 When moving through a fluid, rotating systems carry an efficiency advantage only at extremely low Reynolds numbers (i.e., viscosity-dominated flows) such as those experienced by bacterial flagella, whereas oscillating systems have the advantage at higher (inertia-dominated) Reynolds numbers. Whereas ship propellers typically have efficiencies around 60 % and aircraft propellers up to around 80 % (achieving 88 % in the human-powered Gossamer Condor), much higher efficiencies, in the range of 96 % – 98 %, can be achieved with an oscillating flexible foil like a fish tail or bird wing.
