I guess that the trend of propeller sizing is to choose the littlest diameter in order to obatain the desired thrust, spinning the propeller up to around mach 0.7.
It seems that the weakest point of electric motor is to rotate low rpm and high torque (Condition suitable to spin big diameter and efficenty propeller), at least with cheaper setup.
But my question is: if you digit propeller in google, it happers lots of sites from RC models, up to agricolure drones…I guess that in these applications rpm never reaches mach 0.7 … Some manufacturer specifics the max thrust… So, is it safe to spin these propellers up to mach 0.7 ?
I am not an expert; I just didn’t want to leave you hanging even longer. 0.7 mach (at the tip) is not a magical number, but there seems to be a lot of advice (not related to electric) that exceeding approximately 0.7 (or maybe a bit less) starts to cause too many problems. Noise is the big one, but stability and safety are affected. Some of the advice assumes airplane speeds, though, so I wouldn’t assume it all applies to the much slower forward motion of a ppg.
My understanding is that only efficiency and noise are affected. Maybe by going transonic you could see some shock wave development which damages the prop tips?
AFAIK, the airplane speed is added to the prop speed by taking the norm. So tip speed = sqrt((prop_radius * rotational_velocity_in_rad_per_sec^2) + (forward_velocity)^2)