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With reference to this answer

We read in the above that accelerating a larger mass of air by a smaller amount is (theoretically) more efficient. This is an advantage of a larger propeller than is current practice.

However, we also read above that at some point in increasing size, parasitic drag begins to outweigh the benefits of a larger airmass.

So, what if we used propeller/fan blades with larger surface area, but did not maintain the very high aspect ratios we see today? This would increase Reynolds number, decreasing skin friction drag coefficient. In fact, large-chord fan blades are already being rolled out, though more for simplicity than the above reason.

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  • $\begingroup$ I'm not sure I entirely understand what you are asking. Maybe you could draw a picture of current design vs what you have in mind? Also props vs ducted fans are similar, but different. Maybe narrow down to one or the other to make the question more clear. Also not sure about "In fact, large-chord fan blades are already being rolled out," they have been around since at least the mid 1990s, and they are anything but simple. A single blade for a GE9X costs on the order of $100k to manufacture. $\endgroup$
    – Daniel K
    Dec 3 '20 at 23:26
  • $\begingroup$ you might like to read about 'disc area' referring to features of propeller theory. $\endgroup$
    – skipper44
    Dec 4 '20 at 10:37
  • $\begingroup$ Related: aviation.stackexchange.com/a/15383/4108 $\endgroup$
    – Sanchises
    Aug 31 at 12:14
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In the case of propellers, there are a number of operating and design considerations which must be taken into account here.

For example, while it is true that imparting a small impulse to a large mass wastes less energy than adding a large impulse to a small mass using a propeller, there are practical limits to how big you can make the propeller diameter before the ground handling of the airplane becomes unmanageably awkward. A huge prop might be more efficient, but you'd need a boarding ladder to get into the cockpit behind that huge prop and the landing gear struts would need to be longer and stiffer and hence heavier. Oh yes- and the prop itself would be significantly heavier.

In addition, to drive a big prop slowly and still develop full power with a piston engine requires a reduction gear of some sort between the 2400RPM of the crankshaft and whatever the turning speed of the slow prop was. The gearbox adds weight and requires an overhaul when its time is run out, and that adds cost- and a potential failure mode that is absent in direct-drive designs.

Finally, gearing down the engine to turn a slow prop magnifies the transmitted torque to the prop and therefore from the prop to the engine and airframe. That reaction torque has to be countered somehow or else opening the throttle will not just turn the prop but also roll the aircraft backwards around the crankshaft axis.

100 years of experience with geared and direct drive piston engines and all sorts of props has yielded the best solution in the form of a direct-drive engine spinning a variable pitch prop with a diameter that puts the blade tips comfortably subsonic at the max power setting of the engine.

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  • $\begingroup$ More modern piston engines are often geared anyway (e.g. the Rotax 912), because running at higher RPM gives higher power-to-weight ratio and of course all turbines are (PT6 power turbine runs around 30,000 RPM). (Fixed) reduction gearbox isn't really a big problem. $\endgroup$
    – Jan Hudec
    Jan 4 at 8:42

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