123454 THE AVIATION HISTORIAN Issue No 22the heavily-faired Bellanca 28-70, Irish Swoop, that
had been assembled at Eastleigh before the 1934
MacRobertson air race. Shenstone’s connections
with George Klein, in addition to his visit to
Langley at a crucial time, as well as an alleged
meeting with Jack Northrop and von Kármán in
Los Angeles, suggest that the influence was in
fact Shenstone’s.
The Spitfire was an example of exactly the right
fairing to counter the double-aerofoil effect. Not
only did it avoid the interference burble, it also
contributed to the most remarkable aspect of the
Spitfire’s aerodynamics — the aircraft could be
dived to Mach 0·86.The “smoking gun”
Why this was so was laid out in a much-delayed
Langley research paper by James B. Delano, who
showed that poor wing-root design could lower
critical Mach, and, by implication, limit how fast
an aircraft could travel.^15 This research had been
conducted before the war, and was submitted by
Delano on September 11, 1939. There was more
than a three-year delay, however, before it was
published in February 1943; whether this shows
that the critical importance of Delano’s insight
was appreciated at the time, and thus the results
were kept secret, or whether it was just put to the
bottom of the pile remains unknown.
In Delano’s experiment, pressure variations
over a wing with a t/c ratio of 15 per cent in a
classic teardrop fuselage/wing combination
were measured at various speeds. George Klein’s
“Method 1”, a mid-mounted wing position, was
employed to reduce interference effects, but the
fuselage still retained a degree of longitudinal
convex curvature.
Delano stated that “the results indicate that
the [critical Mach] of the fuselage is decreased
because of the velocities induced by the wing.
The fuselage interference will likewise increase
the velocities over the wing and decrease the
critical [Mach] of the wing”. He also made it clear
that the critical Mach of an aircraft could thus bepredicted by looking at the pressure differentials
over the wing root — not the wing.
This was a simple extension of an observation
made as early as the 5th Volta Scientific Confer-
ence at the Royal Academy of Science in Rome in
1935, where Eastman Jacobs of NACA presented
the world’s first scientific observations of super-
sonic shock behaviour (to an audience including
Dr Ing Adolf Busemann, of whom more later).
Jacobs was able to show that “the critical Mach
number could be increased by shape changes
which could be determined through observation
of burble in low-speed tests”. Eastman went on to
show mathematically how low-pressure peaks in
suction on the upper surface were related to
Mach number; essentially, “if it burbles at low
Mach, it will hit critical Mach early”.^16
Delano’s work was the “smoking gun”. Now
root interference, drag, burbles, separation and
compressibility were tied together with the
common theme of pressure gradients. Delano
(about whom little appears to be known) also
went on to show something else. As the Mach
number is increased, so is the pressure gradient
over the wing/fuselage junction. This totally
subsonic effect of compressibility begins to
become visible on a graph as low as Mach 0·3,LEFT The Supermarine Type
224, built to Specification
F.7/30, was fitted with a fillet
and tested with wool tufts to
measure airflow separation.
Curiously, the aircraft’s
anhedral on its inner wing
section and flat sides would
probably have made these
fillets redundant in any case.BELOW While reading
Muttray’s work, Beverley
Shenstone would have seen
this diagram of an “ideal”
wing/body combination
based on a Joukowski
aerofoil. The planform of the
wing probably influenced
PHILIP JARRETT COLLECTION Shenstone’s Spitfire work.
VIA AUTHORVIA AUTHOR