Do engines like the GE90 and Trent 1000 use transmissions (planetary gears?) for RPM reduction on the fans, or are they welded directly to the same shaft as the low pressure turbine?


2 Answers 2


Those with gear reduction are called geared turbofans (GTFs). In general, you can check the Wikipedia article on any engine, see its classification and specifications, and also check the linked citations.

The GE90 and Trent 1000 are not geared turbofans. Wikipedia lists only 5 geared turbofans, with the biggest (a recent engine; first run 2008; entry into service 2016) powering the A320neo family (narrow-body jetliner) and other regional jets.

Because of the immense torque of large turbofans (thrust >200 kN), it is not yet feasible. It took 20 years to develop the ~100 kN PW1000G.$^1$ It took that long to ensure the new components are maintenance free (apart from oil changes). So for the time being GTFs are outliers, and when a turbofan is one, it would be stated.

Prior to the PW1000G, there is the ALF 502/LF 507 from the 80s that still powers the 146/RJ. The ~30 kN GTF was designed/chosen for its relative quietness for operating from noise-sensitive airports.

Note: the fan blades are not welded, rather connected/attached. You can watch a fan removal here (noisy video).

$^1$ http://large.stanford.edu/courses/2014/ph240/suresh1/docs/pwspecs.pdf

  • $\begingroup$ When I used the term "welded," I was really talking more about how the hub is attached to the shaft, not the blades to the hub, and wasn't necessarily being serious anyway. I had read the Wikipedia article on the GE90 before. In the specs, it doesn't say anything about being a geared turbofan. I guess you're saying that if it doesn't explicitly say geared, then I should assume it's not. Lame. It should be explicit. For example, it should say "direct drive" or something like that (just my opinion). Thanks for your answer. $\endgroup$
    – birdus
    Feb 27, 2019 at 19:47
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    $\begingroup$ @birdus: Unless stated, turbofans are direct drive (the most common form). When something becomes an outlier, it is stated. $\endgroup$
    – user14897
    Feb 27, 2019 at 19:51
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    $\begingroup$ @birdus Until relatively recently, GTFs basically didn't exist on airliners (though a few models did exist on light jets and research aircraft.) The P&W GTF is the only GTF currently being used on airliners and, as the answer mentions, it's rather new. Rolls is also working on a GTF design, but it's still several years away from service entry. So you can very safely assume (for the next decade, at least) that any airliner-sized turbofan that doesn't say it's a geared turbofan is a conventional one where the fan is directly driven by the low-pressure spool. $\endgroup$
    – reirab
    Feb 28, 2019 at 8:48
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    $\begingroup$ @reirab: a certain 80s airliner feels left out :D $\endgroup$
    – user14897
    Feb 28, 2019 at 13:58
  • $\begingroup$ @ymb1 haha, fair point, but it was a relatively small airliner that used 4 bizjet-class engines. Even the engines on the CRJ900 are twice as powerful as those things. $\endgroup$
    – reirab
    Feb 28, 2019 at 17:06

To add to what @ymb1 said, and to expand on your question about "welded", @birdus:

The NTSB accident dockets for Southwest 1380 / NTSB# DCA18MA142, Southwest 3472 / NTSB# DCA16FA217, and American 383 / NTSB# DCA17FA021 all show some great internals about turbofan engines and (mostly) what happens when things fail.

Specifically, Page 15 of the NTSB's Powerplant Group Chairman's Factual Report for SWA1380 has a nice description of the blades and how they attach. It's not 100% clear but reading carefully, it does describe how the blades are attached to the fan disk which is then "bolted to the booster by 24 bolts" (which it further clarifies that, at least on the CFM56, the LPC Low Pressure Compressor is sometimes referred to as the "booster")...

That said, the CFM56 and GE90 do share a common characteristic: the LPC (Low Pressure Compressor - aka "N1") actually has compressor stages attached to the same shaft as the fan.

The Trent 1000 is pretty different - the entire Trent series really evolved from the venerable Rolls Royce RB211 and ever since then the entire series has been a triple-spool turbofan engine (as opposed to the twin-spools utilized by every(?) other manufacturer.)

So on a RB211/Trent XXXX, you have 3 independent spools and 3 gauges in the cockpit: N1, N2, and N3 - corresponding to the LP, IP, and HP shafts (low/intermediate/high). The whole idea of this being to decouple all of the core compressor stages from the fan rotor. If you look at the various compressor stage/turbine stage configurations, the notable difference with a Rolls Royce RB211/Trent is that the Fan has its own LP turbine.

The Trent 1000 it has "Compressor: one-stage LP (fan), eight-stage IP, six-stage HP compressor" and "Single-stage HP turbine (13391 RPM), single-stage IP turbine (8937 RPM), six-stage LP turbine (2683 RPM)". So that means that there's a 6-stage turbine at the back that then spins the one-stage big fan at the front.

The entire purpose of the three spools - even though it is far more complicated than having 2 - is to allow the fan at the front to rotate at a different speed than the engine core compressor sections. Doing so allows you to run the core faster/hotter without spinning the fan at the front too fast.

The PW1100G from Pratt & Whitney is their answer to the problem of needing to turn the fan at the front at different speeds. Instead of using 3 spools to accomplish the task, they use a gearbox. This allows for different fan blade geometry and allows for a much quieter engine. (Speaking from personal experience, the PW1100G on the A220/CS series is seriously quiet.)

Not to be left behind, Rolls Royce is developing the "UltraFan" which will apparently vary the pitch of the fan blades during flight.

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    $\begingroup$ It's debatable whether three rotors are more or less complex than two. A three shaft design is shorter, the rotordynamics are better behaved, and two independent speeds for the core compressor improves stall and surge margin. I remember a meeting between Rolls Royce and the FAA, when RR had the only three-shaft engine design, on the subject of in-flight engine shutdowns. The FAA's starting position was "RR must be mis-reporting the data, because you claim no failures in several reporting categories, unlike every other engine type." Wrong - those failures never occurred with a 3 shaft design. $\endgroup$
    – alephzero
    Feb 27, 2019 at 22:20

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