Technote: Bell P-39 Wing Trailing Edge Calculation.
The root wing profile for the P-39 is based on the NACA 0015 (4-digit series).
The Bell P-39 archive contains ordinate data for the fuselage, tail, stabilisers, cowls and so on but sadly the main ordinate plan for the wings is missing. However, we do have some ordinate data including a mid wing profile section and of course the front, rear and aux beams. We also know the root wing profile is based on the NACA 0015 which collectively provides enough core information to develop the wing structure.
The “baseline” NACA 0015 has a non-zero trailing edge thickness relative to the chord length. Just working from the generic geometry formula we end up with a large trailing edge thickness which is greater than that specified by Bell.
The baseline NACA 0015 airfoil is described by the function:
In order to achieve a degree of control over the resulting trailing edge thickness we only need to adjust the fourth coefficient in the polynomial slightly.
The above amendment will give a zero thickness at the trailing edge. The actual value we were looking for was 0.03in radius which was achieved through trial and error with the fourth coefficient value set to 0.1024.
- x = coordinates along the length of the airfoil, from 0 to c (which stands for chord, or length)
- y = coordinates above and below the line extending along the length of the airfoil, generally defined as either yt for thickness coordinates or yc for camber coordinates
The final profile was checked against known ordinates from the fuselage data.
The information here was sourced from a white paper written by WeiHei, Francisco Gomez, Daniel Rodriguez and Vasilis Theoflis.