...model was trimmed so that the lift force balanced
the weight of the model. Pitch trim was maintained as the tunnel speed varied
by remote adjustment of a tab on the horizontal tall. Limited rolling freedom
was provided by looseness in the gimbal joint and flexibility in the mounting
rod. The model was constructed in such a way as to simulate the stiffness
and mass properties of the aircraft and, accordingly, was quite complex
and expensive to design and build.
The technique illustrated in figure 12.7 was successfully
employed in the development of the B-47 as a means for identifying flutter-critical
combinations of speed and altitude and development of design fixes for flutter
avoidance. A detailed description of the technique is given in reference 173. The mounting rod limits the usefulness of the technique to
fairly low subsonic speeds because of aerodynamic interference effects associated
with the formation of shock waves on the rod at high subsonic Mach numbers.
The complete model flutter tests made on the B-47 were carried out in a low-speed
wind tunnel, and the results were then adjusted for estimated Mach number
effects. Later techniques developed by NACA and NASA allow flutter tests of
complete airplane [368] models to be made at high
subsonic and transonic Mach numbers in a wind tunnel especially designed for
high-speed flutter investigations.
Along with this rather general discussion of various
engineering aspects of the B-47, a few more specific features of its design
should be pointed out. The aircraft was manned by a crew of three. Two pilots
sat in a tandem arrangement under a bubble-type canopy in a manner similar
to that of a fighter; a bombardier-navigator sat in an enclosed compartment
located in the nose of the aircraft. Upward-firing ejection seats were provided
for the pilots, and the bombardier was equipped with a downward-firing ejection
seat. Crew compartments were heated, ventilated, and pressurized. As fast
or faster than most fighters, the Stratojet was equipped with only two 20-mm
cannons situated in a remotely controlled turret located in the tail of the
aircraft. Aiming and firing of these guns was the duty of the copilot whose
seat could be rotated 180° to face rearward.
For assistance in the landing maneuver, the B-47 was
equipped with a drag chute that was deployed during the approach. The added
drag of the parachute aided in controlling the speed and the flight-path angle
during this phase of the landing maneuver. Once on the runway, a large braking
chute was deployed to assist in stopping the aircraft. An interesting insight
into the airport performance of the B-47 is provided by a comparison of its
stalling speed of 175 miles per hour with the cruising speed of 182 miles
per hour given in chapter 4 for the World War II B-17G. Not surprisingly, the length of
hard-surface runways at military air fields increased dramatically in the
years following World War II.
Although the B-47 was equipped with six 7200-pound-thrust
(with water injection) turbojet engines, the thrust-to-weight ratio at maximum
gross weight was only 0.22, which, coupled with its high stalling speed, resulted
in a long takeoff ground roll. To reduce the takeoff field length, the aircraft
was initially equipped with 18 short-duration booster rockets. These units
were an integral part of the aircraft and were known by the acronym JATO (jet-assisted
takeoff). Nine JATO nozzles were located on each side of the fuselage; they
may be seen in figure 12.4 just ahead of the national insignia. On some later
versions of the aircraft, weight was saved by replacing the integral JATO
units with a jettisonable external rack containing the rockets. In this installation,
33 rockets of 1000 pounds thrust each were provided. A B-47 taking off with
a boost from the external JATO pack is shown in figure 12.5.
[369] By post-World War II standards,
the B-47 was classified as a medium bomber; but with a gross weight of 198
180 pounds, the B-47E was far heavier than any bomber flown in World War II
(the gross weight of the B-29 was 120 000 pounds), and it ranked second only
to the 357 500-pound B-36D as the heaviest aircraft operated by the USAF in
the early 1950's. Designed as a strategic bomber, the B-47 also filled various
other roles such as photoreconnaissance. In its design role as a strategic
bomber, the B-47 could deliver a 10 845-pound weapons load at a mission radius
of 2013 miles. Ferry range was 4035 miles (table VI). With air-to-air refueling, which became standard operating
procedure following the close of World War II, both the mission radius and
the ferry range were greatly increased, and targets in Eastern Europe could
be reached from bases in the United States with sufficient range potential
to allow safe return to friendly territory.
In the strategic bombing role for which the B-47 was
designed, weapons delivery at the target was originally intended to take place
from high altitudes. By the mid-1950's, however, the increasing effectiveness
of methods for detecting aircraft at high attitudes, as well as the growing
capability of surface-to-air missiles and fighter aircraft, required the development
of new methods of weapons delivery. As a means of avoiding detection by radar,
penetration of enemy airspace was to take place at high speed and at an altitude
of only a few hundred feet. At the target, the aircraft was to execute an
Immelmann turn with weapons delivery taking place in near vertical flight.
(An Immelmann turn consists of a half loop followed by a half roll from inverted
to normal flight attitude at the top of the loop. A change of 180° in
direction coupled with a gain in altitude are accomplished during the maneuver.)
This method of weapons delivery was known as LABS (lowaltitude bombing system)
and was intended to provide the aircraft a means for escaping destruction
from the blast effects of its own weapon.
Constant practice of the LABS technique subjected the
B-47 fleet to the severe gust-load environment of high-speed low-altitude
flight, as well as the maneuver loads associated with weapons delivery. The
aircraft was not designed for this type of service. As a consequence structural
fatigue problems were encountered, and several aircraft were lost as a result
of structural failure. At one point, the entire B-47 flee was grounded for
inspection and incorporation of necessary design modifications.
[370] Both the structural fatigue
problem and the much greater capability of the Boeing B-52, which began entering
the inventory in 1955, played a part in the retirement of the B-47 from first-line
service. Its life with the Strategic Air Command began in 1951 and ended 15
years later in 1966.
The Boeing B-47 should be viewed as a landmark aircraft
that established a new technical plateau for large, multiengine, jet-powered
aircraft.