Bob Kuykendall wrote:

Since you mentioned it, here's my take pundit's-eye-view of the last
30 years of airfoil development:

The biggest changes have been not in the airfoils themselves, but
rather in the degree of difficulty of developing custom airfoils for
specific applications. With airfoil development packages now available
for desktop computers (or even online), it is now practical to develop
six or eight different sections for a wing panel that optimize the
section for the constraints at that part of the wing. None of those
sections would be anything that a dedicated airfoil designer couldn't
have developed over the course of a few weeks or months in 1975.
However, the fact that you can develop each section over the course of
only a few hours or even just minutes, and have a high degree of
confidence in its effectiveness, can make a noticeable difference in
overall performance.

Beyond that, there has also been sort of a wave of realignment, in
which airfoil design has started to take more account of real-world
conditions. For example, when it comes to low drag at cruising speed
in smooth conditions, there isn't much in the sailplane realm that
beats the old FX-67 sections. However, when you add in turbulence,
construction and fabrication defects, paint chips, dust, bugs, and
rain, the FX-67 degrades rapidly; especially in the 17% thickness
common on second-generation composite gliders. Later airfoils such as
the FX-81 will tend to be more conservative, and try for less in the
way of laminar run, but lose less of their performance as surface
conditions deteriorate and so come out ahead.


How enlightening to have one's profession summarized in two, short
paragraphs. Apparently, we airfoil designers have been remiss in
explaining our work.

It seems illogical that the sailplane factories would expend so much
effort guarding their proprietary airfoils if these shapes can be
developed in minutes using publicly available software. I am often
asked by amateur aircraft designers if they should obtain an airfoil
code and design their own airfoils. My advice is "Go for it and, while
you are at it, stop by your local medical supply house and pick up a
scalpel so you can perform brain surgery on your kids." The safety
issues alone are comparable. Interestingly, almost all aircraft
manufacturers, and all the sailplane factories, employ specialists to
design airfoils for their new aircraft designs.

Indeed airfoil design continues to be an area of intense research
because airfoils are the single largest contributor to the aerodynamic
performance of almost all aircraft. For example, the wing profile drag,
which is a function of the airfoil alone, contributes approximately 60
percent of the entire drag of the sailplane at high speeds. Thus, even
small gains in airfoil performance produce significant improvements in
sailplane performance.

The theoretical methods available today are unquestionably invaluable
tools. It takes as long or longer now to design an airfoil than it did
30 years ago, however, because our understanding of aerodynamics is
deeper. Also, with these methods, we can explore many more variations
than was previously possible. Although the methods, specifically XFOIL
and the Eppler Code, are powerful, they are not perfect, requiring
experimental verification of the predicted characteristics. Wind-tunnel
testing of airfoils also takes longer today than it did in the good old
days. During World War II, NACA tested three airfoils per day (one per
eight-hour shift) in their Low-Turbulence Pressure Tunnel. They used
manometers to measure the wall and wake pressures for three Reynolds
numbers plus one roughness condition. They had a staff of about 75,
most of whom manually integrated the pressures to obtain the lift and
drag coefficients. Today, a typical single-element airfoil test takes
about two weeks, directly involving about five people. We measure
airfoil and wake pressure distributions at more angles of attack for
more Reynolds numbers and roughness conditions using precision transducers.

The past five decades have seen a dramatic shift in airfoil design
philosophy, but not necessarily toward more conservative shapes. NACA
and later F. X. Wortmann developed catalogs of airfoils for classes of
aircraft. Today, because of the ever widening range of applications and
the availability of calibrated codes, pioneered by Richard Eppler,
airfoils are tailored to specific aircraft. Thus, airfoils are designed
for the Discus 2 and different airfoils are designed for the ASW 28. In
the future, this trend will continue as airfoil/aircraft design
integration intensifies, leading to further increases in performance.
If this is not enough to dispel the myth that we have reached a plateau,
concepts such as laminar-flow-control and slotted, natural-laminar-flow
airfoils hold promise for even larger increases in performance.

We glider pilots are fortunate that the four individuals who, in my
opinion, are good low-speed airfoil designers practice their craft more
in pursuit of their passion than of money. Not surprisingly, all four
are glider pilots and good friends. So the next time you see Richard
Eppler, Karl-Heinz Horstmann, or Loek Boermans at the gliderport, offer
to pay for their launch--they have contributed much more than that to
the sailplane you are flying. (They will almost certainly graciously
decline, in any event.)

Dan Somers
Port Matilda, Pennsylvania USA