6:11 Lena: We keep talking about "the fan," but for anyone who’s walked down a jet bridge and looked into the engine, that fan is massive. I was reading that on some engines, like the ones from the Pratt & Whitney GTF family, this fan is responsible for the "main thrust." It’s not just a tiny part of the process; it’s the star of the show.
6:31 Miles: Oh, absolutely. In a modern turbofan, the fan rotor is the first and largest stage of the compressor. Its primary job is to accelerate a massive amount of air. But here’s the kicker: not all that air goes into the "fire" of the engine. Most of it actually goes *around* the core.
6:47 Lena: This is the "bypass" air I’ve heard about, right? Like a cold stream of air that just gets shoved out the back without ever being burned?
4:14 Miles: Exactly. Think of the engine as having two parts: the core, where the "turbo" part happens—compression, combustion, turbines—and the bypass duct, which is basically a giant ring around that core. The fan sits at the front of both. It’s like a giant, high-tech propeller inside a shroud.
7:11 Lena: Why bother with the shroud? Why not just have a big propeller like an old-school plane?
7:17 Miles: Efficiency and speed. The shroud, or the "nacelle," allows the fan to operate in air that’s been slightly slowed down by the inlet. If you’re flying at Mach 0.8, which is standard for a jetliner today in 2026, that air is moving way too fast for the fan to handle efficiently. The inlet slows it down to maybe Mach 0.4 or 0.5 before it hits those blades.
7:40 Lena: So the engine is actually "shaping" the air before the fan even touches it. That makes sense. But I’m still stuck on the sheer scale. Those GE90 blades we mentioned—twenty-five kilograms each. If you have twenty or more of those spinning at high speeds, the centrifugal force must be insane.
8:00 Miles: It is. That’s why the materials matter so much. Older engines used titanium, which is strong but heavy. To keep the weight down, many modern fans use composite materials—like carbon fiber—often with a metal leading edge to protect against bird strikes or debris. Some designs, like the ones MTU works on, use "blisks," where the blades and the disk are manufactured as a single solid part.
8:23 Lena: "Blisks"—blade plus disk. That sounds like it would save a lot of space and weight, but I imagine it’s a nightmare to repair if one blade gets chipped.
8:33 Miles: It’s definitely a high-tech trade-off. Blisks are great for aerodynamics and weight, which is why you see them in high-performance settings like the EJ200 engine in the Eurofighter. But for the giant front fan of a commercial jet, you often still see individual blades because of that sheer 1.25-meter scale.
8:50 Lena: So, the fan is pulling in this "large mass flow," as the technical sources put it. It’s moving a huge volume of air relatively slowly, compared to the hot jet exhaust which moves a small amount of air very fast. Why is that "big and slow" approach better for efficiency?
9:07 Miles: It’s a basic principle of propulsion. Moving a big mass of air slowly is generally more efficient than moving a small mass very fast. It’s why a rowing oar is wide and flat rather than a thin stick. You want to grab as much "stuff" as possible and give it a firm push. The bypass ratio tells us exactly how much of that "stuff" is going around the core versus through it.
9:27 Lena: And as we’ve seen over the last few decades, those fans just keep getting bigger and bigger, meaning our bypass ratios are climbing. We’re moving toward engines that are mostly "fan" and very little "jet" in the traditional sense.
2:24 Miles: Precisely. We’ve gone from bypass ratios of one or two in the early days to ten or even higher today. It makes the engines quieter and much more fuel-efficient. But it also means that the fan has to be perfectly optimized. If that fan isn't doing its job of "sucking" and "blowing" exactly right, the whole engine becomes a very expensive paperweight.