Why is it that i.c. props are stiff and electric props are floppy flexible? Shouldn't they all be stiff?
Because the torqrue impulses are much, much higher on an IC engine. That makes the hub get ahead of, then behind, the tips. For instance, an ST46 puts out around 70 in-ounces of torque on average, but the positive torque peaks are something like 1700 in-oz (peak during the firing stroke), and the negative torque peaks (as it comes up on compression) at -650 in-ln once per rev in a 2-stroke, and once ever other rev when in a 4-stroke. That's a cyclic torque of *2350 in-oz* peak to peak, for a relatively wimpy engine at 70 in-oz If you get the blade whipping back and forth "in-plane", that will incur lots of fatigue. Try holding the tips while someone applies 12 ft-lb of torque to it, an electric prop might deflect an inch on a 12-in prop, the figure it's doing the same thing and building up, 220 times a second.
An electric has a 7, 9, or more poles, the torque (called "cogging torque", usually) goes up and down depending on how the rotor and stator lines up, 7,9, etc times per rev, but each one is much lower in torque,
Based on experience, you can get away with running "electric" props on some IC engines for at least a while. But when I first did this calculation, it scared a bunch of aerospace engineers away from doing it, just because the torque spikes were just so large.
Brett