The same “tractor” propeller used on standard front-engine R/C airplanes. In orange in the diagram above.
These contra-rotating props exactly cancel out motor torques during stationary level flight. Opposite pitch gives downdraft. In dark grey in the diagram above.
Usually a brushless electric “outrunner” type, which is more efficient, more reliable, and quieter than a brushed motor (Figure 3-1).
Sometimes built into combination fittings with landing struts (Figure 3-2).