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Flight
History
(Part Two)
October 24th, 1964,
represented the Valkyrie's last flight for a while.
Returning to North American's Palmdale facility at the end
of this flight, the XB-70 was scheduled to go through a
series of structural tests that would not return her to the
flight line until February of 1965. For the first time, the
wingtips were lowered to the middle position of 25 degrees.
Reaching a maximum speed of Mach 1.4, the
XB-70 remained
supersonic for 40 continuous minutes, establishing a new
world's record for sustained supersonic flight. The overall
time of the flight was cut short, as fuel consumption proved
to be higher than expected.
Upon returning to
Palmdale, engineers puzzled over why the Valkyrie had been
shedding herself of paint on all but her first flight.
Although not overly concerned, no one was especially happy
with unexplainable things on a plane that was to test the
limits of jet-powered flight! Although some observers
wondered if the Air Force was being entirely truthful about
the XB-70's top speed, it was finally determined that
too-thick paint caused by several re-paintings (in order to
pretty the plane up for various VIPs) was being cracked as
the Valkyrie flexed in flight, and was then torn away by the
airstream. During her winter stay at Plant 42, the XB-70 was
carefully repainted with just a single, thin coat of white
paint.
AV/1 returned to
flight on February 16th, 1965. This flight would return the
XB-70 to Edwards AFB (about 30 miles by air from North
American's Palmdale facility). For the first time, the
wingtips were lowered to the full 65 degree mark.
Accelerating to Mach 1.6, Joe Cotton then adjusted the AICS
(Air Induction Control System) -- The series of ramps inside
the intakes which would expand and contract to manipulate
airflow to the engines and protect them from the deadly
onslaught of high-speed air. After another 40 minutes of
supersonic flight, the Valkyrie landed, but with the drag
chutes failing to deploy, the landing run was over 11,100
feet.
The sixth flight was
the first time someone other than Al White and Joseph Cotton
were at the controls. Lt. Col. Fitzhugh "Fitz" Fulton (later
to be NASA's Chief Test Pilot) flew as copilot with Al White
as pilot. But again, hydraulic leaks cut this flight short.
Engineers at North American worked continuously to modify
the hydraulic systems (which operated at 4000psi, more than
35% greater pressure than any other aircraft's system) to
end the constant problems of leakage that had disrupted
almost every flight. Although these changes were never
totally effective on AV/1, AV/2 benefited greatly from the
misfortunes suffered by the first plane.
The seventh flight,
on March 4th, White and Fulton shattered records again,
sustaining supersonic flight for 60 minutes, reaching a top
speed of Mach 1.85. The eighth flight introduced the last of
the four pilots, North American's Van Shepard, to the
aircraft, and saw Mach 2 fall to the six J93 engines. On the
tenth flight, the Valkyrie sustained 74 minutes of
supersonic flight, including 50 minutes beyond Mach
2!
May 7th, 1965 was
AV/1's 12th flight, with Al White and Fitz Fulton at the
controls. Travelling at Mach 2.58 (almost 1700 mph!), a
'thump' was heard in the cockpit, soon followed by a number
of engine-related alarms.
Engines three, four,
five, and six were shut down right away. As the chase planes
caught up, they reported that the horizontal splitter (the
very apex of the delta wing) had torn away. Obviously, the
debris had gone into the intakes and done severe damage to
the engines. For the final approach, the number five engine
was restarted to provide some thrust from the right side,
and the XB-70 landed on the long lake bed without major
incident. All six engines, nearly one-sixth of the 38 ever
built, were destroyed beyond repair. After this flight, the
splitter itself was replaced with a single solid piece in
place of the honeycomb unit that had failed. At this point,
concerns about the integrity of the honeycomb skin began,
and the next 4 flights concerned themselves with "heat
soaking" the skin for sustained periods of time. For the
first time, the XB-70s ability to reach Mach 3 was
questioned.
AV/2 (tail number
20207) made its first flight on July 17th, 1965. Lacking the
gremlins of AV/1's first flight, the wingtips were lowered
65 degrees, and a top speed of Mach 1.4 was reached before
landing at Edwards AFB. AV/2 differed from her older
sibling, inside and out. Internally, a more flexible system
of hydraulic fittings were used, to reduce the leaking that
continued to trouble AV/1. The techniques to build the
honeycomb skin and improved with experience, and hopefully
would not suffer any of the troubles AV/1 was undergoing.
Learning from experience, the fuel tanks on AV/2 were
inspected very carefully, to avoid the internal leakage
problems which ultimately rendered AV/1's number five
(centreline fuselage) unusable. An automatic control for the
AICS was installed, in place of AV/1's manual system, which
used a grid of speed and altitude that the pilots used as a
reference to set the actual ramp position.
Externally, the
difference was obvious -- the wings had been given 5 degrees
of dihedral. This change was a result of testing that showed
AV/1 to have poor roll stability at high speed with the
wingtips fully lowered. The tradeoff was that, at low speeds
with the wingtips up, AV/2 suffered from severe "dihedral
effect" -- a situation where sideslip causes the plane to
drop one wing. The pilot, sensing the low wing but not
spotting the sideslip, would use the ailerons to try and
bring the wing back up -- which actually caused MORE
sideslip, forcing the wing to drop lower! (this is a very
difficult thing to visualise -- it took me several hours to
get it straight in my head) One pilot remarked "it felt like
being backed into a corner." The only solution was for
pilots to closely watch the sideslip indicator.
Another
instrumentation problem cropped up during high-speed flight.
At Mach 3, just one degree of pitch would send the XB-70
into a 3,000 foot per minute change of altitude. Combined
with raising the nose ramp (windscreen assembly) to the
"supersonic" position, which blocked the pilots' view of the
horizon (and tended to reflect the ground below, leading to
several complaints of vertigo), the Valkyrie was virtually
impossible to keep at a fixed altitude. Eventually, a highly
sensitive rate-of-climb meter from a helicopter was
installed to help (but not entirely fix) this
problem.
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