, ducts are awesome.
subhro 4y ago
Great question. Turbo props and turbo fans, both produce thrust, but very differently. In a turbofan, the turbine spends most of its effort turning the compressor blades. In a modern turbofan (TF) engine like GE 9x, you are looking at a 80-20 bypass ratio. This means, 80% of the air drawn from the front just flows through. Now, as the "FAN" (duh!) rotates, a huge volume of air is drawn in and compressed. As the volume moves deeper into the engine, the air pressure increases because the engine tapers down, as well as the compressor works in "compressing" the voume of air. As the air pressure increases, so does the speed and temperature. Now, as this compressed air exits from the back of the engine, it provides a forward thrust. The 20% air that DID get into the core, carrys on the combustion in the engine that provides the energy required to rotate the compressor blades. Now this is definitely simplified as there are many additional pieces, like stator vanes, equipment to shape and correct the velocity of the air, fuel injectors, igniters, afterburners (not in civilian airplanes of course) and many more contraptions. While not completely comparable, a turbo prop (TP) behaves a lot like its piston counterpart. The turbine is simply replacing a traditional 4 stroke IC engine in this case and just tasked with rotating the blades. Each of the blade is a mini (well not so mini) wing that has the shape of an airfoil. As these rotate, they generate lift but since the rotational axis is parallel to the fuselage, it is actually thrust. > Is it just cheaper to build a turboprop, and turbofans are the ultimate in terms of performance? It is definitely cheaper to build a TP. And TPs can take a lot of abuse and needs miniscule maintanance compared to TFs. But if you are talking of raw performance, turbojets (TJ) actually run (fly?) around in circles around TFs. TJs are just incredibly wasteful, inefficient and needs a lot of maintenance.
- Toutouxc 4y ago
  It sounds like you know your way around a turbofan, but you wrote some confusing stuff, mainly this: > Now, as this compressed air exits from the back of the engine, it provides a forward thrust. This is something you generally don't want to happen in a reaction engine. If you eject above-atmospheric-pressure out of your engine, the expansion takes place downstream, in the wake, and that energy is mostly wasted (some pressure thrust is still created, but it's minuscule). You always want all the expansion to happen inside your engine, in a properly designed propelling nozzle, which should always result in a flow of low-pressure high-speed air leaving the engine. If the nozzle fails to expand the air (choked flow, nozzle too short, wrong shape), you're losing thrust (and efficiency).
  - subhro 4y ago
    You are right. I was mostly simplifying the description for NOT aeronautical engineers. But you are absolutely right.
- the__alchemist 4y ago
  So, to confirm for the comparison, in a nutshell (Some of the upstream posts in this sub-thread imply the bypass section of a TF provides its thrust directly, which I believe your post is showing is incorrect): - Treat TJ as base model - TF uses the bypass section to provide additional power to the compressor - TP generates its thrust from a prop on the shaft, vice the core's output (is this called exhaust?). TJ and TF both use the core's output for thrust. Is this right?
credit_guy 4y ago
Energy is proportional with the speed squared, momentum with the speed. You have a fixed budget of energy, initially stored as chemical energy in jet fuel. The engines have a more or less fixed thermodynamic efficiency, so you transform the chemical energy in a fixed amount of kinetic energy of the air being pushed out of the engine. You want that air to have as much momentum as possible (Newton’s action-reaction principle: the momentum the plane gives to the air is the momentum the air gives to the plane). Now, for a given energy, you can push some mass of air with speed v or 4 times as much with speed v/2. The momentum of the air in the latter case is double the first case: 4 m v/2 = 2mv. You always want to move more air at lower speed. You achieve that with higher and higher bypass ratios. Of course, with a higher bypass ratio the engine becomes larger and heavier (and draggier), so there are limits. But if you could increase the bypass ratio without increasing the engine weight and drag, you would always do it. As a bonus, with lower air speed come lower vibrations and noise.
- mlsu 4y ago
  This is also why planes with large wings (think of a glider vs. a fighter jet) are more efficient; they are pushing a larger volume of air slower.
  - nautilius 4y ago
    It’s a bit more complicated than that: glider wing area ~10m2, F16 ~30m2, F22 ~80m2
- TylerE 4y ago
  Also a limiting factor on turboprops (due to their larger diameter) run at a much lower rpm… they’re geared down quite a bit m. This is a complicated area, but in general basically: Supersonic flow is bad. At best you get a ton of noise (like the Russian Tu-95). Efficiency also goes to hell. This is also why the SR-71 had those crazy moving brake cones… they manipulated the shockwave so the air actually entering the engine was subsonic.
  - adgjlsfhk1 4y ago
yread 4y ago
The other comments have some good explanations, I just want to to confuse you further by pointing out the "open rotor engines" https://en.m.wikipedia.org/wiki/Propfan https://en.m.wikipedia.org/wiki/Propfan exist and are different from both
Gibbon1 4y ago
They're both exploiting the same principal. It's more efficient to generate thrust by accelerating a large amount of air a little than a little amount of a air a lot. Figure of merit is the bypass ratio. Props are 50-100, fans are 3-10. So big advantage of a turboprop is a much higher bypass ratio than fans. Downside is props can't spin as fast and so need a high speed gearbox to drive them. And props loose efficiency at high speeds. I think for large fast aircraft turbofans are a win. For medium sized slower aircraft turboprops are a win.
- sien 4y ago
  The turbofan wikipedia page has a very nice graph that shows the propulsive efficiency of turbofans, turboprops and turbojets at different speeds. https://en.wikipedia.org/wiki/Turbofan#Efficiency https://en.wikipedia.org/wiki/Turbofan#Efficiency
salty_biscuits 4y ago
Basically this plot is why. Efficiency versus speed characteristics due to aerodynamics of propellors versus cowled rotors. Turboprop for short low flights, high bypass turbofan for long and high flights. https://en.m.wikipedia.org/wiki/Propulsive_efficiency#/media/File%3APropulsive_efficiency_for_different_engine_types_and_Mach_numbers.png https://en.m.wikipedia.org/wiki/Propulsive_efficiency#/media...
mnw21cam 4y ago
The main difference between a turboprop and a turbofan is the speed of sound. A propeller needs to avoid breaking the speed of sound, otherwise you get obscene noise, a reduction in efficiency, and potentially damage to the propeller (although there are some rare propellers that do regularly break the sound barrier). This means that propeller-driven planes must have a speed limit that is quite a lot lower than the speed of sound, to avoid the propeller tips breaking that speed barrier. A turbofan however is typically installed on planes that want to fly around 80% of the speed of sound. That's really close to the limit, and there isn't enough leeway there for a propeller to get away with it. A turbofan manages by using lots of blades to more gently push the air back, and then the cowling helps by preventing blade-tip vortices, and by constricting the airflow slightly, which increases the air's backwards speed. So a propeller and a turbofan are suitable for different plane speed regimes. When you're travelling slowly, it is most efficient to push a very large amount of air back just a little bit faster than the plane is moving, and a propeller is great at doing that. As you approach the speed of sound, that becomes infeasible, and a turbofan becomes more efficient. If you're trying to break the speed barrier, then you need a lower bypass ratio, or you may even need to switch to a turbojet.
- deleted 4y ago
  [deleted]
- kloch 4y ago
  Virtually all turbofan engine blades exceed the speed of sound at high N1% even when moving slow (like during takeoff). This is part of what generates the distinct "buzzsaw" sound at or near max thrust when seated forward of the engines. The pitch/tone of the buzzsaw sound depends on the number of fan blades and rpm of the main fan (and likely stator design). In this video of an Iran Air 747SP the buzzsaw effect is particularly isolated/exaggerated due to the camera/mic placement, the use of 1970's design (louder) JT9D engines, and the shorter length of the 747SP. https://youtu.be/3lhHwKK-6ms https://youtu.be/3lhHwKK-6ms On a 737 the pitch is higher due to the smaller diameter fan (higher max RPM), while a 777 has the lowest buzzsaw pitch due to having the largest fan diameter. The most recent jet engines (like on the 747-8) have highly suppressed this sound with contoured fan blades.
- 4y ago

neilpanchal 4y ago

> In a turbofan, the turbine mostly rotates the various stages and creates very little thrust. That undersells the turbine. Turbine powers not only the various compressor stages, but also the turbofan itself. All power comes from the turbine and it is is arguably the most challenging part of a jet engine. Turbine blades are tiny and fit in the palm of your hand with fingers closed [1]. Temperatures through the roof, it needs compressor bleed air to keep the turbine blades cool or the blades would melt. I was part of the team that did mechanical design of turbine blades and one of interesting challenges I remember was the trade off between the pressure-side bleed vs. trailing-edge dump techniques of cooling turbine blades. Aero engineers prefer razor thin trailing edge for better efficiency (pressure-side bleed as you can see in the picture). These are tiny holes/slots on the edge of the blade that bleed compressor air from various stages which is at 800F and trying to cool 2200F blade edge. So mechanical engineers (us) would prefer a trailing-edge-dump technique of cooling where we would make the edge thicker and drill the holes at the apex of the edge for better cooling performance. Pressure-side bleed usually won because higher fuel efficiency requirements and pushed us to do other things to cool the blade, even if that requires pulling air from a cooler compressor stage, which impacts efficiency too. Another fun fact, the whole thing leaks hot gas like a sieve until engine reaches temperature and the tip of the turbine blades seal the hot gas path through elongation along the radial direction. Reminds me of the famous SR-71 fuel tank seals. Jet engines are insane. So much complexity. [1] https://www.theengineer.co.uk/media/4kgoz1mg/rr-turbine-blade.jpg https://www.theengineer.co.uk/media/4kgoz1mg/rr-turbine-blad...

subhro 4y ago
Very true. I never said the turbine is not a marvelous piece of engineering. But it DOES provide very little thrust.
- Toutouxc 4y ago
  A turbine, by definition, provides zero thrust, because it works the other way (it extracts energy from the flow). What you meant to say was that the air flowing through the engine core provides very little thrust, because most of its energy is being extracted by the turbine almost immediately after combustion.

globular-toast 4y ago

I think what you mean is very little thrust exits the turbine. In other words, the air expansion from burning the fuel is used almost entirely by the turbine which, in turn, powers both the compressor and a huge fan which provides the actual thrust used for flight.