AA5BY

The nagging question for me is my personal observations that seem to indicate that a plane with a symmetrical fore section (round cowl or box as in a stick) seem not to yaw much when going straight up (a condition where there is neutral angle of attack). Contrasting those, is the fore section that lacks symmetry in the horizontal plane with a prop shaft high. They seem to yaw left on vertical climb.

If horizontal asymmetry is the cause, then I'm thinking that pressures on the right fore section are greater than the left causing the plane to yaw. It seems quite reasonable that in close proximity that a spiral effect exist and that on the right side the spiral collides with the fore section and on the left side the spiral escapes to free space. The result is greater pressure on the right side and the plane yaws left.

It can be further noted, that as the plane slows its vertical climb, the yaw force increases. My thought is that the reason is because the spiral flow is washed less aft as the plane slows and thus has more pressure up front where the lever arm is then stronger to inflict yaw.

Edit note: On second thought... the greater yaw force as the plane slows could more likely be explained by less rudder control at slower speeds.