4 ms·

Isn't that also what a propeller does?

moonrobin 7y ago
IANAAE (I Am Not An Aerospace Engineer), but I think the key difference here is the compression that happens with turbines as opposed to propellers which only move the air.
- larkeith 7y ago
  I've always wondered what exactly differs a jet from a ducted fan - this makes a lot of sense, thank you!
  - dTal 7y ago
    Modern "high bypass" jet engines are actually ducted fans, with the shaft powered by a "gas turbine" whose role is mostly to provide torque. All propulsion units compress the air to some extent, even propellers - if they didn't, they would be silent. However, in a gas turbine, the function of the "compressor stage" (which is NOT the "turbine" part) is to get a lot of oxygen in one place, where it can be combined with a lot of fuel, which makes it very hot, which causes it to adiabatically expand a lot, driving the turbine at the back (this is what a 'turbine' does - extract power). If the air weren't compressed, you wouldn't be able to burn much fuel with it, and it wouldn't get so hot and expand so much. (By the way, this is the same reason for superchargers and turbochargers in piston engines. More air = more fuel = more heat and expansion) This process isn't actually very efficient at moving air though, which is why gas turbines are now mostly used to power ducted fans, propellors, and helicopter rotors.
    - deleted 7y ago
      [deleted]
- semi-extrinsic 7y ago
  In a propeller aircraft, basically you waste the energy left in your exhaust gas. But in a turbojet you make some use of it. Also there is the distinction between just propeller, a turboprop, a turbojet and a turbofan. What we call jet airliners are typically high bypass turbofans. Turbojets (without bypass) have low efficiency at subsonic speeds, but they were used on Concorde and on fighter aircraft.
dTal 7y ago
A propellor turns shaft torque into air momentum. You need some kind of motor to drive it. A ducted fan also does this, though its aerodynamics are a bit different. Like a propellor, there is little compression. A 'turbine' turns flow into torque. A traditional jet engine turns fuel into torque with a compressor and a turbine. Confusingly it's sometimes called a 'gas turbine'. It also incidentally moves air, but not very efficiently. A turboprop drives a propellor with a 'gas turbine'. A modern 'high bypass' jet engine (or "turbofan") drives a ducted fan with a 'gas turbine'. An 'electric jet' drives a ducted fan with an electric motor. No turbine is involved.
- jansan 7y ago
  These definitions better be correct, becaus I will use your post as a reference from now :)

IANAAE (I Am Not An Aerospace Engineer), but I think the key difference here is the compression that happens with turbines as opposed to propellers which only move the air.

larkeith 7y ago
I've always wondered what exactly differs a jet from a ducted fan - this makes a lot of sense, thank you!
- dTal 7y ago
  Modern "high bypass" jet engines are actually ducted fans, with the shaft powered by a "gas turbine" whose role is mostly to provide torque. All propulsion units compress the air to some extent, even propellers - if they didn't, they would be silent. However, in a gas turbine, the function of the "compressor stage" (which is NOT the "turbine" part) is to get a lot of oxygen in one place, where it can be combined with a lot of fuel, which makes it very hot, which causes it to adiabatically expand a lot, driving the turbine at the back (this is what a 'turbine' does - extract power). If the air weren't compressed, you wouldn't be able to burn much fuel with it, and it wouldn't get so hot and expand so much. (By the way, this is the same reason for superchargers and turbochargers in piston engines. More air = more fuel = more heat and expansion) This process isn't actually very efficient at moving air though, which is why gas turbines are now mostly used to power ducted fans, propellors, and helicopter rotors.
  - deleted 7y ago
    [deleted]
semi-extrinsic 7y ago
In a propeller aircraft, basically you waste the energy left in your exhaust gas. But in a turbojet you make some use of it. Also there is the distinction between just propeller, a turboprop, a turbojet and a turbofan. What we call jet airliners are typically high bypass turbofans. Turbojets (without bypass) have low efficiency at subsonic speeds, but they were used on Concorde and on fighter aircraft.

dTal 7y ago

A propellor turns shaft torque into air momentum. You need some kind of motor to drive it. A ducted fan also does this, though its aerodynamics are a bit different. Like a propellor, there is little compression. A 'turbine' turns flow into torque. A traditional jet engine turns fuel into torque with a compressor and a turbine. Confusingly it's sometimes called a 'gas turbine'. It also incidentally moves air, but not very efficiently. A turboprop drives a propellor with a 'gas turbine'. A modern 'high bypass' jet engine (or "turbofan") drives a ducted fan with a 'gas turbine'. An 'electric jet' drives a ducted fan with an electric motor. No turbine is involved.

jansan 7y ago
These definitions better be correct, becaus I will use your post as a reference from now :)