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I'm looking at the 100 people project and I am looking at clean airplanes. Why did we stop using a clean source of lift, the propeller?

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    $\begingroup$ Turbofans (B737/A320) are basically large propellers, so we continue with propellers optimized for M 0.8 speed, and also continue with traditional propellers for slower aircraft flying at lower altitude (e.g. ATR, or Q400). For propellers, the actual source of energy is often a turboprop, that is a (jet fuel) turbine, like other airliners. This is not cleaner in any way. $\endgroup$ – mins Oct 18 '17 at 19:08
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    $\begingroup$ Unless you're talking about a rotary-wing craft (i.e. helicopter), I don't really think the propeller creates lift - it's the airflow around (and most specifically below) the wing that creates lift. Also, in what way do you think it is a "clean source" of lift where a jet engine is (by inference) a "dirty source" of lift? $\endgroup$ – FreeMan Oct 18 '17 at 19:30
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    $\begingroup$ How are propellers cleaner than jet engines? $\endgroup$ – zeta-band Oct 18 '17 at 19:35
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Look into Propulsive efficiency (and maybe also this one). Turboprops have a speed of peak efficiency, which is around mach 0.6 or so. If you try to go faster, it gets significantly less efficient. A high bypass turbofan, on the other hand, is most efficient at a higher speed, maybe mach 0.8 - 0.85. So the simple answer is: we switched from props to jets because people wanted to fly faster but didn't want to burn a lot of extra fuel to do that.

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