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The only recent engine using GTF technology is the PW1000G, however some old designs like ALF502 or TFE731 use this technology. I understand the basic concept of the benefit obtained by the engine introducing this technology (basically giving another parameter to optimize RPMs at low pressure turbine and fan) and that benefit is compensated by the increase in weight.

What is blocking this technology to be further included in modern turbofans? Is it only a business case for P&W architecture?

What made it possible in PW1000G that blocked PW8000?

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    $\begingroup$ The TFR731 is 3500 lb thrust, and the ALF502 is 7000 lb thrust. The PW1000G is in the 20,000 lb thrust class. This is a big difference. $\endgroup$ – fooot♦ Feb 17 '15 at 23:29
  • $\begingroup$ Could you develop that argument further, finally I guess that the key question is the ratio between both optimum RPMs, how that changes with the size (in terms of thrust) of the engine? $\endgroup$ – Trebia Project. Feb 17 '15 at 23:54
  • $\begingroup$ Nobody wants to risk putting 50,000 hp through some groovy wheels? $\endgroup$ – Martin James Feb 20 '15 at 19:01
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The factors that are relevant here are:

  • Increasing the bypass ratio is very desirable in order to improve the efficiency of the engine, and for the same amount of thrust, this means increasing the fan diameter.
  • In order to keep the fan mostly subsonic, the fan rotational speed has to decrease with increasing fan diameters. Supersonic fan blade tips are possible and in use, but you lose efficiency and increase noise, so it's desirable to limit this to the outermost area.
  • For the same efficiency, a low-speed turbine requires more stages and therefore more weight compared to a high-speed turbine. Compare the PW1100G's 3-stage LPT (geared) with LEAP-1A's 7-stage LPT (not geared).
  • The only way to decouple the rotational speed of the LPT from the rotational speed of the fan is by using a gearbox.

In the long run, this is therefore a question of whether the gearbox is heavier than a slower-turning LPT and whether a possible increase in maintenance costs due to the gearbox is acceptable for the given gain in efficiency or reduction in engine weight. With further increases in engine bypass ratios, the gearbox becomes increasingly attractive.

In the short run, this is also a question of whether a company wants to risk developing a completely new component, with potential problems before and after entry into service. Additionally, with a given amount of money and manpower to develop a new engine, management will ask: Where are the engineering hours going to make a bigger difference, in improved fan, compressor, turbine and combution chamber designs/materials/... or in a gearbox design.

So while, for example, Rolls-Royce does not manufacture a geared turbofan right now, their "UltraFan" research engine project does include a gearbox. I think it's safe to say that it is only a matter of time until most commercial turbofans will include a gearbox.

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  • $\begingroup$ I think this is the answer I was looking for. Quick question, why slow moving turbines require more stages? Is because energy transfer is proportional to the square of the rpms? $\endgroup$ – Trebia Project. Mar 1 '15 at 16:51
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    $\begingroup$ Assuming an Euler turbine, the energy transfer in one rotor stage is: P = mp * u * dwu, where mp is the mass flow, u is the rotor speed and dwu is the difference in radial flow speed of the fluid before and after the rotor, i.e. the amount by which the fluid "changed direction". Since dwu is usually in the proximity of u, for a given mass flow and axial speed, I would agree that your statement is approximately correct. Corrections are welcome. $\endgroup$ – JulianHzg Mar 1 '15 at 17:27
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In completion to @JulianHzg's anwser:

Increasing the bypass ratio is very desirable in order to improve the efficiency of the engine, and for the same amount of thrust, this means increasing the fan diameter

Increasing the bypass ratio is surely a way to reach a higher efficiency (and maybe the most developed topic at the moment in aircraft engines), but only if you can provide the engine with a powerful enough gas generator, so increasing the bypass ratio is constrained by other parameters such as combustion chamber temperature and turbine efficiency.

In order to keep the fan mostly subsonic, the fan rotational speed has to decrease with increasing fan diameters. Supersonic fan blade tips are possible and in use, but you lose efficiency and increase noise, so it's desirable to limit this to the outermost area.

Also, blades permanently submitted to shock waves tend to have a very short lifespan because of very high cycle fatigue. Therefore, people tend to avoid supersonic flow conditions.

For the same efficiency, a low-speed turbine requires more stages and therefore more weight compared to a high-speed turbine. Compare the PW1100G's 3-stage LPT (geared) with LEAP-1A's 7-stage LPT (not geared).

Basically, the rotational speed of every element must be adapted to the fluid velocity. So if every rotating element is driven by the same shaft, the flow must be adapted adding stages in the turbine or compressor modules. If you have two shafts, one slow (good for the fan) and one fast (good for the turbine) you get a better compromise in terms of efficiency. However, this poses other issues (rotordynamic instabilities for instance) which explains why Rolls-Royce is the only manufacturer to propose three shafts (even better in terms of efficiency but also very complex).

The gearbox allows to keep using two shafts while improving adaptation between fluid and rotating element.

The question about the weight of the gearbox is very important, is must be recalled that a gearbox mut be able to transmit the power of the LPT to the fan, so it's not a simple gearbox from your car. Also, although epicycloïdal gearboxes have a very good efficieny transmitting power using such trains costs energy, creates heat, requires lubrication and maintenance.

To conclude, this is a very well know technology that is even well known to the aeronautic industry (how do you think helicopters fly !), but that was never used on high-power aircraft engines. Another application of gearboxes are the open-rotor engines which involve even more complex transmissions du to counter rotating fans.

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  • $\begingroup$ You say that "blades permanently submitted to shock waves tend to have a very short lifespan because of very high cycle fatigue". This would be caused by oscillating shocks - is that really common on fan blades? As long as the shock location stays reasonably fixed, there should be no fatigue issues from supersonic operation. $\endgroup$ – Peter Kämpf Jan 3 '19 at 7:27
  • $\begingroup$ @PeterKämpf, I agree that if the schocks location is fixed, there should be no HCF. I am just wondering about the problems brought by the coupling of other phenomena with schock waves, especially flutter, which lead to HCF $\endgroup$ – BambOo Jan 3 '19 at 12:10

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