The Aviation Historian — January 2018

52 THE AVIATION HISTORIAN Issue No 22

on convection for meteorologists throwing in a

line explaining that thunder is just the sound of

clouds bumping together.

Many of these “Occam’s Razor” failures are re-

interpretations of a 1928 paper by the eminent Dr

Ing H. Muttray of the University of Göttingen.^10

Muttray was working in a windtunnel that

was essentially a room in a large townhouse in

Bunsenstrasse. Muttray’s original 1928 paper,

published in the journal Luftfahrtforschung,

presented a mainly circular argument about

fuselages needing to mimic wings in order to

reduce interference drag. Nevertheless, a 1929

English version published by NACA as Tech Note

517 — Investigation of the Effect of the Fuselage on the

Wing of a Low-wing Monoplane — includes a clear

passage by Muttray referring to the “diffuser”

effect of a converging fuselage over a wing,

and an early diagram of fillets that increased or

decreased this effect.

In January 1932 a different Klein, the USA’s

Arthur L., published The Effect of Fillets on Wing-

Fuselage Interference, in which he demonstrated

that an “expanding fillet” that worked to close

the gap and prevent sudden divergence and

longitudinal change in pressures was a solution to

drag and buffet.^11 In this he was clearly working

from Muttray’s 1929 paper, using the newly-

constructed 10ft windtunnel in Pasadena, which

had been sponsored by millionaire philanthropist

Harry Guggenheim. The latter had decided to

invest in what he called the “long-haired men” of

the California Institute of Technology (Caltech) as

the future of aviation, rather than the “practical

men” who had gone before.

Designing the future

Arthur Klein collaborated with departmental

head Clark Millikan (son of the co-founder of

Caltech) and Hungarian aerodynamicist Theodore

von Kármán (also paid for by Guggenheim) to get

the windtunnel approved and built, the tunnel

being based on the principles of the Göttingen

Windkanal-Haus and ideas imported with von

Kármán, who had worked there. As Caltech

was working alongside industry, this had an

immediate effect on design (unlike anything

produced by Ower and the NPL in Britain).

The model Klein used was a simplified

Northrop Alpha (with a thick Clark Y aerofoil

identical to that of the Hurricane). Klein was

quick to discover that it had what he termed the

“wrong” fillet (based on preventing collisions

of air) and was equally quick to diagnose the

“right” or optimum kind, based on the reduction

of longitudinal pressure changes. This was first

“flown” in the Caltech windtunnel in May 1931,

and was attached to the Northrop Beta that June.

The next project for Arthur Klein and Millikan

was the well-filleted Douglas Commercial series,

beginning with the DC-1. Jack Northrop and many

of the Gamma design team collaborated on this

ABOVE With a wing and fuselage

geometry similar to the Junkers-F 13, the

one-off McDonnell Doodlebug, the wing/

fuselage junction of which is seen here

with no modification, was selected by

NACA to explore the effect of various

interference-reducing measures.

LEFT The Doodlebug mounted in the
full-scale windtunnel at NACA’s Langley
establishment in 1934. As predicted,
the large wing fillets seen here reduced
buffeting considerably. The improvement
in the aircraft’s drag was considered
VIA AUTHOR almost incidental.

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