propellors et al.

any medium our crafts pass thru should remain as undisturbed as much as possible, be it air, water, or space-time

e.g. vortices in air after an aircraft passes thru mean we wasted energy by creating them

propellors should help us move our craft forward, but any other effect happening would mean they are not 100% efficient

simple cones on aircraft fan shafts should be replaced by functional ones, that help the airflow thru the turbine

large grills on automobiles are a testament of our failure to engineer efficient propulsion systems!