It was obvious that a computer-controlled, fly-by-wire system was needed.
There was no manual backup system because it was impossible for a pilot to control the plane without the computer. To reduce risks, it was decided to use a proven off-the-shelf system. The F-16's fly-by-wire system was selected; actuators, flight-control computer chassis, and power supply were modified slightly. New computer programs had to be developed.
The control system was designed so the Senior Trend would handle like "an ordinary plain vanilla aircraft." Programming was tested on an NT-33, "by real pilots flying in a real airplane in real turbulence." Some flight tests assumed the directional stability of the Senior Trend was even worse than predicted.[408] It would prove to be a wise precaution.
Despite the Skunk Work's best efforts, by the summer of 1980 the project was behind schedule, and the first flight was nowhere in sight. Each day seemed to bring new problems and no solutions. Ben Rich said years later that this was the low point of his life. The meetings went from before dawn and continued long after dark. In the midst of this, Ben Rich's wife, Faye, died of a heart attack, leaving him emotionally devastated. When he returned to work, Alan Brown, the Senior Trend program director gave him a note. Written on it was Rich's next birthday "June 18, 1981." When Rich asked him about it, Brown said, "That's the date we test-fly the airplane."
He continued, "The date is firm. In granite. Count on it."[409]
By the fall of 1980, aircraft 780 was beginning final assembly at Burbank. By early June 1981, final checkout was completed. The wings were removed and crated. The fuselage was covered in a shroud, and a wooden framework was added to the nose to further hide its shape. Under cover of darkness, number 780 was loaded aboard a C-5 and flown to Groom Lake.
Upon arrival, 780 was taken to a hangar at the south end of the flight line and reassembly began. Even here, security remained paramount: a camouflage net was stretched across the open hangar door. Once assembly was complete, static engine test runs were made. The plane was kept inside the hangar, with the exhaust vented out the open door.[410]
When delivered, 780 was a dark gray color. Before the first flight, patterns of light blue and light brown were painted on the aircraft. As with Have Blue 1001, this three-color finish was meant to hide the faceting. (780 was the only one of the FSD aircraft to be camouflaged.) The paint finish was ragged looking and appeared to have been hurriedly done.[411]
On June 18, 1981, just as Brown had predicted during the dark days of nearly a year before, everything was ready. As dawn broke, 780 was rolled out of its hangar for the first time. Hal Farley ran up the engines, and 780 started down the runway. As with the first hops of both the U-2 and A-12, it was to be an eventful flight.
It was a difficult task to design an air-data system that was stealthy. Four probes extended from the nose, along with a conventional boom. Because the air-data probes had shown erratic readings during ground vibration testing, it was decided to ballast the aircraft to a far forward center of gravity point, turn off the angle of attack, and sideslip measurements to the flight-control system.
Immediately after 780 lifted off the runway, it became apparent to Farley that the directional stability was much worse than predicted. Farley immediately switched on the sideslip feedback to the flight-control computer. The plane's handling "stiffened up," and the rest of the flight was routine. Subsequent analyses indicated the Senior Trend's directional stability and directional-control power were less than predicted. The solution was to increase the area of the fins by 50 percent. The new fins were installed by the fall of 1981. This cured the instability but would cause other problems later in the test program.[412]
Two more test pilots soon joined the program — Skip Anderson (air force) and Dave Ferguson (Lockheed). After a few months, 780's desert camouflage was removed, and it was repainted light gray. The plane had no national markings (in common with most of the other Groom Lake aircraft), but "Hal," "Skip," and "Dave" were painted on the canopy rail.[413]
Between mid-1981 and early 1982, the other four FSD Senior Trend aircraft were delivered to Groom Lake. While the first two (780 and 781) were aerodynamic test aircraft, the other three (782, 783, and 784) were systems aircraft. As such, they had the full set of cockpit displays, just as on the operational aircraft. It has been reported that 782 and 783 had the Skunk Works emblem on their tails. The fifth FSD aircraft (784) reportedly sported a full-color painting of Elliott, the dragon from the Disney film Pete's Dragon. This was in honor of Col. Pete Winter, air force commander at Groom Lake. (For the uninitiated, Elliott was invisible to everyone except Pete.)[414]
Initially, the FSD aircraft were painted gray. The commanding general of the Tactical Air Command then ordered they be painted black. (Although most of the test flights were done in daylight, the aircraft's operational missions were flown at night.) Markings became more formaclass="underline" national insignia, "USAF," and the aircraft number on the tail.[415]
FLYING ON THE EDGE
With the full complement of FSD aircraft on hand, along with additional test pilots, the test program got under way in earnest. The flight-test program explored a number of unknowns related to the Senior Trend's stealth design.
Many test hours were required to calibrate the air-data system for angle of attack, sideslip, airspeed, and altitude. A critical part of the flight control system was the angle of attack limiter. Angle of attack (AOA) refers to the angle between a plane's wing and the direction of the airflow. Wind-tunnel tests and free flights of unpowered models indicated the Senior Trend would pitch up at high angles of attack. (The nose would abruptly and uncontrollably rear up.) The aircraft would then enter a "deep stall" and would not be recoverable. The AOA limiter would have to automatically move the control surfaces to prevent the aircraft from exceeding the critical value.
Because of the risk, the AOA testing was done in slow steps. There were literally hundreds of individual tests run.
Validation of this approach came on May 23, 1983. One of the FSD aircraft was on final, with its left wing low, when a strong wind gust hit it.
This caused the AOA and sideslip to instantaneously reach levels higher than any tested — higher, in fact, than could be tested in the wind tunnel.
The AOA limiter countered with full down eleven in less than 0.4 seconds and moved the fins 90 percent of their full travel. The plane successfully recovered.[416]
Flutter testing was also prolonged. The early tests showed no problems, but during a weapons compatibility test, an air force test pilot put the aircraft into a sideslip while flying at near maximum speed. The left fin underwent "explosive flutter" and disintegrated. The pilot made it back to a successful landing despite very poor stability. Farley called it, "a very professional response by a real pro."
The problem was traced to the redesign of the fins — the added area had reduced the fin's stiffness. The problem had been hidden during the earlier tests by the friction of the fin bearing.[417]
One of the more unusual problems was testing the Senior Trend's inlet grids. There was some concern they could distort the air flow to the engines. In fact, they acted like "flow straighteners," giving the engines a constant flow of air.
More serious was grid icing. Tunnel tests indicated that, in Farley's words, "the inlet grids not only looked like a giant ice cube tray, but acted like one as well." A wiper system and alcohol dispenser was developed.
408
Harold C. Farley Jr. and Richard Abrams, "F-117A Flight Test Program,"
410
"We Own the Night,"
414
Bill Sweetman and James Goodall,
415
"We Own the Night," 13. Ben Rich, who preferred gray, explained that, "The Skunk Works plays by the Golden Rule: he who has the gold sets the rules! If the General had wanted pink, we'd have painted them pink."