"Part of the appeal of flapping wing micro air vehicles is that (unlike helicopters) they offer some resilience against crashing into obstacles. Crashing is still bad, though, and (with some exceptions) significant damage to things that fly generally keeps those things from continuing to be useful. To mitigate this, Harvard University has developed an itty bitty differential to keep a pair of wings (say, on arobot bee or robot fly) generating the same amount of torque, even if one of those wings is damaged. The beauty of the PARITy differential (Passive Aeromechanical Regulation of Imbalanced Torques) is that it’s completely mechanical, and simply due to its design it will (for example) increase the flapping speed of a damaged wing to match the torque output of a paired, undamaged wing. Basically, it’s the same kind of thing that you have controlling the power to the wheels in your car, except about a million times smaller."