Helical Propwash

As the propeller rotates, it produces thrust, drag and torque. It is the propeller’s drag component that causes the propwash vortex sheet emanating from each prop blade to be whirled around on a helical path, or corkscrew fashion. This leads to a loss in propeller efficiency of just under two percent. The slipstream, due to the aircraft’s motion through the air, will be flowing straight back over the fuselage. In effect, it can be considered as sliding over the tailplane unnoticed, whereas the helical vortex propwash will strike the tailplane in a series of pulsations. At a low air speed, the tail will experience more pulsations per unit time than at a high speed due to the vortex coils being closer together. This results in the propwash vortex sheets striking the fin and rudder at a greater angle of attack causing an increase in yaw to the left. At high air speeds the coils will be relatively elongated and the angle of attack on the tail and fin will be reduced resulting in less yaw. The rotating propwash will also strike the underside of the port main wing and stabiliser at an increased angle of attack causing an increase in lift. At the same time, the rotating propwash will strike the top surface of the starboard wing and stabiliser at a reduced angle of attack, resulting in less lift. The net result is a rolling moment, this time to the right, which under some conditions could counteract the induced yaw to the left, caused by the propwash striking the fin and rudder, thus aiding stability.