X-4 Bantam 46-676 was viewed by some as the black sheep of the X Plane family because she was so small and flew relatively slowly. The first of two X-4 aircraft, she was built to explore the transonic speed regime, as were many research aircraft of the day. However, this was different; she lacked a horizontal stabilizer. Engineers thought that removing the conventional horizontal surface from the tail might solve the problem of shock stall and drastically reduce parasite drag. They were wrong. Northrop did (accidentally) discover some important aeronautic properties during later X-4 flight test. To control pitch, elevons on the wings replaced the missing elevators, which would conventionally be housed on the (missing) horizontal stabilizer. These elevons could split upward and downward into speedbrakes/flaps. This design was trusted to Northrop because of their work on a series of tailless flying wing aircraft including the YB-49.
Some folks saw a black sheep, but when Northrop Test Pilot Charles Tucker first laid eyes on the aircraft while it was being built, he thought it was beautiful. This initial impression would be spoiled as he watched engineers build flaws into the aircraft. To control the elevons, the X-4 contained the same hydraulic actuator system used in the enormous YB-49 bomber. The system was not optimized for a small research aircraft and caused huge amounts of stick friction which made fine control of the aircraft completely impossible. These fine stick movements would be essential on the first flight, because Northrop engineers unknowingly misplaced the center of gravity (CG) of the aircraft; they forgot to account for the weight of fuel and crew. This flaw created an enormous longitudinal instability which would cause the aircraft to pitch wildly. Of course, this was all unknown to Charles Tucker, who was slated to conduct the first flight. He was preoccupied by another engineering problem that cropped up during taxi tests. Northrop used an electric motor to drive the rudder, which responded far too slowly for decent yaw control at low speed. Tucker was forced to use the differential wheel brakes to keep the aircraft running in a straight line on the ground. If he tried to use this crippled rudder to steer the aircraft, he would end up in a slow oscillation back and forth, swerving as the control surface tried to keep up with his corrective inputs.
The day finally came for Tucker to fly this mess; December 15, 1948. Tucker began rolling across Rogers Dry Lake at Muroc Air Force Base (now Edwards). He pulled back on the stick to lift off and the aircraft pitched up wildly, shooting up high into the air. To avoid a stall, Tucker pitched back down, but the aircraft entered a steep dive, screaming toward the ground. He recovered from the dive, only to find himself climbing again. The X-4 was extremely sensitive in pitch, which is characteristic of any aircraft with an incorrect CG. The problem was compounded by Tucker’s inability to apply fine control to the aircraft due to the extremely heavy stick friction. Tucker considered ejecting, but he was miraculously able to null this oscillation. Tucker used both hands on the stick, jamming his knees against his hands to keep the aircraft stable. Any slight movement would send the aircraft into another oscillation.
Luckily, two elements came together to save this flight. First, Tucker was an experienced test pilot who kept his cool in the face of danger. Nobody would have criticized Tucker for bailing out of a completely unstable aircraft. Second, the test was operating over Rogers Dry Lake, which is a natural aerodrome, a safe surface for landing stretching across the ground in all directions. Tucker turned around (which took 30 minutes of intensive concentration to refrain from over-controlling this unstable bird), heading back toward Rogers Dry Lake. He reluctantly pulled one hand away from the stick to gradually reduce the throttle. The power was reduced bit by bit until the X-4 settled on the lakebed, safely back on the ground. Tucker informed the engineers that they had some work to do (this is a very diplomatic way of saying that he reamed them a new one).
Changes were made to the aircraft including the addition of lead weights in the nose to correct the CG. The elevon control cable tension was reduced to lighten stick friction and the rudder’s electric motor was replaced with a conventional cable control system. These corrections fixed the instability, but problems cropped up in the engine and fuel system in later missions. Because of these collective issues, X-4 46-676 was permanently grounded after making only ten flights. She was used for spare parts when the second X-4 began to operate. After the program was over, she was put on display at the Air Force Academy in Colorado Springs, Colorado. Because this aircraft was so small and light, students would commonly move the aircraft at night, leaving it in different areas on campus as a joke. She was stored outside for decades and the weather took its toll. In August 2012, the aircraft was restored with a fresh coat of paint. Our X-4 was transported to the Air Force Flight Test Museum on Edwards Air Force Base in California, where she stands with more dignity now, never again be tormented by engineers, maintenance problems, weather or students.