Elliptical blades are wide and thin: designed specially for electric flight. Two styles: white nylon with fiberglass reinforcement and stronger black nylon with carbon reinforcement. 6x5 and 6.5x4 are for SPEED 400 direct drive. 7x7, 8x5 are for SPEED 400 with gearboxes and 7 to 8 cells and for direct drive on SPEED 500 and 600. 8.5x6 through 9.5x7 work well on SPEED 600 with gearboxes. and 10 to 12 cells. Carbon fiber types can accept about 500 greater rpm than the fiberglass props.