MODERN AIRFOIL DEVELOPMENTS

Systematic series of airfoils have*given way, at least in part, to speci­alized airfoils designed to satisfy particular requirements. These airfoils are synthesized with the use of sophisticated computer programs such as the one described in Reference 3.5, which will be discussed in more detail later. One such special purpose airfoil is the so-called supercritical airfoil reported on in References 3.6 and 3.7. This airfoil has a well-rounded leading edge and is relatively flat on top with a drooped trailing edge. For a constant thickness of 12%, wind tunnel studies indicate a possible increase of approximately 15% in the drag-divergence Mach number for a supercritical airfoil as compared to a more conventional 6-series airfoil. In addition, the well-rounded leading edge provides an improvement in CJ^ at low speeds over the 6-series, which has sharper leading edges.

A qualitative explanation for the superior performance of the super­critical airfoil is found by reference to Figure 3.9. At a free-stream Mach number as low as 0.7 or so depending on the shape and CJ, a conventional airfoil will accelerate the flow to velocities that are locally supersonic over the forward or middle portion of its upper surface. The flow then decelerates rapidly through a relatively strong shock wave to subsonic conditions. This compression wave, with its steep positive pressure gradient,/causes the boundary layer to thicken and, depending on the strength of the shock, to separate. This, in turn, causes a significant increase in the drag. The minimum value of the free-stream Mach number for which the local flow becomes

supersonic is referred to as the critical Mach number. As this value is exceeded by a few hundredths, the shock wave strengthens sufficiently to cause the drag to rise suddenly. This free-stream Mach number is known as the drag-divergence Mach number.

The supercritical airfoil also accelerates the flow to locally supersonic conditions at free-stream Mach numbers comparable to the 1- or 6-series airfoils. However, the supercritical airfoil is shaped, so that around its design lift coefficient, the flow decelerates to subsonic conditions through a dis­tribution of weak compression waves instead of one strong one. In this way the drag-divergence Mach number is increased substantially.

Although the possibility of such airfoils was known for some time, their successful development in modern times is attributed to R. – T. Whitcomb. A Whitcomb-type supercritical airfoil is pictured in Figure 3.7.

Tested at low speeds, the supercritical airfoils were found to have good values as well as low Cd values at moderate lift coefficients. As a result, another family of airfoils evolved from the supercritical airfoils, but for low-speed applications. These are the “general aviation” airfoils, designated GA(W) for general aviation (Whitcomb). The GA(W)-1 airfoil is the last of the

airfoils pictured in Figure 3.7. Test results for this airfoil are reported in Reference 3.8, where its CtmtX values are shown to be about 30% higher than those for the older NACA 65-series airfoils. In addition, above Q values of around 0.6, its drag is lower than the older laminar flow series with standard roughness. These data are presented in Figure 3.10 for the GA(W)-1 airfoil. Comparisons of Qmax and Cd for this airfoil with similar coefficients, for other airfoils are presented in Figures 3.11 and 3.12.

Observe that the performance of the GA(W)-1 airfoil is very Reynolds number-dependent, particularly Clm„, which increases rapidly with Reynolds number from 2 to 6 million. At the time of this writing, the GA(W) airfoil is beginning to be employed on production aircraft. The same is true of the supercritical airfoil. Indeed, the supercritical airfoil is being used on both the Boeing YC-14 and McDonnell-Douglas YC-15 prototypes currently being tested for the advanced medium STOL transport (AMST) competition. At the time of this writing, NASA is adopting a new nomenclature for the GA(W) airfoils. They will be designated by LS (low speed) or MS (medium speed) followed by four digits. For example, the GA(W)-1 airfoil becomes LS(1)-0417. The (1) designates a family. The 04 refers to a design lift coefficient of 0.4, and 17 is the тяхітпт

thickness in percent of chord. For more information on modern airfoils, consult Reference 3.36.