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I haven't been able to locate any engines that use steel for the bypass fan. All sources suggest Ti and composites are used for blade construction. Apart from the weight steel could partially make up for it by offering a thinner blade, cost/ease of construction and more resistance to foreign objects.

The reason I ask is that I am trying to design a large ducted fan (800mm) that will be as cheap as possible to manufacture by using a thin sheet of stainless steel for the blade material. The cut to shape sheet will be pressed in a mold to achieve the designed AoA across the blade length and then a 3D printed airfoil will slide over it.

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    $\begingroup$ None of these statements are backed by evidence. Steel doesn't offer better per-volume strength but is considerably heavier than titanium. Ease of construction is marginal because it's the shape that makes things hard, not the machining characteristic of the material. As for resistance to foreign material, steel is probably the worst. As for cost, development cost is the big part, material cost is negotiable. Go head and do your design and make sure the math and physics checks out, so that even if it's heavy and inefficient it's at least safe. $\endgroup$ – user3528438 Sep 14 '20 at 23:46
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    $\begingroup$ The organisations that buy this sort of engine don't care how much it costs to make - they care about the total cost of ownership. It doesn't matter if your 'cheap' fan is half the price of the competition if the engine uses 10% more fuel. To an airline spending tens of millions per year on fuel, even 2% saving is a huge amount. $\endgroup$ – CatchAsCatchCan Sep 15 '20 at 4:39
  • $\begingroup$ I'm curious how you will achieve enough strength by using what is basically a spar + airfoil configuration, if you make the spar out of sheet metal. Or is the 3D printed part going to be load-bearing? $\endgroup$ – Sanchises Sep 15 '20 at 6:59
  • $\begingroup$ Also, out of genuine curiosity: how does your cost estimate of 3D printing your blades compare to milling individual blades? $\endgroup$ – Sanchises Sep 15 '20 at 7:00
  • $\begingroup$ @ CatchAsCatchCan It's an electric ducted fan not meant for any sort of commercial use. The reduction in percentage points of efficiency doesn't matter as much when the range is so short anyways. $\endgroup$ – usernamechecksout Sep 15 '20 at 14:29
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The tickness of the Fan blade is not only defined by the structural requirement but rather for aerodynamic purpose, thus the geometry of the blade won't change much depending on the material used. Most of the metallic blades in use are hollow.

The price of the blade is quite irrelevent in modern engines. If you can decrease the wheight of the fan by 10% for double the price you'll do it as it will ensure so much fuel saving over the life of the engine. Thus I don't see the advantage of using steel over titanium for a fan blade.

Looking at other comments I understand it is for prototyping and needs to remain cheap. I would then recommend to build the core with 3D printed material which will give you the airfoil shape for aerodynamic and then enclose it with a metal sheets in order to obtain a sandwich like structure. This could greatly increase the stiffness and strength without much added weight.

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