# What is the moving air in a jet engine supposed to look like when the fan blades have a wavy leading edge?

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A lot of different jet engines I see will have fan blades with really complex and curvy shapes. When an aerospace engineer designing a jet engine comes up with that fan blade shape: What’s the moving air (in simulation or air-tunnels) supposed to look like? And why?

For that matter what is the engineer who designs a simple box fan doing when they make the blades curve to be wider at the edge and stuff? I’m assuming these designs move the air better then a straight blade but why? What’s the airflow look like?

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One factor is that the end of the blade is moving faster than the part nearer the hub. So different parts of the blade need to be designed for different airspeeds. That will possibly need variation of both the chord, the distance from leading to trailing edges, and the angle of attack that the blade meets the air, along the length going from hub to tip. The design also needs to take into account the centrifugal effect of air being flung outwards by the spin so the housing factors in too.

What they’re actually designing for is “isentropic efficiency” but that’s not very ELI5-y, so let’s go deeper.

The fan is powered by a driveshaft. The driveshaft is powered by burning fuel and turbines, so any energy we waste is fuel we burned that we didn’t have to…we want as much energy going into the fan to go into accelerating the air as we can (and as little into heat or vibration or tubulence or anything else that isn’t accelerating air) because this gives us the best efficiency, hence best performance and lowest fuel burn.

For complicated thermodynamic reasons we don’t have to discuss unless you care, there’s a theoretical upper limit of how efficiently you can turn work into compressed air in a fan-like device. We measure fan performance relative to that theoretical ideal by something called “isentropic efficiency” (higher is better, you can’t get over 100%).

In practice, this means each part of the fan generating as much lift as it can (fans are just wings going in circles) without any excess drag or turbulence or flow that isn’t in the direction we want (backwards).

The geometry is really complicated because the flows are really complicated, for all the reasons /u/varialectio lists…different parts of the fan blade are going different speeds, different parts are sweeping different areas in the same time, the stress on the blade gets higher as you move away from the hub, there are enough blades that their wakes interfere with each other, etc.

Then layer on some real world constraints like you can’t go (very) supersonic, it needs to be durable (get hit by birds, etc.), you need to be able to actually build it and have it last for thousands of hours, etc.