It takes a crooked tail to fly a straight line

In a single-engine airplane with a propeller turning clockwise, several properties of physics come into play that want to push the nose left unless control inputs negate the tendency. Torque simply wants to rotate the airplane in a counterclockwise direction.

P-factor is noticeable when the aircraft is not in level flight, and is a result of the propeller disc being out of vertical alignment, with some blades producing more thrust and causing the airplane to yaw to the left. Add to that the spiraling slipstream from the propeller that wraps around the fuselage, pushing on the left side of the vertical stabilizer, leading to yawing the airplane to the left.

Control inputs can negate the effects, but for decades some airframe designers have built in offset for the vertical fin to help compensate for these phenomena that move the airplane’s direction of flight to the left.

Take a close look at an F4U Corsair fighter, and it becomes evident the vertical fin is offset at an angle on the fuselage to accommodate the physics imposed by a Pratt and Whitney R-2800 engine capable of producing more than 2,400 horsepower.

The dorsal fin fillet of a Bell P-39 Airacobra looks accidentally bent and asymmetrical when viewed from the front, but that’s just the way its sloping sides meet the top of the fuselage to accommodate the vertical fin’s built-in offset.

With an offset vertical fin acting as a permanent answer to some left-turning phenomena, the Corsair and Airacobra share another aerodynamic device common to many aircraft — a rudder trim tab. A trim tab extends into the slipstream at an angle at the trailing edge of the rudder, causing the rudder to deflect in the opposite direction.

Trim tabs have come in a variety of styles, from crudely simple bolted-on aluminum tabs that are semi-permanently bent to achieve the desired rudder offset, to hinged mechanical tabs that can be adjusted by the pilot in flight to produce a range of rudder offset as needed.

The pilot’s handbook for the F4U-5 Corsair says to set the electrically operated rudder trim tab to 6° nose right for takeoff.

And pilots of the P-39N were told to use a scale on the rudder trim to dial in four graduations of right rudder for takeoff.

Look at a classic Fairchild Model 24 cabin monoplane, and it appears as if the rudder is devoid of a trim tab. That’s because Fairchild employed a trick in the rudder’s construction. The bowing of a rudder rib by turning a screw on the ground achieves a variation of the rudder’s airfoil that trims the Fairchild.

Some sources say the Macchi Castoldi series of World War II Italian fighters used wings of slightly different span, left and right, as a counter to the left-turning forces of physics.

And slight engine thrust offset has been employed in some aircraft.

The classic North American P-51D Mustang fighter has a 1° vertical stabilizer offset built in.

Take a look next time you walk among a ramp filled with single engine propeller-driven aircraft. How many have visible vertical fin-and-rudder offsets built into them, giving slight asymmetries that would be easy to overlook?