Aero-News HISTORY by Kevin R.C. "Hognose" O'Brien
Most everybody knows the
story of the P-51 Mustang, at least in broad
brush: asked by British buyers to commit to production of more
Curtiss Kittyhawks, or Curtiss's P-46 improved version (which found
no room on the New York company's production lines), the brash
executives and engineers of LA-based North American Aviation
sneered at Curtiss's data package.
They said they could design a better plane and build it -- and
do it in 100 days.
And in the process they incorporated aerodynamic and
manufacturing improvements that would make the Mustang, as the
grateful British would name this gift horse, the finest single-seat
fighter of the war -- once a British engine was added to the
stew.
Like most legends, this is mostly true. OK, they actually took
102 days -- and the plane had a wooden engine because, when they
ordered the engine from Allison. the nice folks in Indiana made
some jokes about California nutcases and forgot about it -- after
all, nobody designs and builds a fighter in three months.
(North American had to air-mail Allison pictures of the
completed-less-engine plane to get their attention... they had the
engine in another 18 days in that pre- airfreight world).
But after the war, aerodynamicist Ed Horkey discovered that he
might not be the original designer of the Mustang's airfoil... nor
were the NACA aerodynamicists who wrote the papers on laminar flow
that got his and chief designer Ed Schmued's attention.
It took a book by
Theodor von Karman (yes, THAT von Karman, of JPL space fame) in
1954 reviewing a paper by George Cayley (yes, THAT George Cayley,
the father of aerodynamics--pictured right) from 1837 to give the
guys that thought they were the originators of laminar-flow
airfoils a clue as to who the ultimate designer was.
Now, "laminar" means "smooth" as opposed to "turbulent," but the
term "laminar-flow airfoil" is a term of art with a specific
meaning in the aerodynamics world. A laminar-flow airfoil, like the
modified 45-100 on the prototype Mustangs, has its greatest
thickness further back than earlier designs. The "45" indicates
that the greatest thickness is at 45% of the chord; the earlier
airfoils used in US fighters like the P-40, and considered for the
Mustang, had their thickest point at about 23% of chord. This was
longstanding practice, developed empirically and reinforced by
wind-tunnel data.
But the laminar-flow foil could keep the air attached
considerably further back on the chord line of the wing. This
translated into less drag and more lift, speed, and so on -- the
Holy Grail of fighter design.
Horkey related: "Russ Robinson was a NACA aerodynamicist.... He
came by with [NACA's] Ed Hartman.... They said that NACA had tested
in a wind tunnel what was called a laminar-flow low-drag airfoil."
Horkey found that, if the NACA airfoil was made a little more
slender, it worked even better -- on paper. Finally he was able to
A/B compare his modified NACA laminar-flow airfoil to a model wing
made using the NACA 23000 family -- and after many iterations and
visits to various wind tunnels, the decision was done.
The laminar-flow airfoil along with the Meredith-effect radiator
made the Mustang outperform its peers on critical measures of speed
and range. And Ed Schmued (NAA's chief designer and the conceptual
designer of the Mustang), Ed Horkey, Ed Hartman and Russ Robinson,
along with the original NACA researchers, all had a right to be
proud. After all, they had found something totally novel and made
practical use of it.
Or had they?
Horkey, again, on his discovery in von Karman's book,
*Aerodynamics: Selected Topics* (Ithaca, 1954): "Sir George Cayley
in 1937... had taken a trout fish, cut it into sections, and taken
the ordinates of the periphery and divided them by three. When you
took the ordinates of the laminar airfoil and then put the trout
numbers as Xs, you found they were identical. God had invented the
laminar airfoil way before us! Did that ever make us humble!"
(Cayley had actually referred to what he called the "spindle" or
profile of the trout as early as 1809 -- even before the 1837
research cited by von Karman).
Ultimately then, the designer of the P-51 Mustang wing and the
Designer of the trout, (whether you take that to be the direct hand
of God or the impersonal hand of evolution) took separate paths,
perhaps, but arrived at the same destination: the laminar-flow
airfoil.
It's really hard not to close with: "And now you know... the
rest of the story!" But that closing is not ours.
So instead, we'll see you at the airport!