The camber-line designation for the 3-digit-reflex camber line is the same as that for the 3-digit camber line except that the last digit is changed from 0 to 1 to indicate the reflex characteristic. The three digit reflex camber line was designed to have a zero pitching moment coefficient about the quarter-chord position. The principal application was to be sections for rotorcraft main rotors.
The camber line is made up of two equations so that the second derivative decreases linearly to zero at a point r aft of the maximum camber position and remains zero from this point to the trailing edge.
y'' = k*(x-r) if x < r and y'' = n*k*(x-r) if x ≥ r y(0)=0 and y(1)=0
These differential equations may be integrated to show that
y=(k/6)*(x3 -(k2/k1)*(1-r)3*x +r3*(3-r)*x + r3) if x < r and y=(k/6)*(n*(x-r))3 - n*(1-r)3*x - r3*x + r3) if x ≥ r
The constant n is given by:
n = (3/(1-r))*(r-m)2 - r3
This camber line family was reported in NACA Report 537 and the following values of k and r for selected values of m were given as:
| Camber-line designation | m | r | k | n |
|---|---|---|---|---|
| 221 | 0.10 | 0.1300 | 51.990 | 0.000764 |
| 231 | 0.15 | 0.2170 | 15.793 | 0.00677 |
| 241 | 0.20 | 0.3180 | 6.520 | 0.0303 |
| 251 | 0.25 | 0.4410 | 3.191 | 0.1355 |