The Supercritical Airfoil
Let us return to a consideration of two-dimensional airfoils. A natural conclusion from the material in Section 11.6, and especially from Figure 11.11, is that an airfoil with a high critical Mach number is very desirable, indeed necessary, for high-speed subsonic aircraft. If we can increase Mcr, then we can increase T/drag-divergence, which follows closely after Mcr. This was the philosophy employed in aircraft design from 1945 to approximately 1965. Almost by accident, the NACA 64-series airfoils (see Section 4.2), although originally designed to encourage laminar flow, turned out to have relative high values of Mcr in comparison with other NACA shapes. Hence, the NACA 64 series has seen wide application on high-speed airplanes. Also, we know that thinner airfoils have higher values of Mcr (see Figure 11.7); hence, aircraft designers have used relatively thin airfoils on high-speed airplanes.
However, there is a limit to how thin a practical airfoil can be. For example, considerations other than aerodynamic influence the airfoil thickness; the airfoil requires a certain thickness for structural strength, and there must be room for the storage of fuel. This prompts the following question: For an airfoil of given thickness, how can we delay the large drag rise to higher Mach numbers? To increase Mcr is one obvious tack, as described above, but there is another approach. Rather than increasing Mcr, let us strive to increase the Mach number increment between Mcr and T/dr-ag-divcrgcncc ■ That is, referring to Figure 11.11, let us increase the distance between points e and c. This philosophy has been pursued since 1965, leading to the design of a new family of airfoils called supercritical airfoils, which are the subject of this section.
The purpose Of a Supercritical airfoil is tO increase the Value Of Mdrag-divergence> although Mcr may change very little. The shape of a supercritical airfoil is compared with an NACA 64-series airfoil in Figure 11.19. Here, an NACA 642-A215 airfoil is sketched in Figure 11.19a, and a 13-percent thick supercritical airfoil is shown in Figure 11.19c. (Note the similarity between the supercritical profile and the modem
low-speed airfoils discussed in Section 4.11.) The supercritical airfoil has a relatively flap top, thus encouraging a region of supersonic flow with lower local values of M than the NACA 64 series. In turn, the terminating shock is weaker, thus creating less drag. Similar trends can be seen by comparing the Cp distributions for the NACA 64 series (Figure 11.19A>) and the supercritical airfoil (Figure 11.19c/). Indeed, Figure 11.19a and b for the NACA 64-series airfoil pertain to a lower freestream Mach number, Мж = 0.69, than Figure 11.19c and d. which pertain to the supercritial airfoil at a higher freestream Mach number, M<*, = 0.79. In spite of the fact that the 64-series airfoil is at a lower M^, the extent of the supersonic flow reaches farther above the airfoil, the local supersonic Mach numbers are higher, and the terminating shock wave is stronger. Clearly, the supercritical airfoil shows more desirable flow – field characteristics; namely, the extent of the supersonic flow is closer to the surface, the local supersonic Mach numbers are lower, and the terminating shock wave is weaker. As a result, the value of T/drag-divergence will be higher for the supercritical airfoil. This is verified by the experimental data given in Figure 11.20, taken from Reference 32. Here, the value of Afdrag-divergence is 0.79 for the supercritical airfoil in comparison with 0.67 for the NACA 64 series.
Because the top of the supercritical airfoil is relatively flat, the forward 60 percent of the airfoil has negative camber, which lowers the lift. To compensate, the lift is increased by having extreme positive camber on the rearward 30 percent of the airfoil. This is the reason for the cusplike shape of the bottom surface near the trailing edge.
The supercritical airfoil was developed by Richard Whitcomb in 1965 at the NASA Langley Research Center. A detailed description of the rationale as well as some early experimental data for supercritial airfoils are given by Whitcomb in Reference 32, which should be consulted for more details. The supercritical airfoil, and many variations of such, are now used by the aircraft industry on modem highspeed airplane designs. Examples are the Boeing 757 and 767, and the latest model Lear jets. The supercritical airfoil is one of two major breakthroughs made in transonic airplane aerodynamics since 1945, the other being the area mle discussed in Section
11.8. It is a testimonial to the man that Richard Whitcomb was mainly responsible for both.