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!

FMI: www.mustangsmustangs.com, www.stallion51.com, www.aerodyn.org, www.ctie.monash.edu.au/hargrave/cayley.html