Would large, low aspect ratio fan/propeller blades be more efficient? Why not?

Question

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.

Answer 1

Low aspect ratio means more chord. More chord means

1. lower lift coefficient. This will move the operating point of the blade airfoil to a less efficient polar point. Since torque and RPM stay constant, a wider blade cannot absorb more energy and compensates for the higher chord with a proportionally lower lift coefficient.
2. more area. You rightly note that the higher blade Reynolds number reduces the friction drag coefficient, but drag is coefficient times area (times dynamic pressure), so drag will still rise compared to a more narrow blade.

Higher chords are sensible for high-speed propellers which are designed at operation close to Mach 1, where a high lift coefficient will cause higher overspeed and earlier shocks. The most notable example is the Aerosila SV-27 (СВ-27 in Cyrillic) contra-rotating propeller of the D-27 propfan engine powering the Antonov An-70:

SV-27 propellers on the An-70 (picture source). Eight blades in the forward disc and six in the rear, running at only 1200 RPM. In order to reduce Mach effects, all blades have a swept tip and deep chord. Another example is the Junkers VS-9 propeller from 1944.

Junkers VS-9 propeller (picture source). Since this was powered by the same Jumo 213 engine which powered the FW-190D, the deeper chord meant a proportionally lower lift coefficient when compared to the regular FW-190 propeller.

Answer 2

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.