The Stinson 108 uses an airfoil generally identified as the NACA 4412, a well-known NACA four-digit airfoil that was also used on a number of light aircraft of the period, including the Aeronca Chief, Luscombe Model 8, Funk Model B, and later Citabria variants.

The NACA 4412 designation describes the basic geometry of the airfoil:

First digit (4) – Maximum mean camber is 4 percent of the chord.

Second digit (4) – Maximum camber occurs at 40 percent of the chord, measured aft from the leading edge.

Last two digits (12) – Maximum airfoil thickness is 12 percent of the chord.

The chord is the straight-line distance between the leading and trailing edges of the airfoil.

Although the Stinson wing is commonly identified as a NACA 4412, the actual production wing geometry should be considered the controlling configuration. The lower surface of the Stinson wing appears considerably flatter than the theoretical NACA 4412 profile, particularly through much of the aft portion of the chord.

Why the wing slot?

One of the distinctive features of the Stinson 108 wing is its fixed, partial-span leading-edge slots, located on the outer portion of each wing directly ahead of the ailerons.

At high angles of attack, air passing through the slots helps maintain energetic airflow over the upper surface of the outer wing. This delays airflow separation in the area of the ailerons and allows them to remain effective as the aircraft approaches and enters a stall.

As the aircraft approaches the stall, the unslotted inboard portion of the wing can stall while the slotted outer sections continue to maintain attached airflow. The result is improved lateral control and the characteristically docile stall behavior for which the Stinson 108 is known. The FAA specifically notes that the partial leading-edge slots allow the ailerons to remain unstalled at high angles of attack.

Unlike the trailing-edge flaps, the slots do not increase wing camber. Their primary purpose is to delay boundary-layer separation and increase the usable angle-of-attack range of the outer wing.

Fixed slots do produce additional aerodynamic drag, particularly at higher airspeeds, and therefore impose some penalty in cruise speed and efficiency. The Stinson design limits this penalty by using slots only on the outboard portion of the wing where they are needed to protect aileron effectiveness.

NACA Video on Slots

Jeff