Turboprop vs turbojet

[Jackonicko]

How much more environmentally friendly is a modern turboprop than an equivalent sized turbojet, in terms of environmental impact?

Roughly 30% lower fuel burn per passenger seat/mile?
Lower flying and less likely to contrail?
Lower flying and releasing emissions at a 'better' height?
A better 'neighbour' in terms of noise?

Is there an environmental argument for making greater use of turboprops for short haul?

How do the new generation of Open Rotor Fans/Unducted fans/Propfans compare?

Any thoughts?
What am I missing?

[speedrestriction]

Originally Posted by Jackonicko

How much more environmentally friendly is a modern turboprop than an equivalent sized turbojet, in terms of environmental impact?

Roughly 30% lower fuel burn per passenger seat/mile?
Lower flying and less likely to contrail?
Lower flying and releasing emissions at a 'better' height?
A better 'neighbour' in terms of noise?

Is there an environmental argument for making greater use of turboprops for short haul?

How do the new generation of Open Rotor Fans/Unducted fans/Propfans compare?

Any thoughts?
What am I missing?

The word "turbojet" is more usually used to describe aero jet engines with low bypass ratios. A pure turbojet accelerates a small volume of gas to a very high velocity (think military fighters and early jet airliners). All modern commercial types use turbofan engines which compared to a turbojet accelerate a large volume of air but nowhere near as fast as a turbojet engine. This is the reason for the much larger engine diameters these days compared to forty years ago - as an aside this is, indirectly, the reason for disasters with the 737 Max.

From my own experience, the latest narrowbody engines when flown at low cost indices at similar altitudes are burning roughly the same amount of fuel per seat kilometer as large turboprops albeit there has been more progress and research in turbofan engines compared to commercial turboprop engines. I would expect the next generation of airliner turboprop engines might widen the gap again.

[Less Hair]

Turboprops and Turbofans might merge somehow with new unducted fans open rotors or similar coming up. Watch out for new T-tails.

[Jackonicko]

Originally Posted by speedrestriction

From my own experience, the latest narrowbody engines when flown at low cost indices at similar altitudes are burning roughly the same amount of fuel per seat kilometer as large turboprops .....

Really?

That's astonishing, to me, at least.

[msbbarratt]

It'd be quite helpful if performance data for the A400M were published. It's a big aircraft, with big turboprops, goes fairly quickly, flies quite high. It's an aircraft that might in a parallel universe have been given turbofans. It's not directly comparable to any turbo-fanned commercial airliner, but it'd be interesting to see the differences. It looks like it carries less payload for less distance but for disproportionately more fuel compared to, say, an A330-300, but then it's a whole lot stockier in shape.

Plus it comes with the exciting passenger option of sky-diving down to one's destination!

[Private jet]

Over the years there has been a "convergence" of turboprops and jet engines as bypass ratios have got bigger and propulsive efficiencies are more matched. However, turbofans of high bypass ratio ( these days up to 10:1) will always be more effective/efficient at higher speeds due to the fan duct itself, reducing "tip losses" and concentrating the airflow to give an optimum duct pressure ratio of 1.6 and therefore a more useful acceleration.

[tdracer]

The latest generation of high bypass turbofans have really blurred the distinctions between turbofans and turbo props. Both in noise and fuel burn they have gotten rather close - to the point where other factors come into play as to which is better.
Propulsion theory says that you get the maximum efficiency when you accelerate an infinite mass of air an infinitesimal amount. Obviously, an infinitely large fan (or prop) isn't practical, but that's why you see ever increasing bypass ratios on the current generation of turbofans w - most have a bypass ratio around 10-12 (by contrast the early JT9D was around 4).
To a first approximation, propellers are a constant power device, while pure jets are close to a constant thrust device. Since thrust equals power/speed, that means that props are most efficient at low speed, while jets are better at high speed. Hence props have very good takeoff performance and the early pure jets struggled to get off the ground, but happily cruised over Mach 0.8. Turbofans sort of split the difference - they lose efficiency as they go faster, but are still better than pure jets (at least subsonic).

Two things that hurt props compared to fans is that fan duct. Props don't need to carry the weight of a fan duct, but fans have exit guide vanes that take out most of the swirl (improved efficiency), and that duct provides acoustic treatment to lessen the noise footprint. Counter-rotating props can improve the efficiency by taking out most of the swirl, but they create tremendous noise.

Unlike Less Hair, I don't think unducted fans are the future. Noise is a major issue, as is a blade release since there is no containment. I know people who worked the unducted fan 7J7 project - apparently the noise from those counter-rotating props was deafening. As for a blade release - I spent some time working a turboprop project, using the same Allison engine/prop as the C-130J. A prop blade release was considered catastrophic - not because the blade might go through the fuselage, but because the resultant out-of-balance vibration would structurally fail the wing.

[speedrestriction]

Originally Posted by Jackonicko;[url=tel:[phone]

11322748]Really?

That's astonishing, to me, at least.

I was surprised too. Interestingly the target IAS (actually flown as a Mach no.) at low single digit cost index on LEAP equipped narrowbody in the mid twenties on a short sector is only a few knots higher than the speeds flown by the larger TPs.

On the TPs I used to fly we would descend at a higher IAS than I do now on a LEAP equipped narrowbody. As mentioned above, technological convergence for the most frequent use case. The newest LEAP equipped narrowbodies are proving to be real Swiss Army knives, doing everything from ETOPS to domestic sectors very efficiently.