Above, In the late 1930s, the Special Committee on Future
Research Facilities proposed the construction at Langley of a wind tunnel with a 16-foot diameter test section that could
evaluate the cowling and cooling of full-sized aircraft engines and propellers. Approval for construction was granted in 1939,
and the new 16-Foot High Speed Tunnel (HST) became operational on December 5, 1941, just two days before the Japanese attack
on Pearl Harbor. Later in the war, in addition to engine cooling work, testing focused on wing/aileron/elevator flutter problems
and bomb/bomb fuse aerodynamics (possibly including the first atomic bombs according to several sources).
While the 16-Foot HST was never Langley's largest or fastest
wind tunnel, it did play an important role in the postwar evolution of tunnel design. In the late 1940s, Langley physicist
Ray H. Wright observed that the interference caused by wind tunnel walls could be minimized by placing slots in the test section
throat, a concept that came to be known as "slotted throat" or "slotted wall tunnel" design. Testing this new design in the
16-Foot HST, Langley engineers found that it allowed for transonic speeds (up to and beyond the speed of sound, Mach 1, approximately
761 mph at sea level). Retrofitted with a new slotted test section throat and re-powered to 60,000 hp, the facility was re-designated
the 16-Foot Transonic Tunnel (TT) in December 1950. Work on the slotted test section in the 16-Foot HST was instrumental in
Langley winning the Collier Trophy in 1951.
In late 1941, the first test in the 16-Foot High Speed Tunnel
was initiated on the Douglas XA-26 Invader configuration, as shown in the photo above.
One of the first life-threatening problems of heavier-than-air flying
machines demanded early attention from designers and pilots of the day—the dreaded tailspin. The international aeronautical
engineering community knew very little about the primary factors that influenced the spin or the relative effectiveness of
piloting methods to recover from spins. Langley’s staff initially evaluated the use of catapult-launched models in an
airship hangar which was about 105 feet high. After using the method to study spins, the Langley staff concluded that the
catapult technique was time consuming and resulted in frequent damage to the fragile models. They then constructed a 5-ft
diameter vertical tunnel in the late 1920s to provide for measurements of the aerodynamic loads on aircraft models during
simulated spinning motions. The model was not in free-flight, but was driven by an electric motor. Airflow in this early tunnel
was vertically downward, and tests were directed at the aerodynamics encountered in the spin.
Langley then proceeded to successfully design and construct a 15-ft
free-spinning tunnel in 1934 which permitted researchers to observe the spin and recovery motions of small free-flying balsa
models. Operations began in 1935. The airflow in the 15-ft tunnel was vertically upward to simulate the downward velocity
of an aircraft during spins. At the beginning of a typical test, the model was mounted on a launching spindle at the end of
a long wooden rod held by a tunnel technician. A tunnel operator increased the airspeed in the vertical tunnel so that the
air forces on the model equaled its weight. At this point, the model automatically disengaged from the spindle and continued
to float as the airspeed was continuously adjusted to maintain the model’s position at eye level in the test section.
The model’s control surfaces were moved from pro-spin settings to pre-determined anti-spin settings by a clockwork mechanism,
and the rapidity of recovery from the spin was noted. After the test was completed the tunnel airspeed was decreased and the
model was allowed to settle into a large net at the bottom of the test section. The model was then recovered with a long-
handled clamp and prepared for the next test.
In 1941 Langley began operations of a new 20-foot spin tunnel which
replaced the 15-ft tunnel and has been in continuous testing to the present day. The facility features a vertically-rising
test airstream into which free-flying unpowered aircraft models are hand-launched to evaluate spinning and spin recovery behavior,
tumbling resistance, and recovery from other out-of-control situations. The tunnel is a closed-throat, annular return wind
tunnel with a 12-sided test section 20-ft across by 25-ft tall. The test section air speed can be varied from 0 to approximately
85 ft/sec. Airflow in the test section is controlled by a 3-bladed fan powered by a 400-hp direct current motor located in
the top of the facility. The airspeed control system is designed to permit rapid changes in fan speed to enable precise location
of the model in the test section.
The models for free-spinning tests in the Langley 20-foot Vertical
Spin Tunnel are hand launched with a spinning motion into the vertically-rising airstream and a tunnel operator controls the
airspeed to stabilize the spinning model in view of observers. Data measured includes angle of attack, spin rate, and control
positions. Spin-recovery characteristics are determined by remotely actuating the control surfaces of the model. Images of
the stabilized spinning motions and the number of turns required for recovery are documented on high-resolution video. This
versatile testing facility is also used to determine the size and effectiveness of emergency spin-recovery parachute systems
for flight-test aircraft by deploying various parachute configurations from the spinning model. In addition, unconventional
maneuvers such as uncontrollable nose-over-tail gyrations known as “tumbling” can be investigated. In its recent
history the tunnel has also been used to evaluate the stability of vertically descending configurations including munitions
and parachute/capsule systems. The spin tunnel has supported the development of nearly all U. S. military fighter and attack
aircraft, trainers and bombers during its 68-year history, with over 600 projects to date.
The Douglas A-26 Invader (designated B-26 from 1948-1965) was a light
bomber and attack aircraft. It was retired from US military in 1972, and finally from the Colombian Air Force in 1980. The
project made use of the then-new NACA 65-215 laminar flow airfoil for the wings. The aircraft first flew in July 1942 but
there were problems with engine cooling which led to cowling changes and omission of the propeller spinners on production
aircraft, plus modification of the nose landing gear after repeated collapses during testing.
Project #98 in September 1942.
Above, In a concerted effort to achieve drag
reduction extended engine nacelles and spinners were installed.
These Invaders were
tested at Langley for three months in 1944.