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GE90 with swept blades and CF6 with straight blades

(Picture sources: CF6 and GE90)

Aircraft have swept wings for two reasons:

  1. It reduces the thickness-to-chord ratio of the effective chord, which is ideal for flight at higher Mach numbers.

  2. At supersonic speeds, if the wing is swept behind the Mach cone, then the need for a sharp leading edge is eliminated, and a conventional round leading edge can be used instead - perfect for good takeoff/landing performance.

However, fan blades have an incredibly low t/c ratio as well as a sharp leading edge to begin with. So what's the point of having them swept?

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    $\begingroup$ I think it has indeed to do with the fact that the tips are supersonic (or with the fact that just before the tips the airflow is transonic). The design is proprietary so I don't know if there are papers or books around explaining that. Let's see if someone can give a decent answer :⁠-⁠) $\endgroup$
    – sophit
    Commented Dec 16, 2022 at 7:43
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    $\begingroup$ Instead the proprietary, the designs can be patented. See: patents.google.com/patent/US6071077A/en?oq=US6071077A and patents.google.com/patent/EP2543818B1/en?oq=EP2543818B1 for example $\endgroup$
    – Adam
    Commented Dec 16, 2022 at 15:30
  • $\begingroup$ @Adam Thank you for sharing that, didn't nearly read it cover to cover, but I can already tell it's really interesting! $\endgroup$
    – Aditya Sharma
    Commented Dec 16, 2022 at 22:53
  • $\begingroup$ Even helicopters have sweep tips now. $\endgroup$
    – user3528438
    Commented Dec 18, 2022 at 3:33
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    $\begingroup$ I want to throw out that sometimes the answer is "just because." Aerodynamics is the unholy spawn of observations and best available explanations. "Me n' Ernie was working late one night at the wind tunnel and started farting around with nonsensically-curved blade ideas, and stumbled across an 8% performance gain." The older I get the more I appreciate how flight is the mixing of observation with the best barely adequate explanations we have so far. We were a hundred years into powered flight before we realized that our understanding of how a wing actually works was deeply flawed. $\endgroup$
    – Max R
    Commented Dec 24, 2022 at 4:06

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See also this question Why do some turbofan blade have forward swept tips?, Although the answer given is a little vague and the question specifically around the tips. I will do my best to explain, but I am not a turbomachinery expert. A compressor (or a fan) rotor stalls due to the breakdown of the tip leakage vortex or TLV (a bit like a wingtip vortex on an aircraft, caused as flow 'leaks' between the blade tips and the casing, spilling from the pressure to the suction side of the blade) at a certain blade, when this vortex breaks down its wake is incident upon the following blade, producing nasty recirculation zones, and stalling that blade. This leads to the development of stall cells, and potentially a compressor surge: the loss of steady mass flow through the rotor, (and therefore the thrust from the engine). Obviously, this is to be avoided. A modern turbofan engine however must operate over a wide range of flow velocities and densities, and so without the use of a variable geometry nozzle, keeping stall margins sufficiently wide is a huge challenge. Because the blade tips in a fan are supersonic, there is a shockwave at the blade tip, which can exacerbate breakdown of the TLV, and so help induce a stall. It was discovered (see Bergner et al 2005) that sweeping the tips forward increased the distance between the shock and the TLV origin, decreasing their interaction and delaying breakdown of the vortex (hence, increasing stall margin). There are further effects to do with the boundary layer (forward sweep reduces the interaction between shock and BL), but that is well beyond my understanding. In terms of back sweep nearer the hub on blades, this achieves the same ends as on transonic airliner wings: reducing the effective mach number of the incident flow, and delaying the formation of shockwaves on the fan blades, however, the downside is it adds a radial element to the flow velocity, and tends to push air outwards towards the tips, which could be detrimental to tip stall, so as always, everything is a balance. The reason this has only been done recently, is simply: computing. High fidelity 3D CFD for rotors has only been around for a few decades, and so only now can turbomachinery engineers consider the aerodynamics of an entire blade, and even an entire rotor, rather than simply optimising airfoil sections in 2D. For a good overview of the various different factors around blade design this pdf is very helpful: https://rb.gy/kw5de hope this is helpful.

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