The massive success of Boeing in the 1930s, which has been followed by
almost a century of the company being a household name in the field of Aviation,
can trace itself back to a little known aircraft known as the Boeing Monomail.
In the late 1920s Boeing was faced with a big problem: they were falling behind in
the development of aviation technology. For most of the decade, aircraft design had changed little
from those that were pioneered during the First World War. In the post-war commercial industry,
any attempts to innovate were often considered prohibitively expensive, and the more conservative
branches of the military preferred the tried-and-tested design of the biplane.
However, things were slowly changing. The concept of the monoplane was gradually gaining
more favour, and this was considerably increased in the latter half of the decade by a series of
record-setting flights, ones that proved without question the greater efficiency of the monoplane
design. While the manufacturers who were mainly in the commercial sector rushed to this new design
principal, those who catered to the military were left behind: Boeing was among them.
Though trimotor monoplanes had begun to enter service in 1926, it would not be until 1929
that Boeing had enough influence to propose its own monoplane design to the Army. Now three years
doesn’t seem like a lot, but during the interwar period, when aircraft were evolving rapidly,
this was a considerable amount of time to fall behind. Boeing’s proposal was the model 96,
a single-engine fighter that was to demonstrate the benefits of an all-metal monoplane,
and it was promising enough to have a prototype ordered as the XP-9.
The construction of the XP-9 was entirely new to Boeing practice. The fuselage was the
most advanced feature. It used a semi-monocoque structure of sheet duraluminiun over metal formers
for the rear section, and welded steel tubing from the point of the undercarriage forward. The
two-spar wing was simpler, using a metal framework with a fabric covering, and its high-mounted
arrangement made it look a bit like a biplane that had had its lower wing ripped off.
The XP-9 was the first Boeing monoplane to start construction in the factory, but various delays
postponed its scheduled delivery date from April 1929 to September 1930. Because of this it would
not become the first Boeing monoplane to fly. During this time, construction had already
progressed rapidly on the Boeing Model 200, and much of the lessons learned from building the
XP-9 were absorbed into its construction. Still, the XP-9 flew for the first time on
November the 18th 1930…and it was immediately disappointing. Problems with lateral control were
quickly observed, and the aircraft was quickly modified with a larger tail. Though an option to
build five more as the YIP-9 for service testing had been considered, it was felt that the aircraft
was good for an experimental exercise, but not so much for the Army Air corps. And so The XP-9
was relegated to use as a non-flying instructional airframe after accumulating just 15 flying hours.
The Boeing Monomail, on the other hand, had considerably better luck.
Known internally as the Model 200, the Monomail, as suggested rather obviously by its name,
was designed to join Boeings fleet of mail carriers, though it was also designed with
regular cargo transport in mind as well. When compared to the Model 40, which was Boeing’s
principal mail transport at the time, the Monomail was a rapid departure from anything
that had come before it. In the quest for improved performance, the aircraft introduced
several radical design changes that, while not unique to Boeing, would take the company from
the archaic designs of the first world war and propel it towards the first so-called
Golden Age of air travel that was the 1930s. Though the switch to a monoplane configuration was
the most obvious design change, this alone would not give the Monomail the performance increase
that set it apart from its predecessor. Extra performance was obtained through a variety of
other aerodynamic refinements. An anti-drag ring (or cowling) was added to the engine
-something that was rapidly becoming the norm-, the wing was built as a low-mounted cantilever,
removing the need for the thick struts of the XP-9s parasol-style wing, and a
retractable undercarriage meant that only half of each wheel protruded into the airstream.
Along with these innovations, the construction methods used to build the monomial also helped
to improve streamlining as well. The use of a semi-monocoque, all-metal fuselage allowed the
use of a more circular cross-section, and while this fuselage was somewhat based on the XP-9,
the rest of it took lessons from various other designs as well. Most importantly, the wing,
though a monoplane configuration, shared much of its construction with the tried-and-tested
Model 80A biplane. The two spars were built up from square-section duraluminiun tubing,
which themselves were riveted together to form a warren truss. These were joined by
additional Warren truss ribs that supported lower and upper sheets of corrugated metal,
which themselves supported a smooth layer of metal that formed the skin of the wing.
Now, all of this meant that the Monomail should have blown away the competition with its improved
performance, but during its first flight on May the 6th 1930, its performance could
best be described as a modest improvement at best. And there were a few reasons for this.
While the Monomail was a radical design, when compared to those that immediately preceded it,
it was also still remarkably conservative. This was partly due to ingrained design
thinking in the aviation industry, and partly due to the limitations
of the current state-of-the-art technology. The monomial still adhered to the old-school
template of the standard cargo plane. Its pilot was still situated in an open cockpit,
free to enjoy the wonderful comforts of icy winds in winter, and bug-filled skies in summer,
the cargo space was relatively small and situated over the centre of gravity,
and it only made use of a single engine. Though this engine was relatively powerful,
being a 575hp Pratt & Whitney Hornet, it was severely limited by the aircraft’s fixed pitch
propeller. This could only be configured on the ground, which meant that if the monomial was to
be configured for fast, efficient, level-flight, the propeller was woefully inadequate for take-off
performance at ground level. Unfortunately for the monomail,
the technology required to achieve its full potential was not quite ready. Not only were
things still being developed, but at such an early juncture it was also incredibly expensive,
which would have made the monomail unappealing to potential buyers. It was briefly considered
by the Army as a potential light transport, receiving the temporary designation Y1C-18,
but its poor take-off performance soon removed it from consideration. Because of this only
two Boeing Monomails would be built. The second Monomail was built not long after
the first, and it flew for the first time in August of 1930. Known as the Model 221,
It differed from the first in that it was designed primarily as a passenger transport, with cargo
transport being a secondary consideration. Its fuselage was slightly extended by 8 inches (20
cm) to allow the installation of a cabin that could accommodate six passengers. Following
its completion and certification, it was put into service on Boeing’s air transport routes.
Not much is known about its first year of use, but at some point the 221 was taken back to
the facotory and modified so that it could carry an extra two passengers. The fuselage
was lengthened by 27 inches (68cm) to allow the installation of the additional seats, and at the
same time the original model 200 Monomail was also converted to this new configuration.
For a while, the original Monomail was used for a brief experimental test program.
Here it would help develop key technologies that would become commonplace in the aviation industry
in the following years; it completed several tests that compared the effectiveness of streamlined
fixed undercarriages, against the new retractable types; and more importantly, it was used to test
new trailing edge tabs for elevators, better known as trim tabs. The current practice was
to mechanically alter the setting of the entire horizontal stabilizer, but these new tabs greatly
simplified the problem of trim correction, and it was to be a system that became standard for most
aircraft until the dawning of the Jet age. Eventually, both monomails found themselves
in air service once again, flying various trans-continental air routes,
but this service was over after just a few short years. Not only did the Monomail still present a
challenge during take-off and landing, something that was distinctly exacerbated by the lengthening
of the fuselage, which put even more of the aircraft in front of the cockpit, but it was
also being made rapidly obsolete. Variable-pitch propellers were becoming available, which would
have helped its performance, but these were going to be installed on newer, multi-engine transports
instead, and the Monomails time was at an end. They were retired in 1933, after just two years
of service. But they were replaced by another Boeing design that had learned much from their
development. The Boeing 247 represented a true leap-forward in aircraft design, featuring a
semi-monocoque construction, retractable landing gear, trim tabs, de-icing systems,
and even an autopilot. The lessons of the Monomail were learned so quickly that Boeing
went from lagging behind aviation technology, to leading many of the design trends of the 1930s,
which in turn brought the company great success and international prestige.
As always, thank you all so much for watching, and a big thank you of course to the patrons.
Just a friendly reminder that I’ll be mostly away from the 4th – 20th of January, but hopefully
there should be a couple of videos going up in the meantime, but if anything changes, I’ll post
an update on here anyway. A big thank you as well to our Wing Commander tier patreon supporters, and
big shoutout to Michael Grajek and iamthestorm29, who are the newest members of the channel’s
highest tier supporters. Thank you all so, so much, and I’ll catch you all next time. Goodbye.